The M1BT method performs well when either or is located near , where represents the optimal motion vector determined by the FS method. This is usually the case, but sometimes, both and can become quite large. On the other hand, all of the search points in the M1BT method are located within very small areas around and . To be more precise, for every search point around , is always less than or equal to 4, as can be seen in Fig. 1. The same is true for and every search point around . Thus, when both and are quite large, will also be quite large, leading to significant PSNR degradation, where represents the final motion vector determined by the M1BT method.