The screen has only one
canvas attached to it. This is canvas number 0 and all drawing
operations in this canvas are applied to the screen.
Other canvases can be
opened and drawing operations can be performed in them. Drawing
can be performed in one of these 'invisible' canvases and then
copied to the screen.
The reason for doing this
is that images can be built without the user having any knowledge
of them. They can then be copied to the screen in one move.
Copying the image is probably
much faster than building it. This means that if the same image
needs to be drawn to the screen repeatedly it can be stored in
a background canvas and copied to the screen as often as it is