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

[perryh at pluto.rain.com]

Mel Flynn <mel.flynn+fbsd.questions at mailing.thruhere.net> wrote:
> On Monday 07 September 2009 05:09:53 Michael David Crawford wrote:
> > > M> I'm looking for a pseudo-checksum tool for use with
> > > M> cataloging images.
> > One way you could approach it might be to use a blur filter ...
> > Small differences in individual pixels would be blurred away.
> ... the above does not work, because of compression anyway.
> Just because you think of an image as a bitmap, does not mean
> it's stored as such.

Certainly it is the decompressed payloads of the JPEG etc. files
that are to be compared, rather than the files themselves.  It
would never have occurred to me that anyone participating in the
discussion might have thought otherwise.

However, thinking about this inquiry and JPEG in the same sentence
has given me an idea that might help the OP:  JPEG is a "lossy"
compression, with the degree of loss related to the chosen image
quality, so two "similar" images might become identical -- or at
least more similar -- if compressed to a sufficiently low quality
using the JPEG algorithm.