Tammy Lee

Block Partitioning Structure in the HEVC Standard

High Efficiency Video Coding (HEVC) is the latest joint standardization effort of ITU-T WP 3/16 and ISO/IEC JTC 1/SC 29/WG 11. The resultant standard will be published as twin text by ITU-T and ISO/IEC; in the latter case, it will also be known as MPEG-H Part 2. This paper describes the block partitioning structure of the draft HEVC standard and presents the results of an analysis of coding efficiency and complexity. Of the many new technical aspects of HEVC, the block partitioning structure has been identified as representing one of the most significant changes relative to previous video coding standards. In contrast to the fixed size 16 × 16 macroblock structure of H.264/AVC, HEVC defines three different units according to their functionalities. The coding unit defines a region sharing the same prediction mode, e.g., intra and inter, and it is represented by the leaf node of a quadtree structure. The prediction unit defines a region sharing the same prediction information. The transform unit, specified by another quadtree, defines a region sharing the same transformation. This paper introduces technical details of the block partitioning structure of HEVC with an emphasis on the method of designing a consistent framework by combining the three different units together. Experimental results are provided to justify the role of each component of the block partitioning structure and a comparison with the H.264/AVC design is performed.

Improved Video Compression Efficiency Through Flexible Unit Representation and Corresponding Extension of Coding Tools

This paper proposes a novel video compression scheme based on a highly flexible hierarchy of unit representation which includes three block concepts: coding unit (CU), prediction unit (PU), and transform unit (TU). This separation of the block structure into three different concepts allows each to be optimized according to its role; the CU is a macroblock-like unit which supports region splitting in a manner similar to a conventional quadtree, the PU supports nonsquare motion partition shapes for motion compensation, while the TU allows the transform size to be defined independently from the PU. Several other coding tools are extended to arbitrary unit size to maintain consistency with the proposed design, e.g., transform size is extended up to 64 × 64 and intraprediction is designed to support an arbitrary number of angles for variable block sizes. Other novel techniques such as a new noncascading interpolation Alter design allowing arbitrary motion accuracy and a leaky prediction technique using both open-loop and closed-loop predictors are also introduced. The video codec described in this paper was a candidate in the competitive phase of the high-efficiency video coding (HEVC) standardization work. Compared to H.264/AVC, it demonstrated bit rate reductions of around 40% based on objective measures and around 60% based on subjective testing with 1080 p sequences. It has been partially adopted into the first standardization model of the collaborative phase of the HEVC effort.

Quadtree Based Nonsquare Block Structure for Inter Frame Coding in High Efficiency Video Coding

A concept of a quadtree based nonsquare block coding structure is presented in this paper for the emerging High Efficiency Video Coding standard, which includes a quadtree based asymmetric motion partitioning scheme and a nonsquare quadtree transform (NSQT) algorithm. Nonsquare motion partitioning in inter frame coding provides the possibility of getting more accurate prediction results by splitting one coding block into two nonsquare prediction blocks. Contrary to the traditional symmetric motion partitions (SMP), asymmetric motion partitions (AMP) are proposed to improve the coding efficiency, especially for the coding blocks with irregular object boundaries. NSQT is designed for nonsquare prediction blocks (SMP and AMP), which combines square and nonsquare transform blocks in a unified transform structure. It exploits the directional characteristic of an image block to improve the transform efficiency. The combination of nonsquare partitions and NSQT provides high coding flexibility and low implementation cost for both encoder and decoder design. Simulation results show that about 0.9%-2.8% bit-rate saving can be achieved in terms of different configurations, and subjective quality can also be improved.

Coding efficiency improvement of HEVC using asymmetric motion partitioning

In this paper, coding efficiency improvement of HEVC using asymmetric motion partitioning (AMP) is provided based on HM-6.0. AMP allows asymmetric shape partition mode of prediction unit (PU) for inter prediction. AMP improves the coding efficiency, since irregular image patterns, which otherwise would be constrained to being represented by a smaller symmetric partition, can now be more efficiently represented without requiring further splitting. For encoder speed up, additional conditions are checked before doing motion estimation for each motion partitions. If the certain conditions are met, additional motion estimation, which is main source of encoder complexity for AMP, can be skipped. Experimental results demonstrate that AMP with encoding speed-up shows 0.8% coding efficiency improvement with 14% encoding time increase. Especially for videoconference sequences, coding efficiency improvement reaches to 1.4%.

Bi-directional optical flow for improving motion compensation

New method improving B-slice prediction is proposed. By combining the optical flow concept and high accuracy gradients evaluation we construct the algorithm which allows pixel-wise refinement of motion. This approach does not require any signaling for decoder. According to tests with WQVGA sequences bit-saving of 2%–6% can be achieved using this tool.

Inter-layer intra mode prediction for scalable extension of HEVC

This paper introduces a novel inter-layer intra mode prediction method for scalable extension of High Efficiency Video Coding (HEVC) standard, which is being developed by the Joint Collaborative Team on Video Coding (JCT-VC). In HEVC and its scalable extension, thirty-five intra prediction modes are adopted to reduce the spatial redundancy of luma texture within one frame, which may introduce noticeable overhead of delivering the intra mode information in the scalable bit-stream. In this paper, the correlation of intra modes between different layers is exploited to improve the efficiency of intra mode coding in enhancement layer. The intra modes in enhancement layer are mainly predicted with the ones of collocated blocks at base layer. Two techniques are presented in this paper with some differences in terms of the derivation of intra mode predictor from base layer and the selection of three most probable modes at enhancement layer. Compared to Test Model version 1.0 of HEVC scalable extension, 0.4% and 0.1% bit-rate reductions can be achieved with technique 2 in All Intra 2x spatial scalability and All Intra 1.5x spatial scalability respectively, while technique 1 can achieve 0.2% and 0.1% reductions. There is no obvious running time increase for both encoder and decoder.

Large and various shapes block processing in HEVC

Recently, Joint Collaborative Team on Video Coding (JCT-VC) which is joint team by ITU-T SG 16 Q.6 (VCEG) and ISO/IEC JTC1/SC29/WG11 (MPEG) was established and started to define new video coding standard called as High Efficiency Video Coding (HEVC). This paper introduces block partitioning structure of HEVC standard and presents its analysis results. Among many technical aspects of HEVC, the block partitioning structure has been considered as a key factor of its significant coding efficiency improvement. Compared with the macroblock structure of the fixed size 16x16 in H.264/AVC, HEVC defines three flexible size units according to their functionalities. Coding unit (CU) defines a region sharing the same prediction scheme between spatial and temporal predictions and it is represented by the leaf node of the quadtree structure. Moreover, prediction unit (PU) defines a region sharing the same prediction information and transform unit (TU), which is specified by another quadtree, defines a region sharing the same transformation. This paper introduces technical details of the block partitioning structure of HEVC with emphasis on the consistently designed framework by combining three different units together. Provided experimental results justifies each component of the block partitioning structure.

New fast DCT algorithms based on Loeffler's factorization

This paper proposes a new 32-point fast discrete cosine transform (DCT) algorithm based on the Loefflers 16-point transform. Fast integer realizations of 16-point and 32-point transforms are also provided based on the proposed transform. For the recent development of High Efficiency Video Coding (HEVC), simplified quanti-zation and de-quantization process are proposed. Three different forms of implementation with the essentially same performance, namely matrix multiplication, partial butterfly, and full factorization can be chosen accord-ing to the given platform. In terms of the number of multiplications required for the realization, our proposed full-factorization is 3~4 times faster than a partial butterfly, and about 10 times faster than direct matrix multiplication.