Emacs caches images so that it can display them again more
efficiently. When Emacs displays an image, it searches the image
cache for an existing image specification equal to the desired
specification. If a match is found, the image is displayed from the
cache. Otherwise, Emacs loads the image normally.
This function removes the image with specification spec from the
image cache of frame frame. Image specifications are compared
using equal. If frame is nil, it defaults to the
selected frame. If frame is t, the image is flushed on
all existing frames.
In Emacs’s current implementation, each graphical terminal possesses an image cache, which is shared by all the frames on that terminal (see Multiple Terminals). Thus, refreshing an image in one frame also refreshes it in all other frames on the same terminal.
One use for image-flush is to tell Emacs about a change in an
image file. If an image specification contains a :file
property, the image is cached based on the file’s contents when the
image is first displayed. Even if the file subsequently changes,
Emacs continues displaying the old version of the image. Calling
image-flush flushes the image from the cache, forcing Emacs to
re-read the file the next time it needs to display that image.
Another use for image-flush is for memory conservation. If
your Lisp program creates a large number of temporary images over a
period much shorter than image-cache-eviction-delay (see
below), you can opt to flush unused images yourself, instead of
waiting for Emacs to do it automatically.
This function clears an image cache, removing all the images stored in
it. If filter is omitted or nil, it clears the cache for
the selected frame. If filter is a frame, it clears the cache
for that frame. If filter is t, all image caches are
cleared. Otherwise, filter is taken to be a file name, and all
images associated with that file name are removed from all image
caches.
If an image in the image cache has not been displayed for a specified period of time, Emacs removes it from the cache and frees the associated memory.
This variable specifies the number of seconds an image can remain in the cache without being displayed. When an image is not displayed for this length of time, Emacs removes it from the image cache.
Under some circumstances, if the number of images in the cache grows too large, the actual eviction delay may be shorter than this.
If the value is nil, Emacs does not remove images from the cache
except when you explicitly clear it. This mode can be useful for
debugging.
This function returns the total size of the current image cache, in bytes. An image of size 200x100 with 24 bits per color will have a cache size of 60000 bytes, for instance.