I think it's usually enough to compare the images byte-to-byte. You can do this by storing the md5 sums of all the images somewhere, and when someone wants to upload an image, calculate its md5 sum, compare to all existing ones, and if it's new, add the image and store the md5 sum.

You can calculate md5 sums either with the Digest::MD5 module (which is now distributed with perl) or with the md5sum program found on most unices, or with the GnuPG program.

If you really want to compare the images pixel-by-pixel, that is, compare different formats of the same image as equal, then you have to recompress all the images to a certain format of your choice (also removing all metadata). I don't really think this is really neccessary.

In reply to Re: Comparing images by ambrus
in thread Comparing images by Anonymous Monk

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