[Bug 89649] New: dEQP-GLES3: line color interpolation does not have enough precision

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89649

            Bug ID: 89649
           Summary: dEQP-GLES3: line color interpolation does not have
                    enough precision
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: idr at freedesktop.org
          Reporter: itoral at igalia.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

Affected tests:

dEQP-GLES3.functional.rasterization.interpolation.basic.lines
dEQP-GLES3.functional.rasterization.interpolation.basic.line_strip
dEQP-GLES3.functional.rasterization.interpolation.basic.lines_wide
dEQP-GLES3.functional.rasterization.interpolation.basic.line_strip_wide
dEQP-GLES3.functional.rasterization.interpolation.basic.line_loop_wide
dEQP-GLES3.functional.rasterization.interpolation.projected.lines
dEQP-GLES3.functional.rasterization.interpolation.projected.line_loop
dEQP-GLES3.functional.rasterization.interpolation.projected.lines_wide
dEQP-GLES3.functional.rasterization.interpolation.projected.line_strip_wide
dEQP-GLES3.functional.rasterization.interpolation.projected.line_loop_wide
dEQP-GLES3.functional.rasterization.fbo.texture_2d.interpolation.lines
dEQP-GLES3.functional.rasterization.fbo.texture_2d.interpolation.lines_wide
dEQP-GLES3.functional.rasterization.fbo.rbo_singlesample.interpolation.lines
dEQP-GLES3.functional.rasterization.fbo.rbo_singlesample.interpolation.lines_wide

The problem with these are small differences in the interpolated color computed
for certain pixels across lines. dEQP computes a range of acceptable color
values for each pixel and them compares that with the actual rendering result
(as provided by glReadPixels). In many cases (at least for lines that are just
1px wide), the difference is off by just one unit in just one component of a
pixel. For wide lines it seems that more pixels are affected but the
differences are so small that they seem invisible to the naked eye in any case.

Attribute interpolation is done in hardware. The FS receives barycentric
coordinates and deltas, computed by hardware in the payload and then, as far as
I can see, computing interpolated values amounts to a PLN instruction passing
these values as parameters. If this is not producing correct results I would
guess that it is an issue in the precision used by the hardware to do these
computations (be it the PLN, the values delivered in the FS payload or both),
and software can't do much about it. I have also reviewed the state
configurations for various hardware units that are involved in interpolation
and rasterization calculations and they all seem okay to me.

So, my conclusion is that it is not clear to me if dEQP's expectations are 100%
accurate, but even if they are I am not sure that the driver can do much about
this since all the aspects of interpolation seem to be on the side of the
hardware.

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