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<base href="https://bugs.freedesktop.org/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incorrect results with dFdx / dFdy"
href="https://bugs.freedesktop.org/show_bug.cgi?id=94341">94341</a>
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<th>Summary</th>
<td>Incorrect results with dFdx / dFdy
</td>
</tr>
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<th>Product</th>
<td>Mesa
</td>
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<th>Version</th>
<td>11.1
</td>
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<th>Hardware</th>
<td>Other
</td>
</tr>
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<th>OS</th>
<td>Linux (All)
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>medium
</td>
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<th>Component</th>
<td>Mesa core
</td>
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<th>Assignee</th>
<td>mesa-dev@lists.freedesktop.org
</td>
</tr>
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<th>Reporter</th>
<td>ikerr@box.com
</td>
</tr>
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<th>QA Contact</th>
<td>mesa-dev@lists.freedesktop.org
</td>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=122036" name="attach_122036" title="Correct result (left) and incorrect result (right)">attachment 122036</a> <a href="attachment.cgi?id=122036&action=edit" title="Correct result (left) and incorrect result (right)">[details]</a></span>
Correct result (left) and incorrect result (right)
Context: I am using Chrome and osmesa to perform "headless" WebGL rendering.
Many of the Three.js examples (<a href="http://threejs.org/examples/">http://threejs.org/examples/</a>) do not render
correctly. I think I have narrowed the problem down to incorrect results
produced by dFdx / dFdy. Three.js uses these functions for several things
including: flat-shading and bump / normal mapping.
I have created a JSFiddle that demonstrates the problem:
<a href="https://jsfiddle.net/iangkerr/08x72ge9/">https://jsfiddle.net/iangkerr/08x72ge9/</a>
The fiddle renders two triangles, rotated 45 degrees, with a perspective
projection. The fragment shader attempts to compute the surface normal and
assigns the result to gl_FragColor. I expect a purplish colour, due to the
normal having positive X and Z components (R and B), and I get this result on
my Mac. However, when running Chrome + osmesa on Ubuntu, I get a bluish colour
(positive Z component). I have attached a screenshot of the correct result and
the incorrect result.</pre>
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