bug#68006: 30.0.50; Image-mode speed

[Eli Zaretskii <eliz@gnu.org>]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Cc: 68006@debbugs.gnu.org
> Date: Mon, 25 Dec 2023 11:34:49 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Look at its Git history, and look up discussions and bugs from those
> > times.
> >
> > AFAIR, one problem is that images can change behind our backs.  But
> > maybe there are other reasons.
> 
> So I trace this call back to dc255b24e30a (Chong Yidong, 2007-05-21).  At
> that time 'image-flush' was called 'image-refresh' with the same
> functionality of uncaching an image given its spec.
> 
> Before that, the whole image cache was cleared (fcfc4732e0d9, Chong
> Yidong, 2006-02-09).  I did not find a bug reference and I still don't
> understand why it is/was needed.

I think the reason is what I mentioned above.

You seem to interpret the purpose of the image-cache differently from
its real purpose.  The real purpose of the image cache is not to hold
the image in memory for displaying it again and again, the purpose is
to allow the display engine to generate the pixels once, then reuse
those pixels during the current redisplay cycle or a few following
redisplay cycles.  That's why we can so easily evict images from the
cache when some time has passed: the cache is ephemeral anyway, and is
intended to serve only as a short-term memory.

When working with external image files, flushing the image cache is a
good measure to make sure we always display the file on disk, not some
previous copy of it.

If you have a specific application in mind where this cache must be
kept for longer times, we could perhaps arrange for that, but we will
then also need to make sure images are not evicted from the cache
during that time.