In this chapter, we will discuss feature detectors and descriptors, along with various applications of different types of feature detectors/extractors in image processing. We will start by defining feature detectors and descriptors. We will then continue our discussion on a few popular feature detectors such as Harris Corner/SIFT and HOG, and then their applications in important image processing problems such as image matching and object detection, respectively, with scikit-image and python-opencv (cv2) library functions.

 The topics to be covered in this chapter are as follows:

• Feature detectors versus descriptors, to extract features/descriptors from images
• Harris Corner Detector and the application of Harris Corner features in image matching (with scikit-image)
• Blob detectors with LoG, DoG, and DoH (with scikit-image)
• Extraction of Histogram of Oriented Gradients features
• SIFT, ORB, and BRIEF features and their application in image matching
• Haar-like features and their application in face detection