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Perspective correct texture mapping

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I have an interesting problem that I thought I could work out here. I''m writing a little 3D software engine in Java and right now I''m writing the scan-line texture mapping routines. Somehow I occasionally get a texture coordinate that is slightly over the bounds (i.e. i''ll get 256 or 257 for a 256x256 tex map). I put in some temporary ''fudge'' code to bound these and you can''t really tell when it''s displayed, but obviously it bugs me. My current theory is it''s just a floating point precision problem. :/ Has anybody else had this problem when writing a software engine? Heres a brief summary of what I''m doing in case it may help: For each vertex of a triangle I have (u,v) and the transformed coordinates (x,y,z,w) with w saved before the perspective division/view-port transform. So along one edge I get u=lerp(u1/w1, u2/w2) and iw=lerp(1/w1, 1/w2) and the same for the other edge for each y. Then along each scan line I lerp these u and iw values and my tex coordinate comes out as u/iw. Sounds right to me, but that is a lot of interpolating and dividing. I could be getting round off errors. Not to mention I''m clipping to the view fustrum before hand and that''s a further interpolation of the texture coordinates. Yep, I just checked and right now it only seems to happen when I clip to the fustrum (at least from the angles my camera is at). Sigh. Any ideas?

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EDIT: No, I was wrong. Check your clipper is stable and you are getting good clipped w values, floating point might be the problem, depends on how large/small values we're talking.

[edited by - JuNC on December 6, 2002 12:03:49 PM]

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