[cairo] Problem with unaligned clip regions

[Uli Schlachter]

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Am 17.03.2011 19:33, Tristan Schmelcher wrote:
> To add some more information, the black area is created by
> combine_src_u in pixman, and it is black because the corresponding
> mask values are all zero. The mask comes from
> _composite_unaligned_boxes in cairo-image-surface.c, which is called
> from _composite_boxes, which is in turn called from
> _clip_and_composite_boxes. That's about the limit that I can debug
> this though since I don't really understand the box compositing code.

Hi,

I took a closer look at this and it seems to me that _composite_unaligned_boxes
cannot handle more than one box with some operators, e.g. "source".

Attached is the original test case wrapped into a format that cairo-test-suite
can understand (but still needs a good name, some comments, a copyright header,
...).

The original clip is optimized away by _cairo_clip_to_boxes, so this code is hit
without a clip mask and two boxes.

Also, there is an attempted fix attached. No idea if this is correct, but it
works for me. All the existing tests in the test suite hit this code path with
boxes->num_boxes == 1. The only exception is image-bug-710072-unaligned which
has 6 boxes with operator "over". I only have choosen this "if" condition
because it worked for all cases that I found, not because I think it is correct.

Could someone who actually has a clue take a closer look at this?

Cheers,
Uli

- -- 
Sometimes the people that know software the best are the ones
that hate it the most.                                      -- infogulch
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