Shading in computer graphics ppt

shading models in computer graphics ppt and shading techniques in computer graphics ppt
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Dr.ShaneMatts,United States,Teacher
Published Date:23-07-2017
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Shading & Material Appearance © ACM. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/. 1 MIT EECS 6.837 – Matusik Lighting and Material Appearance • Input for realistic rendering – Geometry, Lighting and Materials • Material appearance – Intensity and shape of highlights – Glossiness – Color – Spatial variation, i.e., texture (next Tuesday) Slide Addy Ngan © source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/. 2 Unit Issues - Radiometry • We will not be too formal in this class • Issues we will not really care about – Directional quantities vs. integrated over all directions – Differential terms: per solid angle, per area – Power? Intensity? Flux? • Color – All math here is for a single wavelength only; we will perform computations for R, G, B separately • Do not panic, that just means we will perform every operation three times, that is all 3 Light Sources • Today, we only consider point light sources – Thus we do not need to care about solid angles • For multiple light sources, use linearity – We can add the solutions for two light sources • I(a+b) = I(a) + I(b) – We simply multiply the solution when we scale the light a b intensity • I(s a) = s I(a) Yet again, linearity is our friend 4 Intensity as Function of Distance 2 • 1/r fall-off for isotropic point lights – Why? An isotropic point light outputs constant power per solid r 2 angle (“into all directions”) r 1 – Must have same power in all concentric spheres 2 2 • Sphere’s surface area grows with r = energy obeys 1/r • … but in graphics we often cheat with or ignore this. – Why? Ideal point lights are kind of harsh • Intensity goes to infinity when you get close – not great 2 – In particular, 1/(ar +br+c) is popular 5 Incoming Irradiance • The amount of light energy received by a surface depends on incoming angle – Bigger at normal incidence, even if distance is const. • Similar to winter/summer difference n • How exactly? θ – Cos θ law – Dot product with normal Surface 6 Incoming Irradiance for Pointlights 2 • Let’s combine this with the 1/r fall-off: – I is the irradiance (“intensity”) at in n surface point x θ – I is the “intensity” of the light light – θ is the angle between light direction l and surface normal n l – r is the distance between light and x. Surface x 7 Directional Lights • “Pointlights that are infinitely far” – No falloff, just one direction and one intensity n l – I is the irradiance at surface point x in θ from the directional light – I is the “intensity” of the light light – θ is the angle between light direction l and surface normal n • Only depends on n, not x Surface x 8 Spotlights • Pointlights with non-uniform directional emission • Usually symmetric about a central direction d, with angular falloff – Often two angles • “Hotspot” angle: No attenuation within the central cone • “Falloff” angle: Light attenuates from full intensity to zero intensity between the hotspot and falloff d angles • Plus your favorite distance falloff curve 9 Spotlight Geometry (direction d) hotspot angle Adapted from POVRAY documentation 10 Questions? 11 Quantifying Reflection – BRDF • Bidirectional Reflectance Distribution Function • Ratio of light coming from one direction that gets reflected in © ACM. All rights reserved. This content is excluded from another direction our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/. – Pure reflection, assumes no light scatters into the Incoming direction material Outgoing direction • Focuses on angular aspects, not spatial variation of the material • How many dimensions? 12 BRDF f r 13 BRDF f r 14 BRDF f r • Relates incident irradiance from l = light direction (incoming) every direction to outgoing light. v = view direction How? (outgoing) 15 BRDF f r • Relates incident irradiance from l = light direction (incoming) every direction to outgoing light. v = view direction How? (outgoing) • Let’s combine with what we know already of pointlights: 16 2D Slice at Constant Incidence • For a fixed incoming direction, view dependence is a 2D spherical function – Here a moderate specular component © ACM. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/. highlight incoming incoming Courtesy of Mitsubishi Electric Researh Laboratories, Inc. Used with permission. Example: Plot of “PVC” BRDF at 55° incidence 17 Demo 18 Isotropic vs. Anisotropic • When keeping l and v fixed, if rotation of surface around the normal does not change the reflection, the material is called isotropic • Surfaces with strongly oriented microgeometry elements are anisotropic • Examples: – brushed metals, – hair, fur, cloth, velvet Westin et.al 92 19 Demo 20

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