RD Optimized Coding for Motion Vector Predictor Selection

The H.264/MPEG4-AVC video coding standard has achieved a higher coding efficiency compared to its predecessors. The significant bitrate reduction is mainly obtained by efficient motion compensation tools, as variable block sizes, multiple reference frames, 1/4-pel motion accuracy and powerful prediction modes (e.g., SKIP and DIRECT). These tools have contributed to an increased proportion of the motion information in the total bitstream. To achieve the performance required by the future ITU-T challenge, namely to provide a codec with 50% bitrate reduction compared to the current H.264, the reduction of this motion information cost is essential. This paper proposes a competing framework for better motion vector coding and SKIP mode. The predictors for the SKIP mode and the motion vector predictors are optimally selected by a rate-distortion criterion. These methods take advantage from the use of the spatial and the temporal redundancies in the motion vector fields, where the simple spatial median usually fails. An adaptation of the temporal predictors according to the temporal distances between motion vector fields is also described for multiple reference frames and B-slices options. These two combined schemes lead to a systematic bitrate saving on Baseline and High profile, compared to an H.264/MPEG4-AVC standard codec, which reaches up to 45%.