Is there any simple way to test two PNGs for equality?

Question

I've got a bunch of PNG images, and I'm looking for a way to identify duplicates. By duplicates I mean, specifically, two PNG files whose uncompressed image data are identical, not necessarily whose files are identical. This means I can't do something simple like compare CRC hash values.

I figure this can actually be done reliably since PNGs use lossless compression, but I'm worried about speed. I know I can winnow things down a little by testing for equal dimensions first, but when it comes time to actually compare the images against each other, is there any way to do it reasonably efficiently? (ie. faster than the "double-for-loop checking pixel values against each other" brute-force method?)

Answer 1

1. filter by identical image size (width & height)
2. open file
3. hash uncompressed contents (md5 would do probably)

4. store hash

5. compare hashes to find identical ones

Answer 2

Instead of looping through all pixels in order to check equality, it might be worth while starting from the middle and working your way outwards. Most pictures have the subject in the middle meaning more feature data is located here. Essentially it will be lots quicker to find out if two pictures are different this way.