OpenCV 4 with Python Blueprints - Second Edition

Learning about Haar-based cascade classifiers

Every book on OpenCV should at least mention the Viola-Jones face detector. Invented in 2001, this cascade classifier disrupted the field of computer vision, as it finally allowed real-time face detection and face recognition.

The classifier is based on Haar-like features (similar to Haar basis functions) that sum up the pixel intensities in small regions of an image, as well as capture the difference between adjacent image regions.

The following screenshot visualizes four rectangle features. The visualization works to calculate the value of the feature applied at a location. You should sum up all pixel values in the dark gray rectangle and subtract this value from the sum of all pixel values ...