[Webkit-unassigned] [Bug 52311] [chromium] Add command-line flag to enable composite to offscreen texture.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=52311





--- Comment #57 from Vangelis Kokkevis <vangelis at chromium.org>  2011-02-16 18:48:37 PST ---
Hi James,
  Interesting analysis, however I don't think in this case it's a 23 vs 10 bit precision issue. Even though we pretend to be running OGL ES, on linux we're really running on desktop GL and the mediump qualifier gets ignored (as it's not supported in desktop GL).  In addition, if you're running the tests via DRT I believe that even if you run them on your desktop we will be using MESA and not the real h/w. That said, I agree that the issues you're seeing could very well be related to numerical imprecision when sampling the render target texture to do the final render into the frame buffer.   At this point I don't want to let them block this patch.  




(In reply to comment #56)
> Here's a shot in the dark - feedback is appreciated ...
> 
> Hypothesis: the failing pixels are due to precision limitations in the shader computations (most likely the fragment shader, but possibly a combination of both). This becomes pronounced when trying to sample a large texture in a single chunk.
> 
> For Desktop GL we get max 23 bits (float mantissa) for shader floats, although for OpenGLES we're only guaranteed 2^(-10) precision in fragment shaders using mediump (i.e. 10 bits, highp not guaranteed to be available).
>

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