Re: SVG hack for display engine

[Eli Zaretskii <eliz@gnu.org>]

> From: Anand Tamariya <atamariya@gmail.com>
> Date: Thu, 18 Nov 2021 15:43:01 +0530
> Cc: Alexander Adolf <alexander.adolf@condition-alpha.com>, Emacs Devel <emacs-devel@gnu.org>
> 
>   . does this mean you expect the display engine to tell that
>     display-svg-hook which parts of the display were redrawn?
> 
> No, that won't be necessary. After change hooks and other APIs are enough to derive change and context
> information. In my experience, the characters around the point are usually the most relevant.

But that's not generally true.  Redisplay has its own criteria to
decide which parts need to be redrawn, and those criteria are not
necessarily trivial to guess from buffer contents alone, because
redisplay has access to all kinds of information recorded specifically
for that purpose, and not exposed to Lisp.

>   . what is the "required information" to be passed to
>     display-svg-hook, and how will it be generated from the information
>     that exists in the display engine?
> 
>  "required information" in my sentence above refers to the parameters of (svg-render) API viz. svg x y width
> height. display-svg-hook can be a no-arg function.

What are X, Y, WIDTH and HEIGHT here?

And how will those hooks or svg-render know what to display an where
on the screen?