Computer Science Theory: Image Similarity

So I'm trying to run a comparison of different images and was wondering if anyone could point me in the right direction for some basic metrics I can take for the group of images.

Assuming I have two images, A and B, I pretty much want as much data as possible about each so I can later programmatically compare them. Things like "general color", "general shape", etc. would be great.

If you can help me find specific properties and algorithms to compute them that would be great!

Thanks!

EDIT: The end goal here is to be able to have a computer tell me how "similar" too pictures are. If two images are the same but in one s开发者_运维百科omeone blurred out a face; they should register as fairly similar. If two pictures are completely different, the computer should be able to tell.

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What you are talking about is way much general and non-specific.

Image information is formalised as Entropy.

What you seem to be looking for is basically feature extraction and then comparing these features. There are tons of features that can be extracted but a lot of them could be irrelevant depending on the differences in the pictures.

There are space domain and frequency domain descriptors of the image which each can be useful here. I can probably name more than 100 descriptors but in your case, only one could be sufficient or none could be useful.

Pre-processing is also important, perhaps you could turn your images to grey-scale and then compare them.

This field is so immensely diverse, so you need to be a bit more specific.

(Update)

What you are looking for is a topic of hundreds if not thousands of scientific articles. But well, perhaps a simplistic approach can work.

So assuming that the question here is not identifying objects and there is no transform, translation, scale or rotation involved and we are only dealing with the two images which are the same but one could have more noise added upon it:

1) Image domain (space domain): Compare the pixels one by one and add up the square of the differences. Normalise this value by the width*height - just divide by the number of pixels. This could be a useful measure of similarity.

2) Frequency domain: Convert the image to frequency domain image (using FTT in an image processing tool such as OpenCV) which will be 2D as well. Do the same above squared diff as above, but perhaps you want to limit the frequencies. Then normalise by the number of pixels. This fares better on noise and translation and on a small rotation but not on scale.

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SURF is a good candidate algorithm for comparing images

Wikipedia Article

A practical example (in Mathematica), identifying corresponding points in two images of the moon (rotated, colorized and blurred) :

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You can also calculate sum of differences between histogram bins of those two images. But it is also not a silver bullet...

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I recommend taking a look at OpenCV. The package offers most (if not all) of the techniques mentioned above.