What are integral images?
In order to extract these Haar features, we will have to calculate the sum of the pixel values enclosed in many rectangular regions of the image. To make it scale-invariant, we are required to compute these areas at multiple scales (for various rectangle sizes). Implemented naively, this would be a very computationally-intensive process; we would have to iterate over all the pixels of each rectangle, including reading the same pixels multiple times if they are contained in different overlapping rectangles. If you want to build a system that can run in real-time, you cannot spend so much time in computation. We need to find a way to avoid this huge redundancy during the area computation because we iterate over the same pixels multiple times. To avoid it, we can use something called integral images. These images can be initialized at a linear time (by iterating only twice over the image) and then provide the sum of the pixels inside any rectangle of any size by reading...
