The applet below is a demonstration of an optical illusion. In its original form as conceived by professor Misha Pavel of the Oregon Graduate Institute, a rotating square is occluded by four other squares that leave a cross-like opening to view the motion. Look at the applet. The impression you are supposed to get is that the square periodically expands and then shrinks and then expands again. A similar illusion is achieved when a rotating triangle is being occluded by six triangles forming a hexagon (Suggested by Arthur van Houdt.) The occluding shape may also consist of 4 or 5 triangles and matching rotating shapes. When occluding and rotating shapes do not quite match the effect is very different.
