Re: Emacs canvas support

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> I think it would make more sense to treat the overlaid canvas as
>> a completely separate pixmap: when we get an request to redraw
>> a particular area of the screen, we'd ask the current redisplay code to
>> redraw the corresponding text content and then we'd ask the canvas code
>> to draw on top of it.
>
> Unfortunately, most redisplay cycles don't start from a request to
> redraw a particular area of the screen.

I don't think it makes a big difference.  Upon redisplay, the
glyph-matrix layer will compute which parts of the window needs updates
in the that layer (exactly like it currently does), and the canvas will
do its own computation of which part of it needs updating (presumably
this will be handled by the external canvas library such as Cairo's
rather than by Emacs's own code) and then the compositing layer will
figure out how to combine those two changes.  IIUC libraries like Cairo
already provide functionality to do compositing of layers, so that would
take care of that part as well.


        Stefan