Re: Emacs canvas support

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 29 Apr 2020 21:59:30 +0200
> From: tomas@tuxteam.de
> Cc: emacs-devel@gnu.org
> 
> > The current display engine works by screen lines, so if we want to
> > keep it, we must use the existing framework.
> 
> Now imagine there's a (graphical) line going from (text) display
> line 3 to 7. You are not implying that we have to segment that
> into four chunks, one per text line?

That's one way, but it's ugly and complicated.

> 
> >                                               When a portion of a
> > window is exposed, we redraw all the glyphs in the exposed area(s),
> > and we find the glyphs that need to be redrawn by comparing their
> > coordinates with those of the exposed rectangle(s).
> 
> ... but rather that we intersect the exposed area(s) with each [1]
> of the graphical objects and redraw that (be it before or after the
> text)?

Yes, but we examine what you call "objects" by screen lines.

> [1] conceptually; in reality we'll need some index structure
>    to avoid looking at most of the graphical objects. Quad-
>    trees, whatever.

We don't use them now.  A typical window is not large enough to make
that a worthy complication.