Using a pre-trained detector makes it easy to build a quick prototype, and we are all very grateful to the OpenCV developers for making such useful capabilities as face detection and people detection readily available. However, whether you are a hobbyist or a computer vision professional, it is unlikely that you will only deal with people and faces.
Moreover, if you are like the authors of this book, you will wonder how the people detector was created in the first place and whether you can improve it. Furthermore, you may also wonder whether you can apply the same concepts to detect diverse objects, ranging from cars to goblins.
Indeed, in industry, you may have to deal with problems of detecting very specific objects, such as registration plates, book covers, or whatever thing may be most important to your employer or client.
Thus, the...