[Mesa-dev] [Bug 89624] Drivers, Gallium/legacy swrast glDrawPixels differences

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89624

            Bug ID: 89624
           Summary: Drivers, Gallium/legacy swrast glDrawPixels
                    differences
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Other
          Assignee: mesa-dev at lists.freedesktop.org
          Reporter: daniel.sebald at ieee.org
        QA Contact: mesa-dev at lists.freedesktop.org

In the following two bug reports:

https://bugs.freedesktop.org/show_bug.cgi?id=89586
https://bugs.freedesktop.org/show_bug.cgi?id=89622

I discussed some artifacts concerning pixels at or beyond GL_MAX_TEXTURE_SIZE
in the input image.  In the first bug report I attached a patch that fixes the
vertical lines in legacy swrast.  In the second I simply noted that Gallium
swrast limits the image, and I think it wouldn't be too difficult to correct
that.

Here I want to point out the subtle difference between legacy swrast (post
patch in bug #89586) and the Gallium swrast using images with only 20 x 20
input pixels.

For legacy swrast, the attached image
Screenshot-differences_legacy_swrast-annotated.png shows how the scaled image
can extend past the edge of the boundary in the x-dimension, or not (and it
never falls short of the boundary).  On the other hand, the image will fall
short of boundary in the y-dimension, or not (and it never extends past).  I'm
guessing this behavior has to do with xfactor > 0 and yfactor < 0.

For Gallium swrast, the attached image
Screenshot-differences_gallium-annotated.png illustrates that scaled image
aligns with the x-dimension edge (an it always seems to do so).  However, the
image can extend past the bottom in the y-dimension, or not (and it seems to
never fall short).

Since these two behaviors are different, as subtle as it may be, at least one
of them has a bug.  I suspect that both of them might have a flaw as far as
precisely following OpenGl standard.  It's interesting that the x-dimension for
the Gallium driver seems most consistent.

Granted, the axis box itself may not be exact, so to say exactly what the bug
is at this stage is premature, but at least the behavior of the axis box should
be the same when all that is varied is the graphics driver and not the
application binary.

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