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docs: document issue with Viewperf proe-05 test 6

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Brian Paul 2013-04-23 13:18:41 -06:00
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@ -232,6 +232,36 @@ glClear is called so clearing the depth buffer would be a no-op anyway.
</p>
<h2>Proe-05 test 6</h2>
<p>
This test draws an engine model with a two-pass algorithm.
The first pass is drawn with polygon stipple enabled.
The second pass is drawn without polygon stipple but with blending
and GL_DEPTH_FUNC=GL_LEQUAL.
If either of the two passes happen to use a software fallback of some
sort, the Z values of fragments may be different between the two passes.
This leads to incorrect rendering.
</p>
<p>
For example, the VMware SVGA gallium driver uses a special semi-fallback path
for drawing with polygon stipple.
Since the two passes are rendered with different vertex transformation
implementations, the rendering doesn't appear as expected.
Setting the SVGA_FORCE_SWTNL environment variable to 1 will force the
driver to use the software vertex path all the time and clears up this issue.
</p>
<p>
According to the OpenGL invariance rules, there's no guarantee that
the pixels produced by these two rendering states will match.
To achieve invariance, both passes should enable polygon stipple and
blending with appropriate patterns/modes to ensure the same fragments
are produced in both passes.
</p>
</div>
</body>
</html>