An image processing python code demonstarting basic thing that can be done with haarcascades.
Haarcascade : Intro
Object Detection using Haar feature-based cascade classifiers is an effective object detection method proposed by Paul Viola and Michael Jones in their paper, "Rapid Object Detection using a Boosted Cascade of Simple Features" in 2001. It is a machine learning based approach where a cascade function is trained from a lot of positive and negative images. It is then used to detect objects in other images.
Since, it's its a machine learning algorithm and the harcascade used here is basic, its not detecting some of the faces in the picture.