Comparing images with different resolutions

Question

Is there possible to compare two images with different resolutions?

I mean here some algorithmic/programming approach. For instance, now I calculate hash co

Answer 1

For synthetic images with a few distinct colours I would start with histogram matching.

Basically add up the number of pixels of each colour in each image and divide by the total number of pixels. Then you have a simple float vector as a fingerprint. You can ignore white if you want images with more or less border to count as a match

It's not going to detect the same image with the slices re-arranged, or the text moved down a line but i don't think that is the concern in this case

Answer 2

Two very simple perceptual hashing methods you might give a try before venturing into more complicated territory are based on the Discrete Cosine Transform and the local vs glocal mean of an image:

1. Convert image to grayscale

   1.1 (EDIT) Make your image zero mean

2. Crush your image down to thumbnail size, say [32x32]
3. Run the two dimensional Discrete Cosine Transform
4. Keep the top left [8 x 8], most significant low frequency components
5. Binarize the block, based on the sign of the components
6. Result is a 64 bit hash

And a variant on this theme would be

1. Convert image to grayscale
2. Optionally re-size to a predefined size.
3. Partition the image in a fixed number of blocks
4. Determine the global mean
5. Determine the local mean per block
6. For the hash, write out a 1 or a 0 per block, pending if the local mean was larger or smaller than the global mean.

Also, have a look at phash.