Originally posted by Steven Bell:
There is the java.awt.Robot.getPixelColor(int x, int y). It returns a Color object. I'm not sure how it will work in an applet though.
Rather than trying to detect where an empty space is it might be easier, and less expensive, to hold all the drawn objects and redraw from scratch on each 'frame', if that is close to what you are doing. Then you would determine what is empty by keeping track of where you have drawn.