Extended direct mode for hierarchical B picture coding
In 3D subband coding, motion-compensated temporal filtering (MCTF) technique, especially with the adaptive 5/3 wavelet kernel, efficiently exploits the temporal correlation among neighboring pictures. It can be achieved alternatively by the hierarchical B picture coding technique with the same decomposition as the temporal 5/3 lifting structure but without update step. This technique has been supported by H.264/AVC (2003) with stored B technique. The performance of hierarchical B obviously outperforms that of the conventional IBBP...GOP structure. Direct mode which derives motion vectors from the co-located block of backward reference and does not require any bits for coding motion vectors is a very efficient bi-prediction technique for B pictures coding. Considering that the forward motion vectors in the backward reference is not available when it belongs to block only with backward motion vector, in this paper, we propose an extended direct mode for hierarchical B picture coding to further improve the accuracy of derived motion vectors. It effectively utilizes the forward and backward motion vectors of the co-located blocks in references for scaling.