Cascaded Direction Filtering for Fast Multidirectional Inter-Prediction in H.264/AVC Main and High Profile Compression

Early direct mode decision is a popular technique to improve the execution speed of inter-predictions in a B slice. However, the improvement is limited when this technique is applied to a hardware-based pipelined architecture or to the video sequences where the ratio of macroblocks (MBs) encoded as the direct mode is low. This paper proposes a novel fast inter-prediction algorithm for B slices which increases the encoding speed by early decision of the direction of the inter-prediction using spatial and temporal correlation in a video. A speed-up is achieved by selecting a prediction direction with a lower complexity and discarding a prediction direction with a higher complexity. For the case when early mode decision is not possible, additional speed-up is achieved by partially skipping unidirectional motion estimation (ME) if the estimated cost of the ME is greater than the result of the direct mode prediction that is completed much faster than ME. This proposed sequence of early decisions is referred to as cascaded direction filtering (CDF). The proposed algorithm increases the early mode decision rate, which improves the encoding speed effectively for a hardware-based pipelined architecture as well as videos with a low ratio of MBs encoded as the direct mode. The additional memory size and bandwidth overhead required for the proposed CDF are very small. Experimental results show that the proposed CDF scheme improves the encoding speed by 45% for B slices and 32% for overall sequences. The bitrate is increased by less than 1% with a small peak signal-to-noise ratio degradation of 0.03 dB on average.