Advanced Texture Mapping

1
Advanced Texture Mapping

• May 10, 2007

2
Todays Topics

• Mip Mapping
• Projective Texture
• Shadow Map

3
Texture Resolution

• One size does not fit all
• For example an NxN texture may be OK for a
  polygon about the same size on screen, but not
  for a much smaller or much larger polygon.
• Polygon too large ? blurry texture
• Polygon too small ? wasted texture memory.

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Mip Map

• For an NxN texture, also build textures of N/2 x
  N/2, N/4 x N/4, etc.
• Total memory required 4/3 NxN
• 11/41/16 4/3

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Which Level to Use?

• Naïve answer pick the closest size.
• Note that we dont pick a fixed mip-map level for
  the whole polygon. We will pick a different
  level for each drawn pixel.
• Also, the level does not need to be an integer ?
  In-between levels.

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Filtering

• Here, we only consider individual pixels of a
  scan-converted polygon (on the screen)
• Texture coords(u, v) u, v real number.
• Bi-linear Find the closest mip-map level, fetch
  (?u?, ?v? ), (?u?, ?v? ), (?u? ,?v? ), (?u? , ?v?
  ).
• Tri-linear Also interpolated between two mip-map
  levels.

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Anisotropic Filtering

• Bilinear (and tri-linear) filtering assumes that
  a screen picture is mapped to a circle on the
  texture, which is actually incorrect.
• An ellipse (not necessary axis aligned) should be
  used instead.
• Next slide EWA Ellipse Weighted Average.

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EWA for Texture Mapping

• Paul Heckbert, Survey of Texture Mapping IEEE
  CGA, Nov. 1986. (Figures)
• Green Heckbert, Creating Raster Omnimax Images
  from Multiple Perspective Views Using The
  Elliptical Weighted Average Filter IEEE CGA,
  6(6), pp. 21-27, June 1986.

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Projective Texture

• Mark Segal, et al., Fast shadows and lighting
  effects using texture mapping. SIGGRAPH 92
• Cass Everitt, Projective Texture Mapping
  NVIDIA SDK White Paper.

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Assigning Texture Coordinates to Vertices

• The basic idea
• 1. Move the camera to the projector location.
• 2. Overlay the produced photo on top of the
  texture.
• (i.e. the pixel location becomes the texture
  coordinates)
• Projective Texture Mapping by Cass Everitt
  describes how to use OpenGL texgen to achieve
  that.

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In a Nutshell

• Tools available for generating texture
  coordinates glTexGen() and texture
  transformation matrix. (See todays handout.)
• Use glTexGen() to get
• Then set texture matrix to

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Shadow Map

• Similar to projective texture
• But this time, we will project the depth, instead
  of the slide image.

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Basic Steps of Shadow Maps

• Render the scene from the lights point of view,
• Use the lights depth buffer as a texture (shadow
  map),
• Projectively texture the shadow map onto the
  scene,
• Use texture color (comparison result) in
  fragment shading.

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Eyes View
Lights View
Depth/Shadow Map
Image Source Cass Everitt et al., Hardware
Shadow Mapping NVIDIA SDK White Paper
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Food for Thought

• Why is texture mapping so powerful, useful,
  magical, etc.?
• (Hint a lot of hardware design effort)
• Why are global illumination effects possible now,
  even though we still process one primitive at a
  time?