Re: X performance suffers under emacs 21.1.1

[Richard Stallman]

    > Incidentally, what you describe is already implemented and exactly the
    > way you proposed: clear_image_cache is called from redisplay,
    > indirectly, when it clears the face cache, and clears images that
    > haven't been displayed for 30 minutes.

Why does clearing the face cache clear the image cache?
Is there a reason why that must be done, or is it a heuristic
attempt to clear the image cache more often?