[Bug 25871] New: nearest neighbour samples wrong texel (precision/rounding problem)

[bugzilla-daemon@freedesktop.org]

http://bugs.freedesktop.org/show_bug.cgi?id=25871

           Summary: nearest neighbour samples wrong texel
                    (precision/rounding problem)
           Product: Mesa
           Version: git
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/R600
        AssignedTo: dri-devel@lists.sourceforge.net
        ReportedBy: pierre-bugzilla@ossman.eu


The following fragment program does the wrong thing on r600/r700 hardware:

TEX result.color, 0.498046860099, texture[0], 2D;

(the texture coord is as far as I got before I got bored. I'm not sure the
precision goes much further anyway)

The circumstances is that I have a 4x4 texture with nearest neighbour
interpolation. The above code should be sampling texel 1,1 but is instead
sampling texel 2,2. This causes problems for fragment programs that try to do
things based on texel coords (like bicubic interpolation).

This seems to be a hw bug as I've tried it with fglrx (albeit with a slightly
different card) and it exhibits the same bug. As such, I'm not sure it can be
solved but hopefully there is a workaround.


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