Conceptualizing Haar cascades

When we talk about classifying objects and tracking their location, what exactly are we hoping to pinpoint? What constitutes a recognizable part of an object?

Photographic images, even from a webcam, may contain a lot of detail for our (human) viewing pleasure. However, image detail tends to be unstable with respect to variations in lighting, viewing angle, viewing distance, camera shake, and digital noise. Moreover, even real differences in physical detail might not interest us for classification. Joseph Howse, one of this book's authors, was taught in school that no two snowflakes look alike under a microscope. Fortunately, as a Canadian child, he had already learned how to recognize snowflakes without a microscope, as the similarities are more obvious in bulk.

Hence, having some means of abstracting image detail is useful in producing stable classification and tracking results. The abstractions are called features, which are said to be extracted from...