</head>
<body>
+<div class="header">
+ <h1>The Mesa 3D Graphics Library</h1>
+</div>
+
+<iframe src="contents.html"></iframe>
+<div class="content">
+
<h1>Viewperf Issues</h1>
<p>
</p>
+<h2>Proe-05 test 1</h2>
+
+<p>
+This uses depth testing but there's two problems:
+<ol>
+<li>The glXChooseFBConfig() call doesn't request a depth buffer
+<li>The test never calls glClear(GL_DEPTH_BUFFER_BIT) to initialize the depth buffer
+</ol>
+<p>
+If the chosen visual does not have a depth buffer, you'll see the wireframe
+car model but it won't be rendered correctly.
+</p>
+If (by luck) the chosen visual has a depth buffer, its initial contents
+will be undefined so you may or may not see parts of the model.
+<p>
+Interestingly, with NVIDIA's driver most visuals happen to have a depth buffer
+and apparently the contents are initialized to 1.0 by default so this test
+just happens to work with their drivers.
+</p>
+
+<p>
+Finally, even if a depth buffer was requested and the glClear(GL_COLOR_BUFFER_BIT)
+calls were changed to glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
+the problem still wouldn't be fixed because GL_DEPTH_WRITEMASK=GL_FALSE when
+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>
projects / mesa.git / blobdiff