In air brush you can draw on the screen with only waiving your finger around in the air. Its actually a colour tracking project with a small color bead on the Finger.
This background is always static i.e. there is no addition or subtraction of objects in the background scene. The background-color is always constant. It does not change with time. The object that will be used for writing/painting is of a different color than the background to give us sufficient contrast between foreground and background.