06-CG-Texture Mapping

Texture Mapping

• Adds visual detail to scenes
• Without raising geometric complexity
• A shading trick makes a surface look textured

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Basic idea: use images instead of more polygons to represent fine scale color variation

Q: How do we decide where on the geometry each color from the image should go?

• $1^{st}$ Option: unfold the surface
• $2^{nd}$ Option: make an atlas
• $3^{rd}$ Option: depending on artist's design

Types of projections

▪ Interpolating texture coor

Q: How to map object to texture?

To each vertex ($x,y,z$ in object coordinates), must associate 2D texture coordinates $(s,t)$

Map shapes correspond to various projections

• Planar, Cylindrical, Spherical
• First, map (square) texture to basic map shape
• Then, map basic map shape to object
  • Or vice versa: Object to map shape, map shape to square
• Usually, this is straightforward
  • Maps from square to cylinder, plane, sphere well defined
  • Maps from object to these are simply spherical, cylindrical, cartesian coordinate systems

Planar mapping

Like projections, drop $z$ coord $(s,t)=(x,y)$

Cylindrical Mapping

Cylinder: $r,\theta ,ywith(s,t)=(\frac{\theta }{2\pi },y)$

Spherical Mapping

Convert to spherical coordinates: use latitude/long.

• Singularities at north and south poles

Cube Mapping

Cube mapping uses six square textures that represent the faces of a cube. These are typically labeled as (+X, -X, +Y, -Y, +Z, -Z). The cube is imagined to be surrounding either the object or the entire scene.

• Each face is like a camera looking in one of the six axis-aligned directions.
• The environment or a reflection is “stored” on these six faces.

Interpolating texture coordinates