Haar-like feature

A publication by Papageorgiou et al.[2] discussed working with an alternate feature set based on Haar wavelets instead of the usual image intensities.

Paul Viola and Michael Jones[1] adapted the idea of using Haar wavelets and developed the so-called Haar-like features.

Therefore, a common Haar feature for face detection is a set of two adjacent rectangles that lie above the eye and the cheek region.

The position of these rectangles is defined relative to a detection window that acts like a bounding box to the target object (the face in this case).

A simple rectangular Haar-like feature can be defined as the difference of the sum of pixels of areas inside the rectangle, which can be at any position and scale within the original image.

This was used to increase the dimensionality of the set of features in an attempt to improve the detection of objects in images.

Although the idea is sound mathematically, practical problems prevent the use of Haar-like features at any angle.

In order to be fast, detection algorithms use low resolution images introducing rounding errors.