[Mesa-dev] [Bug 94341] Incorrect results with dFdx / dFdy

[bugzilla-daemon at freedesktop.org]

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 at lists.freedesktop.org
          Reporter: ikerr at box.com
        QA Contact: mesa-dev at 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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