What Is Texture Wrapping?

Article
06/30/2006

In short, texture wrapping changes the basic way that Microsoft® Direct3D® rasterizes textured polygons using the texture coordinates specified for each vertex. While rasterizing a polygon, the system interpolates between the texture coordinates at each of the polygon's vertices to determine the texels that should be used for every pixel of the polygon. Normally, the system treats the texture as a 2-D plane, interpolating new texels by taking the shortest route from point A within a texture to point B. If point A represents the u, v position (0.8, 0.1), and point B is at (0.1,0.1), the line of interpolation looks like the following illustration.

Note that the shortest distance between A and B in this illustration runs roughly through the middle of the texture. Enabling u- or v-texture coordinate wrapping changes how Direct3D perceives the shortest route between texture coordinates in the u- and v-directions. By definition, texture wrapping causes the rasterizer to take the shortest route between texture coordinate sets, assuming that 0.0 and 1.0 are coincident. The last bit is the tricky part: you can imagine that enabling texture wrapping in one direction causes the system to treat a texture as though it were wrapped around a cylinder. For example, consider the following illustration.

This diagram shows how wrapping in the u- direction affects how the system interpolates texture coordinates. Using the same points as in the example for normal, or nonwrapped, textures, you can see that the shortest route between points A and B is no longer across the middle of the texture; it is now across the border where 0.0 and 1.0 exist together. Wrapping in the v-direction is similar, except that it wraps the texture around a cylinder that is lying on its side. Wrapping in both the u- and v-directions is a more complex. In this situation, you can envision the texture as a torus, or doughnut.

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What Is Texture Wrapping?

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