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Editorial Team
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Asked: 2026-05-19T02:19:31+00:00

I’ve been using DirectX (with XNA) for a while now, and have recently switched

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I’ve been using DirectX (with XNA) for a while now, and have recently switched to OpenGL. I’m really loving it, but one thing has got me annoyed.

I’ve been trying to implement something that requires dynamic indexing in the vertex shader, but I’ve been told that this requires the equivilant of SM 4.0. However I know that this works in DX even with SM 2.0, possibly even 1.0. XNA’s instancing sample uses this to do instancing on SM2.0 only cards http://create.msdn.com/en-US/education/catalog/sample/mesh_instancing.

The compiler can’t have been “unrolling” it into a giant list of if statements, since this would surely exceed the instruction limit on SM2 for our 250 instances.

So is DX doing some trickery that I can’t do with OpenGL, can I manipulate OpenGL to do the same, or is it a hardware feature that OpenGL doesn’t expose?

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1 Answer

  1. Editorial Team

    You can upload an array for your light directions with something like glUniform3fv, then (assuming I understand what you’re trying to do correctly) you just need your vertex format to include an index into this array (so there be lots of duplication of these indices if the index only changes once per mesh or something). If you don’t already know, you can use glGetAttribLocation + glVertexAttribPointer to send arbitrary vertex attributes like this to the shader (as opposed to using the deprecated built-in attributes like gl_Vertex, gl_Normal, etc).

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