image signatures

[Dave Long]

> I think no signature function such as you describe in (1) exists,
> except for the domain of one-pixel images.

No need to preserve spatial information.  Probably better to ditch it if you 
want to catch flips and rotations.

Assuming your 16x16 reduction, here are some 1-d signature functions:  (which 
may be useless except for argument.  I'd need to know more about your 
application to be able to say anything useful)

1) are there more 1 or 0 bits?
2) how many more are there?
3) how many bit transitions are there along the principal diagonals/the 
principal orthogonals/around circular paths?

One would need a very wide fudge window with all of these, but they should map 
similar images into similar values.

You should check the literature.  I recall people at a Seybold conference in 
94 or 95 with a "find more like this" database product for images; no doubt 
the state of the art has advanced somewhat since then.

-Dave