Most existing background subtraction algorithms fail to work with low-bandwidth compressed videos mainly due to spatial block compression that causes block artifacts, and temporal block compression that causes abnormal distribution of encoding (random spikes). Fig. 4(a) is an image extracted from an MPEG video encoded at 70 kbits/s. Fig. 4(b) depicts 20-times scaled image of the standard deviations of blue(B)-channel values in the training set. It is easyto see that the
distribution of pixel values has been affected bythe blocking effects of MPEG. The unimodal model in Fig. 4(c) suffers from these effects. For the compressed video, CB eliminates most compression artifacts—see Figs. 4(c)–(f).