Among the many reasons that make computer vision a fascinating subject is the fact that computer vision makes very futuristic-sounding tasks a reality. One such feature is face detection. OpenCV has a built-in facility to perform face detection, which has virtually infinite applications in the real world in all sorts of contexts, from security to entertainment.

This chapter introduces some of OpenCV's face detection functionalities, along with the data files that define particular types of trackable objects. Specifically, we look at Haar cascade classifiers, which analyze contrast between adjacent image regions to determine whether or not a given image or subimage matches a known type. We consider how to combine multiple Haar cascade classifiers in a hierarchy, such that one classifier identifies a parent region (for our purposes, a face) and other classifiers identify child regions (eyes, nose, and mouth).

We also take a detour into the humble but...