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fred_em
05-04-2011, 09:09 AM
Hi,

I don't understand very well how GL_PATCH and the primitive generator are supposed to work.

I am currently manipulating GL_PATCH primitives with 3 vertices. I basically have 1 origin vertex, 1 U-axis vertex and 1 V-axis vertex.

My patches are entirely generated off uniform values, that is, patch vertices are not even used. So I don't need any actual patch vertex at all. 1 would be sufficient as I need to "declare" the GL_PATCH primitive.

Problem: when I try to only have 1 patch vertex, my code doesn't work. I can't understand why, conceptually speaking.

The primitive generator doesn't use patch vertices, right? I mean, it just generates U,V parametric values (all sitting between 0.0 and 1.0) and calls the evaluation shader with them, right?

Patch vertices are just there for my own convenience, or am I missing something?

Cheers,
Fred

hound
05-05-2011, 05:05 AM
What do your shaders look like?

(I'm not actually sure whether one is a valid parameter to GL_PATCH_VERTICES or not. The specs just seem to say that anything less than 0 is invalid.)

fred_em
05-05-2011, 05:54 AM
Nevermind, I figured it out - this was a bug in my code.
Zero doesn't work. One unique patch vertex works fine.
Thanks for your reply anyway. I can't understand why I end up figuring out the problem once I post a message to a forum :(