Hi,
I noticed some strange rendering artifacts with the Savage that are caused by very large texture coordinates on GL_REPEAT'ed textures. Very large means that it gets noticeable with texture coordinates >255 or <-256. A real world example that exhibits this problem is Torcs with the "Spring" track. Right at the start the effect can be seen very nicely. The track before the start line shows artifacts, directly after the start line everything looks fine.
My question is, should I consider such problems an application bug or would it be wise to implement a workaround? I was thinking of implementing a TNL pipeline stage that normalizes texture coordinates.
That could be tricky. If you're thinking of doing something like t' = t MOD maxValue, that'll often cause incorrect interpolation.
I was also wondering if other hardware has similar problems. I'm
attaching a small test program that demonstrates the effect and a
screenshot of what I get on my ProSavageDDR. With software rendering the
output is almost correct. Compile with
cc -lGL -lGLU -lglut teximage.c -o teximage
If the hardware implements texcoord interpolation with fixed point you can imagine how the bits are used.
If the largest texture dimension is 2048, you'd need at least 11 bits to address all texels.
Then you need some sub-texel bits for accurate interpolation and for computing the weighting for linear sampling. You probably need 10 bits there.
Allocate another bit for the sign.
Finally, whatever bits are left in the interpolator will limit the maximum coordinate range. If the interpolator is 32 bits, then 32 - 11 - 10 - 1 = 10. So the max coordinate would be 2^10 or 1024.
Maybe the savage hardware has a narrow interpolator, or allocates the bits differently.
I believe one of the differences between "pro" and "consumer" cards is the accuracy and range of interpolators.
-Brian
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