[cairo] Rendering error in cairo-1.5.10 and up

[Carl Worth]

On Thu, 24 Apr 2008 16:54:47 +0200, Franz Schmid wrote:
>
> sorry to bother you with another cairo bug. I discovered that
> in cairo-1.5.10 and up (including 1.6.4) the cairo_set_dash function
> causes rendering errors with dashed closed polygons.
>
> Please see http://bugs.scribus.net/view.php?id=6971
> for screenshots and comments.

Yikes! That looks like quite a bug.

> The only workaround for that problem is not to close the polygon,
> but that is simply not acceptable.
>
> The last working version of cairo is 1.5.8

Cairo 1.5.10 did include a stroking optimization:

	http://cgit.freedesktop.org/cairo/commit/?id=afbd82671fe5ebe

and also something specific to dashed stroking:

	http://cgit.freedesktop.org/cairo/commit/?id=4471e58c12c2072

It's not an exactly an easy change to simply revert for testing,
though, since it had several followups to fix some problems in the
original change:

	http://cgit.freedesktop.org/cairo/commit/?id=1d93e148766ded8
	http://cgit.freedesktop.org/cairo/commit/?id=bf1f7f70b61b5bb
	http://cgit.freedesktop.org/cairo/commit/?id=f2d21e73827d81d

All five of those changes made it into 1.5.10. But the fact that 3
followups were necessary does lead me to suspect that this new bug
might be yet another problem exposed by this stroking
optimization. (And, Vlad, if I'm wrong in my guess I apologize in
advance for pointing at your commits!)

Franz, What's really going to help us here if you can provide for us a
sequence of cairo calls to replicate the bug. Perhaps you've got easy
facilities in scribus for dumping out a path? (along with scale and
stroke parameters like line width, join style, etc.)

Otherwise the libcairowrap utility (or maybe it's named cairo-trace
these days) can be helpful by using an LD_PRELOAD to dump cairo calls
while running scribus live. Here's one version of that:

	git clone git://annarchy.freedesktop.org/~ickle/cairo-trace

Everyone, is Chris's version here the most up to date thing? We should
put this into /git to make it more clear which version is
official. And we could really use a wiki page on cairographics.org
explaining how to use this.

Thanks for the report, Franz. Sorry about the bug, and I hope we'll be
able to get it fixed quickly.

-Carl
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