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User Clip Plane Aliasing Issue


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#1 andur   Members   -  Reputation: 765

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Posted 27 January 2009 - 11:41 AM

I've been having some weird aliasing-type issues the past couple of days with my stencil mirrors and their associated user clip planes. In the particular scene in question, there is a floor and mirrored wall that are flush against each other, forming a perfect right angle. Both the wall and floor are perfectly planar. The reflection renders fine, except for a weird 0-2 pixel border at the bottom of the mirror that flickers wildly with the camera angle. I can reproduce it with only this one floor and wall in a scene, so, its not something external to that. I've investigated in PIX and found that the stenciled off region is exactly the pixels that it should be, forming a line without gaps along the wall. The floor always renders completely ok, the deviant pixels in question are on the mirror itself. The pixel history also seems to indicate that these pixels are being discarded by the clip plane, and not as the result of a depth/stencil test failure. I have a user clip plane defined to match the mirror plane perfectly. If I disable this user clip plane, then the weird edge artifacts go away. Though, this hurts performance a lot in some of my mirror heavy scenes and introduces clipping artifacts in other scenes. I tried implemented Near-Clip Plane Shearing as per a paper I found on Nvidia's website, and it suffers exactly the same problem as the user clip planes do. Has anyone encountered anything similar to this and/or have any advice on what to try next? I could put a frame around the mirror which would hide the problem, but, that isn't completely to the spec of the scene.

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