indexing - Why does OpenGL not support multiple index buffering?

Question

Why does OpenGL not support multiple index buffers for vertex attributes (yet)?

To me it seems very useful, since you could reuse attributes and you would have a lot more control over the rendering of your geometry.

Is there a reason why all attribute arrays have to take the same index or could this feature be available in the near future?

Answer 1

OpenGL (and D3D. And Metal. And Mantle. And Vulkan) doesn't support this because hardware doesn't support this. Hardware doesn't support this because, for the vast majority of mesh data, this would not help. This is primarily useful for meshes that are predominantly not smooth (vertices sharing positions but not normals and so forth). And most meshes are smooth.

Furthermore, it will frequently be a memory-vs-performance tradeoff. Accessing your vertex data will likely be slower. The GPU has to fetch from two distinct locations in memory, compared to the case of a single interleaved fetch. And while caching helps, the cache coherency of multi-indexed accesses is much harder to control than for single-indexed accesses.

Hardware is unlikely to support this for that reason. But it also is unlikely to support it because you can do it yourself. Whether through buffer textures, image load/store or SSBOs, you can get your vertex data however you want nowadays. And since you can, there's really no reason for hardware makers to develop special hardware to help you.

Also, there are questions as to whether you'd really be making your vertex data smaller at all. In multi-indexed rendering, each vertex is defined by a set of indices. Well, each index takes up space. If you have more than 64K of attributes in a model (hardly an unreasonable number in many cases), then you'll need 4 bytes per index.

A normal can be provided in 4 bytes, using GL_INT_2_10_10_10_REV and normalization. A 2D texture coordinate can be stored in 4 bytes too, as a pair of shorts. Colors can be stored in 4 bytes. So unless multiple attributes share the same index (normals and texture coordinate edges happen at the same place, as might happen on a cube), you will actually make your data bigger by doing this in many cases.