Bug 26010

Summary:	cairo_line_to optimizes away path segments
Product:	cairo	Reporter:	Alois <alois>
Component:	general	Assignee:	Carl Worth <cworth>
Status:	RESOLVED FIXED	QA Contact:	cairo-bugs mailing list <cairo-bugs>
Severity:	critical
Priority:	high	CC:	alois
Version:	1.9.4
Hardware:	All
OS:	All
Whiteboard:
i915 platform:		i915 features:

Description Alois 2010-01-12 08:07:36 UTC

If I draw (with cairo_line_to() calls) a path which contains segments which
are draw twice, these segments are not rendered when the path is stroked.

Example:
cairo_move_to(1,1)
cairo_line_to(1,2);
cairo_line_to(2,3);
cairo_line_to(1,2);
cairo_stroke();

The same happens when I use rel_line_to().

This bug did not exist in cairo 1.8.8

Because of improved PDF handling I need to switch to a newer cairo version
than 1.8.8 and wouldbe very happy if this bug could be checked and fixed.

Thank you

Comment 1 Chris Wilson 2010-01-12 09:09:00 UTC

Thanks for the bug report!

commit 74ea4c908067f5579b51d3dbaea387da8f343671
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Jan 12 17:00:25 2010 +0000

    path: Do not remove anti-parallel line segments in case we are stroking
    
      Bug 26010 - cairo_line_to optimizes away path segments
      http://bugs.freedesktop.org/show_bug.cgi?id=26010
    
    As exercised by path-stroke-twice, we incorrectly optimise away a line
    segment if the path doubled back upon itself. This is very reminiscent
    of the optimisation bug for replacing curve-to with line-to.

commit a5dd5a6069c0d7435260a954d8d98483f5fbcc1c
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Jan 12 16:32:09 2010 +0000

    test: Add path-stroke-twice
    
    Exercises a bug found by alois@astro.ch, whereby we inadvertently remove
    a line segment when the path doubles back upon itself.
    
      Bug 26010 - cairo_line_to optimizes away path segments
      http://bugs.freedesktop.org/show_bug.cgi?id=26010

Comment 2 Alois 2010-01-12 14:35:23 UTC

Thanks for the very quick fix. I have backported the patch into my copy of 1.9.4 snapshot, it works fine now.

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