What are integral images?

In order to extract these Haar features, we need to calculate the sum of the pixel values enclosed in many rectangular regions of the image. To make it scale invariant, we need to compute these areas at multiple scales (that is, for various rectangle sizes). If 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 this, we can use something called integral images. These images can be initialized in a linear time (by iterating only twice over the image) and can then be provided with the sum of pixels inside any rectangle of any...