Multiple Texture Coordinates Per Vertex (DirectX11)

Question

The models my artists are making in maya have multiple different texture coordinates per vertex depending on what face the vertex happens to be a part of.

Originally, to fix this problem, I duplicated each vert every time it appears in a face so each vert can have its own texture coordinates / face normals. This worked out well for a while until I started working on skinned skeletal animation.

I could not seem to get skinned skeletal animation working with this duplicate vertex method. There would be serious gaps in the mesh as it animated. In order to try and fix this, I switched back to indexed drawing, where faces share verts instead of each face having its own unique copy of a vert.

This fixed my skinned animation but now I'm at a loss as how to encode the multiple texture coordinates / face normals that are associated with verts. I was hoping there might be a way to use the index buffer and an array of face normals / uvs to fix this issue. Basically, I figure I can easily add an array of normals/tex coords to each vert but I have no idea how to instruct the gpu to properly index into it.

Answer 1

Duplicating vertices is probably the wrong way to go for this particular use case.

If all you have is an extra set of UVs, just send in an extra set of UVs. D3D has attributes named TEXCOORD0, TEXCOORD1, ... for a reason. :)

Accessing them is just part of the shader. Bind your attributes in your shader and use different sets of UVs for your sample calls. In shader pseudo code:

// vertex shader
main(vert_input in) {
  out.pos = transform(in.pos);
  out.texcoord0 = in.texcoord0;
  out.texcoord1 = in.texcoord1; // pass second set of UVs through to pixel shader
}

// pixel shader
main (pixel_input in) {
  diffuse0 = sample(tex0, in.texcoord0);
  diffuse1 = sample(tex1, in.texcoord1); // sample second texture using second set of UVs
  return diffuse0 * diffuse1;
}

Duplicate the vertices only when core attributes need to vary. For example, if you need different normals. You mentioned different normals in your post, but odds are you really don't need those; it makes little sense to have different normals for different textures in the vast majority of cases. You'd only really duplicate verts with different normals on hard edges, which has nothing to do with multi-texturing.

You can require that the art uses textures which share a UV space. Needing multiple UVs is something that should be relatively rare outside of particular techniques. If your artists are making content that requires multiple UVs for most content, the problem might be that your artists are doing something questionable that you should not try to support; instead you should educate them how to achieve the same visual effect using a more efficient and less hacky method. (A joke that applies: a man walks into a doctor's office and says, "every time I bend my finger backwards it hurts!" and the doctor replies, "then stop bending your finger backwards.")

The skinning bugs you mention is unrelated to your primary question. It's just a bug. If that's your primary problem, fix your skinning code.