[Mesa-dev] [Bug 48545] New: LLVMpipe glReadPixels Firefox hits the slow path (WebGL rendering)

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https://bugs.freedesktop.org/show_bug.cgi?id=48545

             Bug #: 48545
           Summary: LLVMpipe glReadPixels Firefox hits the slow path
                    (WebGL rendering)
    Classification: Unclassified
           Product: Mesa
           Version: 8.0
          Platform: x86 (IA32)
        OS/Version: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Mesa core
        AssignedTo: mesa-dev at lists.freedesktop.org
        ReportedBy: cosinusoidaly at yahoo.co.uk


I've reported this bug to Mozilla
(https://bugzilla.mozilla.org/show_bug.cgi?id=743585), but I think Mesa may be
needlessly going through the glReadPixels slow path so I thought I'd best
report the bug here too.

I think what's happening is Firefox is creating a RGBA backing store of some
kind, and then attempting to read it back BGRA. This is then in turn causing
fast_read_rgba_pixels_memcpy (in /src/mesa/main/readpix.c) to fail and instead
fall back to the slow path. As Firefox will read back the whole frame buffer
each frame, this signifcantly damages performance.

I assume another fast path could be added which does the read back with a
swizzle? In the mozilla bug report I included a patch to mesa that does this
(which is a horrible hack). I'd be happy to try and turn that hack into a more
general solution. Is that worth doing? If so, what's the best line of attack?

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