Lecture notes Cmputer graphics

what is computer graphics and its application and what is computer graphics explain its applications in different fields. how computer graphics is useful in education and entertainment pdf free download
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UNIT - I LESSON – 1: A SURVEY OF COMPUTER GRAPHICS CONTENTS 1.1 Aims and objectives 1.2 Introduction 1.3 History of Computer Graphics 1.4 Applications of Computer Graphics 1.4.1 Computer Aided Design 1.4.2 Computer Aided Manufacturing 1.4.3 Entertainment 1.4.4 Medical Content Creation 1.4.5 Advertisement 1.4.6 Visualization 1.4.7 Visualizing Complex Systems 1.5 Graphical User Interface 1.5.1 Three-dimensional Graphical User Interfaces 1.6 Let us Sum Up 1.7 Lesson-end Activities 1.8 Points for Discussion 1.9 Model answers to “Check your Progress” 1.10 References 1.1 Aims and Objectives The aim of this lesson is to learn the introduction, history and various applications of computer graphics The objectives of this lesson are to make the student aware of the following concepts a. History of Computer Graphics b. Applications of Computer Graphics c. Graphical User Interface 1 1.2 Introduction Computer Graphic is the discipline of producing picture or images using a computer which include modeling, creation, manipulation, storage of geometric objects, rendering, converting a scene to an image, the process of transformations, rasterization, shading, illumination, animation of the image, etc. Computer Graphics has been widely used in graphics presentation, paint systems, computer-aided design (CAD), image processing, simulation, etc. From the earliest text character images of a non-graphic mainframe computers to the latest photographic quality images of a high resolution personal computers, from vector displays to raster displays, from 2D input, to 3D input and beyond, computer graphics has gone through its short, rapid changing history. From games to virtual reality, to 3D active desktops, from unobtrusive immersive home environments, to scientific and business, computer graphics technology has touched almost every concern of our life. Before we get into the details, we have a short tour through the history of computer graphics 1.3 History of Computer Graphics In the 1950’s, output are via teletypes, lineprinter, and Cathode Ray Tube (CRT). Using dark and light characters, a picture can be reproduced. In the 1960’s, beginnings of modern interactive graphics, output are vector graphics and interactive graphics. One of the worst problems was the cost and inaccessibility of machines. In the early 1970’s, output start using raster displays, graphics capability was still fairly chunky. In the 1980’s output are built-in raster graphics, bitmap image and pixel. Personal computers costs decrease drastically; trackball and mouse become the standard interactive devices. In the 1990’s, since the introduction of VGA and SVGA, personal computer could easily display photo-realistic images and movies. 3D image renderings became the main advances and it stimulated cinematic graphics applications. Table 1: gives a general history of computer graphics. 2 1.4 Applications of Computer Graphics We have a short tour through the applications of computer graphics. 1.4.1 Computer Aided Design Computer-aided design (CAD) is use of a wide range of computer based tools that assist engineers, architects and other design profession in their design activities. It is the main geometry authoring tool within the Product Lifecycle Management process and involves both software and sometimes special-purpose hardware. Current packages range from 2D vector base drafting systems to 3D solid and surface modellers. The CAD Process CAD is used to design, develop and optimize products, which can be goods used by end consumers or intermediate goods used in other products. CAD is also extensively used in the design of tools and machinery used in the manufacture of components, and in the drafting and design of all types of buildings, from small residential types (houses) to the largest commercial and industrial structures (hospitals and factories). CAD is mainly used for detailed engineering of 3D models and/or 2D drawings of physical components, but it is also used throughout the engineering process from conceptual design and layout of products, through strength and dynamic analysis of assemblies to definition of manufacturing methods of components. CAD has become an especially important technology, within the scope of Computer Aided technologies, with benefits such as lower product development costs and a greatly shortened design cycle. CAD enables designers to layout and develop work on screen, print it out and save it for future editing, saving time on their drawings. 4 The capabilities of modern CAD systems include (a) Wireframe geometry creation, (b) 3D parametric feature based modelling, Solid modeling, (c) Freeform surface modeling, (d) Automated design of assemblies, which are collections of parts and/or other assemblies, (e) create Engineering drawings from the solid models, (f) Reuse of design components, (g) Ease of modification of design of model and the production of multiple versions, (h) Automatic generation of standard components of the design, (i) Validation/verification of designs against specifications and design rules, (j) Simulation of designs without building a physical prototype, (k) Output of engineering documentation, such as manufacturing drawings, and Bills of Materials to reflect the BOM required to build the product, (l) Import/Export routines to exchange data with other software packages, (m) Output of design data directly to manufacturing facilities, (n) Output directly to a Rapid Prototyping or Rapid Manufacture Machine for industrial prototypes, (o) maintain libraries of parts and assemblies, (p) calculate mass properties of parts and assemblies, (q) aid visualization with shading, rotating, hidden line removal, etc..., (r) Bi-directional parametric association (modification of any feature is reflected in all information relying on that feature; drawings, mass properties, assemblies, etc... and counter wise), (s) kinematics, interference and clearance checking of assemblies, (t) sheet metal, (u) hose/cable routing, (v) electrical component packaging, (x) inclusion of programming code in a model to control and relate desired attributes of the model, (y) Programmable design studies and optimization, (z) Sophisticated visual analysis routines, for draft, curvature, curvature continuity... Originally software for CAD systems were developed with computer language such as Fortran, but with the advancement of object-oriented programming methods this has radically changed. Typical modern parametric feature based modeler and freeform surface systems are built around a number of key C programming language modules with their own APIs. Today most CAD computer workstations are Windows based PCs; some CAD systems also run on hardware running with one of the Unix operating systems and a few with Linux. Some CAD systems such as NX provide multiplatform support including Windows, LINUX, UNIX and Mac OSX. CAD of Jet Engine CAD and Rapid Prototyping Parachute Modeling and Simulation 5 virtual 3-D interiors (Virtual Environment) CAM(jewelry industry) CAM CAD design CAM CAD robot Generally no special hardware is required with the exception of a high end OpenGL based Graphics card; however for complex product design, machines with high speed (and possibly multiple) CPUs and large amounts of RAM are recommended. The human-machine interface is generally via a computer mouse but can also be via a pen and digitizing graphics tablet. Manipulation of the view of the model on the screen is also sometimes done with the use of a spacemouse/SpaceBall. Some systems also support stereoscopic glasses for viewing the 3D model. 1.4.2 Computer Aided Manufacturing Since the age of the Industrial Revolution, the manufacturing process has undergone many dramatic changes. One of the most dramatic of these changes is the introduction of Computer Aided Manufacturing (CAM), a system of using computer technology to assist the manufacturing process. Through the use of CAM, a factory can become highly automated, through systems such as real-time control and robotics. A CAM system usually seeks to control the production process through varying degrees of automation. Because each of the many manufacturing processes in a CAM system is computer controlled, a high degree of precision can be achieved that is not possible with a human interface. The CAM system, for example, sets the toolpath and executes precision machine operations based on the imported design. Some CAM systems bring in additional 6 automation by also keeping track of materials and automating the ordering process, as well as tasks such as tool replacement. Computer Aided Manufacturing is commonly linked to Computer Aided Design (CAD) systems. The resulting integrated CAD/CAM system then takes the computer- generated design, and feeds it directly into the manufacturing system; the design is then converted into multiple computer-controlled processes, such as drilling or turning. Another advantage of Computer Aided Manufacturing is that it can be used to facilitate mass customization: the process of creating small batches of products that are custom designed to suit each particular client. Without CAM, and the CAD process that precedes it, customization would be a time-consuming, manual and costly process. However, CAD software allows for easy customization and rapid design changes: the automatic controls of the CAM system make it possible to adjust the machinery automatically for each different order. Robotic arms and machines are commonly used in factories, but these do still require human workers. The nature of those workers' jobs change however. The repetitive tasks are delegated to machines; the human workers' job descriptions then move more towards set-up, quality control, using CAD systems to create the initial designs, and machine maintenance. 1.4.3 Entertainment One of the main goals of todays special effects producers and animators is to create images with highest levels of photorealism. Volume graphics is the key technology to provide full immersion in upcoming virtual worlds e.g. movies or computer games. Real world phenomena can be realized best with true physics based models and volume graphics is the tool to generate, visualize and even feel these models Movies like Star Wars Episode I, Titanic and The Fifth Element already started employing true physics based effects. Entertainment Games 1.4.4 Medical Content Creation Medical content creation has become more and more important in entertainment and education in the last years. For instance, virtual anatomical atlas on CD-ROM and DVD have been build on the base of the NIH Visible Human Project data set and 7 different kind of simulation and training software were build up using volume rendering techniques. Volume Graphics' products like the VGStudio software are dedicated to the used in the field of medical content creation. VGStudio provides powerful tools to manipulate and edit volume data. An easy to use keyframer tool allows to generate animations, e.g. flights through any kind of volume data. In addition VGStudio provides highest image quality and unsurpassed performance already on a PC Images of a fetus rendered by a V.G. Studio MAX user. 1.4.5 Advertisement Voxel data can be used to visualize the most fascinating and complex facts in the world. The visualization of the human body and medical content creation is an example. Voxel data sets like CT or MRI scans or the exciting Visible Human data show all the finest details up to the gross structures of the human anatomy. Images rendered by Volume Graphics 3D graphics software are already used for US TV productions as well as for advertising. Volume Graphics cooperates with companies specialized on Video and TV productions as well as with advertising agencies. Neutron Radiography of a car engine 1.4.6 Visualization Visualization is any technique for creating images, diagrams, or animations to communicate a message. Visualization through visual imagery has been an effective way to communicate both abstract and concrete ideas since the dawn of man. 8 Visualization today has ever-expanding applications in science, engineering Product visualization, all forms of education, interactive multimedia, medicine etc. Typical of a visualization application is the field of computer graphics. The invention of computer graphics may be the most important development in visualization. The development of animation also helped advance visualization. Visualization of how a car deforms in an Computer aided Learning asymmetrical crash using finite element analysis. Visualization is the process of representing data as descriptive images and, subsequently, interacting with these images in order to gain additional insight into the data. Traditionally, computer graphics has provided a powerful mechanism for creating and manipulating these representations. Graphics and visualization research addresses the problem of converting data into compelling and revealing images that suit users’ needs. Research includes developing new representations of 3D geometry, choosing appropriate graphical realizations of data, strategies for collaborative visualization in a networked environment using three dimensional data, and designing software systems that support a full range of display formats ranging from PDAs to immersive multi-display visualization environments. 1.4.7 Visualizing Complex Systems Graphic images and models are proving not only useful, but crucial in many contemporary fields dealing with complex data. Only by graphically combining millions 9 of discrete data items, for example, can meteorologists track weather systems, including hurricanes that may threaten thousands of lives. Theoretical physicists depend on images to think about events like collisions of cosmic strings at 75 percent of the speed of light, and chaos theorists require pictures to find order within apparent disorder. Computer- aided design systems are critical to the design and manufacture of an extensive range of contemporary products, from silicon chips to automobiles, in fields ranging from space technology to clothing design. Computer systems, on which we all increasingly depend, are also becoming more and more visually oriented. Graphical user interfaces are the emerging standard, and graphic tools are the heart of contemporary systems analysis, identifying and preventing critical errors and omissions that might otherwise not be evident until the system is in daily use. Graphic computer-aided systems engineering (CASE) tools are now used to build other computer systems. Recent research indicates that visual computer programming produces better comprehension and accuracy than do traditional programming languages based on words, and commercial visual programming packages are now on the market. Medical research and practice offer many examples of the use of graphic tools and images. Conceptualizing the deoxyribonucleic acid (DNA) double helix permitted dramatic advances in genetic research years before the structure could actually be seen. Computerized imaging systems like computerized tomography (CT) and magnetic resonance imaging (MRI) have produced dramatic improvements in the diagnosis and treatment of serious illness, and a project compiling a three-dimensional cross-section of the human body provides a new approach to the study of anatomy. X-rays, venerable medical imaging tools, are now being combined with expert systems to help physicians identify other cases similar to those they are handling, suggesting additional diagnostic and treatment information relevant to patients. Sociologists and social psychologists use graphic tools extensively in their research programs. They often turn to sociograms and other visual tools to present and explain concepts extracted from complex statistical analyses and to identify meaningful patterns in the data. Graphic depiction of exchange networks permits the study of changes among groups over time. Another useful approach is Bales's Systematic Multiple Level Observation of Groups (SYMLOG), which provides a three-dimensional graphic representation of friendliness, instrumental-versus-expressive orientation, and dominance in small groups. Graphic visualization has demonstrated utility for organizing information effectively and coherently in a broad range of fields dealing with complex data. Social work deals with similarly (and sometimes more) complex patterns and contextual situations, and, in fact, social work and related disciplines have discovered the utility of images for conceptualizing and communicating about clinical practice. 10 1.5 Graphical user interface A graphical user interface (GUI) is a type of user interface which allows people to interact with a computer and computer-controlled devices which employ graphical icons, visual indicators or special graphical elements called "widgets", along with text, labels or text navigation to represent the information and actions available to a user. The actions are usually performed through direct manipulation of the graphical elements. The precursor to graphical user interfaces was invented by researchers at the Stanford Research Institute, led by Douglas Engelbart. They developed the use of text- based hyperlinks manipulated with a mouse for the On-Line System. The concept of hyperlinks was further refined and extended to graphics by researchers at Xerox PARC, who went beyond text-based hyperlinks and used a GUI as the primary interface for the Xerox Alto computer. Most modern general-purpose GUIs are derived from this system. As a result, some people call this class of interface a PARC User Interface (PUI) (note that PUI is also an acronym for perceptual user interface). Following PARC the first commercially successful GUI-centric computer operating models were those of the Apple Lisa but more successfully that of Macintosh System graphical environment. The graphical user interfaces familiar to most people today are Microsoft Windows, Mac OS X, and the X Window System interfaces. IBM and Microsoft used many of Apple's ideas to develop the Common User Access specifications that formed the basis of the user interface found in Microsoft Windows, IBM OS/2 Presentation Manager, and the Unix Motif toolkit and window manager. These ideas evolved to create the interface found in current versions of the Windows operating system, as well as in Mac OS X and various desktop environments for Unix-like systems. Thus most current graphical user interfaces have largely common idioms. Graphical user interface design is an important adjunct to application programming. Its goal is to enhance the usability of the underlying logical design of a stored program. The visible graphical interface features of an application are sometimes referred to as "chrome". They include graphical elements (widgets) that may be used to interact with the program. Common widgets are: windows, buttons, menus, and scroll bars. Larger widgets, such as windows, usually provide a frame or container for the main presentation content such as a web page, email message or drawing. Smaller ones usually act as a user-input tool. The widgets of a well-designed system are functionally independent from and indirectly linked to program functionality, so the graphical user interface can be easily customized, allowing the user to select or design a different skin at will. Some graphical user interfaces are designed for the rigorous requirements of vertical markets. These are known as "application specific graphical user interfaces." Examples of application specific graphical user interfaces:  Touch screen point of sale software used by wait staff in busy restaurants 11  Self-service checkouts used in some retail stores..  ATMs  Airline self-ticketing and check-in  Information kiosks in public spaces like train stations and museums  Monitor/control screens in embedded industrial applications which employ a real time operating system (RTOS). The latest cell phones and handheld game systems also employ application specific touch screen graphical user interfaces. Cars have graphical user interfaces in them. For example, GPS navigation, touch screen multimedia centers, and even on dashboards of the newer cars. Metisse 3D Window manager XGL 3D Desktop Residents training in Videoendoscopic Surgery Laboratory Visualization 1.5.1 Three-dimensional graphical user interfaces For typical computer displays, three-dimensional are a misnomer—their displays are two-dimensional. Three-dimensional images are projected on them in two dimensions. Since this technique has been in use for many years, the recent use of the term three-dimensional must be considered a declaration by equipment marketers that the speed of three dimension to two dimension projection is adequate to use in standard graphical user interfaces. 12 Screenshot showing the 'cube' plugin of Compiz on Ubuntu Three-dimensional graphical user interfaces are common in science fiction literature and movies, such as in Jurassic Park, which features Silicon Graphics' three- dimensional file manager. In science fiction, three-dimensional user interfaces are often immersible environments like William Gibson's Cyberspace or Neal Stephenson's Metaverse. Three- dimensional graphics are currently mostly used in computer games, art and computer- aided design (CAD). A three-dimensional computing environment could possibly be used for collaborative work. For example, scientists could study three-dimensional models of molecules in a virtual reality environment, or engineers could work on assembling a three-dimensional model of an airplane. 1.6 Let us Sum Up In this lesson we have learnt about the following a) Introduction to computer graphics b) History of computer graphics and c) Applications of computer graphics 1.7 Lesson-end Activities After learning this lesson, try to discuss among your friends and answer these questions to check your progress. a) The need of Computer Graphics in the modern world b) The use of Computer Graphics in the modern world 1.8 Points for Discussion Try to discuss the following a) Computer aided design b) Computer aided manufacturing 13 1.9 Model answers to “Check your Progress” In order to check your progress, try to answer the following questions a) Discuss about the application of computer graphics in entertainment b) Discuss about the application of computer graphics in visualization 1.10 References 1. Chapter 1 of William M. Newman, Robert F. Sproull, “Principles of Interactive Computer Graphics”, Tata-McGraw Hill, 2000 2. Chapter 1 of Donald Hearn, M. Pauline Baker, “Computer Graphics – C Version”, Pearson Education, 2007 3. Chapter 1, 2, 3 of ISRD Group, “Computer Graphics”, McGraw Hill, 2006 4. Chapter 1 of J.D. Foley, A.Dam, S.K. Feiner, J.F. Hughes, “Computer Graphics – principles and practice”, Addison-Wesley, 1997 14 LESSON – 2: OVERVIEW OF COMPUTER GRAPHICS CONTENTS 1.11 Aims and Objective 1.12 Introduction 1.13 Computer Display 1.14 Random Scan 1.15 Raster Scan 1.15.1 Rasters 1.15.2 Pixel Values 1.15.3 Raster Memory 1.15.4 Key attributes of Raster Displays 1.16 Display Processor 1.17 Let us Sum Up 1.18 Lesson-end Activities 1.19 Points for Discussion 1.20 Model answers to “Check your Progress” 1.21 References 2.1 Aims and Objectives The aim of this lesson is to learn the concepts of computer display, random scan and raster scan systems. The objectives of this lesson are to make the student aware of the following concepts a) Display systems b) Cathode ray tube c) Random Scan d) Raster Scan and e) Display processor 2.2 Introduction Graphics Terminal: Interactive computer graphics terminals comprise distinct output and input devices. Aside from power supplies and enclosures, these usually connect only via a computer both connect to. 15  output: A display system presenting rapidly variable (not just hard-copy) graphical output;  input: Some input device(s), e.g. keyboard + mouse. These may provide graphical input: o A mouse provides graphical input the computer echoes as a graphical cursor on the display. o A keyboard typically provides graphical input located at a separate text cursor position.  There may be other I/O devices, e.g. a scanner and/or printer, microphone(s) and/or speakers. A Display System typically comprises:  A display device such as a CRT (cathode ray tube), liquid crystal display, etc. o Most have a screen which presents a 2D image; o Stereoscopic displays show distinct 2D images to each eye (head-mounted / special glasses); o Displays with true 3D images are available.  A display processor controlling the display according digital instructions about what to display.  memory for these instructions or image data, possibly part of a computer's ordinary RAM. 2.3 Computer display A computer display monitor, usually called simply a monitor, is a piece of electrical equipment which displays viewable images generated by a computer without producing a permanent record. The word "monitor" is used in other contexts; in particular in television broadcasting, where a television picture is displayed to a high standard. A computer display device is usually either a cathode ray tube or some form of flat panel such as a TFT LCD. The monitor comprises the display device, circuitry to generate a picture from electronic signals sent by the computer, and an enclosure or case. Within the computer, either as an integral part or a plugged-in interface, there is circuitry to convert internal data to a format compatible with a monitor. 16 The CRT or cathode ray tube, is the picture tube of a monitor. The back of the tube has a negatively charged cathode. The electron gun shoots electrons down the tube and onto a charged screen. The screen is coated with a pattern of dots that glow when struck by the electron stream. Each cluster of three dots, one of each color, is one pixel. The image on the monitor screen is usually made up from at least tens of thousands of such tiny dots glowing on command from the computer. The closer together the pixels are, the sharper the image on screen. The distance between pixels on a computer monitor screen is called its dot pitch and is measured in millimeters. Most monitors have a dot pitch of 0.28 mm or less. There are two electromagnets around the collar of the tube which deflect the electron beam. The beam scans across the top of the monitor from left to right, is then blanked and moved back to the left-hand side slightly below the previous trace (on the next scan line), scans across the second line and so on until the bottom right of the screen is reached. The beam is again blanked, and moved back to the top left to start again. This process draws a complete picture, typically 50 to 100 times a second. The number of times in one second that the electron gun redraws the entire image is called the refresh rate and is measured in hertz (cycles per second). It is common, particularly in lower- priced equipment, for all the odd-numbered lines of an image to be traced, and then all the even-numbered lines; the circuitry of such an interlaced display need be capable of only half the speed of a non-interlaced display. An interlaced display, particularly at a relatively low refresh rate, can appear to some observers to flicker, and may cause eyestrain and nausea. 17 CRT computer monitor As with television, several different hardware technologies exist for displaying computer-generated output:  Liquid crystal display (LCD). LCDs are the most popular display device for new computers in the Western world.  Cathode ray tube (CRT) o Vector displays, as used on the Vectrex, many scientific and radar applications, and several early arcade machines (notably Asteroids - always implemented using CRT displays due to requirement for a deflection system, though can be emulated on any raster-based display. o Television receivers were used by most early personal and home computers, connecting composite video to the television set using a modulator. Image quality was reduced by the additional steps of composite video → modulator → TV tuner → composite video.  Plasma display  Surface-conduction electron-emitter display (SED)  Video projector - implemented using LCD, CRT, or other technologies. Recent consumer-level video projectors are almost exclusively LCD based.  Organic light-emitting diode (OLED) display The performance parameters of a monitor are:  Luminance, measured in candelas per square metre (cd/m²).  Size, measured diagonally. For CRT the viewable size is one inch (25 mm) smaller then the tube itself.  Dot pitch. Describes the distance between pixels of the same color in millimetres. In general, the lower the dot pitch (e.g. 0.24 mm, which is also 240 micrometres), the sharper the picture will appear.  Response time. The amount of time a pixel in an LCD monitor takes to go from active (black) to inactive (white) and back to active (black) again. It is measured in milliseconds (ms). Lower numbers mean faster transitions and therefore fewer visible image artifacts. 18  Refresh rate. The number of times in a second that a display is illuminated.  Power consumption, measured in watts (W).  Aspect ratio, which is the horizontal size compared to the vertical size, e.g. 4:3 is the standard aspect ratio, so that a screen with a width of 1024 pixels will have a height of 768 pixels. A widescreen display can have an aspect ratio of 16:9, which means a display that is 1024 pixels wide will have a height of 576 pixels.  Display resolution. The number of distinct pixels in each dimension that can be displayed. A fraction of all LCD monitors are produced with "dead pixels"; due to the desire to increase profit margins by companies, most manufacturers sell monitors with dead pixels. Almost all manufacturers have clauses in their warranties which claim monitors with fewer than some number of dead pixels is not broken and will not be replaced. The dead pixels are usually stuck with the green, red, and/or blue subpixels either individually always stuck on or off. Like image persistence, this can sometimes be partially or fully reversed by using the same method listed below, however the chance of success is far lower than with a "stuck" pixel. Screen burn-in, where a static image left on the screen for a long time embeds the image into the phosphor that coats the screen, is an issue with CRT and Plasma computer monitors and televisions. The result of phosphor burn-in are "ghostly" images of the static object visible even when the screen has changed, or is even off. This effect usually fades after a period of time. LCD monitors, while lacking phosphor screens and thus immune to phosphor burn-in, have a similar condition known as image persistence, where the pixels of the LCD monitor "remember" a particular color and become "stuck" and unable to change. Unlike phosphor burn-in, however, image persistence can sometimes be reversed partially or completely. This is accomplished by rapidly displaying varying colors to "wake up" the stuck pixels. Screensavers using moving images, prevent both of these conditions from happening by constantly changing the display. Newer monitors are more resistant to burn-in, but it can still occur if static images are left displayed for long periods of time. Most modern computer displays can show thousands or millions of different colors in the RGB color space by varying red, green, and blue signals in continuously variable intensities. Many monitors have analog signal relay, but some more recent models (mostly LCD screens) support digital input signals. It is a common misconception that all computer monitors are digital. For several years, televisions, composite monitors, and computer displays have been significantly different. However, as TVs have become more versatile, the distinction has blurred. Some users use more than one monitor. The displays can operate in multiple modes. One of the most common spreads the entire desktop over all of the monitors, which thus act as one big desktop. The X Window System refers to this as Xinerama. 19 Two Apple flat-screen monitors used as dual display Display systems use either random or raster scan:  Random scan displays, often termed vector displays, came first and are still used in some applications. Here the electron gun of a CRT illuminates points and/or straight lines in any order. The display processor repeatedly reads a variable 'display file' defining a sequence of X,Y coordinate pairs and brightness or colour values, and converts these to voltages controlling the electron gun. A Random Scan Display (outline)  Raster scan displays, also known as bit-mapped or raster displays, are somewhat less relaxed. Their whole display area is updated many times a second from image data held in raster memory. The rest of this handout concerns hardware and software aspects of raster displays. 2.4 Random Scan Systems A two-dimensional video data acquisition system comprising: video detector apparatus for scanning a visual scene; controller apparatus for generating scan pattern instructions; system interface apparatus for selecting at least one scan pattern for acquisition of video data from the visual scene, the scan pattern being selected from a plurality of such patterns in accordance with the scan pattern instructions; and scan-video interface apparatus comprising random scan driver apparatus for generating scan control signals in accordance with the selected scan pattern, the video detector apparatus scanning the visual scene in accordance with the scan control signals to provide an output to the system interface such that an intensity data map is stored therein, the controller apparatus performing data processing of the intensity data map in accordance with a predetermined set of video data characteristics. 20 2.5 Raster Scan A Raster scan, or raster scanning, is the pattern of image detection and reconstruction in television, and is the pattern of image storage and transmission used in most computer bitmap image systems. The word raster comes from the Latin word for a rake, as the pattern left by a rake resembles the parallel lines of a scanning raster. In a raster scan, an image is cut up into successive samples called pixels, or picture elements, along scan lines. Each scan line can be transmitted as it is read from the detector, as in television systems, or can be stored as a row of pixel values in an array in a computer system. On a television receiver or computer monitor, the scan line is turned back to a line across an image, in the same order. After each scan line, the position of the scan line is advanced, typically downward across the image in a process known as vertical scanning, and a next scan line is detected, transmitted, stored, retrieved, or displayed. This ordering of pixels by rows is known as raster order, or raster scan order. 2.5.1 Rasters Lexically, a raster is a series of adjacent parallel 'lines' which together form an image on a display screen. In early analogue television sets each such line is scanned continuously, not broken up into distinct units. In computer or digital displays these lines are composed of independently coloured pixels (picture elements). Mathematically we consider a raster to be a rectangular grid or array of pixel positions: A Raster Pixel positions have X,Y coordinates. Usually Y points down. This may reflect early use to display text to western readers. Also when considering 3D, right-handed coordinates imply Z represents depth. 2.5.2 Pixel Values The colour of each pixel of a display is controlled by a distinct digital memory element. Each such element holds a pixel value encoding a monochrome brightness or colour to be displayed. 21

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