Detect empty shelves using image processing

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Given an image, how can I create an algorithm using image processing techniques to identify the sections where there are no products present. I also need to create a bounding with coordinates for the empty spaces where products are not present. I need to accomplish this using OpenCV. And I cannot use Deep Learning here.

I have used a Canny edge detector, and empty spaces are well identified using this. Should I use a Contour on the results of the Canny edge detector? Any help would be appreciated.