[cairo] [Fwd: Re: Uniform Line Widths]

Carl Worth cworth at cworth.org
Fri Feb 22 12:44:28 PST 2008

On Fri, 22 Feb 2008 10:38:00 -0800, Bill Spitzak wrote:
> I would also like to see this (along with a matching fix for fonts,
> which I consider a good deal more important as it is currently
> impossible to use the Cairo ctm to position labels without also
> transforming the text). Carl certainly discussed it too and was thinking
> of making this change but this was dropped?

It always seems to come up right before a major release, which is
definitely not the time to make a change like this. We'll need to make
it just *after* a release to see how much it breaks and give people
time to react to that.

> 1. It does not match PostScript/PDF. This would require the CTM to be
> applied by the cairo PDF/PostScript backend rather than transforms being
> sent unchanged through.

I think we're already pre-transforming paths all over the place in
these backends so that's not too big a deal, really.

And I think my explorations of this idea *did* show provide for some
behavior in cairo that couldn't obviously be achieved in cairo. That
might require more image fallbacks there, (but fortunately, those
aren't nearly as expensive now than they were before since we've got
all the fine-grained fallbacks in place).

> 2. It is not compatable with the current Cairo (however it is possible
> that the set of things it is incompatable with is very small or that
> they are producing the wrong output now and the users have not noticed).
> Or a switch can be provided as part of the graphics state.

Right, that's the "change just after a major release, not before"
stuff I discussed above.

> 3. Need an api to query and set the line width using a 2x2 matrix.

And this is API that needs a concrete proposal, (though we may have
gotten pretty close in the discussions long ago).

I like the naming idea of using "set/get_pen" for the new API and that
"set/get_line_width" would be convenience functions on top of those.


> Advantages:

Another big advantage would be that "locking" the pen like this would
be semantically similar to the locking we already do for

So I do agree that this is something we want to do.

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