David Luebke1/19/99

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CS 551/651 Antialiasing Wrap-up

• David Luebke
• cs551dl_at_cs.virginia.edu
• http//www.cs.virginia.edu/cs551dl

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Administrivia

• Hand in Assignment 1
• Hand out Assignment 2
• Sampling strategies in RSRT
• Due Tuesday March 9
• Read Sections 10.1, 10.4

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Recap Antialiasing Strategies

• Prefiltering
• Supersampling
• A-Buffer
• Adaptive supersampling
• Stochastic supersampling

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Recap Nonuniform Supersampling

• To be correct, need to modify filtering step

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Recap Nonuniform Supersampling

• Approximate answer weighted average filter
• Correct answer multistage filtering
• Real-world answer ignore the problem

? ? I(i, j) h(x-i, y-j)
I(x,y)
? ? h(x-i, y-j)
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Recap Antialiasing and Texture Mapping

• Texture mapping is uniquely harder
• Potential for infinite frequencies
• Texture mapping is uniquely easier
• Textures can be prefiltered

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Recap Antialiasing and Texture Mapping

• Issue in prefiltering texture is how much texture
  a pixel filter covers
• Simplest prefiltering scheme MIP-mapping
• Assumption approximate filter size, ignore
  filter shape
• Create a pyramid of texture maps
• Each level doubles filter size

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Recap Mip-Mapping

• Tri-linear mip-mapping
• Pixel size ? depth d
• Linearly interpolate colors within 2 levels
  closest to d
• Linearly interpolate color between levels
  according to d

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Antialiasing Mip Mapping

• Q Whats wrong with the mip-map approach to
  prefiltering texture?
• A Assumes pixel maps to square in texture space
• More sophisticated inverse pixel filters (see
  FvD p 828)
• Summed area tables
• Elliptical weighted average filtering

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Backwards Ray Tracing

• Traditional ray tracing traces rays from the eye,
  through the pixel, off of objects, to the light
  source
• Backwards ray tracing traces rays from the light
  source, into the scene, into the eye
• Why might this be better?

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Backwards Ray Tracing

• Backwards ray tracing can capture
• Indirect illumination
• Color bleeding
• Caustics
• Whats a caustic?
• Give some examples
• Where do caustics get the name?

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Backwards Ray Tracing

• Usually implies two passes
• Rays are cast from light into scene
• Rays are cast from the eye into scene, picking up
  illumination showered on the scene from the first
  pass

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Backwards Ray Tracing

• Q How might these two passes meet in the
  middle?

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Backwards Ray Tracing

• Arvo illumination maps tile surfaces with
  regular grids, like texture maps
• Shoot rays outward from lights
• Every ray hit deposits some of its energy into
  surfaces illumination map
• Ignore first generation hits that directly
  illuminate surface (Why?)
• Eye rays look up indirect illumination using
  bilinear interpolation

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Backwards Ray Tracing

• Watt Watt, Plate 34
• Illustrates Arvos method of backwards ray
  tracing using illumination maps
• Illustrates a scene with caustics
• Related idea photon maps

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Distributed Ray Tracing

• Distributed ray tracing is an elegant technique
  that tackles many problems at once
• Stochastic ray tracing distribute rays
  stochastically across pixel
• Distributed ray tracing distribute rays
  stochastically across everything

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Distributed Ray Tracing

• Distribute rays stochastically across
• Pixel for antialiasing
• Light source for soft shadows
• Reflection function for soft (glossy) reflections
• Time for motion blur
• Lens for depth of field
• Cook 16 rays suffice for all of these

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Distributed Ray Tracing

• Distributed ray tracing is basically a Monte
  Carlo estimation technique
• Practical details
• Use lookup tables (LUTs) for distributing rays
  across functions
• See WW Figure 10.14 p 263

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Advanced Ray Tracing Wrapup

• Backwards ray tracing accounts for indirect
  illumination by considering more general paths
  from light to eye
• Distributed ray tracing uses a Monte Carlo
  sampling approach to solve many ray-tracing
  aliasing problems

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Next Up Radiosity

• Ray tracing
• Models specular reflection easily
• Diffuse lighting is more difficult
• Radiosity methods explicitly model light as an
  energy-transfer problem
• Models diffuse interreflection easily
• Shiny, specular surfaces more difficult