Comparing Images

A practical problem that I have is to sort my digital photos. Some of them have been taken with analog cameras and they have been scanned. Some of them have been scanned several times, using different resolution, different providers or different technologies.

So one issue that occurs is having two directories which contain more or less the same images from different scans.

Some of them have been sorted or enriched with useful information or just rotated correctly, others have better resolution. Maybe it is good to use different scans to create a better image than the best scan.

For a few hundred films doing that manually is possible, but a lot of work. So I thought about automating it, at least partially.

There are several issues that make it more difficult. Scans are not very accurate, so they cut of a tiny random bit in the borders. So to match two images exactly it is necessary to scale and crop them.

Colors look quite different. And then of course resolutions are different. And sometimes they have been turned by an angle of 90 or 270 degrees. Some scans miss a few images that another one has.

So, how to start?

First all images are scaled to thumbnails of approximately 65536 pixels. It turns out to be 204×307, but could of course be anything around that size retaining the rough aspect ratio.

Now all thumbnail images from the two directories are read into memory. 80 thumbnail images are no big deal…

Images in portrait orientation are rotated by 90 and 270 degrees and both variants are used. So from here on all images are in landscape format. Upside down is not supported.

All images are scaled to exactly 204×307 in memory to allow comparison.

And the average r, g and b-values from all images from each directory are calculated. The r,g,b values of each pixel are multiplied or divided by a factor, which is the square root of the quotient of these averages and constrained to the range 0..255. So this partially neutralizes the effect of different colors.

Now for each thumbnail from the first directory is compared with each thumbnail from the second directory. This is done by calculating for each pixel the sum of the squares of the differences between the r, g and b values of the two images.

These values are added up and divided by the number of pixels (204×307 in my case). The square root of this is the comparison result. For images that are actually the same, comparison results between 30 and 120 are possible. Now some heuristic is used to find matches based on these values. Also an „interpolation“ is used, if consecutive images in both directories occur with the middle one missing. This usually brings good results. In some cases manual interaction is needed. So it is possible to mark images in an web interface and then the match that was found for them is revoked. Also it is possible to provide „hints“, which means that these images should be matched.

It took about a day and half to write this and it works sufficiently well for the purpose.

But there are much more sophisticated algorithms that really recognize objects. I have a program called hugin that combines a few images to a panorama. Usually it works quite well and it can even rotate, shift and scale images and do amazing things most of the time. Sometimes it just does not work with certain images. Also Google has of course very powerful software to recognize images and compare them and even create panoramas. If we thing about face recognition… There is really good stuff around. But the first approach with some improvements gave sufficiently good results and the program will only run a couple of hundred times, then it will not be needed anymore.

This is heavily inspired by the Perl-library Image::Compare, but since I am doing something slightly different, I do not use this library any more. But subsequently it has been written in Perl. I will happily provide my source code, but I have baked into it assumptions and conventions that I have introduced myself, but that are not universal, concerning the file names and directory structures for the images.

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