** Description changed:
+ [ Impact ]
+
+ OpenSlide (libopenslide0 in Ubuntu) uses Cairo as its rendering backend,
+ always with CAIRO_OPERATOR_SATURATE and sometimes with subpixel
+ positioning (when reading certain file formats), and Cairo in turn
+ invokes this code. As a result, some slides incorrectly render with
+ large blank spaces and with some pixels rendered on top of other pixels.
+ This effectively makes OpenSlide unusable for several of the file
+ formats it supports. No workaround is believed possible within the
+ OpenSlide codebase.
+
+ [ Test Plan ]
+
+ Compile and run the pixman.c test program uploaded to this bug. It
+ should report "OK".
+
+ [ Where problems could occur ]
+
+ Problems would show up as incorrect pixel output from software that
+ renders via Cairo or pixman.
+
+ [ Other Info ]
+
+ While I'm not a pixman expert, it appears that the change should only
+ affect the broken code path, and it seems unlikely that anything else
+ depends on the incorrect math fixed by this patch. This bug hasn't
+ previously been reported in Ubuntu, which may imply that this code path
+ is not exercised by other packages in the distro.
+
+ The affected source file has had no further commits upstream since this
+ patch was applied in April 2019, and some basic commit grepping didn't
+ turn up any followup fixes elsewhere in the tree.
+
+ [ Original message ]
+
pixman 0.38.4-0ubuntu1 in focal (and actually pixman 0.38.x generally)
has a regression that causes incorrect rendering in some circumstances.
This can be triggered by the use of cairo with CAIRO_OPERATOR_SATURATE
and subpixel positioning, and causes OpenSlide <https://openslide.org/>
to produce incorrect output.
The attached test program will print "Failed" if the bug exists, or "OK"
if it doesn't.
This is fixed upstream in
https://gitlab.freedesktop.org/pixman/pixman/-/commit/8256c235, which is
in pixman 0.40.0.
See https://github.com/openslide/openslide/issues/278 for more context.
** Also affects: pixman (Ubuntu Focal)
Importance: Undecided
Status: New
** Changed in: pixman (Ubuntu)
Status: New => Fix Released
** Changed in: pixman (Ubuntu Focal)
Status: New => Triaged
--
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which is subscribed to pixman in Ubuntu.
https://bugs.launchpad.net/bugs/1988796
Title:
Incorrect rendering triggered by cairo CAIRO_OPERATOR_SATURATE with
subpixel positioning
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