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COMPUTER GRAPHICS

COMPUTER GRAPHICS. CS 482 – FALL 2014. OCTOBER 20, 2014. SCATTERING. LIGHT SCATTERING PHYSICALLY BASED SCATTERING SUBSURFACE SCATTERING AMBIENT OCCLUSION. LIGHT SCATTERING. REFLECTANCE MODELS.

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COMPUTER GRAPHICS

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  1. COMPUTER GRAPHICS CS 482 – FALL 2014 OCTOBER 20, 2014 SCATTERING • LIGHT SCATTERING • PHYSICALLY BASED SCATTERING • SUBSURFACE SCATTERING • AMBIENT OCCLUSION

  2. LIGHT SCATTERING REFLECTANCE MODELS When light hits the molecules of an object’s surface, it scatters in various directions, depending on the material properties of the object. Reflective Above the object’s surface Transmissive Below the object’s surface Impulse All in the same exiting direction CS 482 – FALL 2014 OCTOBER 20, 2014: SCATTERING PAGE 196

  3. LIGHT SCATTERING DIFFUSE AND GLOSSY Surface shading characteristics also impact the scattering that takes place on an object’s surface. LAMBERTIAN Pure diffuse reflection PHONG Specular reflection included y y x x CS 482 – FALL 2014 OCTOBER 20, 2014: SCATTERING PAGE 197

  4. SCATTERING MODELS IMAGE QUALITY VS. PROCESSING TIME Improvements in scattering models have been accompanied by corresponding penalties in processing overhead. Lambertian models ignore specular effects, yielding fast processing but drab images. Phong models interpolate to achieve specular effects, but producing less focused highlights. Cook-Torrance models approximate surfaces as a set of microfacets oriented similarly for smooth surfaces and more randomly for rough surfaces. CS 482 – FALL 2014 OCTOBER 20, 2014: SCATTERING PAGE 198

  5. SCATTERING MODELS MICROFACETS The variability of microfacet orientation in rougher surface models produces appropriate BRDF contributions. Some microfacets are occluded from the light, so they are shadowed and receive no light (and thus reflect none). Some microfacets are not visible from the view direction, so any light reflected from them can’t be seen. In reality, shadowed light continues reflecting and some of it would end up in the view direction, but that is usually not modeled. CS 482 – FALL 2014 OCTOBER 20, 2014: SCATTERING PAGE 199

  6. SCATTERING MODELS ANISOTROPIC MATERIALS Some materials (e.g., brushed metal, velvet) have reflective properties that vary as the surface is rotated. Isotropic materials use a single randomized variable to determine each microfacet’s orientation, while anisotropic materials use two variables. The results of doing this are illustrated in the two isotropic images at near left and the corresponding anisotropic images at far right. CS 482 – FALL 2014 OCTOBER 20, 2014: SCATTERING PAGE 200

  7. SUBSURFACE SCATTERING TRANSLUCENT MATERIAL Under certain circumstances, when light hits certain translucent material, it penetrates the material, is scattered by interacting with what is beneath the surface, and then exits the surface at a different location. CS 482 – FALL 2014 OCTOBER 20, 2014: SCATTERING PAGE 201

  8. SUBSURFACE SCATTERING SCATTERING RADIUS By altering the depth to which the penetrating light rays are allowed to go, the material properties of the modeled object may be adjusted. CS 482 – FALL 2014 OCTOBER 20, 2014: SCATTERING PAGE 202

  9. AMBIENT OCCLUSION LOCALLY CONVEX ENVIRONMENT One means of approximating how much light hits a modeled object is ambient occlusion, a measure of how much light reaches a point from a uniformly lit hemisphere. Ambient occlusion is widely used in movie production since it gives a good indication of creases on surfaces and spatial proximity of objects, and is a cheap (but crude) approximation of global illumination. Ambient occlusion is computed by shooting a uniform distribution of rays from each object through the hemisphere to see if outside geometry is encountered. CS 482 – FALL 2014 OCTOBER 20, 2014: SCATTERING PAGE 203

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