[cairo] Finer-grained fallbacks for the PostScript backend

Carl Worth cworth at cworth.org
Mon Aug 27 14:58:42 PDT 2007

On Mon, 11 Jun 2007 22:06:29 +0930, Adrian Johnson wrote:
> One of the planned cairo features is finer-grained image fallbacks
> for backends that use the analysis surface.

This is really fantastic work, Adrian!

> The one remaining problem I have not been able to fix is that gradient
> and surface patterns do not work with both a device scale and device
> offset specified. The patch works correctly if the fallback resolution
> is set to 72dpi as this results in a device scale of 1.

For people who haven't been following the commits to git, nor the
discussion on IRC, here's an update on where this work stands:

Vladimir fixed the device offset and scale bug.

All of Adrian's fine-grained fallback work, as well as the patches to
make the ps and pdf surfaces use it, are all pushed out to cairo now.

The PDF code takes advantage of "knockout groups" within PDF and it
appears that the poppler library does not currently implement them
correctly. We've reported this bug here:

	Poppler does not correctly handle knockout groups

So people using the latest cairo should notice fewer, and more
fine-grained image fallbacks in their PDF output now. But if the
results seem totally wrong in evince, please double-check with
some other PDF viewer to see if the cairo output isn't correct, and
it's just the above poppler bug causing problems.

The new pdf code does also expose some bugs in cairo's current
native-pdf-gradient code. Or rather, the bugs have always been
exposed, but the test suite wasn't noticing them before, but does now
with the radial-gradient and unbounded-operator test, (though again,
the poppler bug makes this hard to see).

It looks like we could use a new test that exercises EXTEND_NONE with
linear gradient patterns.

And Adrian thinks he knows what the gradient bug is and plans to fix
it as soon as he can.

Anyway, this is a fairly exciting new development for cairo, and
projects like mozilla and inkscape should really benefit from
it. (Plus one more upcoming patch from Adrian to allow for cairo-pdf's
create_similar to use meta-surfaces so that using cairo_push_group
doesn't force image fallbacks when generating PDF).

Keep up the great work, Adrian! We really appreciate it.

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