So far, one thing has held true about what we've done on the canvas. Once a pixel makes it way to the screen, that's it. Everything about how it got there, what properties make up the larger shape we were intending to draw, and so on are completely lost. Do you want to modify whatever it is you just drew? Well...you can't. The best you can do is clear the screen and draw a whole lot of new pixels instead. Fortunately (or unfortunately) this behavior is a characteristic of an immediate mode system:
This is a companion discussion topic for the original entry at https://www.kirupa.com/canvas/from_pixels_to_objects.htm