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Re: redisplay system of emacs

From: grischka
Subject: Re: redisplay system of emacs
Date: Tue, 16 Feb 2010 21:56:03 +0100
User-agent: Thunderbird (Windows/20090812)

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

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

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