Hierarchy

If you remember, in the Contours - Basic Detection and Plotting section, we saw that one of the inputs to the cv2.findContours function was named hierarchy. Now, what exactly is this thing? Well, contours can have relationships with one another. One contour might lie inside another larger contour – it will be the child of this larger contour. Similarly, a contour might even have grandchildren and great-grandchildren as well. These are called nested contours.

Let's look at the following diagram:

Figure 4.17: Total contours

How many contours do you see? 1, 2, or 3?

The answer is 3. Remember what we talked about at the start of this chapter. A contour is the boundary of a white object on a black background. The preceding image has a hollow square and a filled circle. You might be sure of the fact that the filled circle is a single blob; however, you might get confused with the hollow square. A hollow square has two outlines: the outer...