I've read in docs for the middleware we use that support for this can be flaky, but the D3D docs don't mention it. We're going as far back as the earliest hardware SM2.0 Intel GMA cards, basically the GMA950 & GMA3100, so I'm trying to find out if these are old enough to have issues, or we're talking even older fixed-function-only chipsets.
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Clipping planes have been supported for a while on most video hardware, including fixed-function stuff. Intel support, however, may be flaky. – ssube Apr 07 '11 at 19:27
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Just don't bother with user clip planes. They are emulated in weird ways across drivers. Why do you really need them? If it is to optimize something you are going to get a small gain on some HW and pay a horrible price on other. If you need it for some algorithm do it yourself using a texkill and it will be reliable everywhere. Probably not the answer you wanted, but in general, just don't use it and rethink your approach.
starmole
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Clipping entire polygons will mean every pixel doesn't have to go through shaders. Those are pretty expensive so why would it be only a small gain if I can reduce 50% the pixels rendered? – Mr. Boy Apr 13 '11 at 08:31
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The issue is that most HW just does not implement user clip planes. So they emulate it. If you want to save fill you need to use basic clipping though projection or maybe screen space clipping through scissor. But user clip is just not free. It sounds good on paper but is one of those rare features that were cool in the 90s but would waste die space nowadays. – starmole Apr 26 '11 at 05:13
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You should really look at how clipping in hw is done: usually only along z, the x/y is done post projection in 2d. It is actually really hard for hw to clip because one triangle can create two triangles by being clipped. That requires expensive caches - every clip plane can double the number of triangles from one input triangle. – starmole Apr 26 '11 at 05:18