https://bugs.freedesktop.org/show_bug.cgi?id=94341
Bug ID: 94341
Summary: Incorrect results with dFdx / dFdy
Product: Mesa
Version: 11.1
Hardware: Other
OS: Linux (All)
Status: NEW
Severity: normal
Priority: medium
Component: Mesa core
Assignee: mesa-dev@lists.freedesktop.org
Reporter: ik...@box.com
QA Contact: mesa-dev@lists.freedesktop.org
Created attachment 122036
--> https://bugs.freedesktop.org/attachment.cgi?id=122036&action=edit
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 (http://threejs.org/examples/) 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:
https://jsfiddle.net/iangkerr/08x72ge9/
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.
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