Use your Arduino and two Potentiometers for drawing on your Processing canvas.

Inspired by the legendary Etch-A-Sketch, two knobs are used to move a drawing tool in the X and Y directions on the computer screen. If you want to draw a new picture, simply shake the base plate and the canvas will be "emptied".

As in the previous example, the program StandardFirmata must first be installed on the Arduino board an you'll have to select the right port in Processing again (see #15, Processing RGB)