Re: Emacs canvas support

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 29 Apr 2020 19:16:19 +0200
> From: <tomas@tuxteam.de>
> 
> > Drawing over normal text, if we don't want to redesign the entire
> > display engine, needs some new kind of "display element" ( a sibling
> > to "character", "image", "stretch", etc.), one that doesn't
> > necessarily have any effect on the metrics of the screen lines it is
> > drawn upon.  I'm not sure I have a clear idea about what features such
> > a drawing will need to support, but it could be possible to add such
> > an element with not too much effort.  Would someone want to come up
> > with a reasonable list of requirements for such a feature?
> 
> That sounds... exciting. Basically, Emacs would have to have a "display
> list" of graphical elements to draw, each one perhaps having a bounding
> box (to discard those from redisplay which aren't currently visible)
> and perhaps a "layer" (more to the background or foreground).

That's not how Emacs controls what's on display.  It basically
represents each window as a 2D array of glyphs, each one of which has
a certain graphical representation.  The representation itself is of
no concern to the display engine (well, almost); the only thing it
cares about is the metrics of each glyph, because that's what it needs
to do layout calculations.

We need to try to fit into this framework, if possible.