CODEWITHSUNDEEP
openglglsl
Sergey
Sergey

Reputation: 8238

GLSL: number of primitives in current draw call

TES shader provides a built-in input variable gl_PrimitiveID​, which is the index of the current patch in the series of patches being processed for this draw call.

How can I know the total number of patches in this draw call, besides setting uniform variable? And why is there no such input? It seems pretty logical to have something like gl_NumPrimitives built-in, if we already have gl_PrimitiveID counter.

Motivation: I want to index a 1D sampler from 0.0 to 1.0 based on gl_PrimitiveID​, so that the first primitive in the draw call corresponds to 0.0 and the last one corresponds to 1.0.

Upvotes: 4

Views: 245

Answers (1)

Yakov Galka
Yakov Galka

Reputation: 72479

How can I know the total number of patches in this draw call, besides setting uniform variable?

You cannot other than setting a uniform variable.

And why is there no such input?

I'm not part of Khronos, but I speculate that:

  • The underlying hardware is very stream oriented. As a matter of fact you can't know beforehand the number of primitives in the general case (e.g. when you are using a primitive restart index), which means that calculating it would require a separate step to be done which wouldn't fit into the current stream-based architecture.

  • They are trying to keep the latest OpenGL versions to the bare minimum. It means that any features that could be as-efficiently implemented by means of the existing features are out.

Upvotes: 4

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