Is there such thing as a 'soft checksum' tool?

[Modulok]

List,

I'm not even sure such a tool exists, but it's worth asking:

I'm looking for a pseudo-checksum tool for use with catalogging
images. For example, a strict checksum algorithm, like the sha family,
will produce a dramatically different checksum for two files which
differ by only a single bit. I'm looking for something where two
images images, which are similar, get a proportionally similar
checksum. When I speak of similarities I'm referring to their image
patterns. i.e two images of differing sizes, which are otherwise
identical, would produce very similar checksums. So the closer the
checksums are, the more similar two given images are.

Does anyone know of anything like this?
-Modulok-