Brian Paul wrote:
It is.Michel Dänzer wrote:First of all, thanks Keith for sharing your insights ( and Jens for the URL about locking ). On Don, 2002-11-28 at 13:31, Keith Whitwell wrote:gloss: artifacts with the initial highlight, goes away with SW TCL, seems to be the same problem as the ice in tuxracer
This is a result of slight differences between geometry generated by software t&l and hardware t&l. The gloss cylendar is drawn in two passes, and the crawling you see is a result of differences between the passes.
Some work could be done tweaking the vertices (z values?) emitted by swtcl to get a closer alignemnt.
Otherwise, this isn't strictly a bug. GL doesn't require invarience at this level.
I see. Do you have an idea on how to go about tweaking the vertices?
I think this might be a red herring. Firstly, I doubt we'd ever get the h/w
and s/w vertices to be identical in all situations. Secondly, any OpenGL
implementation may display this artifact. I've personally seen this problem
using a card which did both passes fully in hardware.
The real answer is to use a polygon offset in the gloss demo. I'll look
into fixing this.
If tuxracer is drawing coplaner geometry with substantialy different
state, it should also be using polygon offset.
However I wouldn't mind bringing things as close together as possible. It may be easier than we think.
The three things I would investigate are:
- small positive and negative offsets to Z
- stripping out least significant bits from Z (this might be a requirement of the spec, in fact), by addding and then subtracting a large number to force the little bits off the end.
- floating point Z buffers (more work).
Keith
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