[Mesa3d-dev] Artifact in Texture Mapped Quad

[Allen B. <ba...@lo...>]

Hi,

I see a slight artifact in the attached snapshot (the brighter, broken,
vertical yellow line above the left edge of the "O"). The image was
generated from two overlapping, texture-mapped quads drawn in the same
plane (z=0). The blue parts are opaque (r=0.,g=0,b=.5,a=1), while the
yellowish parts have alpha = .3 (1,1,0,.3). Blending and the depth test
are turned on. If I turn off the depth test, the artifact goes away. 

If you could give me some idea where in the code (software Mesa 3.5.1
under X) the pixels are computed for a depth-tested, alpha-blended,
texture-mapped quad, I'll be glad to track this down a little further.
It's probably unavoidable round-off somewhere, but I'd like to
understand what it's the round-off of!

(I'd don't see the artifact in DRI/Mesa 3.4.2 in either direct or
indirect rendering modes, though.)

Thanks,
Allen