Re: Emacs canvas support

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 29 Apr 2020 14:34:09 +0800
> From: Po Lu via "Emacs development discussions." <emacs-devel@gnu.org>
> 
> I'd appreciate some feedback on something I came up with during my spare
> time: Emacs canvas support.

I've read the code and the docs, but I don't think I have a clear idea
of what "canvases" are and what would be their intended usage.
Perhaps consider starting the documentation with some introductory
comments and even a small tutorial.

It sounds like you are talking about a way to create images
dynamically, but then I don't understand why we need canvas-from-image,
for example.

Also, is there support for clicking the mouse on a canvas?  I don't
see it.

> For now it only works on X11 + Cairo builds, and I haven't quite figured
> out how to make redisplay work reliably on canvases

Well, that'd be my main comments.  I don't quite understand the parts
of the display code you use for this, they seem like a copy/paste from
other objects, sometimes with comments that weren't updated and still
reference images instead of canvases.  If the drawing on the canvas is
supposed to be modified by Lisp code, then we'd need a much more
elaborate machinery than just one flag to decide when a canvas needs
to be redrawn.

Thanks.