Identify a mosaic picture

Question

I come to you with a strange question today. How would you distinguish programmaticaly a mosaic picture from a normal one ? e.g, this 2 pictures:

enter image description here

Of course, the format of the mosaic picture will often change. Our eyes can distinguish them pretty easily, but how would you do programmaticaly ?

This is not going the be easy in the general case, but it can be quite easy if your mosaic is well defined in format, i.e. you could check that your mosaic image has certain row and colum pixels white, which is a very simple algorithm. If you want a more general algorithm, could probably try line detectors, etc. but it is going to be a very hard thing and almost impossible to get a 100% detection rate. Which is your case? — drodri, Mar 27 '14 at 21:19
That's the problem, the mosaic will not always be defined in format. I basically crawl thepiratebay on each torrent of a category, and download each picture in each post. I then need to identify which one is the mosaic, to display it. The thing is not every user formats his pictures in the same way. But I saw http://stackoverflow.com/questions/11424002/how-to-detect-simple-geometric-shapes-using-opencv, which shows how to identify basic shapes, and a mosaic is composed of a defined number of squares: 4, 6, 9, etc. Could it work if I apply a basic threshold on the mosaic ? — JPFrancoia, Mar 28 '14 at 08:35

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