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I am teaching myself opengl es. I recently figured out how to parse/load a mesh from .OBJ file. I am now trying to apply a texture to it, I know I can only use one set of indices.. other than that I am lost as to the concept and code required to draw a texture onto my cube. A detailed/semi-detailed explanation of what's required, code, and description of what is happening would be soooo much appreciated. I haven't found much for opengl es on this subject and the explanations are a bit vague for me. Please remember I am using opengl es. Thanks in advance. BTW if it helps this is the .OBJ file

enter code here
# Blender v2.62 (sub 0) OBJ File: ''

www.blender.org

mtllib untitled.mtl o Cube_Cube.002 v 1.067472 -0.769912 -1.117719 v 1.067472 -0.769912 0.882281 v -0.932528 -0.769912 0.882281 v -0.932527 -0.769912 -1.117719 v 1.067473 1.230088 -1.117718 v 1.067472 1.230088 0.882282 v -0.932528 1.230088 0.882281 v -0.932528 1.230088 -1.117719 vt 0.951958 0.112804 vt 0.951958 -0.887196 vt -0.048042 -0.887196 vt -0.048042 0.112804 vt 0.058859 0.112804 vt 0.058859 1.112804 vt 1.058859 1.112804 vt 1.058859 0.112804 vt 0.951958 1.112804 vt -0.941141 0.112804 vt -0.941141 -0.887196 vt 0.951958 0.058859 vt -0.048042 0.058859 vt 0.951958 -0.941141 vt 1.951958 -0.941141 vt -0.048042 -0.941141 usemtl Material.001 s off f 5/1 1/2 4/3 f 5/1 4/3 8/4 f 3/5 7/6 8/7 f 3/5 8/7 4/8 f 2/1 6/9 3/4 f 6/1 7/4 3/3 f 1/5 5/6 2/10 f 5/5 6/10 2/11 f 5/12 8/13 6/14 f 8/12 7/14 6/15 f 1/12 2/14 3/16 f 1/12 3/16 4/13

P.S. I understand how to read/parse/understand these aspects of the file. I just need clarification on how I am supposed to use one indexbuffer for the texture and vertices while making only one call to drawelements. Thanks again!

PhaTty-FU
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  • You basically have to create totally new vertex and texcoord buffers so that index *N* matches vertex *N* and texcoord *N*. I think you could probably search a little more because I think this question or similar has probably been asked hundreds of times. (just search for 'vbo glDrawElements') – Tim May 15 '12 at 20:05
  • two questions then. Do I need to have the same amount of V and VTs? Also, by matching do you mean it would be like f 1/1f 2/2 etc..? After I do this.. do I make any changes to the vertpointer and textcoordpointer? Finally, when all is done.. do I just call drawelements one time like normally? These are the pieces I haven't been able to wrap my head around. – PhaTty-FU May 15 '12 at 20:15
  • Yes, your vertex array and texcoord array will need to be the same length. This will mean creating more vertices and texcoord pairs that are not in the OBJ itself. And yes once you do this, it's just a single call to glDrawElements just like any other object. It's the prepping of the arrays that's the hard part. – Tim May 15 '12 at 20:18
  • thanks for the direction, this is what I needed to know to save me days of try/fail. – PhaTty-FU May 15 '12 at 20:20

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I don't know if blender can do it, but some tools exists to export .obj with a single set of indices. This will look like this: 

f 6/6/6 1/1/1 4/4/4 
f 4/4/4 1/1/1 3/3/3 
f 1/1/1 2/2/2 3/3/3 
f 12/12/12 2/2/2 1/1/1 
f 11/11/11 12/12/12 1/1/1 
f 11/11/11 81/81/81 12/12/12

For instance, UDK does this when you export a scene (made of StaticMesh) to .obj format. Using a tool that exports .obj in such way is the easiest solution for you. You can however rebuild the arrays yourself from what you read from the .obj in your code, but it's a boring and unnecessary time consuming operation.

Benlitz
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  • I would like to know how to do it manually in case the need ever arises.. but that can be for a later time. I think I will try to find a way to export them in a more usable way. Thanks for the answer. – PhaTty-FU May 15 '12 at 20:54