[cairo-bugs] [Bug 84044] New: test/cairo-test-suite reports wrong max diff comparing different opaque images

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=84044

          Priority: medium
            Bug ID: 84044
          Assignee: chris at chris-wilson.co.uk
           Summary: test/cairo-test-suite reports wrong max diff comparing
                    different opaque images
        QA Contact: cairo-bugs at cairographics.org
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: sixtysix at inwind.it
          Hardware: Other
            Status: NEW
           Version: unspecified
         Component: general
           Product: cairo

In test/buffer-diff.c (line 80), when two pixels differ, all channels
are considered to compute result.max_diff, this means in the case of
rgb24 images also the meaningless alpha channel.

Similarly pdiff_compare in test/pdiff/pdiff.c uses the (non)alpha
channel to unpremultiply the not premultiplied rgb channels, hardly
giving the expected results.

Probably there should be a test-suite for pdiff to check for false
positive/negative.

This results in many image.rgb24 failures that would be accepted
using the same criterion used for image.argb32 target making the
cairo test-suite output quite confusing.

BTW to slightly speed up the test-suite you could write and use a
'pdiff_compare_fast' that returns 1 the first time pixels_failed is
incremented.

The best way to speed up the test-suite is to have all tests PASS though.

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