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Define similarity. That sounds flippant, but it's the true problem here. If you can define, in a mathematical sense what "similar" means, then it's easy to find the answer, and you don't need our help.

If you can't define what "similar" means, then nobody can help you, because there are far, far too many different ways of measuring that.

The simplest measure is to do an absdiff and sum the result, but this assumes they are perfectly aligned. It's possible to make images that appear identical, but are the worst possible match by this measure.

It would help if you can share a sample pair of images so we know what it is you're comparing. Then we could maybe make suggestions.