The effect of shadows in Figure 4 is a very interesting problem. Normally we will consider the shadows as background layers,but when the object is moving under these shadows, the shadow would not be in the back of the object, instead, it will become a occluding background layer before the moving object and not completely occlud it. In this situation, the background layer model did help to solve this problem, because it can dynamically change the layer order, and assign a background layer along with a foreground layer to this semi-occluded object.