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ramgeis
10-16-2003, 02:39 AM
Hi all,

somebody told me that there is a possibility to set something like a tolerance value for the depth test functions i.e. a GL_EQUAL would pass if the difference is within the tolerance. Unfortunately he couldn't tell me more and I didn't find the functionality yet. Does anybody know what it could be? Any help appreciated. Thanks in advance.

Greetings, Siegmar

PanzerSchreck
10-16-2003, 03:41 AM
Take a look at glPolygonOffset.

cass
10-16-2003, 05:08 AM
There's no such beast in standard OpenGL, and I'm not aware of any vendor-specific extensions for this either.

This would be a significantly more complex depth test though. Instead of just a comparison, you'd need a 24-bit subtraction then a comparison.

For the most part, implementations have gotten pretty good at being depth invariant with a wide variety of state, and for the cases where that's not applicable, glPolygonOffset works well (as mentioned above).

Thanks -
Cass

jwatte
10-16-2003, 06:27 AM
I believe that PolygonOffset() is what you're looking for. If you want the tolerance to be X, simply add X for constant polygon offset. This works as long as the comparision function is open or half open -- it won't work for EQUALS.

"tolerance" is such a loose concept though, because you're "moving the edges out" -- but there's still a finite end where the edge ends; you're just moving that finite end around, which in the end will probably cause the same problems as you had in the first place. I agree with Cass that the best thing to do is to design for invariance (and that's also a strict requirement if you need EQUAL for the depth function).