New Bi-prediction Techniques for B Pictures Coding

In the conventional video codec, B pictures usually employ five prediction modes, including bi-prediction, direct prediction, forward prediction, backward prediction and intra modes. Among them, the direct and bi-prediction modes, whose prediction values are achieved from both forward and backward reference pictures, are more efficient in exploiting the temporal correlation between the reference pictures and the current B picture. In addition, the direct mode does not require any bits for coding the motion vectors. Therefore, the blocks coded with direct or bi-predictive mode are usually much more than the other modes. To further take advantage of the bi-prediction, in this paper we propose a new bi-predictive coding technique, which can achieve the good tradeoff between the bit-rate saving for motion vector coding and the prediction accuracy. Moreover, we also propose the spatial motion vector prediction and motion vector scaling techniques to improve the accuracy of derived direct mode motion vectors. All these techniques have been adopted in AVS video coding standard.