Using Haar cascades to detect things

When we say Haar cascades, we are actually talking about cascade classifiers based on Haar features. To understand what this means, we need to take a step back and understand why we need this in the first place. Back in 2001, Paul Viola and Michael Jones came up with a very effective object detection method in their seminal paper. It has become one of the major landmarks in the field of machine learning.

In their paper, they have described a machine learning technique where a boosted cascade of simple classifiers is used to get an overall classifier that performs really well. This way, we can circumvent the process of building a single complex classifier that performs with high accuracy. The reason this is so amazing is because building a robust single-step classifier is a computationally intensive process. Besides, we need a lot of training data to build such a classifier. The model ends up becoming complex and the performance might not be up to the mark...