Real Time Shaders

1
Real Time Shaders

• CS 319
• Advanced Topics in Computer Graphics
• John C. Hart

2
Real Time Shaders

• Moores law CPU power doubles every 18 months
• Advances in materials
• Advances in methods
• Advances in marketing
• Graphics version GPU power doubles every 6
  months
• Supports more sophisticated shading, though in
  unexpected ways (e.g. strange uses of the texture
  maps)

3
AvailableTechnology

• Texture Shading
• Render shading expressionsusing texture mapping
• Multipass
• Combine shading components using image processing
  combinations
• E.g. Render texture diffuse then render highlight
• Multitexturing
• Store multiple texture coordinates at each vertex
• Textures are combined on the fly
• Fragment lighting
• Compute complex shading per pixel

Slide 22 from Hanrahans Real-Time Shading
Languages
4
Texture Shading

• Heidrich Seidel, SIGGRAPH 99
• Implements shading models using texture maps
• Disables hardware lighting
• Evaluate shading expressions, store in texture
  coordinates
• Precompute remaining expression in texture map

5
Example PhongTexture Shader

• Express model as combination of colors
• C ka Ca kd (N?L) Cd ks (V?R)n Cs
• Plot vertices using texture coordinates
• s N?L
• t V?R
• Precompute texture by evaluating
• C(s,t) ka Ca kd s Cd ks tn Cs
• for 0 ? s,t ? 1

t V?R
s N?L
6
ExampleCook-Torrance
c V?H and g2h2c2-1
7
Example Skin
8
Hair
9
Sampling

• Problem
• Gouraud interpolation of highlights
• Interpolation of N?L and V?R which are
  proportional or power-proportional to intensity
• Solution
• Interpolate vectors instead
• How?

10
EnvironmentMaps

• Spheremap
• reflection into hemisphere
• Singular at silhouette
• Biased samples
• Cubemap
• projection onto cube faces
• Awkward to store

11
Radiance v. Irradiance Maps

• Environment map
• Panoramic picture of surroundings
• Radiance map
• Reflectance equation
• Environment map is an irradiance map Li(wi)
• Radiance map Lr(wr) computed by integrating BRDF
  with irradiance

12
PhongRadiance Map

• Spheremap is reflection of scene on sphere
• Render phong highlight on sphere
• Use rendered sphere as an environment map
• Texture coordinate interpolation interpolates wr

13
MeasuredRadiance Maps
Peercy, Olano, Airey , Ungar, "Interactive
Multi-Pass Programmable Shading", SIGGRAPH 2000
14
Real-Time Bump Mapping
B
B
B

• Embossing
• Multipass difference betweentexture and shifted
  copy
• Environment-mapped bump mapping
• Perturb environment map index with bump map (map
  of height field differentials)
• Register combiners
• Store vectors in texture map
• Compute per-pixel dot product