CSE 681 Texture Mapping: An OverviewReview

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CSE 681Texture Mapping An Overview/Review
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Why Textures?

• How can we model this scene with polygons?
• Lots of detail means lots of polygons to render
• 100s, 1000s, Millions of polygons!
• Modelling is very difficult and cumbersome

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Why Textures?

• Render a single polygon with a picture of a brick
  wall mapped to it?

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The Quest for Visual Realism
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Textures

• Phong lighting just wont do
• Plastic, rubber, or metallic looking objects
• Add surface detail with real world patterns and
  images
• We get modelled detail at a low cost
• Very useful in games a billion dollar industry
• Provides realism at a low cost
• Graphics hardware vendors work hard to optimize
  texture mapping

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Terminology

• Texture An array of values
• 2D (most common), 1D, and 3D
• Color, alpha, depth, and even normals
• Texel A single array element
• Texture Mapping The process of relating texture
  to geometry

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Texture Sources

• Pixel maps (bitmaps)
• Load from an image file gif, jpg, tiff, ppm,
  etc.
• Procedural textures
• Program generated texel values

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Procedural Textures

• Checkerboard texture with 8 x 8 texels for each
  box

RGB checkerboard(int rows, int cols) pixel
new RGB(3 rows cols) long index 0 for
(int i 0 i lt rows i) for (int j 0
j lt cols j) texelindex.red 1.
texelindex.green 1.
texelindex.blue 1.
index 0 for (i 0 i lt rows i) for
(j 0 j lt cols j) if (((i/8)
(j/8))2) 0) texelindex.green 0.
texelindex.blue 0.
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Texture Mapping

• How do we apply a texture onto an object?
• Construct a mapping between the texture and
  object
• Use the texture to lookup surface attributes

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Texture Mapping

• Problems
• The texture and object are in two different
  spaces
• Where in the rendering pipeline do we specify
  this mapping?
• Object or world space?
• Map onto untransformed surfaces
• Texture filtering A point on the surface maps to
  a location between texels in the texture

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Texture Space

• A texture is defined in a normalized space
• 2D textures (s, t) ? 0 1, 0 1
• Some times (u, v) is used also

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Texture Value Lookup/Filtering

• Normalized space is continuous but the texture is
  a discrete array
• Texel values are located on a cartesian grid

(1,1)
Index (s, t) may not land on a grid value
(0.25,0)
(0.5,0)
(0.75,0)
(0,0)
(1,0)
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Raytracing a Textured Object

• Shoot ray
• Map the intersection point of the visible surface
  to object space
• Map to texture space
• Filter the texture
• Determine pixel color with retrieved texture
  information

Eye or World
World to Object
Object to Texture
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Map 2D Textures To Objects

• Define mapping between object and texture spaces
• For example, a simple quad in object space is
  easy!
• Akin to wall papering or gift wrapping

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Mapping 2D Textures To Objects
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2D Texture Mapping Approaches

• Intermediate Mapping
• Map the texture onto a simple intermediate
  surface
• Map the intermediate surface to the final object
• Intermediate objects
• Plane
• Sphere
• Cylinder
• Cube

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Planar Mapping
Project to an axial plane, e.g. drop z coord
(s,t) (x, y)
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Spherical Mapping
Mapping Polar (u, v) coordinates
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Cylindrical Mapping

Mapping Cylindrical (u, v) coordinates
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Cube Mapping
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Intermediate Mapping
Paul Bourke