Constructing Basic Shapes in OpenCV

One basic functionality offered by OpenCV is drawing basic shapes. OpenCV provides functions to draw lines, circles, rectangles, ellipses, and so on. When building a computer vision project, you usually want to modify the image by drawing some shapes. For example, if you develop a face detection algorithm, you should draw a rectangle highlighting the detected faces in the computed image. Additionally, if you develop a face recognition algorithm, you should draw a rectangle highlighting the detected faces and also write some text showing the identity of the detected faces. And finally, it is a common approach to write some debugging information. You could show, for example, the number of detected faces (in order to see the performance of your face detection algorithm) or the processing time. In this chapter, you are going to see how to draw basic and more advanced shapes using the OpenCV library. 

The following topics will be covered:

• A theoretical introduction to drawing in OpenCV
• Basic shapes—lines, rectangles, and circles
• Basic shapes (2)—clip and arrowed lines, ellipses, and polylines
• Drawing text
• Dynamic drawing with mouse events
• Advanced drawing