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; 
, 
, and 
. As
with 2D textures, texture coordinates range from 
to 
in each dimension.
Filtering is controlled in the same fashion as 2D textures, using texture
parameters and texture environment.