Question What is the difference between baseVertex and firstVertex in SubMeshDescriptor?

What is the difference between baseVertex and firstVertex in SubMeshDescriptor?

They look like completely same, but none of them marked as obsoleted, nor explicitly documented that they are same.

 

I think the documentation is wrong. firstVertex is the index of the first referenced vertex in the vertex buffer.

Even though you run through the index buffer sequentially, the referenced vertex indices could be all over the place (non-contiguous). firstVertex and vertexCount are the range of the vertices in the vertex buffer that could potentially be referenced - but there might be some in between that are not actually referenced.

Code (CSharp):

1. int firstVertex = int.MaxValue;
2. int lastVertex = int.MinValue;
3. for (int i = 0; i < indexCount; ++i)
4. {
5.     int idx = baseVertex + indices[indexStart + i];
6.     firstVertex = min(firstVertex, idx);
7.     lastVertex = max(lastVertex, idx);
8.     Vertex v = vertices[idx];
9. }
10. vertexCount = lastVertex - firstVertex + 1;
11.  

So indexStart and indexCount is the referenced range in the index buffer and firstVertex and vertexCount is the referenced range in the vertex buffer.

That's my understanding. Somebody correct me if I'm wrong.

 

Actually, the documentation is not wrong but ambiguous. Once you understand the concept, you understand how they mean it.

Best explanation is here: https://docs.unity3d.com/ScriptReference/Rendering.SubMeshDescriptor-vertexCount.html

PS: baseVertex and firstVertex are not necessarily the same number either. firstVertex could be greater than baseVertex if there is no indices[indexStart + i] with the value 0.