Re: Emacs canvas support

[Eli Zaretskii]

> From: Po Lu <address@hidden>
> Cc: address@hidden,  address@hidden,  address@hidden
> Date: Sat, 02 May 2020 07:27:07 +0800
> 
> Eli Zaretskii <address@hidden> writes:
> 
> > I think the information needs to be recorded in the glyph structure
> > that represents a canvas.
> 
> I have a canvas field inside `struct glyph' that points to the canvas
> object.

It is not a good idea to have pointers in the glyph structure.  (Yes,
xwidget uses that, but don't take example from that, please.)  A glyph
should be self-describing from the metrics POV.  If you need
additional information (beyond the basic metrics), use the example of
images and faces: we cache the objects and record their cache ID in
the glyph.

> What I'm trying to do right now is track the position of the
> canvases on-screen, so I can draw them separately from redisplay.

My suggestion is not to "draw separately from redisplay".  I don't see
how this could work, unless you also modify extensively how the second
phase of redisplay, the one entered via update_frame and
update_window, works.