[cairo] Avoiding seams (again)

[Owen Taylor]

On Wed, 2006-03-22 at 22:18 -0800, Vladimir Vukicevic wrote:
> On 3/22/06, Timothée Lecomte <timothee.lecomte at ens.fr> wrote:
> > Hi !
> >
> > Do you remember the thread about avoiding seams between paths a months ago ?
> > Well, I am still facing this problem with no ideal solution.
> 
> We're running in to this problem in mozilla-land as well, in
> particular with thick border rendering.  Since CSS borders can have
> different colors/patterns/etc. for each side of the border, they tend
> to get drawn as trapezoidal wedges for solid colors.  Right now, with
> our cairo builds, we end up with a seam there, as seen in the
> testcases in bug https://bugzilla.mozilla.org/show_bug.cgi?id=328241 .
> 
> We don't really have a good solution yet; I don't think SATURATE is an
> option for us, since we're slow enough as it is (though it might be
> acceptable for certain situations where we know we're going to run
> into this, like thick borders).  We've been considering supersampling,
> maybe extending push group/pop group to support supersampling
> internally, but we haven't done any investigation down that path yet
> either.

This particular problem (as with many cases where the geometry is known
in advance) should be pretty manageable by just drawing the border
in a separate group with ADD and compositing the result.

You'd probably want to special case the normal all-borders-the-same
case and draw it with 4 rectangles for efficiency.

						Owen