Three dimensional textures are a logical extension of 2D textures. In 3D textures, texels become unit cubes in texel space. They are packed into a rectangular parallelpiped, each dimension contrained to be a power of two. This texture map occupies a volume, rather than a rectangular region, and is accessed using three texture coordinates, S, T, and R. As with 2D textures, texture coordinates range from zero to 1 in each dimension. Filtering is controlled in the same fashion as 2D textures, using texture parameters and texture environment.