3rd texture coordinate in Wavefront *.OBJ file

Question

What is the meaning of 3rd component in texture coords in Wavefront *.OBJ file ?

File looks like

vt 0.8490 0.1109 -0.2498
vt 0.8580 0.1235 -0.2498
vt 0.8695 0.1338 -0.2498
vt 0.8828 0.1416 -0.2498
vt 0.9135 0.1474 -0.2360
vt 0.8981 0.1457 -0.2360
vt 0.8835 0.1409 -0.2360 

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UPDATE Model was created in 3DS Max and source texture was PNG file. So texture is 2D

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UPDATE 2

When I flip second coordinate then texture looks properly when I render it with OpenGL. 3rd coordinate

u1 = u v1 = 1 - v

w - is not used at all.

Answer 1

3rd coordinate should be ignored. Only first 2 are used to map texture.

Answer 2

The model is apparently using a volume texture.

Answer 3

vt specify the fact that the coordonates are texture coordonates.

Read more about the Wavefront .obj format. http://www.fileformat.info/format/wavefrontobj/egff.htm