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 parallelepiped, each dimension constrained 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 0 to 1 in each dimension. Filtering is controlled in the same fashion as 2D textures, using texture parameters and texture environment.