RD Optimized Coding for Motion

The H.264/MPEG4-AVC video coding standard has achieved a higher coding efficiency compared to its predecessors. The significant bit rate reduction is mainly obtained by efficient motion compensation tools, as variable block sizes, multiple ref- erence 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 bit stream. To achieve the performance required by the future ITU-T challenge, namely to provide a codec with 50% bit rate reduction compared to the current H.264, the reduction of this motion infor- mation 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 be- tween motion vector fields is also described for multiple reference frames and B-slices options. These two combined schemes lead to a systematicbitratesaving onbaselineandhighprofile, comparedto an H.264/MPEG4-AVC standard codec, which reaches up to 45%. allowed to improve the motion compensation efficiency. This has generated a compression gain with an increase of the pro- portion of bits dedicated to the motion information which can reach up to 40% of the total bit rate. Moreover, efficient non normative choices (2) based on rate-distortion schemes have been integrated in the reference software (3). These schemes give the optimal choices, in the rate distortion sense, among many new competing encoding modes. With the finalization of the H.264 standardisation, the Video Coding Expert Group (VCEG/ITU-T SG16 Q6) has a new challenge, namely, to provide a 50% compression gain for an H.264 equivalent quality. As it was proposed for H.264/AVC Key Technical Area (KTA) software in (4), more accurate motion models seem to be an interesting approach for this challenge with probably a larger bit rate reduction of luminance block residue. This paper proposes the use of competing coding tech- niques to improve these schemes: first, a competition-based spatio-temporal scheme for the prediction of motion vectors is introduced, including a modification of the rate-distortion criterion (RD-criterion). Second, the amount of skipped mac- roblocks is increased using a competition-based SKIP mode. These two modifications are implemented and tested into the H.264 reference software for the recommended Baseline and High profile. Moreover these schemes are now included (5) in the KTA software. The remaining of this paper is organized as follows: a state of the art on motion vector coding with a summary of H.264 mo- tion vector selection and coding are presented in Section II. The competing framework and the proposed predictors for the mo- tion vectors and SKIP mode are described in Section III, espe- ciallytheadaptationofthesepredictorsforreferenceframesand for B-slices. Section IV comments the impact of the proposed method on the complexity. Finally Section V presents simula- tion results for Baseline and High profile in which the average compression gains are respectively 7.7% and 4.3% (and up to 45% for one of the 720p test sequences) with equivalent quality to H.264.