Skimming “Build OpenCV Mini-Projects” by PyImageSearch: Contours

Getting a taste of OpenCV color operations were interesting, but I didn’t really understand what made OpenCV more powerful than other image processing libraries until we got to contours, which covers most of the second half of PyImageSearch’s Start Here guide Step 4: Build OpenCV Mini-Projects.

This section started with an example for finding the center of a contour, which in this case is examining a picture of a collection of non-overlapping paper cut-out shapes. The most valuable concept here is that of image moments, which I think of as a “summary” for a particular shape found by OpenCV. We also got names for operations we’ve seen earlier. Binarization operations turn an image into binary yes/no highlight of potentially interesting features. Edge detection and thresholding are the two we’ve seen.

Things get exciting when we start putting contours to work. The tutorial starts out easy by finding the extreme points in contours, which breaks down roughly what goes on inside OpenCV’s boundingRect function. Such code is then used in tutorials for calculating size of objects in view which is close to a project idea on my to-do list.

A prerequisite for that project is code to order coordinates clockwise, which reading the code I was surprised to learn was done in cartesian space. If the objective is clockwise ordering, I thought it would have been a natural candidate for processing in polar coordinate space. This algorithm was apparently originally published with a boundary condition bug that, as far as I can tell, would not have happened if the coordinate sorting was done in polar coordinates.

These components are brought together beautifully in an example document scanner application that detects the trapezoidal shape of a receipt in the image and performs perspective correction to deliver a straight rectangular image of the receipt. This is my favorite feature of Office Lens and if I ever decide to write my own I shall return to this example.

By the end of this section, I was suitably impressed by what I’ve seen of OpenCV, but I also have the feeling a few of my computer vision projects would not be addressed by the parts of OpenCV covered in the rest of PyImageSearch’s Start Here guide.

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