Re: Enriched/Org is a colorful Org

[Carsten Dominik <carsten.dominik@gmail.com>]

On 12 apr. 2013, at 08:41, Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Carsten Dominik <carsten.dominik@gmail.com>
>> Date: Fri, 12 Apr 2013 00:49:32 +0200
>> Cc: "emacs-orgmode@gnu.org List" <emacs-orgmode@gnu.org>
>> 
>>> Overlays should be OK as long as they aren't too many, and as long as
>>> you don't move them around too much, particularly in post-command-hook
>>> or some such.
>> 
>> This explanation sounds to me as if the display engine is building
>> some kind of tree of overlays to find properties changes quickly.  Too bad I don't have time to understand this stuff in more detail, it sounds interesting.
> 
> Just search xdisp.c for "overlay", you will see the story quite
> clearly, I think.

My Sunday pleasure reading project.

> 
> There are actually 2 aspects that make display engine's job harder
> with overlays.  One is indeed that finding overlays that "cover" a
> given buffer position is not a very efficient operation.  The display
> engine tries to overcome this to some extent by "centering" the
> overlay data base around the place in the buffer it is rendering.
> This is done for every screen line that is being examined by the
> display code.
> 
> The other problem is that there's no easy way of finding out which
> parts of the buffer are being affected by changes in overlays.  The
> consequence of this is that redisplay cannot easily determine whether
> a given portion of what's on the glass needs to be redrawn when
> overlays change.  The analysis of which part(s) of a window need to be
> redrawn is at the heart of redisplay optimizations, whereby Emacs
> tries very hard to reuse the portions of display that are already up
> to date.  Changes in overlays defeat that.  Therefore, changing _any_
> overlays in a buffer disables most, if not all, redisplay
> optimizations, meaning that the entire portion of the buffer that is
> displayed will be completely redrawn each time overlays _anywhere_ in
> the buffer are changed.

Wow.  OK.  I need to think to that, and I will try to take a
fresh look at overlay use in Org.

So the reason that the combination with hi-line is slow is because
hl-line is using post-command-hook to move its overlay, and redisplay
of a full window with org-mode is slow because so much stuff is
hidden and Emacs makes a full re-evaluation of what needs
to be displayed?

This makes sense.

Thanks for taking the time to get me this far.

- Carsten