Most of the graphics drawing routines require information about how the
drawing is to take place.  For this reason, most graphics drawing routines
use a data structure called a RastPort, that contains pointers to the
drawing area and drawing variables such as the current pen color and font
to use.  In general, you pass a pointer to your RastPort structure as an
argument whenever you call a drawing function.

 The RastPort Structure 
 Using the Graphics Drawing Routines 
 Performing Data Move Operations