Hand shape analysis

Now that we know (roughly) where the hand is located, we aim to learn something about its shape.

Determining the contour of the segmented hand region

The first step involves determining the contour of the segmented hand region. Luckily, OpenCV comes with a pre-canned version of such an algorithm—cv2.findContours. This function acts on a binary image and returns a set of points that are believed to be part of the contour. As there might be multiple contours present in the image, it is possible to retrieve an entire hierarchy of contours:

def _find_hull_defects(self, segment):
    contours, hierarchy = cv2.findContours(segment, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

Furthermore, because we do not know which contour we are looking for, we have to make an assumption to clean up the contour result. Since it is possible that some small cavities are left over even after the morphological closing—but we are fairly certain that our mask contains only the segmented area of interest...