As we can see on the pictures above, the Otsu thresholding did its work quite well - we got black quadrilaterals surrounded by white areas. Using blob counter it is possible to find all the black objects on the above binary images, perform some checks to make sure these objects are quadrilaterals, etc. etc. It is really possible to get everything working starting from this point, however it may have some issues. The problem is that Otsu thresholding worked for the above images and it actually works for many other images. But not for all of them. Here is one of the images, where it does not work as supposed and all the idea fails.