how to work with z coordinates in a texture?

Question

how to work with z coordinates in a texture in iphone openGl.

Answer 1

z coordinates are only useful for 3d textures (which are rather unusual) or for cubemaps, which are far more popular (to create environment mapping, reflections, etc.).

for the latter case, the three coordinates represent an unit vector addressing a point on the unit sphere; the correct texel is automatically retreived.