Assimp faces indices

Question

I have been using Assimp for a while and now I'm trying to load a .obj file. It loads perfectly, but I would like to manipulate the face data after loading it.

Basically I have this in the simple cube.obj file (Full file - http://pastebin.com/ha3VkZPM)

# 8 Vertices
v -1.0 -0.003248 1.0
v 1.0 -0.003248 1.0
v -1.0 1.996752 1.0
v 1.0 1.996752 1.0
v 1.0 -0.003248 -1.0
v -1.0 -0.003248 -1.0
v 1.0 1.996752 -1.0
v -1.0 1.996752 -1.0


# 36 Texture Coordinates
vt 1.0 0.0
vt 0.0 0.0
...

# 36 Vertex Normals
vn 0.0 0.0 1.0
vn 0.0 0.0 1.0
...

f 1/1/1 2/2/2 3/3/3
f 2/4/4 4/5/5 3/6/6
f 5/7/7 6/8/8 7/9/9
f 6/10/10 8/11/11 7/12/12
f 3/13/13 6/14/14 1/15/15
f 3/16/16 8/17/17 6/18/18
f 7/19/19 2/20/20 5/21/21
f 7/22/22 4/23/23 2/24/24
f 3/25/25 7/26/26 8/27/27
f 3/28/28 4/29/29 7/30/30
f 2/31/31 6/32/32 5/33/33
f 2/34/34 1/35/35 6/36/36

And as I understand face entry is V/T/N (Vertex indicies, tex coord indices and normal indices).

so f 1/1/1 2/2/2 3/3/3 represents a triangle of vertices (1,2,3) - right?

From this face entry - I want to extract only the vertex indices.

Now enters Assimp - I have this now - Where Indices is a stl::vector

for (uint32 i = 0; i < pMesh->mNumFaces; i++) {
    const aiFace& Face = pMesh->mFaces[i];
    if(Face.mNumIndices == 3) {
        Indices.push_back(Face.mIndices[0]);
        Indices.push_back(Face.mIndices[1]);
        Indices.push_back(Face.mIndices[2]);

}

Here are the values of pMesh->mNumFace = 12 - So thats correct.

(for 1st face) 
Face.mindices[0] should probably point to 1/1/1
Face.mindices[1] should probably point to 2/2/2
Face.mindices[2] should probably point to 3/3/3

Now how do I extract only the vertex indices? And when I check the values of Face.mIndices[0] its index as 0,1,2...respectively. Why so? Assimp Faces all have indices (0,1,2)

I searched on Google and StackOverflow - here are some similar question but I cant seem to figure it out.

https://stackoverflow.com/questions/32788756/how-to-keep-vertex-order-from-obj-file-using-assimp-library

Assimp and D3D model loading: Mesh not being displayed in D3D

Assimp not properly loading indices

Please let me know if you need more info. Thanks.

Answer 1

OpenGL and DirectX use a slightly different way of indexing vertex data then the obj format does. In contrast to the file format where it is possible to use different indices for positions/texcoords etc, the graphic card requires one single index buffer for the whole vertex.

That beeing said: Assimp passes the obj format and transforms it into a single-index-buffer representation. Basically this means, that each unique vertex-texcoord-normal combination will give one vertex while the indexbuffer points to this new vertex list.

As far as I know, it is not possible to access the original indices using Assimp.