OpenCV comes preinstalled with a range of sophisticated classifiers for general-purpose object detection. Perhaps, the most commonly known detector is the cascade of Haar-based feature detectors for face detection, which was invented by Paul Viola and Michael Jones.
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), which sum up the pixel intensities in small regions of an image, as well as capture the difference between adjacent image regions. Some example rectangle features are shown in the following figure, relative to the enclosing (light gray) detection window:
Here, the top row shows two examples of an edge feature, either vertically oriented (left) or oriented at a 45 degree angle (right...