For the current block in Fig. 1, is not available until the ME process has been performed across the whole search area, so we must set a predictor for before it can be used. The influence of this predictor on coding quality and efficiency is briefly analyzed as follows. In Eq. (2), if a larger predictor is selected for , a smaller Dif value will be obtained. According to Fig. 2, this should result in a smaller SR size. Although a higher coding speed can be achieved with a smaller SR, the coding quality may be lost if the true MV falls outside this smaller SR. In contrast, a smaller predictor gives a larger SR and a higher coding quality. Hence, our purpose is not to get an accurate estimate for but to achieve a tradeoff between the coding quality and the coding efficiency. Therefore, we replace with a smaller predictor: Display Formula
(3)where , , , and are the SADs computed from the MVs of the spatially neighboring blocks (A is the left block, B is the upper block, C is the upper-right block) and the colocated block (E).