Comparing bitmap images for differences?

[Terry Reedy]

<lanwrangler at gmail.com> wrote in message 
news:1178024505.768505.262270 at h2g2000hsg.googlegroups.com...
|I know it's a long shot but does anyone have any pointers to generic
| algorithms - or, even better, Python code - for comparing images and
| computing a value for the "difference" between them?

If PIL and the other posted ideas are not enough, another approach is to 
convert the image from PIL format to NumPy array format.  (pygame has 
functions to do this, I believe, since it uses NumPy (actually the older 
Numerical Python at present) for its surface arrays.)  You can then do most 
any calculation you want.

tjr