Descriptors describe local image regions and are invariant to image transformations such as rotation, scale or translation. They provide a measure and distance function for a small patch around an interest point. Therefore, whenever the similarity between two image patches needs to be estimated, we compute their descriptors and measure their distance. In OpenCV, the basic Mat type is used to represent a collection of descriptors, where each row is a keypoint descriptor.

There are the following two possibilities to use a feature descriptor extractor:

(See the following diagram where the descriptors used in this chapter are indicated in red color.)

The common interface allows us to switch easily between different algorithms. This can be very useful when choosing an algorithm to solve a problem, as the results of each algorithm can be compared with no effort. On the other hand, depending on the algorithm...