[cairo] color fonts and CAIRO_OPERATOR_SOURCE

Behdad Esfahbod behdad at behdad.org
Tue Sep 12 05:39:56 UTC 2017

It was actually not that complicated:

On Mon, Sep 11, 2017 at 1:44 AM, Uli Schlachter <psychon at znc.in> wrote:

> Hi everyone,
> yesterday I was asked to comment here:
> https://github.com/i3/i3/pull/2925
> The issue seems to be: With operator SOURCE, drawing a color glyph
> clears the entire surface. So, while "normal" glyphs are supposed to be
> a mask that is then filled, color glyphs seem to be handled as an
> unbounded source. This doesn't make a difference for OVER, but for lots
> of other operators (at least SOURCE) it obviously matters.
> I didn't investigate this at all. I did not even try to reproduce.
> Hence, this ping. Could one of you please look at this, confirm this
> really happens, and say what should be done about this? Thanks.
> And yes, in the thread on color fonts, Matthias Clasen answered one of
> my questions with:
> >> Okay... so what is the new model? What happens when I draw a color glyph
> >> with operator XOR and a red source?
> >
> >
> > The red source is ignored for color glyphs because they are used as the
> > source.
> So apparently this behaviour is by design, meaning that glyphs can only
> really be used with operator OVER any more (well, and some others). So
> let me ask this another why: Is this really a good behaviour?
> Oh and one more thing: Who updates cairo's docs and all the explanations
> on the web page? ("glyphs work like this, except when they do not").
> Cheers,
> Uli
> P.S.: The list of recipents is copied from the recent thread on color
> fonts. I have no overview of how people ended up in this list. Sorry if
> $YOU are the wrong recipent.
> P.P.S: Yes, I also included Gtk-devel-list. I vaguely remember someone
> saying that this stuff is relevant there. Dear moderator, sorry for the
> work that this causes you.
> --
> 99 little bugs in the code
> 99 little bugs in the code
> Take one down, patch it around
> 117 little bugs in the code
>   -- @irqed

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