Alexander Alshin

Sample Adaptive Offset in the HEVC Standard

This paper provides a technical overview of a newly added in-loop filtering technique, sample adaptive offset (SAO), in High Efficiency Video Coding (HEVC). The key idea of SAO is to reduce sample distortion by first classifying reconstructed samples into different categories, obtaining an offset for each category, and then adding the offset to each sample of the category. The offset of each category is properly calculated at the encoder and explicitly signaled to the decoder for reducing sample distortion effectively, while the classification of each sample is performed at both the encoder and the decoder for saving side information significantly. To achieve low latency of only one coding tree unit (CTU), a CTU-based syntax design is specified to adapt SAO parameters for each CTU. A CTU-based optimization algorithm can be used to derive SAO parameters of each CTU, and the SAO parameters of the CTU are inter leaved into the slice data. It is reported that SAO achieves on average 3.5% BD-rate reduction and up to 23.5% BD-rate reduction with less than 1% encoding time increase and about 2.5% decoding time increase under common test conditions of HEVC reference software version 8.0.

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.

Motion Compensated Prediction and Interpolation Filter Design in H.265/HEVC

Coding efficiency gains in the new High Efficiency Video Coding (H.265/HEVC) video coding standard are achieved by improving many aspects of the traditional hybrid coding framework. Motion compensated prediction, and in particular the interpolation filter, is one area that was improved significantly over H.264/AVC. This paper presents the details of the interpolation filter design of the H.265/HEVC standard. First, the improvements of H.265/HEVC interpolation filtering over H.264/AVC are presented. These improvements include novel filter coefficient design with an increased number of taps and utilizing higher precision operations in interpolation filter computations. Then, the computational complexity is analyzed, both from theoretical and practical perspectives. Theoretical complexity analysis is done by studying the worst-case complexity analytically, whereas practical analysis is done by profiling an optimized decoder implementation. Coding efficiency improvements over the H.264/AVC interpolation filter are studied and experimental results are presented. They show a 4.0% average bitrate reduction for the luma component and 11.3% average bitrate reduction for the chroma components. The coding efficiency gains are significant for some video sequences and can reach up to 21.7%.

Rotational transform for image and video compression

To improve video coding efficiency, the Rotational Transform (ROT) was proposed for adaptive switching between different transforms cores. The Karhunen Loeve Transform (KLT) is known to be optimal for given residual but requires much side information to be signaled to the decoder. The Discrete Cosine transform (DCT) is known to be close to optimal but for strongly directional components, it is sub-optimal. The main idea of ROT is that small modification of DCT coefficients can improve energy compaction. The ROT is implemented as a secondary transform applied after the primary DCT. The ROT matrix is sparse and thus enjoys relatively small computational complexity and memory usage increment. The encoder tries every rotational transform from the dictionary. Only one number, the ROT index, needs to be signaled to the decoder. Because the ROT is an orthogonal transform, encoder search is greatly simplified: distortion can be estimated in frequency domain and no inverse transformation is needed. This makes the ROT an efficient way to improve image/video compression. The ROT Coding gain for Intra slice is 2–3% in the HM 1.0 software implementation.

High precision probability estimation for CABAC

Entropy coding is the main important part of all advanced video compression schemes. Context-adaptive binary arithmetic coding (CABAC) is entropy coding used in H.264/MPEG-4 AVC and H.265/HEVC standards. Probability estimation is the key factor of CABAC performance efficiency. In this paper high accuracy probability estimation for CABAC is presented. This technique is based on multiple estimations using different models. Proposed method was efficiently realized in integer arithmetic. High precision probability estimation for CABAC provides up-to 1,4% BD-rate gain.

Interpolation filter design in HEVC and its coding efficiency - complexity analysis

Coding efficiency gains in the High Efficiency Video Coding (H.265/HEVC) standard are achieved by improving many aspects of the traditional hybrid coding framework. Motion compensated prediction, and in particular the interpolation filter, is one of the areas that was improved significantly over H.264/AVC. This paper presents the details of the motion compensation interpolation filter design of the H.265/HEVC standard and its improvements over the interpolation filter design of H.264/AVC. These improvements include discrete cosine transform based filter coefficient design, utilizing longer filter taps for luma and chroma interpolation and using higher precision operations in the intermediate computations. The computational complexity of HEVC interpolation filter is also analyzed both from theoretical and practical perspectives. Experimental results show that a 4.5% average bitrate reduction for the luma component and 13.0% average bitrate reduction for the chroma components are achieved compared to interpolation filter of H.264/AVC. The coding efficiency gains are significant for some video sequences and can reach up to 21.7%.

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 filtering for scalable extension of HEVC

This paper introduces inter-layer filters for the scalable extension of High Efficiency Video Coding (SHVC) standard, which is being developed by the Joint Collaborative Team on Video Coding (JCT-VC). The major new coding tool in SHVC is inter-layer texture prediction. It provides about 18% average BD-rate reduction compared with HEVC two-layer simulcast. In the case of spatial scalability, base layer reconstructed pictures are up-sampled to the enhancement layer resolution to generate inter-layer texture prediction. A set of 2D separable 8 taps (luma) and 4 taps (chroma) DCT based interpolation filters, which follow the design principles of HEVC motion compensation interpolation filter, are used in the up-sampling process. In the case of SNR scalability, the up-sampling process is not needed since the reference layer has the same spatial resolution as the current layer but encoded with lower quality. This paper proposes a novel inter-layer filter with denoising effect for SNR scalability to improve enhancement layer coding efficiency and equalize the number of stages in inter-layer processing between SNR and spatial scalabilities. Experimental results show that the usage of inter-layer de-noising filter in SNR scalability provides up to 7.5% BD-rate reduction and has observable improvement on subjective visual quality.

Coding efficiency improvements beyond HEVC with known tools

In this paper, several coding tools are evaluated on top of the HEVC version 1. Among them there are straightforward extension of HEVC coding tools (such as Coding Unit size enlarging, fine granularity of Intra prediction angles) and algorithms that have been studied during HEVC development (such as secondary transform, multi-hypothesis CABAC, multi-parameter Intra prediction, bidirectional optical flow). Most of them improve performance of Intra coding. Minor adjustment to the final version of HEVC standard was done for efficient harmonization of the proposed coding tools with HEVC. Performance improvement observed from investigated tools is up to 7,1%, 9,9%, 4,5% and 5,7% in all-intra, random access, low-delay B and low-delay P test scenario (using HEVC common test conditions).

DCT based interpolation filter for Motion Compensation in HEVC

High Efficiency Video Coding (HEVC) draft standard has a challenging goal to improve coding efficiency twice compare to H.264/AVC. Many aspects of the traditional hybrid coding framework were improved during new standard development. Motion compensated prediction, in particular the interpolation filter, is one area that was improved significantly over H.264/AVC. This paper presents the details of the interpolation filter design of the draft HEVC standard. The coding efficiency improvements over H.264/AVC interpolation filter is studied and experimental results are presented, which show a 4.0% average bitrate reduction for Luma component and 11.3% average bitrate reduction for Chroma component. The coding efficiency gains are significant for some video sequences and can reach up 21.7%.