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popsoftheyear

Clipped Polys

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I have a problem with my gouraud shaded renderer. Say there was a quad that gets clipped against the viewport, creating a 6 sided polygon. When split into to triangles in the very last state for rendering, a lot of times (depending on which edge(s) were clipped against the viewport), the colors will look like that shift, since they get interpolated along different vertices. I tried different methods for splitting the poly into triangles, and it still happens. The only solution I can think that would fix the problem is to split the polys into triangles before clipping... but that could potentially slow down the clipper a bit. What is the best compromise or solution for this problem? Also I do my back-face culling at the last stage, after all vertices have been projected. When I cull before this, it always makes polys dissappear just before they should, and reappear just before they should...I think it would look right in a parallel projection. Is this a common problem that happens for people? Thanks. -Scott

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If you're not using perspective correction for the gouraud shading then you'll get a very slight shift in the colouring when tris are clipped in 3d but it shouldn't really be a problem, if it's quite noticable then it sounds like there's something wrong with your clipping code.
And backface culling shouldn't do that, either if you're using the normal or the winding it should work fine.

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Thanks. I fixed the coloring problem by just splitting all quads into triangles during initialization, instead of right before they get drawn to the screen. It kept shifting because after a quad was clipped, when the renderer splits them into triangles for rendering sometimes the triangles wouldn't link the original vertices that made triangles when the quad wasn't clipped...

As for the backface culling...I don't know! The winding (after perspective projection) works fine though...so I'll just stick with that for the moment.

I had another question regarding cache misses but I'll post another thread.

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