Introduction - If you have any usage issues, please Google them yourself
This program creates a rectangle when you click the mouse on the window, and stretches the rectangle until you release the mouse. This requires the creation of a mouse callback. For the main function we use a while loop that keeps redrawing each box as its created. Boxes are drawn and windows are created as learned in previous tutorials. The main new thing is the creation of the mouse callback. This is initialized with cvSetMouseCallback. The callback itself is an arbitrarily named function of the form my_mouse_callback( int event, int x, int y, an int flags, void* param) where x and y are the mouse position and event is a code representing what mouse action occured