JPEG differences

[extremis]

I already know that compare command-line tool can find the differences between 2 images but what image is actually better (quality)? If one had the original bitmap then he could compare with it and the less different jpeg would be the better. But if one does not have the original bitmaps (as in the case of jpegs from the internet) what other way there is?
I usually assume that the larger file (or the one with the most unique colors) is the better one, but is there a more sophisticated way?

[anthony]

I can say with certainaty, Their is no way to be sure of the quality of any image from the internet.

I have seen the same image shrunk down, then expanded, and transformed though multiple quaility settings by people who just do not understand anything about JPEG or general lossiness of image transformations.

The only thing you can do is to try get the image about the same size
(with directly, or by matching up some sort of markers), then do a comparision
of the image to find out how different they are.

Comaprision of edge detected images may be an even better way of determining simularity.

If you have any ideas of image comparision I would love to here of them. This is a long term interest of mine. Some of my notes are located in http://www.cit.gu.edu.au/~anthony/graph ... e/#compare
and http://www.cit.gu.edu.au/~anthony/info/ ... _comparing