'Finding similar images with different intensities/brightness

Suppose I have images as follows:

What would be my options to compare the similarity between the two images? Obviously they are the same image just with different brightness. I couldn't find any plausible way for this and currently my best bet would be to train a cnn or autoencoder and compare the feature vectors of the outputs, but that just seems a bit overkill for this. Any tips would be appreciated.

Solution 1:^[1]

Pretty robust working solution (I tested) is to check correlation of brightness sign changes between pixels.

I.e. assuming images A and B, loop for significant number of pixels:

IF (
  (brightness of pixel 1 from A IS LARGER than brightness of pixel 2 from A)
  AND
  (brightness of pixel 1 from B IS LARGER than brightness of pixel 2 from B)
) {
  COUNTER++;
}

And vice versa for opposite relationship. The higher the COUNTER the more similar are the images.

IMPORTANT: I tested the method on (inter-area) scaled-down images, not full size, as full size images may contain some compression artifacts. My intuition suggests it will work on full size images anyway, just with a different threshold. If not, resize by area to preserve well average brightness values (similar to INTER_AREA in OpenCV), and it will do the trick.

Related ideas can be found at image comparison algorithm.

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Solution 1

'Finding similar images with different intensities/brightness

Solution 1:[1]

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Solution 1:^[1]