It's time to pick a simple example to gain more confidence! I am able to finish 'drawing.c' example. It goes through the openCV drawing functions, lines, rectangles,... They are all pretty rich APIs, able to go beyond the basic drawing primitivesAPI. The PolyLine API is a bit difficult to understand at the beginning. Turns out it is able to draw a number of contours with a single API. The angle arguments of ellipse are confusing.
OpenCV uses a 'liberal font' called Hershey. It's a vector font. Interesting story behind this:
The example uses the random number generator APIs given by OpenCV. Equally impressive, it is able to specify distribution type (Uniform or Normal). Useful for generating an array of numbers, such as 'noisy screen'.