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Shading

Shading. CMSC 435/634. RenderMan. Displacement. Surface. Light. Volume. Imager. Displacement. Given Position Normal Compute Position Normal. Displacement. Surface. Light. Volume. Imager. Surface color. Given Position Compute Color Opacity. Displacement. Surface. Light.

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Shading

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  1. Shading CMSC 435/634

  2. RenderMan Displacement Surface Light Volume Imager

  3. Displacement • Given • Position • Normal • Compute • Position • Normal Displacement Surface Light Volume Imager

  4. Surface color • Given • Position • Compute • Color • Opacity Displacement Surface Light Volume Imager

  5. Lighting • Given • Surface Position • Compute • Light Direction • Light Color Displacement Light Surface Volume Imager

  6. Volume Effects • Given • Position • Color • Compute • New Color Displacement Surface Light Volume Imager

  7. Image Effects • Given • Pixel Color • Compute • Pixel Color Displacement Surface Light Volume Imager

  8. Non-real time vs. Real-time • RenderMan • GPU Application Application Texture/ Buffer Displacement Vertex Surface Light Geometry Volume Imager Fragment Displayed Pixels Displayed Pixels

  9. RenderMan vs. GPU • RenderMan • Developed from General CPU code • Seconds to hours per frame • 1000s of lines • “Unlimited” computation, texture, memory, … • GPU • Developed from fixed-function hardware • Tens of frames per second • 1000s of instructions • Limited computation, texture, memory, …

  10. History (not real-time) • Testbed[Whitted and Weimer 1981] • Shade Trees [Cook 1984] • Image Synthesizer [Perlin 1985] • RenderMan[Hanrahan and Lawson 1990] • Multi-pass RenderMan[Peercy et al. 2000] • GPU acceleration[Wexler et al. 2005]

  11. History (real-time) • Custom HW [Olano and Lastra 1998] • Multi-pass standard HW [Peercy, Olano, Airey and Ungar 2000] • Register combiners [NVIDIA 2000] • Vertex programs [Lindholm et al. 2001] • Compiling to mixed HW [Proudfoot et al. 2001] • Fragment programs • Standardized languages • Geometry shaders[Blythe 2006]

  12. RenderManBrick mortar brick gap height width gap

  13. Brick Shader surface brick( uniform float width = .2, uniform float height = .1, uniform float gap = .05, color brick = color(1,0,0), color mortar = color(.5,.5,.5) ) { varying color bc; /* compute brick color */ normal Nf = faceforward(normalize(N),I) Oi = Os; Ci = Oi*bc*(ambient()+diffuse(Nf)); }

  14. Where am I in my brick? “brick coordinates” Brick Color bs bt varying float bs, bt; /* compute brick coordinates */ if (bs < width && bt < height) bc = brick; else bc = mortar;

  15. Brick Coordinates bt = mod(t, height+gap); bs = s; if (mod((t-bt)/(height+gap), 2) == 1) bs += (width+gap)/2; bs = mod(bs, width+gap);

  16. RenderManShader Variables Cs, Os u, v, du, dv, s, t time, dtime P, dPdu, dPdv, dPdtime, N, Ng E, I L, Cl, Ol (In illuminance) Ci, Oi

  17. RenderMan Outputs • Displacement • P, N • Set DisplacementBounds! • Surface, Volume, Imager • Ci, Oi • Light • Cl, Ol

  18. Shading Methods • Repeating Patterns • mod, sin • Divide and floor • Shapes • Implicit form: is this pixel inside • Color tables • Noise or computed patterns

  19. Noise Subtleties • Many noise functions based on a lattice • Piecewise function between integer coordinates • Hash of integer coordinates  control points • Interpolating values easy but poor • Even with higher-order interpolation • Perlin’snoise • Passes through 0 at each integer • Hash gives gradient

  20. Noise Characteristics • Repeatable • Locally continuous but distant points uncorrolated • values • RenderMan [0,1], average 0.5 • Perlin’s [-1,1], average 0 • 1/2 – 1 cycle per unit • Versions for 1D-4D input

  21. Perlin Noise in RenderMan surface noisetest(float sc=1) { Ci = float noise(floor(1/t)*sc*P); }

  22. Fractional Brownian Motion (fBm) // Combine octaves, scaled by 1/f for(f=1; f<=floor(1/t); f*=2) Ci += (float noise(f*sc*P)-.5)/f; Ci += .5;

  23. Turbulence // fBm using abs(noise) for(f=1; f<=floor(1/t); f*=2) Ci += abs(float noise(f*sc*P)-.5)/f; Ci += .5;

  24. Advanced Shading Methods • Perturbed patterns • Adjust position, radius, etc. with noise • Bombing • Divide space into cells • Compute random position in each cell • Check if pixel is inside shape • Blending • Fade effects in and out with smoothstep

  25. RenderManShader Debugging • printf • Format codes for colors and points • Use conditionals to just print a few pixels! • Render as color • Map intermediate values to 0-1 • Interpret results

  26. GPU Shading Application Texture/ Buffer Vertex Geometry Fragment Displayed Pixels

  27. GPU Shading Choices • OS: Windows, Mac, Linux • API: DirectX, OpenGL • Language: HLSL, GLSL, Cg, … • Compiler: DirectX, OpenGL, Cg, ASHLI • Runtime: CgFX, ASHLI, OSG (& others), sample code

  28. GLSL / HLSL • Vertex, Geometry & Fragment/Pixel • C-like, if/while/for • Structs & arrays • Float + small vector and matrix • Swizzle & mask (a.xyz = b.xxw) • Common math & shading functions

  29. Vertex Demo:Blend Positions

  30. Vertex + Fragment Demo:Fresnel Environment Map

  31. Noise • Controlled, repeatable randomness • Still spotty implementation • Can use texture or compute

  32. Modified Noise [Olano 2005] • Three relatively independent modifications • New computable hash • Change gradient computation • Reorder computation • Variety of computation/texture options • Can just store in a texture • Can compute with some texture accesses • Can compute with no texture accesses

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