Re: redisplay system of emacs

[Eli Zaretskii]

> Date: Tue, 16 Feb 2010 20:20:06 +0100
> From: grischka <address@hidden>
> Cc: address@hidden
> 
> >> Redisplay_internal does lots and lots of computations to detect which part
> >> of the buffer should be redisplayed at a given moment. All these
> >> computations can be avoided using a system of subwindows (1 for every
> >> char).
> >
> > Those computations are complicated because the buffer contents (or window
> > width, or scroll amount, or...) might have changed, not just because the
> > window might have been damaged in some complicated fashion.
> 
> It's not _that_complicated.  Basically it just draws the new screen to
> memory, then compares it with the current screen and then updates the
> differences.

That's only a small part of the story.  It draws only part of the
screen, not all of it (unless optimizations fail), and it goes to
great lengths to minimize that part of the screen.

The reason is that delivering to the screen is not the heaviest part
of the redisplay; it's that ``drawing to memory'' that is.