3D Texture mapping

In an .obj, file it is possible to specif开发者_开发技巧y 3 values for a vt line.

vt 0.769645 0.729072 0.00000000

The .obj spec says its for "depth". What does this actually do and when is it useful?

If you have a 3D Texture, it specifies the third coordinate. You can actually have an image that has pixels in 3D instead of 2D, and use this for a texture map. This is very common with procedural texture generation, since it allows much better looking "materials" to be generated than a 2D image, with far less "seams" or "repeated" sections on the texture map.