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Lecture notes Information Theory Coding
information theory and source coding lecture notes and information theory and coding lecture notes nptel pdf free download
Source Encoding and Compression
Department of Information Technology
University of Turku
Table of Contents
1. Introduction ...................................................................................................................... 3
2. Codingtheoretic foundations ............................................................................................ 6
3. Informationtheoretic foundations ................................................................................... 12
4. Source coding methods ................................................................................................... 15
4.1. ShannonFano coding ............................................................................................... 15
4.2. Huffman coding ........................................................................................................ 17
4.2.1. Extended Huffman code ......................................................................................... 20
4.2.2. Adaptive Huffman coding ...................................................................................... 20
4.2.3. Canonical Huffman code ........................................................................................ 23
4.3. Tunstall code ............................................................................................................ 26
4.4. Arithmetic coding ..................................................................................................... 29
4.4.1. Adaptive arithmetic coding .................................................................................... 35
4.4.2. Adaptive arithmetic coding for a binary alphabet .................................................... 36
4.4.3. Viewpoints to arithmetic coding ............................................................................. 39
5. Predictive models for text compression ........................................................................... 40
5.1. Predictive coding based on fixedlength contexts....................................................... 42
5.2. Dynamiccontext predictive compression .................................................................. 48
5.3. Prediction by partial match ........................................................................................ 52
5.4. BurrowsWheeler Transform .................................................................................... 58
6. Dictionary models for text compression .......................................................................... 62
6.1. LZ77 family of adaptive dictionary methods ............................................................. 63
6.2. LZ78 family of adaptive dictionary methods ............................................................. 66
6.3. Performance comparison ........................................................................................... 71
7. Introduction to Image Compression ................................................................................ 72
7.1. Lossless compression of bilevel images .................................................................... 72
7.2. Lossless compression of greyscale images ................................................................ 75
7.3. Lossy image compression: JPEG ............................................................................... 77
· T. C. Bell, J. G. Cleary, I. H. Witten: Text Compression, 1990.
· R. W. Hamming: Coding and Information Theory, 2 ed., PrenticeHall, 1986.
· K. Sayood: Introduction to Data Compression, 3 ed., Morgan Kaufmann, 2006.
· K. Sayood: Lossless Compression Handbook, Academic Press, 2003.
· I. H. Witten, A. Moffat, T. C. Bell: Managing Gigabytes: compressing and indexing
documents and images, 2 ed., Morgan Kaufmann, 1999.
· Miscellaneous articles (mentioned in footnotes)3
This course is about data compression, which means reducing the size of source data
representation by decreasing the redundancy occurring in it. In practice this means choosing a
suitable coding scheme for the source symbols. There are two practical motivations for
compression: Reduction of storage requirements, and increase of transmission speed in data
communication. Actually, the former can be regarded as transmission of data ‘from now to
then’. We shall concentrate on lossless compression, where the source data must be
recovered in the decompression phase exactly into the original form. The other alternative is
lossy compression, where it is sufficient to recover the original data approximately, within
specified error bounds. Lossless compression is typical for alphabetic and other symbolic
source data types, whereas lossy compression is most often applied to numerical data which
result from digitally sampled continuous phenomena, such as sound and images.
There are two main fields of coding theory, namely
1. Source coding, which tries to represent the source symbols in minimal form for storage or
2. Channel coding, the purpose of which is to enhance detection and correction of trans
mission errors, by choosing symbol representations which are far apart from each other.
Data compression can be considered an extension of source coding. It can be divided into two
1. Modelling of the information source means defining suitable units for coding, such as
characters or words, and estimating the probability distribution of these units.
2. Source coding (called also statistical or entropy coding) is applied to the units, using their
The theory of the latter phase is nowadays quite mature, and optimal methods are known.
Modelling, instead, still offers challenges, because it is most often approximate, and can be
made more and more precise. On the other hand, there are also practical considerations, in
addition to minimizing the data size, such as compression and decompression speed, and also
the size of the model itself.
In data transmission, the different steps can be thought to be performed in sequence, as
Source Channel Channel Source
encoding encoding decoding decoding
We consider source and channel coding to be independent, and concentrate on the former.
Both phases can also be performed either by hardware or software. We shall discuss only the
software solutions, since they are more flexible.4
In compression, we usually assume two alphabets, one for the source and the other for the
target. The former could be for example ASCII or Unicode, and the latter is most often
binary. There are many ways to classify the huge number of suggested compression methods.
One is based on the grouping of source and target symbols in compression:
1. Fixedtofixed: A fixed number of source symbols are grouped and represented by a fixed
number of target symbols. This is seldom applicable; an example could be reducing the
ASCII codes of numbers 09 to 4 bits, if we know (from our model) that only numbers
occur in the source.
2. Variabletofixed: A variable number of source symbols are grouped and represented by
fixedlength codes. These methods are quite popular and many of the commercial com
pression packages belong to this class. A manual example is the Braille code, developed
for the blind, where 2x3 arrays of dots represents characters but also some combinations of
3. Fixedtovariable: The source symbols are represented by a variable number of target
symbols. A wellknown example of this class is the Morse code, where each character is
represented by a variable number of dots and dashes. The most frequent characters are
assigned the shortest codes, for compression. The target alphabet of Morse code is not
strictly binary, because there appear also intercharacter and interword spaces.
4. Variabletovariable: A variablesize group of source symbols is represented by a variable
size code. We could, for example, make an index of all words occurring in a source text,
and assign them variablelength binary codes; the shortest codes of course for the most
The first two categories are often called block coding methods, where the block refers to the
fixedsize result units. The best (with respect to compressed size) methods today belong to
category 3, where extremely precise models of the source result in very high gains. For
example, English text can typically be compressed to about 2 bits per source character.
Category 4 is, of course, the most general, but it seems that it does not offer notable
improvement over category 3; instead, modelling of the source becomes more complicated, in
respect of both space and time. Thus, in this course we shall mainly take example methods
from categories 2 and 3. The former are often called also dictionary methods, and the latter
Another classification of compression methods is based on the availability of the source
1. Offline methods assume that all the source data (the whole message ) is available at the
start of the compression process. Thus, the model of the data can be built before the actual
encoding. This is typical of storage compression trying to reduce the consumed disk space.
2. Online methods can start the coding without seeing the whole message at once. It is
possible that the tail of the message is not even generated yet. This is the normal situation
in data transmission, where the sender generates source symbols onebyone and
compresses them on the fly.
We shall use the word message to represent the unit which is given as input to one execution of the com
pression program. In storage compression, the message usually means a file.5
Still another classification, related to this one, is based on the emergence and status of the
1. Static methods assume a fixed model, which is used by both the encoder and decoder, and
which is common for all messages from the source. This is acceptable, if there is not much
variation between distinct messages, for example, if they are all English texts from the
same application domain.
2. Semiadaptive methods construct a model (for example a codebook) separately for each
source message. The model is based on the whole message and built before starting the
actual encoding. Thus, two passes through the data are needed. The encoder must first
send the model and then the encoded message to the decoder. This presupposes an offline
method. In some data communication situations this kind of delay is not acceptable.
3. Adaptive methods construct the model on the fly. When encoding a certain input symbol
(or group of symbols), the model is based on the previous part of the message, already
encoded and transmitted to the decoder. This means that the decoder can construct the
same model intact. Both partners gradually learn the properties of the source. The process
starts with some initial model, for example a uniform distribution for all possible symbols,
and then changes the model to converge towards the actual distribution.
Even though semiadaptive methods can in principle create a more precise model of the
source, it has turned out that adaptive methods can achieve the same compression efficiency.
One reason is that they do not have to transmit the model from the encoder to the decoder.
Another advantage is that they may adapt to local changes in the statistical properties within
the message. Adaptive methods are quite common today in generalpurpose compression
Compression efficiency is normally measured by the compression ratio, i.e. the ratio of the
source message size to its compressed size, both measured in bits. In text compression, an
objective measure is bits per source character (bpc), because it is independent of the original
coding. In image compression, bits per pixel is a suitable measure.
Earlier, character codes were normally of fixed length, such as ASCII (7 or 8 bpc), and ISO
8859 character sets (8 bits per character). The more recent Unicode has been adopted in
many contexts (Java, XML). Some Unicode versions have a fixed number of bits per
character: UCS2 with 16 bpc, and UCS4 with 32 bpc. The others are of variable length:
UTF8 with 8, 16, 24 or 32 bpc, and UTF16 with 16 or 32 bpc. Even though our source text
characters may have a variable number of bits, we can still talk about e.g. fixedtovariable
coding, if we encode one source character at a time because a character is logically a fixed
2. Codingtheoretic foundations
Here we study the coding of messages, consisting of symbols from a given source alphabet,
denoted S = s , ..., s . In practice, the actual alphabet, used in coding, can be an extension
of the original, so that also groups of symbols are assigned their own codes. A codebook
contains a codeword w for each (extended) symbol s. Here we assume that the codewords
can be of variable length. In adaptive compression, the codebook can also be dynamic, so that
it may change during the coding of the source message. However, for each position of the
message, there is a unique, discrete sequence of bits for every possible symbol.
The codebook may also be implicit if the code is a computable function of the source
symbols. The function is determined from our model of the source. As mentioned, it is
possible to encode a group of source symbols as a unit. Since the codebook grows
exponentially with the group size, large groups can only be coded with implicit methods. An
extreme is a method which treats the whole message as one group, and computes a single
codeword for it. In such coding, there is no precise correspondence between individual source
symbols and coded bits: For a certain bit in the coded sequence, we cannot say exactly, which
source symbol has produced it. Arithmetic coding, to be discussed later, is an example of this
kind of method.
Let us study the desirable properties what the codebook (explicit or implicit), denoted W =
w , ..., w , should have. The first necessary condition is, of course, uniqueness: For each
s , s Î S: s ¹ s Þ C(s) ¹ C(s), where C is the coding function. This condition would be
i j i j i j
sufficient if we should transmit only a single symbol (and if endofcode can be signalled by
some means). However, this is not usually the case. For multisymbol messages (and variable
length codewords) we have two options:
(a) use some special symbols (so called commas) between the codes,
(b) develop selfpunctuating codes.
The former would be inefficient use of the special symbols, thus we concentrate on the latter.
Let X and Y be any two sequences of source symbols. If X¹ Y Þ C(X) ¹ C(Y), then code C is
called uniquely decodable (decipherable). The following is an example of an illdesigned
a = 0
b = 01
c = 11
d = 00
Now consider the encoded sequence “00110”. It can result either from message “dca” or
“aaca”. The codebook (or code, for short) is not uniquely decodable. The following codebook
is, instead, such:
a = 0
b = 10
c = 110
d = 1117
Here the only interpretation of “00110” is “aac”. This code is uniquely decodable and,
moreover, a prefixfree code (prefix code, for short): No codeword is the prefix of any other
Codebooks are often illustrated by binary trees (called also decoding trees or decision trees).
The latter of the above codebooks can be visualized as follows:
It is easy to see that a prefixfree code is represented by a binary tree where each source
symbol corresponds to a leaf node. Most often this kind of coding trees are also complete, i.e.
all internal nodes have two children. The prefixfree code is always an instantaneous code, i.e.
we know the source symbol as soon as all its code bits have been received. Instantaneous
decoding is extremely useful, but not absolutely necessary for unique decodability. For
example, the following code is uniquely decodable, even though it is not prefixfree:
a = 0
b = 01
c = 011
d = 111
When decoding the bit string 00110, we cannot say after the first bit, whether it is part of ‘a’,
‘b’, or ‘c’. After the second bit we can, since there is no code with “00”. The next source
symbol can be decided only after the last ‘0’ has been seen. A more general worst case is the
sequence “0111...111”, for which the first source symbol can be decided only after seeing the
very last bit. The reader might notice that each codeword is the reverse of the corresponding
prefixfree codeword in the earlier codebook. Unique decodability results from the property
that the coded sequence can be decoded backwards as if the codebook were prefixfree
(actually it is suffixfree).
If the decoding tree is fully balanced, it means that all codewords are of the same length, i.e.
we have a block code. If the size of the source alphabet is not a power of 2, at least the lowest
level is incomplete. If the difference between the longest and shortest codeword length is 1,
then the code is called shortened block code. For example, the 5symbol alphabet a, b, c, d,
e can be encoded as
a = 00
b = 01
c = 10
d = 110
e = 111
A rather simple condition has been proved about the lengths l , ..., l of an instantaneous
code, namely the Kraft inequality:8
Theorem 2.1. An instantaneous codebook W of q codewords exists if and only if the lengths
l , ..., l of the codewords satisfy K = £ 1.
Proof. For an instantaneous code we have the symbols in the leaves of the decoding tree, and
any binary tree with q leaves can be interpreted into an instantaneous code. Thus we show
that for any binary tree the depths of leaves satisfy the inequality. We apply induction: For
q=1, there is one leaf with depth = 1, so the sum K is equal to ½ £ 1. (Note that the code
length cannot be zero, so a rootonly tree is illegal.) For q=2 we have two leaves with depth
1, so the sum is ½ + ½ £ 1. Now assume an arbitrary tree with q nodes, with sum K £ 1.
(a) Extend any leaf (with depth d) with one new leaf (making the old leaf internal). The new
leaf has depth d+1. Thus the sum of the theorem becomes K1/2 +1/2 £ K £ 1.
(b) Extend any leaf (with depth d) with two new leaves (making the old leaf internal). The
new leaves have depth d+1. Now the number of leaves becomes q+1, and the sum value is
d d+1 d+1
K1/2 +1/2 +1/2 = K £ 1.
Since any binary tree can be constructed step by step using extensions (a) and (b), the
In general form, the Kraft inequality has radix r in the denominator, instead of 2, but we
restrict ourselves to the binary target alphabet. Moreover, it can also be shown that, for binary
target alphabet, strict equality should hold; otherwise the code is inefficient.
Let us examine the validity of the theorem by studying the previous reduced block code, with
codeword lengths 2, 2, 2, 3, 3. We obtain the sum ¼ + ¼ + ¼ + 1/8 + 1/8 = 1. Thus it is
easy to check for a given set of lengths, whether a codebook can be constructed or not, and if
yes, whether the code is inefficient. Consider the set of lengths 1, 3, 3, 3, which produces
the sum 7/8. We can directly say that the code is inefficient. The related tree has a leaf with no
brother; therefore the leaf can be omitted, and the related symbol gets one bit shorter code
word (corresponding to the parent). Now, the set 1, 2, 3, 3 produces K = 1.
The Kraft inequality holds for instantaneous codes. The MacMillan inequality says that the
same holds for any uniquely decodable code. The proof is not quite as straightforward as
above. Important is that, when choosing a codebook, we can calmly restrict ourselves to
instantaneous codes, because for any set of lengths in a uniquely decodable code, we can
derive an instantaneous code with the same lengths.
In some applications, the source alphabet can in principle be infinite (but yet countable). One
such situation is runlength coding, where we should encode the number of similar successive
symbols in a sequence. Of course, all runs are of finite length, but no upper bound can be
given. In this case we need a general coding system for all integers; the traditional fixedlength
representation cannot be used. A coding system is in a sense universal, if it works for all
possible sources in some class. Usually, an additional requirement is set for true universality,
such that the code satisfies a certain efficiency constraint (to be explained later). Since the
alphabet is infinite, we cannot store an explicit codebook, but we must provide an algorithm
(function), which produces the code for any integer. Two desirable properties for such a code
· Effectiveness: There is an effective procedure that can decide whether a given sequence of
bits belongs to the codebook or not.
· Completeness: adding any new code would create a codebook that is not uniquely de
There are various ways of designing the universal code; Peter Elias has described and
analysed potential coding approaches for positive integers. A simplified description of them is
1.acoding: This is unary coding, where number n is represented by n1 zeroes plus an end
ing onebite: 1®1; 2®01; 3®001; 4®0001; 5®00001; ... Actually,acoding is not
universal in the strict sense, because it does not satisfy the efficiency constraint.
2.bcoding: This is the traditional (positional) representation of numbers, excluding the
leading zeroes. Since its codes are not selfpunctuative, we need an end symbol ‘’, and
thus our code alphabet is ternary. The leading 1bit can be omitted: 1®; 2®0; 3®1;
4®00; 5®01; 6®10; 7®11; 8®000; ... This can be transformed to the normal
binary representation e.g. by further coding: 0 ®0; 1®10; ®11. The final encoding
would thus be 1®11; 2®011; 3®1011; 4®0011; 5®01011; 6®10011; 7®101011;
8®00011. Notice that the lengths of the codewords are not monotonically increasing with
the number to be encoded.
3.gcoding: To avoid using the end symbol, the positional representation is prefixed by an a
coded length. Again, the leading 1bit is omitted from the number actually we can think
that the 1bit that ends the unary code is simultaneously the leading 1bit. The small
integers are now coded as follows: 1®1; 2®010; 3®011; 4®00100; 5®00101;
6®00110; 7®00111; 8®0001000; ...
4.dcoding: similar as gcoding, but the length is not represented with unary coding, butg
coding. Examples: 1®1; 2®0100; 3®0101; 4®01100; 5®01101; 6®01110; 7®01111;
dcoding is theoretically better thangcoding for large numbers (cf. next section).
There are many other useful properties of codes. One such property is robustness against
transmission errors. In the worst case, inverting a single bit in a sequence of variablelength
codes may scramble the whole tail of the coded sequence. The synchronization property
refers to the behaviour of the decoding process in the context of errors (add/drop/complement
a bit). Synchronization means, how fast the decoder will catch again the correct codeword
An example of an elegant, robust, universal coding scheme for integers is so called
Zeckendorf representation (called also Fibonacci representation) . Its simplest form encodes
Peter Elias: “Universal Codeword Sets and Representations of the Integers”, IEEE Trans. on Inf. Theory,
Vol. IT21, No. 2, 1975.
A. S. Fraenkel, S. T. Klein “Robust Universal Complete Codes for Transmission and Compression “,
Discrete Applied Mathematics, Vol. 64, 1996, pp 31–55.10
numbers by bitweighted sums of Fibonacci numbers …, 34, 21, 13, 8, 5, 3, 2, 1. For
example, the value 17 can be represented by the sum 0·34 + 0·21 + 1·13 + 0·8 + 0·5 + 1·3 +
0·2 + 1·1. The robustness of Zeckendorf code results from the fact that every number can be
represented with a code with no pairs of adjacent 1bits (results immediately from the
Fibonacci system). Thus, by extending the code with two additional 1bits we can make the
code selfdelimiting. Moreover, by transmitting the least significant bits first, up to the most
significant 1bit, only a single extra 1bit is needed to delimit the code. The codebook for
small integers looks like follows.
Number Weights for Code
8 5 3 2 1
1 0 0 0 0 1 11
2 0 0 0 1 0 011
3 0 0 1 0 0 0011
4 0 0 1 0 1 1011
5 0 1 0 0 0 00011
6 0 1 0 0 1 10011
7 0 1 0 1 0 01011
8 1 0 0 0 0 000011
9 1 0 0 0 1 100011
10 1 0 0 1 0 010011
11 1 0 1 0 0 001011
12 1 0 1 0 1 101011
… … …
Since no two consecutive 1bits can exist within a codeword, excluding the end, singlebit
errors can only propagate to the next codeword, but not further. What is more, it has been
shown that basic arithmetic operations can be performed directly with Zeckendorfcoded
However, transmission errors are not our main concern in this course, so we shall later only
mention some results and observations from this topic.
As an example of a parametric code, we present the so called Golomb code , which is optimal
for specific types of numeric sources. As a preliminary step, we introduce a code for uniform
distribution of a finite set of symbols (i.e. all q symbols are equally likely). If q is a power of
two (q = 2), then the codes for the alphabet naturally consist of all possible kbit
combinations. However, if q is not a power of 2, a fixedlength code ofélog qù bits would be
redundant (some code values unused). In this case we choose a socalled semifixedlength
code, where some of the codewords have lengthëlog qû, and the rest have length élog qù .
The following tables shows semifixedlength codebooks for q in 2…8.
x q = 2 q = 3 q = 4 q = 5 q = 6 q = 7 q = 8
0 0 0 00 00 00 00 000
P. Fenwick: “Zeckendorf Integer Arithmetic”, Fibonacci Quarterly, Nov 2003, pp. 405 – 413.
S. W. Golomb: “RunLength Encodings”, IEEE Trans. on Inf. Theory, Vol. 12, 1966, pp. 399401.11
1 1 10 01 01 01 010 001
2 11 10 10 100 011 010
11 110 101 100 011
4 111 110 101 100
5 111 110 101
6 111 110
Golomb coding is applied to numeric data (i.e. integers). It has one parameter m, which is
used in encoding of an integer x as follows. Encode the quotient a = ëx/mû with unary
coding, and the remainder x ma with semifixedlength coding, where the alphabet size q =
m. The two parts are concatenated to codewords. An example with m=3 is given in the table
below. The parts of the codewords are separated by a space, for clarity. In the special case
where m is a power of 2 (denote 2 ) we obtain a so called Rice code , which is structurally
simpler, as shown in the table for m = 2.
x Golomb Rice
m = 3 m = 2
0 0 0 00
1 0 10 01
2 0 11 100
3 10 0 101
4 10 10 1100
5 10 11 1101
6 110 0 11100
7 110 10 11101
8 110 11 111100
9 1110 0 111101
Both are very efficient in practice, if the distribution of source numbers is close to geometric.
Such a distribution results e.g. from repeating binary events (sequences of ‘success’/’failure’)
where a success is more probable than a failure, and we encode the number of successes up to
the next failure. Another interpretation of the same situation is runlength encoding of a binary
sequence. If p = P(0) P(1), then m should be chosen such that the probability of m 0bits
before the next 1bit is about 0.5. From this it follows that m » 1/log p is the best choice.
The next chapter studies the connection between symbol probabilities and optimal codes more
R. F. Rice: “Some Practical Universal Noiseless Coding Techniques”, Jet Propulsion Laboratory, JPL
Publication 7922, Pasadena, CA, 1979.12
3. Informationtheoretic foundations
Information theory, mainly founded by Claude Shannon at Bell Labs, studies both noise
protection and efficient use of the communication channel. It gives bounds for the ultimate
transmission rate of the channel, and for the ultimate data compression. Of these two we
concentrate on the latter. First we shall define a quantitative measure of information for both
single symbols and the whole alphabet. Notice that the word ‘information’ is here a quite
concrete technical concept, and bears only loose analogy with the ‘normal’ concept of
information in human communication.
Suppose that we have a source alphabet S = s , ... s , with probabilities P = p , ... p . The
1 q 1 q
question is, how much information the receiver gets when seeing a certain symbol s. The
principle is that a less frequent character gives more information than a more frequent one.
Thus, the amount of information measures the degree of surprise (uncertainty). If some p = 1
then all symbols to be sent are = s , and there is no surprise no information. For constructing
an information function I(p), three properties should be satisfied:
1. I(p)³ 0,
2. I(p p ) = I(p ) + I(p ) for independent events,
1 2 1 2
3. I(p) is a continuous function.
From condition 2 it follows that I(p ) = nI(p). We see that I satisfies the same properties as
the logarithm function. In fact, the solution is essentially unique, and we define I(p) = k log p.
From property 1 (and noticing that 0 £ p £ 1), we deduce the value k = 1. Hence we have:
I(p) = log p = log (1/p)
The base of logarithm can be chosen. Normally we take base = 2, and define the unit of
information to be a bit. This word has now two meanings: a digit in 2base representation and
a unit in measuring information. Of course, this is not a coincidence, as seen from the
following example: Consider tossing a (fair) coin with P(heads) = P(tails) = ½. Then I(heads)
= I(tails) = log ½ = 1 (bit), which is consistent with the idea that with one binary digit we
can tell the outcome. If the coin is unfair, such that P(heads) = 1/8 and P(tails) = 7/8, then we
have I(heads) =log (1/8) = 3 and I(tails) =log (7/8)» 0.193 bits. Thus getting heads is a
much bigger surprise than getting tails. Notice also that the information amount is a real
valued quantity, and thus more general than the binary digit. For studying the additive pro
perty, consider tossing a fair coin three times. The information amount of the sequence:
headstailsheads (or any other 3sequence), is: I(½½½) = I(1/8) =log (1/8) = 3 bits = I(½)
+ I(½) + I(½), as expected.
Next we define the average information when receiving a symbol from alphabet S, with
probability distribution P. Since we get amount I(p) with probability p, the average (ex
pected) information per symbol, called entropy, is a weighted sum:
HP ()= pIp()= p logç÷
i i i
C. E. Shannon: “A Mathematical Theory of Communication”, Bell System Technical Journal, Vol. 27, pp.
379423, 623656, 1948.13
Example. Let P = 0.1, 0.2, 0.3, 0.4. Then H(P) = 0.1 log 0.1 0.2 log 0.2 0.3 log 0.3
0.4 log 0.4» 0.332 + 0.464 + 0.521 + 0.529 = 1.846. Compare this with uniform distribution
P = 0.25, 0.25, 0.25, 0.25, which has entropy H(P) = 4 ´ 0.25 log 0.25 = 2. It is easy to
prove the observation generally: a uniform distribution gives the maximum entropy. Notice
the analogy with physical phenomena e.g. in thermodynamics, where the same word is used.
Shannon has proved the following fundamental “noiseless source encoding theorem”:
Theorem 3.1. Entropy H(S) gives a lower bound to the average code length L for any
instantaneously decodable system. (Proof skipped)
Even though precise symbol probabilities are seldom known, entropy is an excellent reference
measure when studying the compression power of a source coding method. The difference
LH(S) between the average codeword length and entropy is called redundancy of the code.
The equality in Theorem 3.1 holds if the probabilities p are powers of 2, i.e. p= 1/ 2 . In
this case we reach the theoretic optimum with redundancy = 0. Even if this were not the case,
L can always be limited to at most H(S) + 1 by choosing
æ 1ö æ 1ö
logç÷£ l logç ÷+ 1
2 i 2
èø è ø
A code is called universal if L £ c H(S) + c asymptotically for some constants c and c and
1 2 1 2
for nonzero entropy H(S). The code is asymptotically optimal if c = 1.
Consider the encoding of natural numbers n = 1, 2, etc. Often we do not know their exact
probabilities, and they are hard to find out experimentally. However, the following
fundamental result has been proved :
Theorem 3.2. There exist codes, which satisfy the above universality condition for any
distribution satisfying P(1)³ P(2) ³ P(3)³ … ³ P(n)³ … (Proof skipped).
The codeword lengths for such encodings are nondecreasing with n. Of the coding methods
for numbers (Chapter 2), it can be shown that acode is not universal in the above sense,
whereasg and dcodes are. Moreover,gcode has constant c = 2, while dcode has c = 1,
the latter being asymptotically optimal. Zeckendorf code (called also Fibonacci code) is also
universal, but not asymptotically optimal; its c value is about 1.440. Nevertheless, for a large
subset of integers, the Zeckendorf codewords are actually shorter thandcodes.
Even thoughacode (= unary code) is not universal, it is optimal if the probabilities are:
P(1) = 1/2, P(2) = 1/4, P(3) = 1/8, …, P(n) = 1/2 , … In fact, this kind of distribution is not
uncommon. Note that the sum of probabilities 1/2 + 1/4 + 1/8 + … = 1, as required.
Above we assumed the successive symbols of the message to be independent. In practice this
is not the case, so we should extend the concept of entropy. This leads us to the modelling
part of compression, which we shall study later more deeply. Here we mention the natural
extension to conditional probabilities. Suppose that we know m previous symbols before s ,
See: Peter Elias: “Universal Codeword Sets and Representations of the Integers”, IEEE Trans. on Inf.
Theory, IT21, No 2, pp. 194203, 197514
namely s,, L s , and the related conditional probability P(s s,, L s ). The conditional in
i i i i
1 m 1 m
formation of s in this mmemory source is naturally log (1 P(ss ,L,s )) and the conditional
2 ii i
H(Ss ,L,) s = P(ss ,L,s )log
i i ii i 2
1 m 1 m
P(ss ,L,) s
S è ø
The situation can be described by an m’th order Markov process, which is a finite automaton
with states for all mgrams of symbols, and transitions corresponding to successors of m
grams, with the conditional probabilities. In an ergodic Markov process the state probabilities
approach the so called equilibrium. The global entropy is
HS ()= Ps ( ,L,s )P(ss ,L,s )log
i i ii i 2
1 m 1 m ç ÷
m P(ss ,L,) s
S è ii i
= Ps ( ,L,ss , )log
i i i 2
m+1 P(ss ,L,) s
S ii i
There is another measure for the complexity of a message which is called Kolmogorov com
plexity, and which can be considered more general than entropy. It is defined as the length of
the shortest (binary) program for generating the message. If the sequence of symbols is drawn
at random from a distribution that has entropy H, then the Kolmogorov complexity is
approximately equal to H. However, consider a pseudo random number generator, which
produces a seemingly random sequence of symbols. The generating algorithm can be
regarded as the compressed representation of the message, because the sequence can be
recovered by applying it.
Unfortunately, Kolmogorov complexity has been shown to be noncomputable, i.e. there is no
algorithm to determine it (cf. Gödel’s incompleteness theorem). This makes the concept
rather impractical. However, a somewhat related idea is used in fractal compression of
images, where the compressor tries to find selfrepeating components from the image, i.e.
tries to find the ‘hidden algorithm’ that has generated the image. However, this can only be
done approximately (i.e. the method is lossy), unless the image is a pure fractal.15
4. Source coding methods
In this section we shall study the second part of compression, namely source coding, and
leave the first, ‘harder’ part (= modelling) to the next sections. Source coding is based on the
assumed probability distribution (P) of the source symbols (S). For a finite alphabet it is
usually possible to give at least estimates of the probabilities. For an infinite alphabet the
distribution is often implicit. For example, we could assume that the probability of a number is
inversely proportional to its magnitude. At least, it must hold that p ® 0 as i ® ¥, because
Sp = 1. We shall concentrate on finite alphabets.
4.1. ShannonFano coding
The theory of Section 3 gives a symbol (s ) with probability p the information log (1/p ) bits.
i i 2 i
An optimal coding solution would give the symbol a code of this length. However, the length
is an integer only if p is a negative power of 2. If this is not the case, we can take as the actual
length the next larger integer, i.e.élog (1/p)ù. It is easy to see that this choice satisfies the
l l l l
i i i i
l³log(1/) pÞ2³1/ pÞ p³12/Þ p³ (12/)Þ³ 1 (12/)
i 2 i i i å i å å
Thus, there is a prefix code with lengths élog (1/p )ù for i = 1, ..., q. Since every codeword
length is at most one larger than the theoretical optimum, it also holds that the average code
length L satisfies:
H(S)£ L£ H(S) + 1
Example. Let p = p = 0.3, and p = p = p = p = 0.1. From log (1/0.3)» 1.737 and
1 2 3 4 5 6 2
log (1/0.1)» 3.322, we get the code lengths 2, 2, 4, 4, 4, 4. The codes can be assigned in
lexicographic order for the list of lengths sorted into ascending order:
s = 00, s = 01, s = 1000, s = 1001, s = 1010, s = 1011
1 2 3 4 5 5
which has the average code length of 2.8, while the entropy is H(S) » 2.371. If we study the
decoding tree, we notice that it is not complete (not succinct): The subtree 11... is missing.
Checking the Kraft inequality confirms the property (the sum is not 1):
1 1 1 1 1 1 1 3
= + + + + + = 1
l 2 2 4 4 4 4
2 2 2 2 2 2 2 4
The situation is more obvious if we take a binary source alphabet with p = 0.99 and p =
0.01, for which the above rule gives code lengths l = 1 and l = 7, although the only sensible
possibility is to choose both lengths to be = 1 (corresponding to codes 0 and 1). Nevertheless,
for lengths 1 and 7, the average code length would yet be only 0.99´1 + 0.01´7 = 1.06, while
the entropy is
0.99×log 0.99 0.01× log 0.01» 0.08 bits
The above description is the theoretical basis for ShannonFano coding (presented in
dependently by Shannon Weaver and Fano in 1949), but the actual algorithm is more
constructive, and also avoids noncomplete decoding trees. Its idea is to divide the
‘probability mass’ (with related symbols) repeatedly into two approximately samesized
halves, and simultaneously construct the decoding tree in a topdown fashion. With no loss of
generality, we assume here that probabilities p , ..., p are in descending order. Sorting may
thus precede the following codebook generation algorithm:
Algorithm 4.1. ShannonFano codebook generation.
Input: Alphabet S = s , ..., s with probability distribution P = p , ..., p , where p ³ p .
1 q 1 q i i+1
Output: Decoding tree for S.
Create a root vertex r and associate alphabet S with it.
If S has only one symbol then return a tree with r as the only node (root).
Choose index j ( ¹ 0 and ¹ q), for which the sums p and p are the closest.
i =1 ij =+1
Find decoding trees, rooted by r and r , for the subalphabets s , ..., s and
1 2 1 j
s , ..., s recursively and add them as subtrees of r. Attach labels 0 and 1 to the
corresponding parentchild relationships rà r and r à r .
Return the tree rooted by r.
It can be proved that the same properties hold for this code as for the previous code based on
direct computation of the codeword lengths, i.e. the redundancy is at most 1. To be precise,
the selection of an optimal split index j is not trivial (generally, optimal partitioning is NP
hard). In practice the suggested condition (based on closest sums) is sufficient.
Example. S = a, b, c, d, e, f, P = 0.3, 0.3, 0.1, 0.1, 0.1, 0.1. The codebook generation
proceeds step by step, as shown in the figure on the next page, with subsets of symbols and
related sums of probabilities marked for nodes.
The first split is done into probability subsets 0.3, 0.3 and 0.1, 0.1, 0.1, 0.1, attached with
bits 0 and 1 (to be prefixes for the codes in the subsets). The former is split again, and
produces singleton subsets 0.3 and 0.3 with final codes 00 and 01 for a and b. Subset
0.1, 0.1, 0.1, 0.1 is split first into 0.1, 0.1 and 0.1, 0.1, producing prefixes (= paths
from the root) 10 and 11. The final splits produce four singleton subsets, each with probability
0.1. The related codes are 100, 101, 110 and 111. For the codebook a=00, b=01, c=100,
d=101, e=110, f=111 the average code length is 2.4 bits, having redundancy » 0.029 bits per
symbol. In fact, the obtained code is optimal (among methods assigning distinct codewords
for symbols), although generally ShannonFano code does not guarantee optimality.
The tree structure is suitable for the decoder: by branching according to the received bits it is
easy to pick up the correct original symbol from the leaf. The encoder, instead, needs some
other data structure, presumably a table with an entry for each symbol and the related
(1) (2) (3)
a,b,c,d,e,f: 1.0 a,b,c,d,e,f: 1.0
0 1 0 1
a,b: 0.6 c,d,e,f: 0.4
a: 0.3 b: 0.3
a,b,c,d,e,f: 1.0 a,b,c,d,e,f: 1.0
0 1 0 1
c,d,e,f: 0.4 c,d,e,f: 0.4
a,b: 0.6 a,b: 0.6
0 1 0 1 0 1 0 1
0 1 0 1
a: 0.3 b: 0.3 c,d: 0.2 e,f: 0.2 a:0.3 b:0.3
c:0.1 d:0.1 e:0.1 f:0.1
4.2. Huffman coding
The bestknown source coding method is without doubt Huffman coding. The generation of a
Huffman code resembles the ShannonFano method, but the direction is opposite: Shannon
Fano generates the decoding tree topdown, while Huffman method does it bottomup. The
average codeword length of the Huffman code reaches the entropy if the probabilities are
integer powers of ½ (cf. Chapter 3). It also holds that, even in cases where the entropy is not
reached, Huffman coding is optimal among the methods that assign distinct codewords for
separate, independent symbols of the source alphabet.
The idea of Huffman code is developed as follows. First assume, without loss of generality,
that the probabilities of symbols are sorted into decreasing order: p³ p³ ... ³ p . Obviously,
1 2 q
this implies that the codeword lengths must satisfy: l £ l £ ... £ l . Since all nonroot nodes
1 2 q
in an efficient (complete) decoding tree have a sibling, and symbol s is on the lowest level
(leaf), also s should be a leaf, more precisely, the sibling of s and have the same codeword
length. (The choice may not be unique, if there are equal probabilities, but then we can make
an arbitrary choice.) Therefore, we can assign suffixes 0 and 1 to the last two symbols; their
final codewords will be of the same length and differ only in the last bit position. The pair can
be considered a single ‘metasymbol’ (= subalphabet s , s ), which has probability p +
q1 q q1
p , and becomes the parent of the last two symbols. From this we can continue by repeating
the process for the reduced alphabet of q1 symbols, and attaching new parents to already ge
nerated subtrees. More formally, the algorithm proceeds as follows:
Algorithm 4.2. Huffman codebook generation.18
Input: Alphabet S = s , ..., s with probability distribution P = p , ..., p , where p ³ p .
1 q 1 q i i+1
Output: Decoding tree for S.
Initialize forest F to contain a distinct singlenode tree T for each symbol s and set
weight(T ) = p .
while F 1 do
Let X and Y be two trees in the forest which have the lowest weights.
Create a binary tree Z, with X and Y as subtrees (equipped with labels 0 and 1).
The root of Z is a new node representing the combined symbol set of X and Y.
Set weight(Z) = weight(X) + weight(Y).
Add Z to forest F and remove X and Y from it.
Return the single remaining tree of forest F.
The resulting code is a prefix code, because the original symbols will be leaves of the final
tree. The proof of optimality is skipped here, but essentially we proved it already in the above
derivation. As Huffman coding is optimal, and ShannonFano satisfies
H(S) £ L £ H(S) + 1
then also Huffman code satisfies it. In fact, it has been proved that an upper bound for the
redundancy is p + 0.086, where p is the highest probability among the symbols. To be
precise, the upper bound is Min(p + 0.086, 1).
The generated codebook is not unique, if there occur equal probabilities during the process;
ties in choosing the two smallest weights can be broken arbitrarily, without affecting the
average length L of codewords. However, if we always favour the smallest trees, we obtain a
code, the longest codeword of which is minimal. This may have practical advantage. When we
still notice that, in each combining step, 0 and 1 can be assigned to subtrees in either way, it is
clear that the number of different optimal codebooks is exponential.
Example. Determine the Huffman code for alphabet S = a, b, c, d, e, f where P = 0.3, 0.3,
0.1, 0.1, 0.1, 0.1. Forest F develops in the steps shown in the picture on the following page.
We obtain the same codebook 00, 01, 100, 101, 110, 111 as with ShannonFano method,
which in this case is also optimal. Technically the Huffman algorithm can be implemented e.g.
in the following ways:
(1) Build and maintain a heap (more precisely, a minheap) structure for the symbols (both
original and metasymbols), ordered by weight. The heap can be built in linear time using
Floyd’s algorithm. Extracting the smallest and adding a new metasymbol need both
O(log q) operations for q nodes, and thus the total complexity is O(q log q).
(2) If the alphabet is already sorted by probability, we can build the tree in linear time by
keeping a queue of the created metasymbols: they will be generated in increasing order of
weight. The two smallest weights are thus found either among the sorted list of original
symbols or from the queue of metasymbols.19
0.3 0.3 0.1 0.1 0.1 0.1
0.3 0.3 0.1 0.1 0.1 0.1
a b c d e f
a b c d e f
(3) (4) 0 0.4
0 1 0 1
0 1 0 1
0.3 0.3 0.1 0.1 0.1 0.1 0.3 0.3 0.1 0.1 0.1 0.1
a b c d e f
a b c d e f
(5) (6) 0 1.0 1
0.6 0.2 0.2
0.6 0.2 0.2
0 1 0 1 0 1
0 1 0 1 0 1
0.3 0.3 0.1 0.1 0.1 0.1 0.3 0.3 0.1 0.1 0.1 0.1
a b c d e f
a b c d e f
There are some special probability distributions worth looking at. First, if all symbols are
equally likely, and if the number of symbols q is a power of two, we get a perfectly balanced
binary tree and a block code with each codeword having length log q. If symbols are equally
likely but q is not a power of two, we obtain a shortened block code, where the lengths differ
by at most one. A somewhat weaker condition is sufficient for this: If the sum of two smallest
probabilities (p + p ) is greater than the biggest probability (p ), then we also end up to a
q1 q 1
(shortened) block code. This is seen immediately from technique (2) above, because all actual
symbols are combined before any of the metasymbols, and the metasymbols are combined in
the order they were generated.
Another extreme is the (negative) exponential distribution p = 1/2 , the Huffman tree of which
is a degenerated tree where each internal node has a leaf child (two leaves on the lowest
level). This kind of code is similar to unary code (11...10), except for one codeword that has
all its bits = 1.
A distribution that often occurs in practice, e.g. for symbols in natural language, is the Zipf
distribution, where the i’th symbol in the alphabet, ordered in decreasing order of probability,
has probability proportional to 1/i. Since the sum of 1/i is not limited when i approaches
infinity, the Zipf distribution is valid only for finite alphabets. A typical compression result is
about 5 bits per character.
In static compression, the Huffman code can be determined once for all inputs. In semi
adaptive coding, however, the code is determined for each input message separately.
Therefore, in transmitting the message to the decoder, one has to send both the codebook and
the encoded message. This reduces the compression gain but, having a finite alphabet, the loss
is negligible for long messages. Anyway, it is worthwhile to consider the compression of the20
codebook (= decoding tree) itself. The shape of the tree can be expressed with 2q1 bits: The
whole tree contains always this number of nodes, and for each node we must tell whether it is
a leaf or not. In addition, we have to transmit the order of symbols in the leaves from left to
right. In total, the representation amounts to 2q1 + qélog qù bits. It is also possible to express
the codebook by transmitting only the lengths of codewords (in alphabetic order), or counts
of different lengths plus symbols in probability order. We shall return to the latter alternative
4.2.1. Extended Huffman code
For large alphabets, with the largest probability not too big, Huffman coding is generally
efficient. However, for very small alphabets with a skew distribution it can be rather bad,
compared to the entropy. For example, if we have S = a, b, c with P = 0.8, 0.18, 0.02,
the optimal codebook will be 0, 10, 11, with average codeword length = 1.2 bits, while the
entropy is only 0.816 bits. The redundancy (0.384 bits per character) can be reduced by
extending the alphabet S to S , consisting of nsymbol blocks, called ngrams. Their count is
q , and the probability of each can be computed as the product of elementary probabilities.
Then we can construct the Huffman code for the extended alphabet. As before, we have the
(n) (n) (n)
following condition for the average codeword length: H(S ) £ L £ H(S ) + 1. Therefore,
the number (L) of bits per original symbol is bounded as follows:
H(S )/n£ L£ (H(S ) + 1)/n Þ H(S) £ L £ H(S) + 1/n
In the limit, we get arbitrarily close to the entropy. In the above example, by taking pairs of
symbols (aa, ab, ac, ba, bb, bc, ca, cb, cc) as the new coding alphabet (n = 2), the average
codeword length will reduce to 0.8758 bits, having a redundancy of only 0.06 bits per symbol.
Of course, the size of the codebook, as well as the effort of computing it, will explode when n
gets larger. This restricts the applicability of the method. Moreover, many of the ngrams will
not even occur in the actual message. It should somehow be possible to compute the needed
codes on the fly, without building an explicit decoding tree. This idea will be used later,
especially in arithmetic coding.
4.2.2. Adaptive Huffman coding
Normal Huffman coding needs knowledge of the symbol probabilities before starting the
encoding. This means that the compression is done offline, in two passes. If this is not
applicable, we should develop a modification where the code adapts to the probabilities
symbol by symbol, while the message is being transmitted. Then the decoder can maintain the
same code synchronously with the encoder, and there is no need to transmit the decoding
A naive solution would be to start with some default, e.g. uniform distribution, maintain
symbol frequencies, and compute the whole code repeatedly; once per each transmitted
symbol. Although this is in a sense optimal, it is clearly too laborious. It should be noticed that
small local changes in the symbol probabilities do not usually cause global changes in the
Huffman tree structure. We shall take advantage of this observation. In what follows we talk
about symbol and node weights (corresponding to frequencies), instead of probabilities. This21
has no effect on the resulting code, only the scaling of numbers is different. Weights are thus
integers, and a parent has always a weight which is the sum of the weights of its children. A
Huffman tree has the sibling property if the following two conditions hold:
(1) Each node, except the root, has a sibling (i.e. the binary tree is complete).
(2) The tree nodes can be listed in nondecreasing order of weight in such a way that each
node is adjacent to its sibling in the list.
Theorem. A binary tree with weights associated with its nodes, as defined above, is a
Huffman tree if and only if it has the sibling property.
The proof is skipped. Obviously, it must be possible to swap nodes during the course of
adaptation, as the order of weights (frequencies) changes. To do this, we maintain a threaded
list of nodes in nondecreasing order. In this list, the adjacent nodes with equal weights are
called a block. When, during encoding, a new symbol x has been transmitted, the tree must be
updated: The related leaf weight is increased by one. Now it may happen that the node is not
anymore in the correct place. Therefore, it is swapped with the rightmost node y of the block;
the next block must have weights at least as large as the increased value, so the order is
correct. If x = y then x was the only node in its block, and no swapping is required. Next
assume that x¹ y and denote the sibling of x by z, and the sibling of y by w. Now we check
that the sibling property holds. Since before the update weight(x) = weight(y), then after the
swap the sibling property holds for y and z. For w, two cases can be distinguished:
1) weight(x) = weight(w) before the update, and the order of nodes is x...wy... After the swap
and increase of weight(x) it holds: weight(w) weight(x) and the order is y...wx... Thus the
sibling property holds.
2) weight(x) weight(w) before the update, and the order of nodes is x...yw... After the swap
and increase of weight(x) it holds: weight(x)£ weight(w) and the order is y...xw... Again,
the sibling property holds.
Notice also that in swapping, the possibly attached subtrees are carried along. The procedure
continues to the parent of x, the weight of which is also increased by one, and the same test
plus possible swap are done. The process stops at the root.
The following example shows the evolution (including swap x « y) of a sample Huffman
tree, after increase of weight(a). Node a maintains its position, but the increase of the weight
of its parent x from 3 to 4 makes it out of order, and relocation of x follows, whereby its
children a and b move along.22
6 w 3 y 10 r
3 x 3 z 6 w 4 x
1 2 3 3 2 2
a b y z a b
List: (a, b, z, x, y, w, r) List: (a, b, z, y, x, w, r)
A modification to the above algorithm has been suggested, where one starts with an empty
alphabet, and a tree with only one node as a placeholder. As new symbols are encountered
during the message, the placeholder splits and the new symbol occupies the other half. Of
course, the code of the placeholder, plus the ASCII (or corresponding) code of the new
symbol must be transmitted to the decoder.
The swap can be made in constant time, but finding the rightmost node in the block of equal
weights can be more timeconsuming. This can be improved by maintaining a second list for
the last nodes of each block of equalweighted nodes. Each tree node must contain a pointer
to the related block node, from which we directly find the last node of the block. If the block
list is twoway linked, it can be updated in constant time per one weight increase. Thus, the
overall complexity is proportional to the depth of the related leaf, and also proportional to the
number of output bits.
It can be proved that the compression result is at most 2h + n bits, where h is the length of the
corresponding result from static twophase Huffman coding, and n is the length of the source
message. In practice, the lengths of the two codes are very close to each other.
In the form presented, adaptive Huffman coding does not adapt well to local changes in the
probability distribution within a (long) message. Some researchers have suggested modifica
tions, where all the weights are periodically changed, e.g. nullified or halved, so that old
symbol occurrences will have less weight than the new ones.
Adaptive Huffman coding has been used also in practice: The Unix compact command is
based on it. Its practical advantage is, however, limited by the fact that in many modelling
techniques for the source, a separate decoding tree is needed for each possible context. For 2
or 3memory sources for ASCII characters, the number of trees (plus related lists) becomes
huge. Arithmetic coding, to be described later, is much more applicable to adaptive coding.23
4.2.3. Canonical Huffman code
The decompression speed is usually more important than compression speed. In Huffman
decoding, the normal procedure is to follow a path in the decoding tree from the root to the
correct leaf. At each node, we have to inspect the transmitted bit and make a choice between
the children. For large alphabets (consisting of multicharacter words, for example), the size
of the tree will be quite large, and the probing may become inefficient. Canonical Huffman
coding is a scheme which avoids the explicit storage of the tree, and has much better
As mentioned before, the number of alternative Huffman codebooks is exponential, due to tie
situations and alternatives in bit allocation. Canonical Huffman coding will use the same
lengths of codewords for the symbols as normal Huffman coding, but may assign the bits in a
different, systematic way. The following table shows three possible Huffman codes for a six
symbol alphabet, with given frequencies.
Symbol Frequency Codebook I Codebook II Codebook III
a 10 000 111 000
b 11 001 110 001
c 12 100 011 010
d 13 101 010 011
e 22 01 10 10
f 23 11 00 11
Codebooks I and II are generated with the normal Huffman technique; their difference is just
in the assignment of bits: the codes are complements of each other. Codebook III is, however,
different. Although it is a prefix code and optimal (the codeword lengths being equal to those
in codebooks I and II), it is not possible to generate it with the Huffman algorithm. Thus, we
suggest the following relaxed definition:
A Huffman code is any prefixfree assignment of codewords, where the lengths
of codewords are equal to the depths of corresponding symbols in a Huffman tree.
What is so special in codebook III is its nice ordering of codeword values: The 3bit code
words are in increasing sequence, and the same holds for 2bit codewords. Moreover, if we
discard the last bit from 3bit codewords, the resulting sequence of 2bit codewords is still
nondecreasing. This property enables simple decoding: If the integer value of the first two
bits is 2 or 3, the codeword has been uniquely identified. If the value is 0 or 1, we know that
the two bits are a prefix of a 3bit codeword, so the next bit is read, and the corresponding
numerical value (0..3) computed, which uniquely identifies the related symbol.
In a general case, we do similarly as above. First we have to determine the length of each
codeword by using the Huffman algorithm. Then we compute the number of codewords per
each length from 1 to maxlength (= maximum depth of the Huffman tree). Using these we can
determine the firstcode value for each length, whereafter the symbols can be assigned their
codewords one by one. A more formal algorithm is given as follows.
Algorithm 4.3. Assignment of canonical Huffman codewords.
Input: Length l for each symbol s of the alphabet, determined by the Huffman method.
Output: Integer values of codewords assigned to symbols, plus the order number of each
symbol within samelength symbols.
Set maxlength := Maxl
for l := 1 to maxlength do
Set countll := 0
/ Count the number of codes for each length /
for i := 1 to q do
Set countll := countll + 1
/ Compute the first code for each length, starting from longest /
/ Shifting to one bit shorter means division of the next code by 2 /
Set firstcodemaxlength := 0
for l := maxlength 1 downto 1 do
Set firstcodel := (firstcodel+1 + countll+1 ) / 2
/ Initialize the assignment of codewords; nextcodel loops within length l. /
for l := 1 to maxlength do
Set nextcodel := firstcodel
/ Assign the codewords to characters and create the symbol table for decoding /
for i := 1 to q do
Set codewordi := nextcodel
Set symboll , nextcodel firstcodel := i
i i i
Set nextcodel := nextcodel + 1
The twodimensional array symbol is used in decoding, which is based on the fundamental
property of the canonical Huffman code: All kbit prefixes of an lbit codewords are always
strictly smaller (of their numerical value) than any kbit codeword. By inspecting the trans
mitted bit sequence, we take one bit at a time until the numerical value of the bit string is
larger than the firstcode of the respective length. Then we have reached a legitimate code, and
the related symbol can be looked up from the created symbol table.
The following picture illustrates the data structures used. Data about an example symbol ‘A’
with code 1236 having length 15 is given. The code is supposed to be the 3 in the group of
codes of length 15. The situation is just after assigning the character i to the symbol table.25
15 1234 2 14
1234 3 13
12361234 = 2
0 1 2 3
Algorithm 4.4. Decoding of canonical Huffman code.
Input: The numerical value of the first code for each codeword length, plus the symbol for
each order number within the set of codewords of equal length.
Output: Decoded symbol.
set value := readbit( ); set l := 1
while value firstcodel do
begin Set value := 2 value + readbit()
Set l := l + 1
return symboll, value firstcodel
The loop is extremely fast; multiplication by 2 can be implemented by shiftleft. The memory
consumption of the decoding process is quite small: The firstcode table has an entry only for
each different length. The symbol array is larger; it is shown as a 2dimensional array, but the
actual implementation can be an array of arrays, requiring only q entries, plus cumulative
counts for each length.
4.3. Tunstall code
All the previous coding methods were of the type ‘fixedtovariable’, i.e. a fixed number
(often 1) of source symbols were mapped to bit strings of variable length. Now we take the
opposite approach, namely map blocks of a variable number of source symbols into fixed
length codewords. Later we shall study many practical algorithms belonging to this category
(‘LZ78 family’, among others), which gather frequent substrings of the source message as
coding units. Here we restrict ourselves to pure source coding, i.e. we assume that only the
symbol probabilities to be given, not probabilities of substrings. The probability of a substring
s s ...s is thus computed as the product p p ...p of probabilities of the (independent)
i1 i2 ik i1 i2 ik
When using fixedlength codewords, the optimum selection for the extended alphabet would
be such where the substrings are equiprobable (i.e. have uniform distribution). In practice, we
may only approximate the optimum situation. Of course, an additional condition is that any
source message can be parsed into the substrings of the extended alphabet. Tunstall has
developed a technique (1968), which produces the required codebook. It can be characterized
as ‘reverseHuffman’ coding, since the two are in a way dual methods. Note also the resem
blance to ShannonFano coding.
The Tunstall method builds a trie, where each branch is labelled with a source symbol, and
each node has a weight equal to the product of probabilities of symbols on the path to the
root. For simplicity, we restrict the discussion to prefixfree substrings, i.e. the trie must be
complete, so that every nonleaf node has precisely q children; one for each symbol of the
source alphabet. This enables extremely simple encoding: we just follow the path of branches
where the labels correspond to those in the source message. The fixedlength codeword is
attached to the leaf. Decoding is still simpler: The transmitted codewords can be used as
indices to a lookup table of related strings. In the following, we denote the codeword length
by k. Thus, the number of different codewords is 2 , and we should assign them to the leaves
of the trie.
The trie is built topdown, by starting from a 1level trie with leaves for all symbols. Then we
repeatedly extend the trie by adding q children for the leaf having the largest probability, until
the number of leaves grows beyond 2q+1. Formally presented, the codebook generation is
done as follows.
Algorithm 4.5. Tunstall codebook generation.
Input: Symbolss,i = 1, ..., q of the source alphabet S, symbol probabilities p , i = 1, ..., q,
and the length k of codewords to be allocated.
Output: Trie representing the substrings of the extended alphabet, with codewords 0, ..., 2 u
attached to the leaves (0£ u£ q 2), plus the decoding table.27
Initialize the trie with the root and q firstlevel nodes, equipped with labels s , ... s ,
and weights p , ..., p .
n := 2 q Number of remaining codewords
while n ³ q 1 do
Find leaf x from the trie having the biggest weight among leaves.
Add q children to node x, equipped with labels s , ... s , and weights
weight(x)×p , ..., weight(x)×p .
Set n := n q + 1
for each leaf l in preorder do
Assign codeword(l ) := i.
Denote path(l ) = sequence of labels from the root to l .
Add pair (i, path(l )) to the decoding table.
If q 2, it may happen that some codewords are left unused. More precisely, the number of
unused codewords is
u=2 (q 1) 1
We could use them by extending some leaf only partially, but that would destroy the prefix
free property of the trie.
When implementing the algorithm, we obviously need a priority queue to effectively find the
largest weight among the leaves. By using a heap for this purpose, the codebook generation
takes time O(2 + k×2 /q), because the depth of the heap is O(k), but we need the heap only
for the O(2 /q) internal nodes. The complexity also includes the assumption that the fixed
length codewords can be manipulated with constanttime operations, i.e. k number of bits in
a machine word. This is a rather nonrestrictive assumption, because the size of the decoding
table can hardly be larger than, say, 2 entries. When walking the final trie in preorder, we
naturally maintain a stack of the ancestors of the current node, so that the path is easily
The last substring of a message may represent an incomplete path of the trie, but this is easily
handled by taking any complete path, and sending the message length before the coded
message. This is possible, because we assume a semiadaptive coding environment.
The average number of bits consumed by an input symbol is
(( P path)Length( path))
Example. Let S = A, B, C, D, P = 0.5, 0.2, 0.2, 0.1, and k = 4.
A B C D
0.5 0.2 0.2 0.1
A B C D A B C D A B C D
0.25 0.1 0.1 0.05 0.1 0.04 0.04 0.02 0.1 0.04 0.04 0.02
A B C D
0.125 0.05 0.05 0.025
The decoding table for this trie is as follows:
0000® AAA 0100® AB 1000® BB 1100® CB
0001® AAB 0101® AC 1001® BC 1101® CC
0010® AAC 0110® AD 1010® BD 1110® CD
0010® AAD 0111® BA 1011® CA 1111® D
All possible codeword values were used in this case, because 2 1 = 15 is divisible by q1 = 3.
The average number of bits per source symbol is 4/(0.25×3+0.65×2+0.1×1)» 1.86, whereas the
entropy is H(S) » 1.76 bits per symbol. One possible Huffman code for this example would be
A=0, B=10, C=110, and D=111, which results in 1.80 bits per character; somewhat better
than with Tunstall coding.
It should be noticed that the Tunstall trie is optimal among prefixfree tries. In chapter 2 we
noticed that in decoding of variablelength codes, the prefixfree property was invaluable,
because it guaranteed unique decoding. Here in encoding, on the other hand, the prefixfree
property is not mandatory: It is also possible to assign codeword values to internal nodes, and
allow the number of children to be q. At each step of encoding, we can take the longest
substring of the remaining part of the source message, so that the substring matches a path in
the trie, and send the attached codeword. This modification may improve the compression
gain somewhat, but it makes codebook generation more complicated, and slows down the
4.4. Arithmetic coding
One of the most popular source coding algorithms (in addition to Huffman code) in modern
compression methods is arithmetic coding. It is in most respects better than the traditional
Huffman coding: It reaches the informationtheoretic lower bound (within used precision),
i.e. entropy, and is very flexible in both static and adaptive coding situations. It represents a
pure separation of source coding from building and maintaining the model of the source.
Therefore, it is used in many of the best compression programs as the final coding technique.
With respect to compression performance, it is superior to Huffman coding especially for
small alphabets and skewed probability distributions. It is computationally relatively efficient,
although usually somewhat slower than Huffman coding. There are, however, faster
suboptimal rearrangements of the basic method, especially for binary source alphabets.
The origins of arithmetic coding can be traced back to the pioneers of information theory and
source coding, namely Shannon and Elias, who invented the basic ideas about 50 years ago.
However, modern arithmetic coding was discovered 1976, independently by two researchers
(Pasco and Rissanen). Since then, several papers have appeared describing the practical
implementation of arithmetic coding. Among the many, we mention here the paper written by
I.H. Witten, R.M. Neal, and J.G. Cleary: “Arithmetic Coding for Data Compression”, in
Communications of the ACM, Vol. 3, No. 6, 1987, pp. 520540. This paper contains also a
detailed Ccode of a simple implementation.
Arithmetic coding is hard to classify among other coding methods. In principle, it generates
only a single codeword for the whole message. Therefore, it can be regarded as an extreme
case of extended Huffman code, except that arithmetic coding does not create any explicit
codebook. The size of the codebook for extended Huffman is exponential in the block size,
and becomes prohibitive for large blocks. It is peculiar to arithmetic coding that there is no
clear correspondence between symbols of the source message and bits of the code; we cannot
say that a certain bit resulted from a certain symbol, as is the case with Huffman code.
The key idea of arithmetic coding is to represent the message by an interval of real numbers
between 0 and 1. Longer messages are represented with smaller intervals (and higher pre
cision) than shorter messages, and vice versa. The size of the interval should be in accordance
with the probability of the message. The encoding process starts with the whole 0, 1)
interval, and successive symbols of the message reduce the interval size in proportion to their
probabilities. At an arbitrary position, the interval is partitioned into subintervals, one for each
symbol of the alphabet, and then the next symbol of the message determines, which sub
interval is selected. Notice that the intervals are halfopen; the upper bound value belongs
always to the next higher subinterval.
In the following example, the alphabet is A, B, C, D with given probabilities 0.4, 0.3, 0.2,
0.1, and the message to be encoded is “BADCAB”. First, the starting interval 0, 1) is
divided into four subintervals: 0, 0.4), 0.4, 0.7), 0.7, 0.9), 0.9, 1), of which we select the
second one, corresponding to the first letter ‘B’ of the message. Interval 0.4, 0.7) is then
divided again into four intervals in the proportion of the original symbol probabilities: 0.4,
0.52), 0.52, 0.61), 0.61, 0.67), 0.67, 0.7). The next message symbol is A, so we select the
first subinterval. This process is repeated up to the last symbol of the message; the final
interval is 0.516784, 0.517072). The process can be graphically described as follows.30
B 0.52 0.52 ...... etc.
Notice how each succeeding interval is contained in the preceding interval, the final interval
size being equal to the product of probabilities of the message symbols. For visibility, the
intervals can be rescaled for each subinterval as follows:
1.0 0.7 0.52 0.52 0.5188 0.51736
D D D D D D
C C C C C C
B B B B B B
A A A A A A
0.0 0.4 0.4 0.508 0.5164 0.5164
After computing the final interval, any real value from that interval is a valid representative of
the whole message. For simplicity, we could take the lower bound 0.516784 of the interval as
the code. The midpoint of the interval is another popular choice, and we shall use it in our
analysis of the method.
Decoding is as simple as encoding, and proceeds as follows: From the first division into
subintervals, we notice that the value 0.516784 belongs to the second one corresponding to
symbol B. The interval is thus reduced to 0.4, 0.7), similarly as in encoding. Then we
compute the next division into four subintervals, and notice that the value 0.516784 belongs
to the first one, corresponding to A, namely 0.4, 0.52), and so on. The encoder and decoder
create the same hierarchical (nested) division, only the basis for choosing the correct
subinterval at each step is different (encoder: symbol at hand; decoder: code value).
In the following tentative algorithms, we assume that the symbols of our alphabet are
numbered from 1 to q, and the numbers are used instead of the actual symbols.31
Algorithm 4.6. Arithmetic encoding.
Input: Sequencex = x , i=1, ..., n; probabilities p , ..., p of symbols 1, ..., q.
i 1 q
Output: Real value between 0, 1) that represents X.
cum0 := 0
for i := 1 to q do cumi := cumi1 + p
low := 0.0
high := 1.0
for i := 1 to n do
begin range := high low
high := low + range cumx
low := low + range cumx1
return (low + high) / 2
Algorithm 4.7. Arithmetic decoding.
Input: v: Encoded real value; n: number of symbols to be decoded;
probabilities p , ..., p of symbols 1, ..., q.
Output: Decoded sequence x.
cum0 := 0
for i := 1 to q do cumi := cumi1 + p
low := 0.0
high := 1.0
for i := 1 to n do
begin range := high low
z := (v low) / range
Find j such that cumj1£ z cumj
x := j
high := low + range cumj
low := low + range cumj1
return x = x , ..., x
These algorithms are not applicable to practical coding as such, for several reasons: They
assume arbitraryprecision realvalued arithmetic, and decoding needs the number of symbols.
The major question is, what representation is chosen for the final code. We can take any
value from the final interval, but its exact representation is not necessary, as long as the
approximation is good enough for determining the correct interval. If we look at the binary
representations of the endpoints of the final interval, the best choice is to take all significant
bits of the midpoint, up to the first position which makes a distinction from both lower and
upper endpoints. In the above example, the binary representations are:
· upper: 0.517072 = .10000100010111101...
· lower: 0.516784 = .10000100010010111...
· midpoint: 0.516928 = .10000100010101010...32
A sufficient code is 1000010001010, i.e. 13 bits; the last bit is needed to ensure a prefixfree
code. From compression point of view, the code takes 2.17 bits per symbol, while the entropy
is only 1.846 bits per symbol. However our input message “BADCAB” did not have the
distribution assumed. Instead, the message “ABCDABCABA” would have had the final
4 3 2
interval length 0.4 ×0.3 ×0.2 ×0.1 = 0.000002764. The amount of information for this
probability is log (1/0.000002764)» 18.46 bits, which is exactly the value determined by the
entropy. However, we round this value up, and add one bit position, to guarantee prefixfree
property, so the final code will be 20 bits long. We give the general result formally below
(proof skipped). Notice especially that we restrict the discussion here to prefixfree messages,
so the intervals of different messages will be disjoint. We return to this issue later.
Theorem 4.2. Let range = upper lower be the final probability interval in Algorithm 4.6
when encoding message x. The binary representation of mid = (upper + lower) / 2 truncated
to l(x) = élog (1/range)ù + 1 bits is a uniquely decodable code for prefixfree messages.
Using Theorem 4.2, we can derive the average number of bits used per source symbol. Notice
that l(x) is the number of bits to encode the whole message x. The average code length of a
message containing n symbols is obtained as follows.
L = Pxlx ()()
é 1 ù
= Px ( ) log +1
å ê ú
ê 2 ú
£ Px ( ) log +2
ê 2 ú
= Px ( )log +2 Px ()
where S the extended alphabet of nsymbol blocks from S. On the other hand, we know
that L³ H(x ). Thus we get the following bounds for the number of bits per source symbol:
Hx() Hx() 2
££ L +
n n n
which for independent source symbols gives
HS()££ L HS()+
Thus, for long messages, the compression efficiency gets extremely close to the entropy. This
is a major result, and with good reason arithmetic coding can be called optimal. The common
conception that Huffman coding is optimal does not hold generally; only among the methods
that give distinct codewords for distinct symbols of the alphabet.
In the above discussion, we assumed that the set of studied source messages was prefixfree,
i.e. message pairs like “TO BE” and “TO BE OR NOT TO BE” were not accepted. The
reason is that the related intervals will not be disjoint, and hence we cannot guarantee the
uniqueness of the code. If we used the lower value as our codeword, then sequences “A”,
“AA”, “AAA”, ... would all result in the same codeword (value 0), if ‘A’ is the first symbol in33
partitioning. There are two possibilities: either transmit the length of the message before the
actual code, or extend the alphabet with an endofmessage symbol. The former is not useful
in adaptive (1phase) coding, so the latter is most often used. Actually, it makes the messages
prefixfree. The only question is, what probability we should assign to the new symbol.
Optimal would be 1/n for an nsymbol message, but generally n is unknown. Fortunately, an
estimate of the average message length is quite satisfactory.
We now turn to the practical implementation of arithmetic coding. The following problems
can be observed in the above tentative algorithms:
· The encoded message is created only when the last input symbol has been received.
Incremental transmission and decoding would be highly desirable in practice.
· The precision of normal floatingpoint arithmetic is insufficient for all but the shortest
messages. Fixedpoint arithmetic should be applied, most preferably integer operations.
The key idea to solving the above problems is to apply scalings (zoomings, renormalizations)
of the interval as soon as we know enough about the next bit to be transmitted. In other
words, the common bits in the front of lower and upper cannot change, anymore, and can
therefore be transmitted. The following cases can be distinguished:
1. If the upper bound of the interval is below 0.5, we know that the next bit in the codeword
will be 0. We can transmit it immediately, and in a way move the binary point one position
to the right in the representation of the interval, i.e.
lower := 2 × lower
upper := 2 × upper
2. If the lower bound gets 0.5, we know that the next code bit is 1, which is transmitted.
Removing the 1bit after the binary point corresponds to subtraction of 0.5, and then
moving the binary point right corresponds to multiplication by 2 (which in practice is
implemented by shiftleft):
lower := 2 × (lower 0.5)
upper := 2 × (upper 0.5)
We call these two operations halfscalings. Observe that we may have to apply halfscaling
operations 1 and 2 possibly in mixed order, until lower 0.5 and upper 0.5. For
example, if lower = 0.4 and upper = 0.45, the scalings proceed as follows:
Transmit 0, lower = 0.8, upper = 0.9
Transmit 1, lower = 0.6, upper = 0.8
Transmit 1, lower = 0.2, upper = 0.6
The stated problems are not, however, completely solved with halfscalings. A situation
may arise, where the nested intervals keep in the middle, so that lower 0.5 and upper
0.5 even though upperlower gets extremely small; possibly creating a floatingpoint
underflow. This problem can be solved with quarterscaling as follows:34
3. If lower 0.25 and upper 0.75, then we know that the next bits to be transmitted are
either 011...1 (if upper finally drops below 0.5), or 100...0 (if lower finally gets 0.5).
Now we scale the interval by
lower = 2 × (lower 0.25)
upper = 2 × (upper 0.25)
but we cannot send any bits yet. Instead, we have to remember, how many pending bits are
waiting, i.e. how many quarterscalings have been done before the next bit b is finally
resolved. The pending bits will be complements of b, and sent after b. Notice that the
interval size is doubled, so not very many repetitions of scaling are required, at a time.
As an example of quarterscaling, consider the interval where lower = 0.45 and upper = 0.55.
The following adjustments are now made:
lower = 0.4, upper = 0.6
lower = 0.3, upper = 0.7
lower = 0.1, upper = 0.9
Thus, three pending bits will wait for transmission. There may become still more, if the same
situation reappears after the next shrinking of the interval (with the next source symbol). If
not sooner, the final code value (midpoint of the last interval) will ultimately decide the value
of the pending bits.
The feature that all bits cannot be immediately sent means that the sender and receiver may
not operate quite synchronously. However, in practice, the delay will not be very long.
Commercial compression programs contain some adjustments to the basic algorithm, to avoid
the worst case (but at the cost of some additional redundancy).
The decoder starts from interval 0, 1), and then restricts it according to received bits. It reads
as many bits as needed to uniquely determine the next symbol: If a 0bit is received, the lower
half of the interval is selected, if a 1bit is received, the upper half of the interval is selected.
This is repeated until the interval is contained in the interval of some symbol. This will be the
next decoded symbol, and the interval is scaled similarly as in encoding. This means that the
‘used’ bits are discarded, and new ones are taken to again narrow down the interval in
searching for the next symbol. Notice also that the problem of pending bits does not occur in
Above we suggested that, due to speed, integer arithmetic would be more preferable than
floatingpoint arithmetic. It is relatively easy to convert the above calculations to operate on
integers. We just replace the 0,1) interval with a suitable 0, 21 integer interval. In the
scalings and other calculations, we just replace 0.5 by 21(= half) 0.25 by 21 (= first
quarter) and 0.75 by 3×21 (= thirdquarter). Instead of probabilities, we should use
frequencies of symbols. In reducing the interval, we have to perform the following com
upper := lower + (range× cumsymbol / totalfreq) 1
lower := lower + (range× cumsymbol1 / totalfreq)
In order to avoid integer overflow in multiplication, we set a restriction on both range and
frequency. For wbit machine words, the maximum cumulative frequency should be 2 .35
Due to scalings, the range is always larger than one quarter. Therefore, to avoid underflow
(lower = upper) for minimum frequency 1, the totalfreq should be less than 2 . Witten,
Neal and Cleary (see reference earlier) suggest the selection 0..2 1 = 0..65535 for the initial
range, and maximally 2 1 for totalfreq, when using 32bit machine words. It makes the
probability model somewhat imprecise for highly skewed distributions, but this is not usually a
Decoding can be similarly modified for integer arithmetic. The process can be started by
reading the first k bits (= value) sent by the encoder. The first symbol x is determined by the
ê(value lower+×1) total freq1ú
cumx ( £ 1) cumx()
Since the precision of value is two bits more than the precision of cum, adding more bits to
value would not change the interval, so the selection of x is unique and correct. After every
scaling, one front bit is dropped from value and the next one read in to the rear, creating a
new value for the next symbol to be decoded.
4.4.1. Adaptive arithmetic coding
Arithmetic coding is especially flexible in the respect that the model, i.e. probability dis
tribution, can be changed at any time, even after every transmitted symbol. This is because the
nested reductions of intervals can be based on any distribution, without affecting the correct
functioning of the method. In principle, adapting the model does not belong to pure source
encoding, but in order to develop a onepass method, which has no knowledge of the source
beforehand, adaptation is needed. We assume that successive symbols of the message are
independent. The approach is analogous to adaptive Huffman coding, see Section 4.2.2.
Basically, adapting the model means maintaining the symbol frequencies; after each trans
mitted symbol we increase its frequency, as well as the total frequency, by one. As this is done
by both encoder and decoder, the two models are kept synchronized. As for the initial model,
there are two alternatives:
(a) Initialize the frequencies of all symbols to one.
(b) Start with an ‘active’ alphabet consisting of only a placeholder symbol, called NYT (Not
Yet Transmitted). When some symbol is encountered for the first time, the NYT is
transmitted (using the distribution of the active alphabet), followed by the actual symbol
(using an even distribution for the ‘passive’ alphabet, consisting of all symbols that are
not yet encountered). This symbol is then transferred from the passive alphabet to the
active, equipped with frequency one.
The former approach is simpler, and sufficient in most situations, at least for long messages.
The latter has the advantage that the unused symbols do not distort the active distribution; i.e.
waste the probability space and cause an unnecessary reduction of intervals. This is especially
appropriate for large alphabets (e.g. consisting of words), where only a small fraction of all
symbols are actually used in a single message. The problem with alternative (b) is, what
frequency we should assign the NYT symbol. Actually, its probability decreases after every
new active symbol. Thus, we should initialize the frequency to the estimated size of the final36
active alphabet, and reduce the frequency by one after every new symbol. However, the value
should not be reduced to zero, unless the whole alphabet becomes active.
Another practical problem with adaptive arithmetic coding is the computation of cumulative
frequencies, which are needed in both encoding and decoding. Keeping a simple array of
cumulative frequencies is quite inefficient; for example, adding 1 to the frequency of symbol
‘A’ will cause addition to all subsequent cumulative frequencies. One possibility is to keep the
alphabet ordered according to increasing probability. Then, adding the frequency of the most
probable symbol would affect only one cumulative frequency, adding the frequency of the
next probable symbol would affect only two cumulative frequencies, and so on. In addition,
we need a mapping table for the order of symbols in the cumulative frequency table.
The above approach is applicable for small alphabets and skew distributions, but generally we
should have a more effective data structure. Here we suggest a balanced tree, which is a
modification of the structure presented by A. Moffat .
We suggest that the frequencies of symbols are maintained in the leaves of a balanced binary
tree. Each internal node contains the sum of frequencies of its children. An example tree looks
like follows; the actual storage could be implicit in an array, such as for heaps.
67 54 81 62
54 13 22 32 60 21 15 47
A B C D E F G H
The cumulative frequency of a symbol s is obtained as follows: Add the leaf frequency of s to
the frequencies of all left siblings on the path to the root. For example, the cumulative fre
quency of ‘G’ is 15 + 81 + 121 = 217. The cost of this computing is obviously at most
O(log q) for an alphabet of size q. Maintaining the structure is as efficient: If some frequency
is increased by one, all nodes up to the root should get an increment of one, as well. Thus, the
complexity is again O(log q).
4.4.2. Adaptive arithmetic coding for a binary alphabet
A binary alphabet (symbols 0 and 1) presents no problem to arithmetic coding; the above
algorithms are directly applicable to this case. However, now we can tailor the method and
develop faster encoding and decoding programs. The first observation is that the cumulative
frequency table has only one relevant entry, expressing the breakpoint between the pro
babilities of 0 and 1. In other words, we do not need any complicated data structure for main
A. Moffat: “Linear Time Adaptive Arithmetic Coding”, IEEE Trans. on Information Theory, 36, 2, 1990,
taining cumulative frequencies. In fact, in some cases it might be reasonable to decompose
longer symbol representations (e.g. ASCII) into bits, and do the encoding bitwise.
The main source of slowness in both arithmetic encoding and decoding is the multiplication
operations needed in reducing the intervals. Scalings, instead, need only shiftleft operations.
For a binary alphabet, several techniques have been developed, which avoid the multi
plications by some kind of approximations. We study here two such approaches.
(1) The first technique (due to Langdon Rissanen, 1981), is based on approximating the
smaller of the two probabilities by the nearest integral power of ½, denoted 1/2 , where
Q is an integer. The related symbol is called LPS (Less Probable Symbol), while the other
one is called MPS (More Probable Symbol). The encoding and decoding routines contain
the following expression:
low + range cum
The multiplication can be calculated by shifting range Q positions to the right. Notice
that we can freely choose either 0 or 1 as the ‘first’ symbol of our alphabet, whichever
happens to have a smaller probability. The order can even be changed during the coding,
as long as both the encoder and decoder apply the same rule of deciding the order.
Now the problem is, how we should choose the value Q for a given probability. Denote
the actual value of the smaller probability by p. The related symbol (LPS) is coded as if it
had probability 1/2 , occupyinglog (1/2 ) = Q bits (based on information theory). MPS
has true probability 1p, and occupieslog (11/2 ) bits. The average number of bits per
symbol is then
pQ (1 p)log(1 )
We should find optimal values of Q for each p in interval 0, 0.5). It is sufficient to
determine subintervals, within which a certain Qvalue is optimal. The breakpoint value
between, say, Q = r and Q = r+1 can be determined by solving the following equation:
pr (1 p)log(1 )= pr ( + 1)(1 p)log(1 )
2 r 2 r+1
p= where z = log
1+ z 1 1/2
The following table gives a few probability ranges, related Qvalues, and approximated
probabilities (1/2 ):
Probability range Q Effective probability
0.0452 0.0905 4 0.0625
0.0226 0.0452 5 0.03125
The maintenance of the actual probability is the responsibility of the model. For a given
probability, supplied by the model, the encoder (and decoder) chooses the correct range,
and applies the related Qvalue in the shiftright operation, avoiding multiplication.
It is surprising how little compression performance is lost due to the approximation. The
loss can be measured by the ratio of entropy to the average number of bits per symbol:
p logp (1 p) log(1 p)
pQ (1 p) log(112/)
The worst case is for the first breakpoint, namely p = 0.369, which gives 95 effective
ness. In practice a typical observed value has been around 98.5. Normally, such a small
loss is quite acceptable.
(2) E.g in the compression of blackandwhite images, another technique is used for ap
proximating the coding formulas. Several variations of the approach have been suggest
ed, e.g. Qcoder (original), QMcoder (refined; studied here), MQcoder (used in JBIG
standard for compressing bilevel images), and Mcoder (used in H.264/AVC video com
pression standard). The QMcoder involves a sophisticated technique for maintaining the
model (= probabilities of LPS and MPS) during the coding. The model is based on a fixed
Markov model (i.e. finite automaton), where certain events in coding trigger a transition
to the next state. Here we concentrate on the coding part of the method.
In QMcoder, MPS is regarded as the first symbol of the alphabet, and LPS is the second.
The roles of the original symbols may have to be changed during the coding. For
example, when coding a blackandwhite image, in mostlywhite areas the ‘white’ symbol
is the MPS, whereas in mostlyblack area the opposite holds. Again, we denote the
probability of LPS by p, and the probability of MPS by 1p.
QMcoder maintains two values for the intervals: lower and range. The ideal subdivision
of the interval can be described as follows:
lower + range
lower + range× (1p)
Again, independent of the chosen subinterval, multiplication is needed to update the
range value. To eliminate multiplication, QMcoder operates (virtually) in the overall
interval 0, 1.5). Scaling (renormalization, zooming) is done each time when the range
drops below 0.75. After scaling, 0.75£ range 1.5. Now, the following crucial approxi
mation is made:
This results in the following approximations of multiplications:39
range× p » p
range× (1p) = range range× p » range p
When coding the next symbol, the interval is updated as follows:
· For MPS, lower is unchanged, and range := range p.
· For LPS, lower := lower + range p, and range := p.
Scalings involve only multiplications by 2, which can be implemented by a shiftleft
operation. Thus all multiplications are eliminated. The practical implementation of QM
coder is done with integer arithmetic, replacing 1.5 by 2 = 65536, and 0.75 by 2 =
32768. Notice also that QMcoder does not use the midpoint of the interval as the code
value, but the lower bound. When the lower value is scaled, we do not have to subtract
32768, even if the value was larger than that (interval in the upper half). The bits that
flow over are just the correct 1bits of the code. They are sent to the output procedure as
8bit chunks, i.e. bytes. The problem of pending bits is solved in lowerlevel routines, and
we skip its discussion here. The details can be found from: W.B. Pennebaker J.L.
Mitchell: JPEG Still Image Data Compression Standard, Van Nostrand Reinhold, 1993.
A slightly different MQcoder is used in JPEG 2000 image compression standard.
4.4.3. Viewpoints to arithmetic coding
There are some practical problems with arithmetic coding. First, since it produces one
codeword for the entire message, the code is not partially decodable nor indexable; we
always have to start from the beginning in order to recover a portion of the encoded message.
Second, arithmetic coding is vulnerable. Onebit errors usually result in a totally scrambled
data for the rest of the decoded message, and usually an incorrect number of decoded
symbols. Codewordbased methods like Huffman tend to be selfsynchronizable, although no
upper bound for the length of the disrupted sequence can be given.
A version called Arithmetic Block Coding (ABC) avoids these problems, assuming that the
symbol distribution is static. Its principle is to apply the original idea of arithmetic coding
within the limits of machine words (32 or 64 bits), and restart a new coding loop when the
bits are used. The coding technique resembles Tunstall code, but does not construct an
explicit codebook. ABCcoding is quite fast, because it avoids the scalings and bitlevel
operations. The code length is not optimal, but rather close to it.
J. Teuhola T. Raita: ”Arithmetic Coding into FixedLength Codewords”, IEEE Trans. on Inf. Theory, Vol.
40, No. 1, 1994, pp. 219223.40
5. Predictive models for text compression
In the previous sections we have studied source encoding methods which are based on a given
probability distribution of symbols. Now we change our focus to the methods, using which
the distribution is determined. This task is the responsibility of the model of the source. The
best models are those which at each step assign the highest possible probability to the symbol
which will actually appear in the text . This is then reflected in the smallest possible number of
bits ‘used’ in actual coding. We can characterize the model as making predictions of what will
follow in the message. The concept of prediction is generalized so that the model does not
predict only the most probable symbol, but gives all possible symbols some probabilities on
which they a predicted to occur. The model can be determined statically before coding, or
dynamically, based on the processed prefix of the message.
Predictive techniques are currently the most powerful models in text compression, with
respect to compression efficiency. Their drawback is relatively high consumption of resources
– both time and working storage (for the model). Many of the practical text compression
programs use so called dictionary methods, where frequent substrings are units of coding.
The subsequent ‘entropy coding’ technique is usually very simple even fixedlength coding
of substrings. Dictionary methods are relatively fast, but their compression power is clearly
lower than with the best predictive methods. This may be somewhat surprising, since one
would assume that taking larger units as the basis for modelling would enable bigger gains in
compression. Dictionary techniques are described in Section 6.
We can distinguish different kinds of predictive models, depending on the information that is
used as the basis of prediction. Three typical approaches are the following:
(1) Finitecontext models
In normal text, especially in natural language, there is a high interdependency between suc
cessive symbols. In English, for example, the probability of ‘u’ is 0.024, but the probability of
‘u’ to occur after ‘q’ is as high as 0.991. Thus, if coding has proceeded to an occurrence of
‘q’, the following ‘u’ will involve only log 0.991 bits» 0.013 bits of information. On the
other hand, the prediction is not certain, and the other symbols must also be assigned non
zero probabilities, and their occurrence would result in a very high number of bits. This idea
can be generalized to more than one preceding symbol. If we have last processed the symbols
‘th’, then there is a high probability for ‘e’ to follow, although other plausible alternatives
exist, as well.
We thus need conditional probabilities of symbols, knowing a few previous symbols. The
lower bound of compression is given by conditional entropy (see Section 2), which is
(almost) reached by arithmetic coding, because it functions as well for distributions that vary
from symbol to symbol. The conditional probabilities are usually determined empirically, by
gathering statistics from the message text itself.
The previous symbols used in prediction are called the context (or prediction block; called
also conditioning class) of the next symbol, and the length of the context is called the order
of the model. Order 0 means a model where no context is used, but the symbols are predicted
Actually, the techniques discussed in this chapter apply to any symbol sequences in general, not just text.41
on the basis of their (global) probabilities. We even define an order ‘ 1’ to denote prediction
of all q symbols with the same probability (1/q). In natural language, the highest correlations
are between adjacent symbols, although some kind of dependencies may extend even over 30
symbols, but weaken considerably at word boundaries. In principle, a longer context should
give a more precise (= more skewed) distribution and thus result in more effective
compression. In practice, it has been observed that extending the order above 5 or 6 will not
usually pay. The problem is that collecting reliable statistics becomes difficult, because longer
substrings occur more seldom. Also the model size grows rather large, although only the
observed contexts need to be recorded.
(2) Finitestate models
There may be dependencies between symbols, which cannot be represented by finitecontext
models. Consider, for example, a source program written in some structured programming
language. If the keyword if has been encountered (in a Pascallike programming language), it
is very probable that the word then will follow sooner or later, even though the number of
intervening symbols may be arbitrarily large. Thereafter, we would expect the word else,
although it may never appear. On the other hand, it is highly exceptional to see then word
then or else if no if has been met. Another example is the matching of parentheses in
arithmetic expressions. The occurrence of ‘(‘, without an enclosing ‘)’ is infrequent. This, as
well as expecting quotes to match, holds also for natural language.
A finitestate model is described by a finitestate machine, with states, transitions, and symbol
labels for transitions, together with transition probabilities. The model is also called a Markov
model or finitestate probabilistic model. Compressing a message means traversing through
the machine from state to state, choosing transitions according to input symbols. A symbol is
encoded (e.g. by arithmetic coding) using the distribution of transitions leaving the current
state. To adapt to the if ... then ... else structure, the machine should lead, after seeing ‘i’ and
‘f’, to such a subset of states which gives the transition sequence ‘t’®‘h’®‘e’®‘n’ a higher
probability than normal. The model thus remembers that if has appeared. Another example
where a finitestate model is useful is such data which contains certain size of repeating blocks
(alignments) of symbols. Such data could be a table, an image, a DNA sequence, etc. A
sketch of a machine for encoding matching (nonnested) parentheses is given below, showing
partitioning of the states.
States with low States with higher
probability for ’)’ probability for ’)’42
Although finitestate models are in principle stronger than finitecontext models, it is very
difficult to take advantage of this possibility, unless the special features of the messages to be
compressed are known beforehand. Notice that the finitestate machine need not be static
during the compression; in a onephase adaptive compression scheme, we can add states and
transitions, as well as update the transition probabilities during compression, as long as the
decoder is able to do exactly the same changes to its own model, intact. An efficient
compression technique using this approach was presented for a binary alphabet by Cormack
Note still that any finitecontext model can be represented by a finitestate model. The related
machine has a state for each possible context, and transitions for all possible successors of
that context. It is clear that finitestate models suffer from the same problem as finitecontext
models, namely high demand of working storage for the model. A machine corresponding to
context length n would in principle have q states and q transitions from each state. In
practice, we only have to create states according to observed contexts. As for speed, walking
from state to state is rather fast, but maintaining the model can be timeconsuming.
(3) Grammar models
Even though finitestate models are able to capture noncontiguous properties of the source,
they are not able to notice arbitrary nestings of grammatical structures. For example, if a
mathematical expression (or a LISP program) can contain an arbitrarily deep nesting of
balanced parentheses, there is no finitestate machine that could ‘check’ the correctness. A
more powerful machine is needed, which has a stack as an auxiliary data structure. The same
holds also for the ‘if ... then ... else’ example above, because these structures can also be
A suitable model for these kinds of formal languages is a contextfree grammar. A syn
tactically correct source program could be compressed by first parsing the program and then
expressing the parse tree in some compact way. There exist compression programs based on
this approach, but they are far from generalpurpose. Syntactically incorrect programs cannot
be handled, nor comments within programs (the proportion of which should be very large). As
for natural language, it is very hard to find a suitable grammar. Automatic induction of the
grammar, on the basis of the source message only, is not possible, because the system should
learn also the features which are not allowed in the language. Moreover, the level of nesting is
not usually very deep in natural language, implying that finitecontext and finitestate models
are sufficient for all practical purposes. Notice that as long as an upper bound for nesting is
known, a finitestate model can be built to capture the structure.
5.1. Predictive coding based on fixedlength contexts
We start the description of predictive methods by introducing three simple techniques which
all use a fixedlength (k) context as the basis of prediction. These techniques are examples of
different kinds of couplings between modelling and coding, and also the tradeoff between
speed and compression efficiency. All three methods are fully adaptive, onepass techniques.
G.V. Cormack and R.N.S. Horspool: “Data Compression Using Dynamic Markov Modelling”, The
Computer Journal, Vol. 30, No. 6, 1987, pp. 541550
T. Raita and J. Teuhola: “Predictive Encoding in Text Compression”, Information Processing Manage
ment, Vol. 25, No. 2, 1989, pp. 151160.43
At each step of coding, we should be able to define an approximated distribution of successor
symbols in any context of length k. Since we cannot have statistics of contexts before we have
seen them, we must resort to some default predictions during the learning phase of coding.
Even after having seen the context earlier, it may be that the current successor has not
occurred before in this context. Therefore, we must have default codings for the notyet
occurred successors, as well.
An important technical question is, what kind of data structure we should use for storing
information about contexts (so called prediction blocks) and their successors. Any data
structure that enables searching by a stringvalued key is sufficient. Most of the predictive
methods, presented in the literature, use a trie structure for storing and searching context
strings. Here we apply a much faster technique, namely hashing: The context string of k
symbols is converted to a number, from which the home address is calculated by a
(randomizing) hash function. The successor information, based on the processed part of the
message, is stored in the home address. There are two aspects that make hashing especially
· The hash table is faster and more compact than a trie, because the contexts need not be
represented explicitly. This means that, given a certain amount of main memory, we can
use longer contexts with hashing, compared to the trie.
· Collisions can be ignored. Their only effect is to deteriorate the compression efficiency
slightly, because two different contexts may be interpreted to be the same. However, if the
table is not too full, collisions are so rare that they do not play a significant role. In
adaptive compression, the table need not be transmitted to the decoder, because the
decoder is able to maintain its own table identically with the encoder.
The three methods are here presented in increasing order of compression gain (and thus in
decreasing order of speed):
A. Simple success/failure prediction
Given a source message X = x x ... x and a prefix x x ... x of x, where i k, we denote the
1 2 n 1 2 i 1
context of x by C = x ... x . We predict that x is the same as the successor of the previous
i ik i 1 i
occurrence of C before this. The success/failure is encoded with one bit; in case of a failure
. If C has not occurred before, a default pre
we also have to transmit the actual successor x
diction is made (e.g. space for English sources). A hash table T0..m1, which is an array of
m symbols, is maintained to keep track of the latest successors. For each i from k+1 to n we
store x into the slot indexed by H(x ... x ). The hash function H maps kgrams into 0..m1.
i ik i 1
The data structure is illustrated as follows:
Thus we do not use any of the ‘entropy coders’ of Chapter 4, but a simple yes/no coding.44
Character string S
A B C D X Y Z
Hash function H
Hash table T
Y X Z
The encoding algorithm is formulated as follows:
Algorithm 5.1. Predictive success/failure encoding using fixedlength contexts.
Input: Message X = x x ... x , context length k, hash table size m, default symbol d.
1 2 n
Output: Encoded message, consisting of bits and symbols.
for i := 0 to m1 do Ti := d
Send symbols x , x , ..., x as such to the decoder
1 2 k
for i := k+1 to n do
addr := H(x ... x )
ik i 1
pred := Taddr
if pred = x
then Send bit 1 / Prediction succeeded /
Send bit 0 and symbol x / Prediction failed /
Taddr := pred
It is obvious that this extremely simple method is exceptionally fast, although its compression
gain is modest. On the other hand, it is able to react to local changes of successors for a given
context. Due to its speed, this technique has actually been used in some network protocols.
Decoding is as simple as encoding, and we skip it here.
One interesting technical detail is the selection of the hash function, and the computation of
numerical representatives of successive (overlapping) kgrams. If we are encoding ASCII
characters, then for k£ 4 we can consider the kgram as an unsigned 32bit integer as such.
Moreover, the next kgram is obtained from the previous by shiftleft and logical or
operations. The common and good choice for the hash function is H(C) = C mod m.
Note that sending the first k symbols as such (to get the process started) can be avoided by
assuming a virtual string of k symbols (e.g. spaces) to be at the front of the message. These45
then constitute an artificial context for the first actual symbol x . The same technique can be
applied in the methods to follow, as well.
B. Universal coding of successor numbers.
Now we generalize the prediction to more than one alternative symbol. For each context,
every symbol of the alphabet is given an order number, in decreasing order of probability
(approximately). However, due to speed, we want to avoid explicit maintenance of
frequencies, and therefore we encode the order number of the actual successor within the list
of successors for the current context. The order of probability in the list is approximated by
applying the movetofront principle: The actual successor is always moved to the front of the
list. This means the same as in method A; the latest observed successor of the context is our
primary prediction. The symbols which have never occurred as successors for a context,
appear virtually at the rear, but in practice they do not have to be stored. In most cases, the
order number of the real successor should be small, and we can apply any of the universal
coding schemes (a to d) for integers, presented in Section 2. Hash table T now contains the
heads of the successor lists. Decoding is again analogous, and skipped.
Character string S
A B C D X Y Z
Hash function H
Hash table T
Y X Z
A A A
The encoding algorithm is given below.
Algorithm 5.2. Prediction of symbol order numbers using fixedlength contexts.
Input: Message X = x x ... x , context length k, hash table size m.
1 2 n
Output: Encoded message, consisting of the first k symbols and gcoded integers.46
for i := 0 to m1 do Ti := NIL
Send symbols x , x , ..., x as such to the decoder
1 2 k
for i := k+1 to n do
addr := H(x ... x )
ik i 1
if x is in list Taddr
r := order number of x in Taddr
Send g(r) to the decoder
Move x to the front of list Taddr
r := order number of x in alphabet S, ignoring symbols in list Taddr
Send g(r) to the decoder
Create a node for x and add it to the front of list Taddr
C. Statisticsbased prediction.
By maintaining the frequencies of successors we get a more precise estimate of the actual
successor probabilities for each context. By extending the data structure of Method B, we can
store the frequencies in the list elements, together with the corresponding characters. The
total frequency of the list items is stored in the list head (field Ti.total of the hash table
entry). Since we now can compute probability estimates for successors, we use arithmetic
coding for them.
The main problem, concerning all predictive methods, is what we should do with the symbols
that have not occurred with the current context C before. We cannot give them zero
probability, because coding would fail if the symbol is met, anyway. Here we assume that
some small initial frequency ( e) is assigned to all symbols in each context. The value of e
should be optimized empirically. When using integer frequencies, we must choosee = 1, but
probably use increments larger that one for the actual successors.
The data structure extended by frequency fields and total frequencies for successor lists is
given below, followed by the encoding algorithm.47
Character string S
A B C D X Y Z
Hash function H
Hash table T
Head of successor list (ptr)
V 3 X 2 Z 4
Y 2 W 3
Ae Ae Ae
Algorithm 5.3. Statisticsbased coding of successors using fixedlength contexts.
Input: Message X = x x ... x , context length k, alphabet size q, hash table size m.
1 2 n
Output: Encoded message, consisting of the first k symbols and an arithmetic code.
for i := 0 to m1 do
begin Ti.head := NIL; Ti.total :=e×q;
Send symbols x , x , ..., x as such to the decoder
1 2 k
Initialize arithmetic coder
for i := k+1 to n do
addr := H(x
... x )
ik i 1
if x is in list Taddr.head (node N)
then F := sum of frequencies of symbols in list Taddr.head before N.
F := sum of frequencies of real symbols in list L headed by Taddr.head.
F := F +e×(order number of x in the alphabet, ignoring symbols in list L)
Add a node N for x into list L, with N.freq = e.
Apply arithmetic coding to the cumulative probability interval48
F / Ti.total), (F+N.freq) / Ti.total)
Ti.total := Ti.total + 1
N.freq := N.freq + 1
end / of for i := … /
Finalize arithmetic coding
The compression efficiency of Method A for English source has been observed to be about
4.5 bits per symbol, while Method B produces from 3.5 to 4 bits per symbol and Method C
from 3 to 3.5 bits per symbol. The ratios of both compression and decompression times are
roughly 1:5:11 for the three methods.
The presented methods exemplify the separation of modelling and coding steps in com
pression, although choosing one may affect the choice of the other. Next we shall study more
sophisticated modelling techniques, resulting in better compression performance.
5.2. Dynamiccontext predictive compression
If we perform adaptive compression, where the model gets more precise all the time during
the process (assuming an ergodic source), then it is clear that we cannot predict with a
context that has not occurred earlier in the text. In other words, we should choose the longest
context that has appeared before. This implies that our contexts tend to grow, when the
coding gets further. A sophisticated implementation of this simple idea has been presented by
R. N. Williams . He made adjustments especially concerning the following questions:
· How to store the observed contexts to enable effective finding of the longest possible
context at each step
· How long contexts should we store
· When is a context considered ‘reliable’ to make a prediction
· What to do if the prediction fails, so that the current successor has not occurred before in
A tailored trie structure is used to store the contexts. The substrings are stored in backward
order, so that the last symbol of the context is stored on the first level, and the first symbol of
the context on the leaf level. Every node stores the frequencies of observed successor symbols
for the context represented by the path from the current node to the root. At a certain
position of coding, we can just scan the message from the ‘cursor’ backwards, and
simultaneously traverse down the tree, directed by the scanned symbols. An example trie, with
maximal context length 3 is presented below. It uses a restricted alphabet of A, D, J, P, and
is based on the string “JAPADAPADAA”. Attached to each node there is an array of four
numbers, representing the frequencies of successor symbols A, D, J, P, in this order.
Ross N. Williams: “DynamicHistory Predictive Compression”, Information Systems, Vol. 13, No. 1, 1988,
A D J P
1,2,0,2 2,0,0,0 1,0,0,0 2,0,0,0
A D J P A A
0,0,0,0 1,0,0,1 0,0,0,1 0,2,0,0 2,0,0,0 2,0,0,0
D A A P D J
0,0,0,0 1,0,0,1 0,2,0,0 2,0,0,0 1,0,0,0 1,0,0,0
The context lengths are increased, and the trie extended, using a controlling parameter, called
extensibility threshold, et. It is the minimum number of occurrences that a node (i.e. the
related context) must have before it can get children (i.e. the context extended backwards).
The value of et is chosen between 2,¥), depending on the desired model size.
There are two other parameters to control the model size:
· Depth limit m of the trie sets also a limit to the context buffer and the time consumed per
symbol, namely O(m).
· Node limit z gives the available number of nodes in memory; the model can be ‘frozen’
after reaching the limit.
The prediction itself can be tuned by a parameter called credibility threshold, ct. It is the
minimum number of occurrences that a context must have before it can be used to make a
prediction. This is based on the plausible belief that a shorter context with more precise
statistics can often make a better prediction than a longer context with very rough statistics.
The value of ct is chosen from 1, ¥).
The coding of symbols can be done e.g. by arithmetic coding, using the frequencies stored in
the predictor node. Now we are faced with a socalled zerofrequency problem. If the actual
successor has not occurred before, then straightforward computation would give it probability
zero, which is illegal. We should assign a small positive probability for each nonoccurred
symbol, but it is not clear what that value should be. Williams suggests the following formula
for the probability of a symbol that has followed the given context x times out of y; parameter
q is the alphabet size:
The formula is sensible, because
·x(x, y) » x/y, for large x and y.
· if x = y = 0, then every symbol is given probability 1/q (case with no statistics),
· if x = y/q then the symbol is given probability 1/q.50
We have now solved all principal problems, and can present the encoding algorithm.
Algorithm 5.4. Dynamiccontext predictive compression.
Input: Message X = x x ... x , parameters et, m, z, and ct.
1 2 n
Output: Encoded message.
Create(root); nodes := 1;
Initialize arithmetic coder
for i := 1 to q do root.freqi := 0
for i := 1 to n do
current := root; depth := 0
next := current.childx / Assume a fictitious symbol x /
while depth m and next¹ NIL cand next.freq³ ct do
current := next
depth := depth + 1
next := current.childx
x(current.cumfreqx , current.freqsum))
Start to update the trie
next := root; depth := 0
while next¹ NIL do
current := next
current.freqx := current.freqx + 1
depth := depth + 1
next := current.childx
/ Continues … /
/ Study the possibility of extending the trie /
if depth m and nodes z and current.freqsum³ et
forj := 1 to q do
newnode.freqj := 0
newcode.childj := NIL
current.childx := newnode
newnode.freqx := 1
nodes := nodes + 1
Finalize arithmetic coder
The decompression program is (again) analogous to compression: At each position, we use
the current context to determine the node which the encoder has used in prediction. The
related frequency vector is fed to an arithmetic decoder, which solves the symbol. The main
tenance of the trie is performed identically with the encoder.
Williams has performed empirical tests to determine the compression efficiency of his method
with different types of source data. Some of his results are as follows:
Text type Source size Bits per symbol
English text (Latex) 39 836 3.164
Dictionary 201 039 4.081
Pascal program 20 933 2.212
While these results are already quite promising (well below the order0 entropy of the studied
sources), we can pinpoint a certain defect in the usage of available prediction information: If
prediction fails, i.e. the actual successor of the context has not occurred before, the symbol is
given a small slice of probability (determined by function x). The same probability is given to
any symbol that has not occurred before with this context. However, some smaller context
might provide us valuable information about the relative proportions of symbols, which have
not appeared with the larger context. This observation leads us to the method to be described
5.3. Prediction by partial match
Now we try to use more of the contextual information collected from the front part of the
message, for predicting the distribution of the next symbol. The initial idea is to apply
blending of contexts of different lengths, as follows: The probability of symbol x is computed
as the weighted sum
Px () = wPx ()
where o goes through all possible orders of contexts, and P (x) is the statistically determined
probability for order o. The heuristically determined weights w should sum up to 1. Clearly,
P(x) 0 if w 0, which is sufficient for correct coding.
In practice, blending of different contexts is done in a different manner, by finding the longest
(£ k) matching context, and then escaping to a shorter context, if the longest gives a zero
probability to the successor symbol. More precisely, we maintain the observed frequencies of
all contexts of orders 0 to k, as well as the frequencies of their successors. The first estimate
for the conditional probability of symbol x in message x is
Freqx( ...) x
P(xx ...) x =
i ik i1
Freqx( ...) x
If this is zero, we encode a fictitious escape symbol (esc), with some small but nonzero
probability P(esc) , repeat the attempt to encode x , but now using context length k1. This is
repeated until the above formula gives a nonzero probability, or the order becomes1, which
gives the probability 1/q. Note especially that the frequency of an empty context
(x ...x ) is the number of processed symbols (= i1). In principle, the encoding of each
i i 1
source symbol consists of encoding zero or more escapes plus the actual symbol.
The procedure can be still improved by applying the so called exclusion principle: When
computing the probability of symbol s in some context with order k, we can exclude all the
symbols which have occurred as successors of longer contexts. We know that the current
symbol is not one of them; otherwise we would not have had to escape to a shorter context.
This idea increases the probabilities and hence improves the compression efficiency. On the
other hand, it has a negative effect on the processing speed. The following notation will be
used for the adjusted probability:
Freq(x ...x x ) Freq(x ...x x )
i j i1 i i j i1 i
P (x x ...x )==
ex i i j i1
Freq (x ...x ) Freq(x ...x s Freq(x ...x s)= 0)
ex i j i1å i j i1 i j1 i1
The arithmetic coder is used as the final encoder, therefore we also have to calculate
cumulative probabilities Cum (x x ...x ). The whole encoding algorithm is as follows:
ex i ij i 1
J.G. Cleary and I.H. Witten: “Data compression using adaptive coding and partial string matching”, IEEE
Trans. Communications, Vol. 32, No. 4, 1984, pp. 396402.
Actually, the probability P(x …) must be slightly decreased, in order to make room for the escape.
Algorithm 5.5. Compression using prediction by partial match.
Input: MessageX = x x ... x , parameter k = longest context length.
1 2 n
Output: Encoded message.
Initialize arithmetic coder
for i := 1 to n do
coded := False
j := Maxm£ k (x
... x ) Î Contexts
i m i 1
while not coded do
if Prob (x x ...x ) 0
ex i ij i 1
arithencode(Cum (x x ...x ), Cum (x1 x ...x ))
ex i ij i 1 ex i ij i 1
coded := True
arithencode(1.0 Prob (esc x ...x ), 1.0)
ex i j i 1
exclusionset := successors(x ...x )
i j i 1
if not coded
j := j 1;
if j = 1
m := z x z Ï exclusionset
q’ := q exclusionset
coded := True
for j := 0 to Min(i, k) do
contexts := contextsÈ x ... x
i j i
freq(x ... x ) := freq(x ... x ) + 1
ij i ij i
Finalize arithmetic coder
Above we have assumed that functions Prob and Cum consult the variables contexts and
exclusionset. Note also that substrings in contexts include also the predicted symbols, i.e.
their length is maximally k+1. There are several implementation details to be solved, e.g. the
escape probability, strategy for maintaining frequencies, and the data structure used to store
the contexts. We shall talk about each of these in turn.54
There is no unique (theoretically optimal) solution to the zero frequency problem, i.e. to the
determination of escape probability. The first idea is to assign the esc symbol always the
smallest possible frequency, i.e. 1, and
Prob (esc x ...x ) =
A i j i 1
Freq (x ...x ) +1
ex i j i
Freq(x ...x )
i j i
Prob (x x ...x ) =
A i i j i 1
Freq (x ...x ) +1
ex i j i 1
This is reasonable in the sense that the escape probability decreases when the context
frequency increases, i.e. it becomes more and more probable that the successor has already
been met before. PPM equipped with this strategy is called PPMA. Another, not so obvious
deduction is that the escape probability is proportional to the escapes already taken with the
present context. In other words, if some context has had many different successors, there will
probably be many more.
Prob (esc x ...x ) =
B i j i 1
Freq (x ...x )
ex i j i 1
dif =s s Î Succ(x ...x )
i k i 1
This technique, named PPMB, also includes a feature that a symbol is not predicted until it
has occurred twice. Therefore, the probability of successor x is obtained as follows:
Freq(x ...x x ) 1
i j i 1 i
Prob (x x ...x ) =
B i i j i 1
Freq (x ...x )
ex i j i 1
The ‘1’ in the numerator stands for the first occurrence, handled as an escape. Still another
formula was suggested by Alistair Moffat . It is similar to PPMB, but begins predicting
symbols as soon as they have occurred once as successors. This approach, called PPMC, has
turned out more successful than PPMA and PPMB in practice. The formulas for esc and the
real successor are as follows:
Freq (x ...x )+ dif
ex i j i1
Freq(x ...x x )
i j i1 i
Prob (x x ...x )=
C i i j i1
Freq (x ...x )+ dif
ex i j i1
Algorithm 5.5 used the rule that, after prediction, the frequency of the predicted symbol was
increased by one for all orders of contexts from 0 to k. The PPMC version, instead, applies so
called update exclusion: The symbol frequency is increased only for the context used to
predict it. This can be rationalized by thinking that the frequencies for lower order contexts
represent only the cases where the symbol cannot be predicted by higherorder contexts. This
modification to the algorithm has been observed to improve the compression efficiency by a
couple of per cent. Moreover, it reduces the time required for updating the frequencies.
A. Moffat: “A note on the PPM data compression algorithm”, Research Report 88/7, Dept. of Computer
Science, Univ. of Melbourne, Australia, 1988.55
PPMC uses only 8 bits for the frequencies; periodic scaling is needed, but the method
becomes more adaptive to local changes in the statistical properties of the message.
Another modification, called lazy exclusion, does not reduce the alphabet when taking an
escape during prediction. This gives somewhat worse compression (about 5), but the
compression process becomes significantly faster (even to the half), because there is no need
to keep track of excluded symbols. This, and some other simplifications are implemented in a
version called PPMC’, which is tuned for high efficiency. There are at least two simple
alternatives for maintaining the contexts (and their successors):
1. Backward tries were described in Section 5.2. Their advantage is that all contexts of
different orders can be found along a single path from the root down to the longest
matching context. Finding the next current context must be started from the root.
2. Forward tries store the contexts in forward order. This is useful in the sense that the
successors can be stored within the same structure, i.e. the height of the trie is k+1. Counts
are maintained for each node (partial context): The count represents the number of
occurrences of the context (represented by the path from the root), and the counts of its
children record how often the corresponding symbols have occurred in the context. Thus,
the parent count should be equal to the sum of child counts (+1 for the last processed
context, whose successor has not been counted yet).
Forward tries are not very efficient as such, because every escape to a shorter context
requires walking a new path from the root. However, they can be made much faster by
equipping the nodes with so called vine (suffix) pointers. Suppose that the path of symbols to
a given node on level m is s , s , ..., s . The vine pointer from this node points to the node
1 2 m
on level m1, representing path s , ..., s . Vine pointers have two advantages. First, after
finding the longest matching context, the possible escapes can be realized by following the
vine pointers to shorter contexts. Second, the next current context can be found by following
the vine pointer from the previous predicted node. Thus, it is never necessary to search for
contexts, but only to follow pointers. Updating the tree may involve more time, unless update
exclusion is applied. New contexts must be added, anyway. An example tree, derived from the
message “cacbcaabca” is given below, for k=2. Vine pointers are shown with dashed arrows.
a (4) b (2) c (4)
a (1) b (1) c (1) c (2) a (3) b (1)
b (1) c (1) b (1) a (2) a (1) c (1) c (1)56
The compression performance of PPMC is excellent, producing about 2 bits per symbol for
long messages in English language. Therefore it is generally used as a reference method.
Several improved versions of PPM have been published. One of them, presented by P.
Howard , suggested a new variant (D) of estimating the escape probabilities: When pro
cessing a new successor after some context, PPMC adds 1 to both escape frequency (variable
dif) and the new symbol frequency. PPMD, instead, adds only ½ to both, so that altogether 1
is added, no matter if the symbol is new or met before in this context. The formula for the
escape probability is simply
u / 2
where u is the number of different symbols in the context and n is the total frequency of the
context. This, together with some other special tunings, improves the compression efficiency
about 5 10, compared to PPMC (see table of comparison below).
The inventors of PPM have later published a version called PPM, the idea of which is to
relax from setting a fixed maximum length for the contexts. Instead, the method chooses
always the longest deterministic context currently in the model. This means that the context
has earlier occurred with only one successor. If there is no fitting deterministic context in the
current model, the longest stored context is used, instead. This approach represents extreme
prediction: choose as skew a distribution as possible, based on the most uncertain statistics.
Practical experiments have shown that, as for compression efficiency, this is a relatively
profitable strategy, although improvements are perhaps smaller than expected.
PPM uses a generalized context trie, where each path from the root to a leaf represents a
context (substring) that has had only a single successor in the message, up to the current
position. Each leaf node has a pointer (= index) to the successor symbol of the context in the
source message. This makes it simple to extend the path if the same context appears again:
add one node for the pointed symbol, and add one to the pointer value. To update the trie, a
linked list of pointers to the currently active contexts (= suffixes of the processed part of the
message) can be maintained. This list in a way replaces the vine pointers suggested in the
earlier implementation of PPM.
The implementation of PPM is obviously more challenging than PPM. The amount of model
data can be much larger, the trie can be much higher, and processing much slower. The
authors suggest a so called PATRICIAstyle trie, where space is saved by collapsing non
branching subpaths into single nodes. This will make the size of the trie linear in the length of
the input message. However, the maintenance of frequency counts for the nodes becomes
PPM uses a similar technique for handling escapes as PPMC. The choice of escape proba
bility is here even more crucial, because the method uses longest deterministic contexts. These
contexts tend to grow and grow, keeping the prediction insecure (= escapes occur)
throughout the message, while for PPMC the proportion of escapes reduces when coding
proceeds. PPM also applies the technique of excluding predictions made by higherorder
contexts, after escaping to lower order. Another tailored improvement in the PPM family,
P.G. Howard: “The Design and Analysis of Efficient Lossless Data Compression Systems”, Technical Report
CS9328, Brown Univ., 1993.
J.G. Cleary, W.J. Teahan, and I.H. Witten: “Unbounded Length Contexts for PPM”, Proc. of the Data
Compression Conference, Snowbird, Utah, 1995, pp. 5261.57
called PPMZ, has been presented by Charles Bloom . He attacked three features that caused
PPM to be less efficient than expected:
1. The unbounded context may produce too imprecise prediction the order should be
decided more locally. In PPMZ, only deterministic (one successor) unbounded contexts are
used, otherwise a fixed limit is set on the context length (8 in the experiments).
2. Different message types may have different optimal local orders. Bloom has noticed that
the best confidence measure is given by the probability of the most probable symbol
(MPS). The context with the highest Prob(MPS) is attempted first in prediction and, in
case of failure, followed by an escape to the next smaller context.
3. The escape probability estimations in PPMA ... PPMD have all been based on heuristic
formulas. Although no theoretically best formula has been presented, it is possible to base
the escape probability on statistics and prediction. Bloom built a 4component synthetic
context for escapes, including PPM order, number of escapes, number of successful
matches, and the previous symbols. This information is squeezed into 16 bits and used as
an index to a table (2 entries) storing the actual number of escapes and matches. In
coding, the escape probability is just looked up from this table.
The following table gives a comparison of PPMC, PPMD+ (a tuned version of PPMD),
PPM and PPMZ for Calgary Corpus, a popular benchmark for compression programs. The
results denote bits per source symbol (character or byte).
File Size PPMC PPMD+ PPM PPMZ
bib 111 261 2.11 1.86 1.91 1.74
book1 768 771 2.48 2.30 2.40 2.21
book2 610 856 2.26 1.96 2.02 1.87
geo 102 400 4.78 4.31 4.83 4.03
news 377 109 2.65 2.36 2.42 2.24
obj1 21 504 3.76 3.73 4.00 3.67
obj2 246 814 2.69 2.38 2.43 2.23
paper1 53 161 2.48 2.33 2.37 2.22
paper2 82 199 2.45 2.32 2.36 2.21
pic 513 216 1.09 0.80 0.85 0.79
progc 39 611 2.49 2.36 2.40 2.26
progl 71 646 1.90 1.68 1.67 1.47
progp 49 379 1.84 1.70 1.62 1.48
trans 93 695 1.77 1.47 1.45 1.24
average 224 402 2.48 2.25 2.34 2.12
The excellent results of PPMZ have been obtained at the cost of extremely high processing
time and large working storage. Thus, the methods cannot be considered candidates for
practical compression software. However, they may have other interesting applications where
powerful predictive capability is needed. Such models have even been used to predict the
C. Bloom: “Solving the Problems of Context Modeling”, California Inst. of Technology,
5.4. BurrowsWheeler Transform
In 1994, M. Burrows and D.J. Wheeler presented a compression method, which at first sight
seems to be totally different from all known techniques. It first performs a transformation of
the original message into a form which contains all the original symbols, but ordered quite
differently, so that symbols are highly clustered. The transformed message is then compressed
by some simple technique. The compression performance is about the same order as with the
best predictive methods, but the speed is much better.
A precondition for any transformation is that it must be reversible, i.e. an inverse
transformation must exist to recover the original message. The BurrowsWheeler transform
has this property, and is done as follows. Given a sequence X0..N1 of symbols from alpha
bet S, form a matrix having all rotations (cyclic shifts) of X as rows. The rows of the matrix
are then sorted into lexicographically increasing order. The last column L of the sorted matrix
M is the result of transformation, plus the index (I) of the row containing the original
message. An example transformation of the message ‘MISSISSIPPI’ is as follows:
0 MISSISSIPPI IMISSISSIPP P
1 IMISSISSIPP IPPIMISSISS S
2 PIMISSISSIP ISSIPPIMISS S
3 PPIMISSISSI ISSISSIPPIM LAST M
4 IPPIMISSISS SORT MISSISSIPPI COLUMN I
5 SIPPIMISSIS PIMISSISSIP P
6 SSIPPIMISSI PPIMISSISSI I
7 ISSIPPIMISS SIPPIMISSIS S
8 SISSIPPIMIS SISSIPPIMIS S
9 SSISSIPPIMI SSIPPIMISSI I
10 ISSISSIPPIM SSISSIPPIMI I
The result of the transformation consists of I = 4 and L = P, S, S, M, I, P, I, S, S, I, I .
Let us now derive the reverse transformation for recovering the original message. The first
observation is that column L contains all symbols of the original message, but in different
order. Therefore, by sorting L, we can recover the first column F of the sorted matrix M.
Since the rows of M are all rotations, we can construct a matrix M’, with L as the first column
and F as the second, and the rest of the columns unknown, but sorted lexicographically on the
second column. Thus, M’ is a cyclic rotation of M to the right by one position.
P I ...
S I ...
S I ...
M I ...
M’ = I M ...
P P ...
I P ...
S S ...
S S ...
I S ...
I S ...
M. Burrows D.J. Wheeler: “A Blocksorting Lossless Data Compression Algorithm”, Research Rep. 124,
Digital Systems Research Center, Palo Alto, California, 1994.59
More precisely, M’i, j = Mi, (j1) mod N. All rotations of the original message appear
once in both M and M’. The original message could be recovered if we could order the
symbol pairs in the first two columns of M’ appropriately (cf. domino pieces):
MI, IS, SS, SI, IS, SS, SI, IP, PP, PI
To conclude the correct order of ‘chaining’, we make the following observation: If we
consider the rows of M’ starting with a certain symbol x, we know that these rows are
mutually in sorted order, because the first symbol is x in all of them, and the rest are sorted
(lexicographically from the second column on). Therefore, these rows appear in M and M’ in
the same order. The corresponding rows of M appear in 1rotated form in rows of M’ with x
as the second symbol. This makes up the connection: The rows of M’ with x as the first
symbol are (in the same order) the 1rotations of rows of M’ with x as the second symbol. For
example, the rows of M’ starting with IM, IP, IS, IS correspond in this order to the rows
starting with PI, SI, SI, MI, so that the symbol ‘I’ is the connecting link.
Formally, we can represent the chain by an index vector T storing the connections: If Lj is
the k’th instance of x in L, then Tj = i, where Fi is the k’th instance of x in F. Thus, it holds
that Lj = FTj. For the above example, the vector T is as follows:
Index L F T
0 P I 5
1 S I 7
2 S I 8
3 M I 4
4 I M 0
5 P P 6
6 I P 1
7 S S 9
8 S S 10
9 I S 2
10 I S 3
The reverse transformation recovers the original message from back to front, starting from the
Lsymbol from row 4, which was the order number of the original message in the sorted
matrix M. Since M’ is its 1rotation (to the right), the last symbol of the message is ‘I’. We
follow the Tlink to row 0, which gives us the symbol ‘P’, preceding the last ‘I’. Then the T
link leads us to row 5, giving the previous symbol ‘P’, and so on.
The construction of array T can be done in linear time: For each symbol, determine its first
occurrence in column F. Then scan through column L, and assign each row the next index
value of the related symbol.
There is a symmetric alternative to set the links, so that the original message is recovered in
forward order, but this feature is not important; by now it should be clear to the reader that it
is question of an offline compression method. In an online situation, the method is still
applicable by dividing the message into blocks, and handling one block at a time. However,
the block size should be rather large to obtain good compression gain.60
By now, we have just transformed the original message into another, equally long form. To
understand the properties of the transformed string, as for compression, we should realize that
subsequent symbols of L are predecessors of very similar contexts, because these contexts are
sorted. We could express it so that we are using the whole message as a context for each
symbol. A rotated message has somewhere an adjacent pair, representing the last symbol and
the first symbol of the original message. Extending the contex over this point probably does
not give any advantage, so in practice we can interpret that the context of a symbol is the
related suffix of the original message. Furthermore, we can mirror the original message, if
wanted, to obtain a context that is the prefix of the message up to the current symbol. This is
in fact quite close to the idea of deterministic contexts applied in PPM.
From the above observation we conclude that the symbols in L have a high probability of
repeating themselves in succession. This can be taken advantage of in the final compression of
the transformed message. Burrows and Wheeler have suggested a simple movetofront
coding, which proceeds as follows. We first initialize a list Z to contain all symbols of the
alphabet S. The symbols of L are encoded according to their position number 0..N1 in this
list. Moreover, after each symbol, we move the related list item in Z to the front of the list.
Because equal symbols tend to cluster, there is a high probability of getting mostly small order
numbers as a result, even sequences of 0’s, when the same symbol is repeated. After this
second transformation, the resulting sequence R of numbers is encoded using e.g. Huffman or
Assuming that Z is initially I, M, P, S, then movetofront coding of L = P, S, S, M, I, P, I,
S, S, I, I produces the sequence R = 2, 3, 0, 3, 3, 3, 1, 3, 0, 1, 0. This example is too small
to see the considerable clustering of symbols in L, and sequences of 0’s in the result.
Now that we are convinced of the correctness of the method, and its potential in compression,
we should study how it can be effectively implemented. The matrix representation was purely
for illustrative purpose, and its actual construction is out of the question. So, the main
problem is, how do we perform the sorting. Burrows and Wheeler describe a sophisticated
variant of Quicksort for suffixes. We take here a simpler approach: Create a vector P0..N1,
containing indexes of the symbols in X. Initially Pi = i for all i. Then we can sort P with any
comparisonbased method, using a sophisticated comparison operator:
Pi £ Pj if the rotation starting at location i is lexicographically £ rotation starting at j.
This means that we compare symbols pairwise from indexes i and j on, until they do not
match. After sorting, P points to the symbols of column F. Column L is obtained from the
previous positions as follows:
Li = X(Pi1) mod N
In practice, the performance can be improved by making wordbased comparisons of 4 ASCII
symbols at a time, but word alignments must be adjusted. Another alternative would be to
apply radix sorting in forward order, so that we first partition the rotations according to their
first symbols, then to their second symbols, and so on. Again, rotations would be represented
by pointers to their starting positions. After a few partitioning levels, the subsets are so small
that they can be sorted with some simple method. Still another (lineartime) alternative is to
build a suffixtree, but its memory consumption is rather high.61
One may wonder if there are other alternatives to movetofront coding that would perform
better. The first idea is to apply any normal text compression method, such as PPM, to the
transformed message. However, in spite of repetitions of symbols, the properties of the
transformed message are quite different. The neighbouring symbols do not give contextual
information; they only ‘increase’ the probability of symbols to reappear.
One clear improvement, suggested also by Burrows and Wheeler, is to encode runs of zeroes
separately, because they are so common; roughly 2/3 of all numbers to be coded are normally
Peter Fenwick has tried to improve on the results of Burrows and Wheeler, and experi
mented several variations of final coding. One of his observations was that unary coding of
numbers from movetofront compression performs surprisingly well. This means that the
distribution of number probabilities is close to negative exponential. The following table
shows the compression performance of the original BurrowsWheeler (BW) method and
Fenwick’s best version (called BZIP), in comparison to the earlier PPMC and PPMZ
programs. It can be seen that the performance of BW and BZIP is between PPMC and PPMZ.
File Size PPMC PPMZ BW BZIP GZIP
bib 111 261 2.11 1.74 2.07 1.95 2.51
book1 768 771 2.48 2.21 2.49 2.40 3.26
book2 610 856 2.26 1.87 2.13 2.04 2.70
geo 102 400 4.78 4.03 4.45 4.48 5.34
news 377 109 2.65 2.24 2.59 2.51 3.06
obj1 21 504 3.76 3.67 3.98 3.87 3.83
obj2 246 814 2.69 2.23 2.64 2.46 2.63
paper1 53 161 2.48 2.22 2.55 2.46 2.79
paper2 82 199 2.45 2.21 2.51 2.42 2.89
pic 513 216 1.09 0.79 0.83 0.77 0.82
progc 39 611 2.49 2.26 2.58 2.50 2.67
progl 71 646 1.90 1.47 1.80 1.72 1.81
progp 49 379 1.84 1.48 1.79 1.71 1.81
trans 93 695 1.77 1.24 1.57 1.50 1.61
average 224 402 2.48 2.12 2.43 2.34 2.70
A variant of BW, called bzip2, is nowadays a standard codec in Unix/Linux. What is
especially interesting is that the speed of BW is amazingly good, thinking of all the phases
needed. Compared to GZIP, the other popular compression program in Unix, the tuned
version of BW is not much slower, but gives clearly better compression, especially for long
texts in natural language (see above). PPMZ, on the other hand, is an order of magnitude
slower. Similar to GZIP, also in BW the decompression is much faster than compression. This
is an important property e.g. in information retrieval. Thus the transformbased techniques
seem to give a good solution to the speed/compression tradeoff.
P.M. Fenwick: “The BurrowsWheeler Transform for Block Sorting Text Compression: Principles and
Improvements”, The Computer Journal, Vol. 39, No. 9, 1996, pp. 731740.62
6. Dictionary models for text compression
Until now we have mostly studied compression methods that encode one symbol at a time.
Next we move to the class of methods that encode longer substrings as units. This is a very
natural approach, considering the parsing of text into words, prefixes, suffixes, phrases, etc.
In general, dictionarybased coding means replacing substrings of the source message by
some kind of pointers or indexes to an implicit or explicit dictionary.
One might assume that by generalizing the coding unit from a single symbol to multiple sym
bols, we should get advantage to the compression efficiency. However, it can be proved that
each dictionary scheme has an equivalent statistical scheme achieving at least the same
compression. In other words, statistical methods are at least as good, and this is also the ob
servation in practice: statistical methods outperform the dictionary methods. The advantages
of the latter are simplicity and speed, which have made them generally more popular. The si
tuation might change in the future, along with the increasing speed of computer processors.
Quite many dictionary methods have been suggested, because there are so many alternative
ways to implement the basic idea. Some of the design decisions are:
· Selection of substrings to the dictionary
· Restricting the length of substrings
· Restricting the window of the dictionary in adaptive methods
· Encoding of dictionary references
One classification is the same as presented before, based on the generation time of the
1. Static dictionaries are fixed in advance, before seeing the message to be compressed. This
is useful only when the type of messages is known precisely in advance. For example, we
could use a normal English dictionary for coding messages in English. However, in most
cases these kinds of dictionaries are too large and too nonspecific.
2. Semiadaptive dictionaries create a specific dictionary for each message before
compression. Since the dictionary must be transmitted to the decoder, the dictionary size is
usually restricted. Finding an optimal dictionary is an NPcomplete problem, so a heuristic
is needed. Typically, one tries to find approximately equifrequent substrings, so that
encoding can be performed using fixedlength codes.
3. Adaptive dictionaries are based on the processed part of the message. As with statistical
techniques, this category is by far the most popular, and both of the large method ‘families’
to be studied (LZ77 and LZ78) are adaptive.
For static and semiadaptive dictionaries, we can distinguish different kinds of parsing
strategies of dividing the message into substrings:
· Greedy: The message is parsed from left to right, and at each step the longest matching
substring is chosen. (In adaptive methods, this is the only reasonable choice.)
· LongestFragmentFirst (LFF): Choose the longest substring somewhere in the unparsed
part of the message, matching with a dictionary substring.
· Optimal: Create a directed graph of all matching substrings and determine its shortest
Jacob Ziv and Abraham Lempel were the pioneers of adaptive dictionary compression. In
years 1977 and 1978 they published two seminal papers , which have been the basis for most
text compression methods currently in practical use. In the literature, methods are usually
classified according to their origin into LZ77 and LZ78 families of methods. The former
applies an implicit dictionary, whereas the latter builds dynamically an explicit dictionary. We
shall study each of them in turn, as well as some representative methods.
6.1. LZ77 family of adaptive dictionary methods
In the LZ77 approach, the dictionary is simply part of the already encoded sequence. ‘Part’
means that we maintain a sliding window of N symbols, divided into two parts: the search
buffer contains a suffix of NF symbols of the processed sequence, and the lookahead buffer
contains the rest F symbols. The sliding can be implemented by using a circular buffer, so that
the oldest symbols are always overwritten by the newest ones, as the encoding proceeds. A
typical search buffer size NF is 416Kbytes. The reasons for restricting the domain of
substrings into a fixedsize search buffer are twofold. First, the references to substrings can be
made shorter, and second, scanning the search buffer can be made faster. Moreover, due to
locality of occurrences, the best matches are often found rather close. The reasons for
restricting the size of the lookahead buffer are similar: representation of the length can be
restricted and finding the match can be made faster. A typical value for F is 16. Longer
matches are so rare that degradation of compression is negligible.
The encoder searches the window for the longest match. After finding it, the encoder encodes
it with the triple offset, length, char where offset is the distance of the match start from the
current position, length is the length of the match, and char is the next symbol in the
lookahead buffer after the match (i.e. the first nonmatching symbol). The reason for sending
the third component is to handle the cases where no match can be found. In this case both
offset and length are 0, and the third component gives the symbol to proceed with. If fixed
length codes are used, coding the triple takesélog (NF) ù + élog Fù + élog qù bits.
2 2 2
Suppose that our message is as follows:
Suppose further that the length of the window N = 13, the lookahead buffer size F = 6, and
the current window contents is as follows:
The lookahead buffer contains the string “dabrar”, so we are looking for “d...” from the
search buffer. Since there is no ‘d’, we transmit the triple 0, 0, d. This seems wasteful, but
in practice the situation is rare. Moreover, practical implementations use somewhat different
techniques (see later). After encoding ‘d’, we move the window one step forward, and obtain
J. Ziv and A. Lempel: “A Universal Algorithm for Data Compression”, IEEE Trans. on Information Theory,
Vol. 23, No. 3, 1977, pp. 337343.
J. Ziv and A. Lempel: “Compression of Individual Sequences via VariableRate Coding”, IEEE Trans. on
Information Theory, Vol. 24, No. 5, 1978, pp. 530536.64
Searching backwards, we first find “ad...”, having match length 1, then we find “ac...”, also
with match length 1, and then finally “abra...”, having the longest match length 4. The
position of the match is encoded as the distance from the current position, i.e. 7. The whole
triple is thus 7, 4, r. The window is moved 5 symbols forward, which produces
The longest match in the search buffer is “rar”, but in this case we can use an even longer
match, namely “rarra”, overlapping the lookahead buffer. It is easy to see that the decoding
succeeds without problems: symbols are copied from the referenced substring one by one; at
the time when the copying passes the end of the search buffer, the next symbols (“ra”) have
already been solved and can be ‘reused’. The following observation holds: The encoder looks
for the longest match with its first symbol in the search buffer. The triple to be sent is thus
3, 5, d. An extreme situation occurs when encoding runs of equal symbols. Assume the
following coding situation (with ‘’ separating the search and lookahead buffers):
… xa aaaaaaaab …
The next triplet will be 1, 8, b, because “aaaaaaaa” matches with “aaaaaaaa”. Thus, the
coding is very effective.
The authors of the LZ77 scheme have shown that asymptotically the performance of this
algorithm approaches the best possible that could be obtained by any (semiadaptive) scheme
having full knowledge of the statistics of the source. However, this is true only asymp
totically, and in practice it is possible to make different kinds of improvements for finite
messages. As for compression efficiency, the composition of the triple and its coding have
been varied, as well as the window size.
· LZR (Rodeh et al., 1981) uses the whole processed part of the message as the search
buffer. It applies universal, variablelength coding of integers, representing the arbitrarily
· LZSS (Storer Szymanski, 1982) avoids the third component of the triple: It sends a flag
bit to tell, whether the next code represents an offsetlength pair, or a single symbol.
· LZB (Bell, 1987) usesgcoding for the match length, so parameter F is no more relevant.
Further, LZB includes some other tunings, concerning e.g. the front part of the message.
· LZH (Brent, 1987) combines LZ77 with Huffman coding.
As for speed, the basic LZ77 encoding is quite slow, due to string searching. On the other
hand, decoding is extremely fast a typical feature of many dictionary methods. Encoding can
be speeded up by auxiliary index structures (e.g. trie or hash).
Popular compression packages, such as PKZip, Zip, LHarc, ARJ and Zoo, all use algorithms
based on LZ77, but are equipped with variablelength coders. We study here a popular
package used especially in Unix environment (part of the GNU software):65
GZip uses a hash table to locate previous occurrences of substrings. The next three symbols
of the lookahead buffer are hashed to a number, used as an index to a hash table. The table
contains heads for lists containing references to the locations where the three symbols have
occurred in the search buffer. To keep searching fast, the lists have limited length, i.e. some
occurrences are forgotten. Keeping the latest occurrences at front slightly improves the
compression efficiency, because the pointer (offset) refers to the closest instance of the
longest match. If no suitable match is found, raw symbols are encoded. The used data
structure is illustrated below. The task is to find the longest match among the substrings
starting with “ABC …”. Note that generally the match is more than three characters.
… ABC … ABC … ABC … ABC …
Search buffer Lookahead buffer
Two Huffman codes are used for representing the substrings:
1. Lengths of matches and raw symbols are considered a single set of values (e.g. symbols as
0..255 and lengths as l + 253, where l = 3, 4, ... These values are Huffmancoded.
2. Offsets are Huffmancoded, so that recent matches, being usually more frequent, are
encoded with a smaller number of bits. If the first code represents a raw symbol, not a
length, then the offset can be omitted.
Notice that the reversed order of the codes is extremely important for the decoder. It may
seem odd to combine match lengths and raw symbols into a single code, but in this way the
flag bit (cf. LZSS) is avoided, and in the long run this will pay off. The Huffman codes are
generated semiadaptively, by compressing blocks of up to 64 Kbytes at a time. A code table
of the canonical Huffman code is placed at the beginning of the compressed block. This
means that GZip is not really a singlepass method, but since 64Kbytes can easily be kept in
the main memory, the program needs to read the input only once (as was done in the block
wise BurrowsWheeler technique).66
The basic parsing technique in GZip is greedy. However, it offers a slower option to check,
whether parsing a raw character + following match would result in better compression than
the direct longest match.
GZip outperforms most other ZivLempel methods in terms of compression effectiveness and
decoding speed. One faster variant, called LZRW1, has been presented by Ross Williams. It
maintains a hash table with only a single pointer per each entry, not a list. It thus remembers
only the last occurrence of each trigram of symbols (disregarding collisions). Moreover, it
updates the table only by the first three symbols of the real match. The compression
performance is, of course, significantly worse than with gzip.
6.2. LZ78 family of adaptive dictionary methods
The drawback of the LZ77 approach is as follows: Choosing a small window does not yield
the longest possible matches, whereas choosing a larger window gives longer matches, but
also larger offset values and thereby longer codes. Another source of inefficiency is the fact
that LZ77 actually reserves many codes for the same substring; one for each instance within
the window. LZ78 tries to overcome these problems by avoiding the restricted window, and
by building an explicit dictionary. It must be built by both encoder and decoder in an identical
manner. The code sequence consists of index, char pairs, where index refers to the dic
tionary entry corresponding to the longest match, and char is the symbol to follow. This pair
will also be the newest entry in the dictionary, i.e. the longest match extended with the next
symbol. Thus the ‘words’ in the dictionary grow one symbol at a time. Index 0 is reserved for
the nomatch case, so that the coded substring consists of only char, which is added to the
dictionary. We study the encoding using the following input message:
Initially, the dictionary is empty, so the first three symbols must be encoded with index 0.
Thereafter, the dictionary will contain these symbols, so that the second b is already found
from there. It is extended with symbol a, and ba will be the next dictionary entry. Then we
again meet a fresh symbol ‘‘, and add it to the dictionary as such. Symbol w is found next,
matching the dictionary entry 1, and extended with a, and so on. The whole sequence is
developed in the table on the next page.
Although LZ78 is able to build the dictionary adaptively, it has the drawback that the size of
the dictionary grows without bound. When the size reaches the reserved memory, then we
either have to continue with the obtained fixed dictionary or prune it, or discard it and restart
from an empty dictionary.
LZ78, in addition to being practical, has the important theoretical property that if the source
message is generated by a stationary, ergodic source, then the compression performance is
asymptotically optimal (i.e. the code size approaches entropy in the limit). An ergodic model
means that any message produced by it becomes entirely representative of the source as the
message length grows longer. Unfortunately, the convergence to the optimum is quite slow.67
Lookahead buffer Encoder output Dictionary index Dictionary entry
wabbawabba... 0, w 1 w
abbawabbaw... 0, a 2 a
bbawabbawa... 0, b 3 b
bawabbawab... 3, a 4 ba
wabbawabba... 0, 5
wabbawabba... 1, a 6 wa
bbawabbawa... 3, b 7 bb
awabbawabb... 2, 8 a
wabbawabba... 6, b 9 wab
bawabbawoo... 4, 10 ba
wabbawoowo... 9, b 11 wabb
awoowoowo.. 8, w 12 aw
oowoowoo 0, o 13 o
owoowoo 13, 14 o
woowoo 1, o 15 wo
owoo 14, w 16 ow
oo 13, o 17 oo
There are many ways to modify the basic LZ78 technique. One of the most famous, called
LZW, was suggested by Terry Welch . It has a similar idea as LZSS had in modifying LZ77,
namely discarding the character component from the code tuples. The encoded sequence in
LZW thus consists of only indices to the dictionary. The dictionary must be initialized with all
symbols of the alphabet. After finding a matching substring x, the next dictionary entry will be
xa where ‘a’ is the first symbol of the next matching substring. In decoding, building of the
dictionary proceeds ‘one step behind’, and may cause a situation where a substring is needed
before it is stored in the dictionary. The problem and its solution are exemplified by encoding
The dictionary is initialized with symbols a and b. The encoder first notices the longest match
for ‘aaba...’ to be ‘a’, and adds ‘aa’ to the dictionary. Then it finds the longest match for
‘abab...’ which is again ‘a’, and ‘ab’ is added to the dictionary. The longest match for
‘baba...’ is ‘b’, and ‘ba’ is added to the dictionary. The longest match for ‘ababaaa’ is ‘ab’,
and ‘aba’ is added to the dictionary, and so on. Thus, the first symbol of each parsed
substring is the last symbol of the previously added entry to the dictionary. The whole
dictionary will be as follows:
T.A. Welch: “A Technique for HighPerformance Data Compression”, IEEE Computer, June 1984, pp. 819.68
Index Substring Derived from
2 aa 0+a
3 ab 0+b
4 ba 1+a
5 aba 3+a
6 abaa 5+a
The decoder gets the sequence 0, 0, 1, 3, 5, 2. Let us now try to follow the decoder’s steps.
After receiving a certain index, the decoder outputs the related substring, and adds an entry to
the dictionary with the last symbol still unsolved. It will be completed only after seeing the
next substring, the first symbol of which is the correct last symbol of the previously added
dictionary entry. The first three indexes are trivial to decode, because they refer to singular
symbols. Thereafter, the decoder’s dictionary is a, b, aa, ab, b. The next index is 3, which
represents the substring ‘ab’. We can thus replace the ‘b’ by ‘ba’, and add substring ‘ab’ as
the 5 entry to the dictionary. Now we have the tricky situation where the index (5) obtained
by the decoder should be used before the last symbol of the related entry is solved. The
solution is, however, simple: We know that the unsolved symbol of the previously added
dictionary entry starts the next substring. The next substring is ‘ab’, so its first symbol is ‘a’,
and this is the correct value for ‘’. Now the decoder adds the entry ‘aba’ to the dictionary
(entry 6). The last code index = 2 is simply decoded to ‘aa’, and the job is finished.
LZW was originally proposed for compressing disk files with the aid of special channel hard
ware. In this situation speed is of utmost importance, and thus many implementations use a
fixed maximum size for the dictionary, typically 4096 entries (= 12 bits for indexes). The
dictionary remains static, after having grown to this size. There are three important practical
variants of the LZW scheme, explained below.
Unix Compress (LZC)
One of the most popular file compression methods in Unix has been the compress program
(uncompress is the decoder), called also LZC, where compressed files are denoted with ‘.Z’
suffix. The indexes to the dictionary of size M are encoded usingélog Mù bits, so that the
code sizes grow with one bit always when the dictionary size exceeds some power of two.
The maximum number of dictionary entries can be selected between 2 and 2 . After reaching
the maximum, the dictionary keeps static, but the system monitors the compression ratio. If it
falls below a given threshold, the dictionary is discarded, and the building process is restarted.
The method is thus also locally adaptive to some extent.
The Graphics Interchange Format (GIF) was developed by Compuserve Information Service
as a method for encoding graphical images. It is very similar to the Unix compress program.
The first byte of the compressed message contains the number b of bits per pixel in the
original image (typically 8). The initial size of the dictionary is 2 entries, of which 0..2 1
are the original pixel values, 2 is the so called clear code, which is used to reset the dic
tionary to the initial state. Another special codeword value is 2 +1, which denotes endof
information. When the dictionary is filled up, its size is doubled, up to the maximum of 409669
entries, after which it remains static. The coded data are represented in blocks of maximally
255 bytes. Each block is surrounded by a header (containing the length) and a terminator. The
endofinformation code should precede the last terminator.
The performance of GIF is not especially good, except for drawings and other graphic
images. The efficiency is close to arithmetic coding with no context information. The reason
for inefficiency for photographic images is that image data is fundamentally different from
text: it does not consist of regular phrases, but rather digitized representations of more or less
continuous 2dimensional surfaces, with some noise included. GIF images use a so called
indexed colour model, i.e. a palette of colours, where visually close tones may have quite
different indexes. Therefore, continuity of tone values is lost. The best image compression
methods (such as JPEG, JPEG 2000) are based on colour models (such as RGB) supporting
(sampled and quantified) continuity, and therefore methods applicable to digital signals can be
used. However, these methods are typically lossy, i.e. part of the image information is lost.
GIF compression is in principle lossless, but the step of converting the original image into
indexed representation is lossy.
The CCITT (now called ITUT) Recommendation V.42 is an errorcorrecting procedure for
data transmission in telephone networks. V.42 bis is the related data compression standard.
Actually, V.42 can operate in two modes, in compressed and in uncompressed (transparent)
modes, because there may be types of data which cannot be compressed at all (and which
would in fact extend in size, if compressed). The compressed mode uses a modified LZW
algorithm. Again, the minimum dictionary size is 512 entries, and the recommendation is
2048, but the actual size is ‘negotiated’ between the sender and receiver. Three special
codeword values are distinguished: 0 = enter transparent mode, 1 = flush the data, 2 = incre
ment codeword size.
V.42 bis contains some modifications to the original LZW scheme: After the dictionary has
grown to its maximum size, the method reuses those entries which are not prefixes to other
dictionary entries. This means that these entries have not been used in coding, and seem to be
of little value. Furthermore, it is wise to reuse first the oldest such entries. Due to creation
order, they are found first when scanning from start to end.
To reduce the effect of errors, there is a certain maximum length for encoded substrings.
CCITT recommends a range of 6 to 250, with default = 6 symbols.
A third modification concerns the special situation in LZW where the latest entry of the dic
tionary is used immediately, before its last symbol has been solved. V.42 bis simply forbids
this kind of reference, but instead suggests to send the constituents of the last entry.
Some other versions of LZ78, suggested in the literature, are the following:
LZT (by P. Tischer, 1987) applies the LRU scheme to dynamically maintain a full dictionary,
replacing the least recently used substrings by new ones.
LZMW (by V.S. Miller and M.N. Wegman, 1984) builds long substrings more quickly: It
creates a new substring always by concatenating the last two matching substrings, not just
adding a single character.70
LZJ (by Matti Jakobsson, 1985) stores in the dictionary all distinct previously occurred
substrings of the input, up to a certain maximum length h (h = 6 is sufficient). The maximum
size M of the dictionary is also fixed, e.g. M = 8192 is suitable. After having reached this
maximum size, the dictionary is pruned by replacing substrings that have occurred only once.
It is peculiar to LZJ that the encoding is faster than decoding, making it suitable for archiving
LZFG (by E.R. Fiala and D.H. Greene, 1989) is one of the most practical LZ variants,
involving many finetuned details. It can actually be considered a combination of the ideas of
LZ77 and LZ78. It maintains a similar sliding window as LZ77, but stores in the dictionary
only substrings that are prefixed by some previously matched substring. When the window is
moved, substrings falling out are pruned from the dictionary. The data structure used for
representing the dictionary is a Patricia trie, where a parentchild arc can represent a sequ
ence of several symbols. The leaf nodes represent the whole remaining part of the window, so
there is basically no upper bound for the match lengths. The encoded sequence consists of
references to the trie nodes. If the match ends to a node whose parent arc represents only a
single symbol, only the node number is needed. If the parentchild arc represents a longer
path, another number must be used to specify the exact endpoint of the match.
There are many details in LZFG, which are out of the scope of the current discussion.
However, the coding of numbers is interesting, and differs from our earlier ‘universal’
variablelength representations (a,g, and dcodes). Indexes of internal nodes (and possible
match lengths) are encoded using a technique called phasingin, where we know the range of
integers 0, m1, but m need not be a power of 2. Two different codeword lengths are used,
so that the numbers from 0 to 2 m1 are encoded withëlog mû bits, and numbers
from 2 m to m1 are encoded withëlog mû +1 bits,
Another number coding system, namely the family of StartStepStop codes, is used in LZFG.
It is a kind of parameterized version ofgcode, and meant for a limited range of integers. It
employs n different lengths of simple binary codes. Parameter start gives the number of bits
used to represent the first 2 numbers. The next set of codewords contains start+step bits
each, with 2 different values. The next contains start+2×step bits, and so on, until the
final length stop. Each codeword is prefixed by the unary representation of the length; the
final length (stop) can be represented without the ending ‘1’ of the unary code. The number of
different codes available is
Since this grows exponentially with stop, relatively high numbers can be represented with
small parameter values. For example, startstepstop (1, 2, 5) code would have codewords
10, 11, 01000, 01001, 01010, 01011, 01100, 01101, 01110, 01111, 0000000, 0000001, ...,
0011111 for integers 0 to 41. The normal klength binary code is a startstepstop code with
parameters (k, 1, k). The gcode, on the other hand, has parameters (0, 1, ¥).
By tuning the three parameters, the startstepstop code can be made to approximate the
Huffman code of some observed distribution. This is just what has been done in LZFG. The
leaf nodes are encoded with (10, 2, 14) code, and the related length components with
(1, 1, 10) code. In addition, phasingin can be applied to shorten the longest codes.71
6.3. Performance comparison
We conclude this section by making a comparison between different methods in the LZ77 and
LZ78 families, as well as the extreme version of statistical coding, namely PPMZ. The test
data is again the Calgary Corpus. The results, in bits per source symbol, are as follows.
File Size LZ77 LZSS LZH GZIP LZ78 LZW LZJ’ LZFG PPMZ
bib 111 261 3.75 3.35 3.24 2.51 3.95 3.84 3.63 2.90 1.74
book1 768 771 4.57 4.08 3.73 3.26 3.92 4.03 3.67 3.62 2.21
book2 610 856 3.93 3.41 3.34 2.70 3.81 4.52 3.94 3.05 1.87
geo 102 400 6.34 6.43 6.52 5.34 5.59 6.15 6.05 5.70 4.03
news 377 109 4.37 3.79 3.84 3.06 4.33 4.92 4.59 3.44 2.24
obj1 21 504 5.41 4.57 4.58 3.83 5.58 6.30 5.19 4.03 3.67
obj2 246 814 3.81 3.30 3.19 2.63 4.68 9.81 5.95 2.96 2.23
paper1 53 161 3.94 3.38 3.38 2.79 4.50 4.58 3.66 3.03 2.22
paper2 82 199 4.10 3.58 3.57 2.89 4.24 4.02 3.48 3.16 2.21
pic 513 216 2.22 1.67 1.04 0.82 1.13 1.09 2.40 0.87 0.79
progc 39 611 3.84 3.24 3.25 2.67 4.60 4.88 3.72 2.89 2.26
progl 71 646 2.90 2.37 2.20 1.81 3.77 3.89 3.09 1.97 1.47
progp 49 379 2.93 2.36 2.17 1.81 3.84 3.73 3.14 1.90 1.48
trans 93 695 2.98 2.44 2.12 1.61 3.92 4.24 3.52 1.76 1.24
average 224 402 3.94 3.43 3.30 2.70 4.13 4.71 4.00 2.95 2.12
The general observations are:
· The LZ77 family seems to be slightly better that LZ78 family.
· GZip represents a compressioneffective implementation of LZ77.
· LZFG represents a compressioneffective implementation of LZ78.
· Both LZ77 and LZ78 families are far behind the best predictive methods.
The tradeoff between compression performance and processing time explains the popularity
of LZmethods in practice. Generally, encoding in the LZ78 family seems to be somewhat
faster, but decoding slower than in the LZ77 family. The primary reasons are that an LZ78
encoder finds the matches faster than an LZ77 encoder, but an LZ78 decoder must rebuild the
dictionary, whereas an LZ77 decoder just copies the substrings referred to. Both, in turn, are
an order of magnitude faster than, say, PPMZ, used for comparison. For example, GZip has
been observed to be about 23 times faster than PPMZ for the Calgary Corpus, even excluding
the ‘pic’ file (which took several hours for PPMZ).72
7. Introduction to Image Compression
The compression methods described in earlier sections can in principle be applied to image
data, as well, by transforming the 2dimensional collection of image elements (most often
pixels) into a 1dimensional sequence of symbols. However, image data has so special char
acteristics that the compression performance would not be too good. An example is the GIF
format for image representation. The problem is with the modelling part of compression.
Huffman and arithmetic coders are, of course, still of utmost importance.
Photographic images are digitized representations of more or less continuous color / bright
ness ‘surfaces’, a kind of 2dimensional signals, where the numeric value of a pixel is highly
correlated with the numerical values of its neighbouring pixels. Among different images, it is
much harder to find unifying characteristics than, say, among English documents. Image
modelling is concerned with geometric properties and local mathematical dependencies in the
source, rather than symbol patterns. In simple terms, text and images represent different scales
of measurement: nominal vs. interval / ratio scale.
Digitized (raster) images can be classified into the following categories:
· Bilevel images contain only two pixel values; 0=white and 1=black. They are often called
also blackandwhite images.
· Greyscale images have typically 256 different levels of greyness from white to black.
Thus, a pixel is represented by an 8bit byte.
· Color images have three color components (e.g. red, green, blue), and each is represented
as a greyscale image. Thus, each pixel takes 24 bits.
Image compression can be divided into two categories: lossless an lossy compression. The
latter means that only an approximation of the original image can be recovered. In what
follows, we discuss some basic methods in both categories, concentrating on the former.
7.1. Lossless compression of bilevel images
There are many applications where blackandwhite images are quite sufficient. Any printed
document can be interpreted as a bilevel image, e.g. in telefax transmission. From com
pression point of view, symbolic representation of the text on the page would yield a much
higher efficiency. On the other hand, the image form is much more flexible; handwritten text is
as easy to process as typewritten.
The basis of compressing bilevel images is twofold:
· The proportions of black and white pixels are usually quite different. Typically, there are
more white than black pixels.
· The pixels of the same color are highly clustered. We can distinguish subareas of the image
that are totally white or totally black.
Any methods developed for bitvector compression are useful here, because they can be
applied to rows (or columns) of the image. One of the simplest such methods is runlength
coding, where the length of each run of zeroes / ones is encoded. Since white and black runs
alternate, only the lengths are needed, assuming some starting color. Actually, the method
should be called ‘runlength modelling’; the coding of lengths can be done using any of the73
aforementioned techniques: Huffman, arithmetic, universal integer encoding, etc. A
considerable improvement to compression efficiency can be obtained by predictive runlength
coding. Each pixel value (0/1) is predicted on the basis of a few of its neighbours above and
left (west, northwest, north, northeast). The original bilevel image is transformed to a so
called error image, containing a bit for each pixel: 0, if prediction was successful, and 1, if
not. According to experiments, the proportion of ones reduces to one third, and compression
improves by about 30. The decoder performs the reverse transformation (while the same
predictive neighbours are known to it, as well).
Another simple method for bitvector compression is block coding, where the vector is
divided into fixedlength blocks. A completely white block is encoded as a single zero,
whereas other blocks are encoded as 1 plus the block contents. This is efficient enough for
almost white images. It can be generalized to hierarchical block coding, where the flag bits
are compressed by the same technique, recursively. The following example with block size = 3
illustrates the twolevel case. Bits in bold are transmitted to the decoder.
100 111 000 011 000 100
101 000 000 101 111 110 000 000 000 000 010 001 000 000 000 111 000 000
The same idea can still be generalized into two dimensions, so that the image area is divided
into rectangular boxes, and completely white boxes are encoded with 0, and other blocks with
1 plus the contained pixels, as follows:
0 1 0100 1 1001 1 1111
This approach can also be generalized hierarchically, for example
0110 0111 1111 1010 1110
If the proportion of black areas is relatively large, it might be more efficient to reserve a short
code also for completely black blocks. Further compression (of the order 10) is achieved by
e.g. Huffman coding of the 2´2 blocks. A static codebook is sufficient in practical74
applications. More improvement can be obtained by applying a similar prediction as was
suggested in predictive runlength coding.
One of the earliest application areas of bilevel image compression has been the facsimile or
fax. The number of pixels per sheet is either 1728´1188 (low resolution), or 1827´2376 (high
resolution). CCITT (now ITUT) has classified the fax equipment into four groups:
· Group 1: Analog scheme, speed£ 6 min / A4 sheet.
· Group 2: Analog scheme, speed£ 3 min / A4 sheet
· Group 3: Digital scheme, applies 1 or 2dim. compression, speed£ 1 min /A4 sheet.
· Group 4: Digital scheme, applies 2dim. compression, speed£ 1 min / A4 sheet.
The 1dimensional scheme of group 3 performs coding independently for each line, using run
length coding. The lengths are Huffman coded, but not as such; they are expressed as two
length = 64 ´ m + t
where t = 0, ..., 63 and m = 0, ..., 27. There are separate Huffman codes for black and white
run lengths. The 2dimensional option uses a relatively complicated scheme of coding a line
on the basis of the previous line. The method is a modification of so called Relative Element
Address Designate (READ); the details are skipped here. To prevent error propagation, the
group 3 compression restricts the number of lines compressed with the 2dimensional scheme.
In normal resolution, every other line is compressed by the 1dimensional scheme, and in high
resolution every fourth. Group 4 uses only the 2dimensional scheme.
The standard for bilevel image compression has been developed by JBIG (Joint BiLevel
Image Compression Group, 1993), which was a joint experts group of ISO (International
Standards Organization), CCITT (Consultative Committee on International Telephone and
Telegraph) and IEC (International Electrotechnical Commission). In JBIG, the encoding of a
pixel is based on a context of either 7 or 10 neighbouring pixels, which have already been
coded (and known to the decoder). In addition, for 10pixel contexts, there are twoline and
There are 128 different 7pixel contexts and 1024 different 10pixel contexts. JBIG method
gathers statistics from all of them, and uses 128 or 1024 different QMcoders for final
encoding. The larger context is in principle more accurate, but converges more slowly.
In addition to the normal sequential mode (row by row), JBIG offers also a so called pro
gressive mode of operation, where first a lowresolution image is sent. Then the resolution is
doubled repeatedly until the highest resolution. The next level takes a few bits from the
previous level to its context. The details of progressive transmission are skipped.
There is a newer version of JBIG, namely JBIG2 (from 2000) which is becoming quite
popular. It is meant, in addition to telefax, also for storage of document images, wireless
transmission, printer spooling, etc. It compresses even more effectively than the old JBIG by
dividing the image into three kinds of regions:75
· Symbol regions contain mainly text, i.e. regular shapes, coded using a dictionary method.
The dictionary entries consist of binary patterns to be matched with the bilevel image
fragment. The method allows approximate matching and thereby lossy compression.
· Halftone regions contain halftone (raster) images, and compressed also with a dictionary
that offers bit patterns for halftone elements.
· Generic regions represent the rest, compressed using the traditional predictive method.
7.2. Lossless compression of greyscale images
The compression of greyscale (and color) images is in most applications allowed to be lossy,
i.e. the decoder does not recover the image into exactly the original form, but into an approxi
mation, which is considered to be similar enough for the human eye. Some critical applica
tions, such as xrays in medicine, demand lossless compression.
The above methods for bilevel images could in principle be used also for greyscale images.
If each pixel is represented by 8 bits, we can distinguish 8 bit planes of the original image,
each of which is a bilevel image. In fact, by sending the planes in order from the most to least
significant, we obtain a progressive compression method for greyscale images. Still better
results are obtained if the pixel codes are transformed into Gray codes before partitioning the
image into bit planes. Gray code has the property that codewords of two successive integers
differ at only one position. Another improvement is obtained by including some pixels from
the previous bit plane in the contexts of the current plane.
Most of the lossless compression methods for greyscale images apply prediction. The pixels
are processed line by line from left to right, in so called rowmajor order (raster scan order).
Since the greylevels tend to change smoothly, a few neighbouring pixels (among the already
processed ones) are sufficient to make a good prediction of the current pixel. The difference
between real and predicted values (called prediction error or residual) is encoded and
transmitted to the decoder, which does the same prediction and recovers the real pixel value
on the basis of the coded difference. If the prediction function is a linear combination of the
neighbour values, the method is said to use a linear prediction model.
The standard for greyscale (and color) image compression has been defined by JPEG (Joint
Photographic Experts Group, 1991). It offers both lossy and lossless modes of compression.
The latter applies the linear prediction model, by offering seven alternative prediction
functions (actually eight, of which the first makes no prediction). By denoting the predicted
pixel by x and its three neighbours in the north, west and northwest by N, W and NW, the pre
diction formulas are:
1. X = N 4. x = N + W NW
2. X = W 5. x = W + (N NW) / 2
3. X = NW 6. x = N + (W NW) / 2
7. x = (N + W) / 2
The prediction errors are coded either by Huffman or arithmetic coding. Typical compression
performance in lossless mode for photographic images is around 50, i.e. 4 bits per pixel.
This seems to be a relatively low gain, compared e.g. to text compression. One reason is that
‘natural’ images usually contain so called texture, some kind of noise, which makes
compression difficult, but which gives the image its natural character. On the other hand,76
lossy JPEG can in most cases achieve compression of less than 1 bit per pixel, while the
decoded result is still practically indistinguishable from the original.
The speed of compression can be improved, if we replace the final coder by some nonoptimal
method, which assumes a static (skew) distribution of prediction errors.
By using a larger context (above and left of the predicted pixel, so that the decoder can repeat
the same prediction), one can get a more precise prediction, at the cost of processing time. A
good example of this kind of extension is CALIC (“Context Adaptive Lossless Image
Compression”, 1994). It uses a sevenpixel context and tries to deduce, which direction
would be best to use in prediction. In other words, it tries to find horizontal/vertical
similarities among the context pixels. If it seems that there is a horizontal line, then the W
neighbour might be best, while for a vertical line N would be a better choice.
NW N NE
A later variant of lossless greyscale image compression is JPEGLS (from late 1990’s),
which can be regarded as a simplification of CALIC. It was originally developed by HP under
the name LOCOI. It applies a twostep prediction with a 4pixel context (W, NW, N, NE).
The first estimate for the predicted symbol is the median of W, N and NW pixels. This is then
refined by the average error made earlier for the same 4pixel context. The compression
results are better than with the old lossless JPEG, but slightly worse than those obtained by
(the more complicated) CALIC.
As a final note about lossless image compression, the modelling differs essential from lossy
image compression (to be discussed next). As we saw, lossless models resemble predictive
methods in text compression, even though closeness of the numerical values of neighbors was
utilized also. The extremely popular image formats GIF (Graphics Interchange Format) and
PNG (Portable Network Graphics) use methods borrowed from text compression: GIF is
based on LZ78 and PNG uses a modification of LZ77, combined with prediction.77
7.3. Lossy image compression: JPEG
Using lossless techniques, it is hard to compress photographic images much below 50. This
is due the small variations (noise) among neighboring pixel values, even in a very smooth area
in the picture. On the other hand, in most applications it is not absolutely necessary to recover
the original image precisely, as long as a human observer cannot tell the difference – or at
least it is not disturbing. By relaxing somewhat from the strict preservation requirement, it is
possible to get much higher compression gains. Because of the large size of images, most
applications use lossy image compression. An example where this is not allowed is medical
imaging, for example xrays.
The bestknown and mostused standard for lossy image compression is JPEG. It is a result of
committee work (Joint Photographic Experts Group, 198692, ISO standard 1994). It uses
quite a different approach to modeling the picture, compared to lossless JPEG, namely
conversion to frequency domain by a discrete, twodimensional cosine transform (DCT). It
resembles the betterknown Fourier transform, which is the standard tool in signal
processing. Since images can be considered 2dimensional signals, it is not surprising that
similar techniques apply. The cosine transformation converts the image to a linear
combination of cosine functions of different wavelengths (frequencies). The reason why this
helps in compression is that the smooth or smoothly changing areas can be represented with
only a few nonzero coefficients for the lowfrequency components. The areas with rapid
changes, on the other hand, can be modeled more roughly (with higher error), because a
human observer is not so sensitive to those. Conclusively, the transform converts the image
pixel matrix into a coefficient matrix of the same size, but with quite skewed element values.
The transform is of course reversible, enabling decoding.
Let us study the technique a bit more detailed, and start with greyscale images, the pixel
values of which are typically 8 bits, with the range 0..255. The image is partitioned into blocks
of 8x8 pixels, which are the units of transform. The reasons for not applying the transform to
the whole image are twofold. First, the transform is a relatively heavy operation, and second,
the regularities in the image are usually local, so that using larger units would not pay.
Nevertheless, the neighbors across block boundaries correlate, which is not taken advantage
of in compression (except for the lowestfrequency component, which is encoded on the basis
of the corresponding component of the previous neighbor.
The 2D discrete cosine transform of NxN block f(x, y) is obtained by the following formula:
N 1 N 1
(2x +1)up (2y +1)vp
C(u,v) = a(u) a(v) f (x, y) ×cos ×cos
and the reverse transformation by a very similar formula:
N 1 N 1
(2x +1)up (2y +1)vp
f (x, y) = a(u) a(v)C(u,v) ×cos ×cos
u=0 v=0 î þ
Above the constants were: N=8,a(u) = 1/ N for u=0, and 2 / N for u0.78
Note that the transformation is in principle lossless, if the calculations are done with infinite
precision. The coefficients C(u, v) represent the weights of the 64 2D basis functions, so that
their linear combination recovers the original block f(x, y). The basis functions are illustrated
A numeric example of a block transform is shown below (example borrowed from
Original: Transform result:
The coefficients in the topleft corner represent the lowfrequency components, and are more
important to the visual quality of the image than the coefficients in the bottomright corner,
representing the highest frequencies. This difference is taken into consideration in the next
step, namely quantization of coefficients. It is the only lossy step in the process, and means
reducing the precision of coefficients by dividing them with suitable integers. The JPEG
standard defines quantization matrices for different image qualities. The common feature is
that the divisors are smallest for the topleft coefficients and largest for the bottomright
coefficients. An example quantization matrix, and related quantization result of the above
block are as follows (example borrowed again from http://en.wikipedia.org/wiki/JPEG):79
Quantization matrix Quantization result
The fact that the largest values are concentrated on the topleft corner is typical. It is also
typical that the majority of the other values are zero, especially with higher compression
ratios. This illustrates concretely the importance of lowfrequency components in the cosine
transform, and the reason why compression really works: the distribution is skew, and zeroes
dominate (cf. the result of BurrowsWheeler transform, followed by movetofront
The quantized numbers could be coded with arithmetic of Huffman coding, of which the latter
is more common. Before coding, the obtained matrix is linearized into a 1dimensional vector
of 64 values. Due to the diagonallyoriented distribution, linearization is not perform rowby
row or columnbycolumn, but diagonally, in a socalled zigzag order:
The previous coefficients constitute the following linearized vector:
−26, −3, 0, −3, −2, −6, 2, −4, 1, −4, 1, 1, 5, 1, 2, −1, 1, −1, 2, 0, 0, 0, 0, 0, −1, −1, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
The first (topleft) coefficient (socalled DCcoefficient) is encoded separately from others,
because it has different statistics. It represents the average greyvalue of the block, and is
correlated with the DCcoefficients of the neighboring blocks. Therefore, differences from
neighbors are encoded, instead, using Huffman method. The 63 other values are not encoded
as such, but runlength coding is applied, in the following way: The alphabet for Huffman
coding consists of pairs of the following components:
· Number of zeroes before the next nonzero element
· Number of significant bits in the nonzero element80
The former is restricted to values 0..15, and the latter to 1..10, resulting in 160 combinations.
Two special pairs are:
· 0, 0 that represents EOB = endofblock
· 15, 0 that represents a sequence of zeroes without an ending nonzero element
The reason for considering the pairs as symbols of the alphabet is that the components of the
pair are highly correlated: A long sequence of zeroes is probably followed by a small (rather
than large) nonzero value. Default Huffman tables are defined in the JPEG standard.
In addition to coding the pairs, also the nonzero values must be represented, but for that no
special code is needed, because the number of bits for them is known from the second
component of the related pair. Simple signed base2 representation is sufficient (values can be
positive or negative).
JPEGdecoding proceeds in the opposite direction. After Huffman decoding, the quantized
coefficient matrices can be recovered by reversing the zigzag scanning of elements.
Dequantization means multiplying the matrix elements by the corresponding elements of the
quantization matrix (see the matrices at the top of page 77). The resulting values represent
approximations of the original coefficients. Finally, the inverse of DCTtransform returns the
block of pixel values, which are close to the original. An example shows the effect (socalled
blocking artefacts) of too large quantization values.
Compression ratio 7:1 Compression ratio 30:1
Until now, we have assumed a monochrome, greyscale image. Color images are based on
some color model, which most often consists of three components. The bestknown color
model is RedGreenBlue (RGB), used in monitors. It is not advantageous to compress the
three component images separately, because there are high correlations between them.
Therefore, JPEG first applies a transform to a socalled YUV model (YIQ in NorthAmerica),
which distinguishes the luminance component from two chrominance components. These
three images are less correlated. An example (Lena) below shows how the ‘energy’ in the
image is concentrated on the luminance component image.81
Moreover, because the human visual system is less sensitive to changes of color than changes
of brightness, the two chrominance components are usually subsampled by 2, i.e. they
constitute only one quarter of the original image size, each. Subsampling is already a lossy
compression step. The three component images are compressed with the above described
method, with certain tunings.
As a conclusion from the studied image compression techniques, we can make the following
general observations and recommendations:
· Digitized representations of continuous physical phenomena need totally different
modeling techniques from texttype data.
· The correlations within the source datasets should be carefully analyzed and localized.
One should try to use the correlations in transforming the dataset into another form where
the distribution of values is as skew as possible, and where the components are
· When developing a lossy method, the loss should be positioned to a place where it is not
· In entropy coding, there are lots of choices for the alphabet. For Huffman coding, one
should choose symbols, possibly artificial groups of symbols/values, which are not
· For Huffman coding, a large (extended) alphabet works often better than small.
· Predefined (static) Huffman tables should be used, if possible, for speed reasons, and to
avoid sending the tables to the decoder.
Image compression is a very large subject area, which would deserve a course of its own. The
same holds for digital video, which would be quite impractical without compression.