bug#68006: 30.0.50; Image-mode speed

[Manuel Giraud via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

[...]

> 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.

Ok, I guess I understand this purpose...

> 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.

... but then this does not seem like a good fit for the usage in
image-mode.  When using 'image-next-file'/'image-previous-file', I
imagine that the idea is browse some pictures seamlessly.  Removing an
image from the cache each time seems to go against this.

About being sure to display the file on disk, maybe we could call
'image-flush' only if the file has changed since its display.  WDYT?
-- 
Manuel Giraud