Re: redisplay system of emacs

[grischka]

Eli Zaretskii wrote:
> > 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.

Sure, thats what I wrote: It updates only the differences.

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

That is probably true nowadays with fast graphical screens (say for PCs
after ~1995).  However the design of emacs' "redisplay" is based on the
contrary assumption that sending the updates to the terminal more expensive
than their calculation.  If you want to it is optimized for _very_ slow
terminals to the expense of code simplicity.  I assume it was working great
with 14400 baud connections, but then again emacs didn't have colors and
var-pitch fonts at that times, either.

--- grischka