Building optimized metasprite from image?

Hi,

I have many large sprites that I want to break down into individual hardware sprites, maximizing sprite coverage while minimizing the number of used sprites. I'm doing it manually but I wonder if there's a tool that can do it automatically. There probably isn't one, but I figured I'd ask

Assuming there isn't one, what problem domain is this kind of image analysis? What solutions are usually used? What should I be looking at in terms of theory?

Thanks!
djcc

There used to be something called Objective that worked by first converting an image as a set of columns, then converting it as a set of rows, and keeping the one with the fewest tiles.
For example, converting as a set of rows turns the top image in this post into the bottom image.
See also Tilificator.