3 index buffers

13
votes

So, in both D3D and OpenGL there's ability to draw from an index buffer.

The OBJ file format however does something weird. It specifies a bunch of vertices like:

v -21.499660 6.424470 4.069845
v -25.117170 6.418100 4.068025
v -21.663851 8.282170 4.069585
v -21.651890 6.420180 4.068675
v -25.128481 8.281520 4.069585

Then it specifies a bunch of normals like..

vn 0.196004 0.558984 0.805680
vn -0.009523 0.210194 -0.977613
vn -0.147787 0.380832 -0.912757
vn 0.822108 0.567581 0.044617
vn 0.597037 0.057507 -0.800150
vn 0.809312 -0.045432 0.585619

Then it specifies a bunch of tex coords like

vt 0.1225 0.5636
vt 0.6221 0.1111
vt 0.4865 0.8888
vt 0.2862 0.2586
vt 0.5865 0.2568
vt 0.1862 0.2166

THEN it specifies "faces" on the model like:

f 1/2/5 2/3/7 8/2/6
f 5/9/7 6/3/8 5/2/1

Where we're v/t/n the first number is an index into the vertices array, the second an index into the tex coord array, the third, into the normals array.

So, in trying to render this with vertex buffers,

In OpenGL I can use glVertexPointer, glNormalPointer and glTexCoordPointer to set pointers to each of the vertex, normal and texture coordinate arrays respectively.. but when it comes down to drawing with glDrawElements, I can only specify ONE set of indices, namely the indices it should use when visiting the vertices.

Ok, then what? I still have 3 sets of indices to visit.

In d3d its much the same - I can set up 3 streams: one for vertices, one for texcoords, and one for normals, but when it comes to using IDirect3DDevice9::DrawIndexedPrimitive, I can still only specify ONE index buffer, which will index into the vertices array.

So, is it possible to draw from vertex buffers using different index arrays for each of the vertex, texcoord, and normal buffers (EITHER d3d or opengl!), or must I create a single interleaved array and then visit IT?

2
opengldirect3d3d
possible duplicate of Rendering meshes with multiple indices – Nicol Bolas

2 Answers

8
votes

OBJ was not designed to map to OpenGL or DirectX.

Everything you said is true, and no, it's not possible to use per-attribute indices.

You have to convert the OBJ representation to something that only uses a single index per vertex (possibly duplicating some vertex data along the way).

2
votes

What you really want to be doing (speaking from the D3D side of the fence, I don't know OpenGL sorry) is loading the OBJ in to an array of custom vertex format--that way you have a struct with the index, vertex, normal and tex coord in it and you can just render it using one DrawIndexedPrimtive.

Am I misunderstanding your problem definition here? I think you need to give a little more detail--you shouldn't need multiple index buffers to render a single OBJ file (unless you are deliberately batching it). If that is the case, that you are deliberately batching it, then you should take a look at the arguments to DrawIndexedPrimitive which allow you to specify an offset in to the VB to use.