Cixun Zhang

High Performance, Low Complexity Video Coding and the Emerging HEVC Standard

This paper describes a low complexity video codec with high coding efficiency. It was proposed to the high efficiency video coding (HEVC) standardization effort of moving picture experts group and video coding experts group, and has been partially adopted into the initial HEVC test model under consideration design. The proposal utilizes a quadtree-based coding structure with support for macroblocks of size 64 × 64, 32 × 32, and 16 × 16 pixels. Entropy coding is performed using a low complexity variable length coding scheme with improved context adaptation compared to the context adaptive variable length coding design in H.264/AVC. The proposals interpolation and deblocking filter designs improve coding efficiency, yet have low complexity. Finally, intra-picture coding methods have been improved to provide better subjective quality than H.264/AVC. The subjective quality of the proposed codec has been evaluated extensively within the HEVC project, with results indicating that similar visual quality to H.264/AVC High Profile anchors is achieved, measured by mean opinion score, using significantly fewer bits. Coding efficiency improvements are achieved with lower complexity than the H.264/AVC Baseline Profile, particularly suiting the proposal for high resolution, high quality applications in resource-constrained environments.

Overview of AVS-video: tools, performance and complexity

Audio Video coding Standard (AVS) is established by the Working Group of China in the same name. AVS-video is an application driven coding standard. AVS Part 2 targets to high-definition digital video broadcasting and high-density storage media and AVS Part 7 targets to low complexity, low picture resolution mobility applications. Integer transform, intra and inter-picture prediction, in-loop deblocking filter and context-based two dimensional variable length coding are the major compression tools in AVS-video, which are well-tuned for target applications. It achieves similar performance to H.264/AVC with lower cost.

Efficient Fixed-Point Approximations of the 8x8 Inverse Discrete Cosine Transform

This paper describes fixed-point design methodologies and several resulting implementations of the Inverse Discrete Cosine Transform (IDCT) contributed by the authors to MPEGs work on defining the new 8x8 fixed point IDCT standard - ISO/IEC 23002-2. The algorithm currently specified in the Final Committee Draft (FCD) of this standard is also described herein.

Video coding using Variable Block-Size Spatially Varying Transforms

In our previous work, we introduced Spatially Varying Transforms (SVT) for video coding, where the location of the transform block within the macroblock is not fixed but varying. In this paper, we extend this concept and present a novel method, called Variable Block-size Spatially Varying Transforms (VBSVT). VBSVT utilizes Variable Block-size Transforms (VBT) in the SVT framework, and is shown to be more preferable for coding prediction error with different characteristics than fixed block-size SVT and also the standard methods that use fixed or adaptive block sizes at fixed spatial locations. In addition, VBSVT has similar decoding complexity with fixed block-size SVT and lower decoding complexity compared to standard methods as only a portion of the prediction error needs to be decoded. Experimental results show that, VBSVT achieves 4.1% gain over H.264/AVC on average over a wide range of test set. Gains become more significant at high quality levels and go up to 13.5%, which makes the proposed algorithm very suitable for future video coding solutions focusing on high fidelity applications.

The Technique of Prescaled Integer Transform: Concept, Design and Applications

Integer cosine transform (ICT) is adopted by H.264/AVC for its bit-exact implementation and significant complexity reduction compared to the discrete cosine transform (DCT) with an impact in peak signal-to-noise ratio (PSNR) of less than 0.02 dB. In this paper, a new technique, named prescaled integer transform (PIT), is proposed. With PIT, while all the merits of ICT are kept, the implementation complexity of decoder is further reduced compared to corresponding conventional ICT, which is especially important and beneficial for implementation on low-end processors. Since not all PIT kernels are good in respect of coding efficiency, design rules that lead to good PIT kernels are considered in this paper. Different types of PIT and their target applications are examined. Both fixed block-size transform and adaptive block-size transform (ABT) schemes of PIT are also studied. Experimental results show that no penalty in performance is observed with PIT when the PIT kernels employed are derived from the design rules. Up to 0.2 dB of improvement in PSNR for all intra frame coding compared to H.264/AVC can be achieved and the subjective quality is also slightly improved when PIT scheme is carefully designed. Using the same concept, a variation of PIT, post-scaled integer transform, can also be potentially designed to simplify the encoder in some special applications. PIT has been adopted in audio video coding standard (AVS), Chinese National Coding standard.

A new approach to compatible adaptive block-size transforms

Adaptive Block-size Transforms (ABT) has been widely used in image/video coding, since it exploits the maximum feasible signal length for transform coding. However, if the transforms in an ABT coding system are Integer Cosine Transforms (ICT), not only separate transform units but also different scaling matrices are required, which consume a vast amount of resources in practical implementations. In this paper, a new approach to compatible ABT is presented, by which 8x8, 8x4, 4x8 and 4x4 transforms can be processed in one transform unit. Furthermore, with Pre-scaled Integer Transform (PIT), the compatibility of scaling matrices especially for 8x4 and 4x8 ICT can be achieved and a single scaling matrix is required. Simulation results and analysis reveal that this approach greatly saves hardware resources and makes the implementation of ABT much easier without loss of performance.

The technique of pre-scaled integer transform

The integer cosine transform (ICT) is adopted by H.264/AVC for its bit-exact implementation and significant complexity reduction compared to the discrete cosine transform (DCT) with an impact in peak signal-to-noise ratio (PSNR) of less than 0.02dB. In this paper, a new technique, named pre-scaled integer transform (PIT), is proposed. With PIT, the implementation complexity is further reduced compared to conventional ICT, especially for low-end processors while all the merits of ICT are kept. Extensive experiments show that no obvious penalty in performance is observed but rather a slight gain in PSNR is obtained by using PIT when the integer transform matrix used meets certain requirements.

Low complexity algorithm for Spatially Varying Transforms

In our previous work, we introduced Spatially Varying Transforms (SVT) for video coding, where the location of the transform block within the macroblock is not fixed but varying. SVT has lower decoding complexity compared to standard methods as only a portion of the prediction error needs to be decoded. However, the encoding complexity of SVT can be relatively high because of the need to perform Rate Distortion Optimization (RDO) for each candidate Location Parameter (LP). In this work, we propose a low complexity algorithm operating on macroblock and block level to reduce the encoding complexity of SVT. The proposed low complexity algorithm includes selection of available candidate LP based on motion difference and a hierarchical search algorithm. Experimental results show that the proposed low complexity algorithm can reduce around 80% of the candidate LP tested in RDO with only marginal penalty in coding efficiency.

Video Coding Using Spatially Varying Transform

In this paper, a novel algorithm called spatially varying transform (SVT) is proposed to improve the coding efficiency of video coders. SVT enables video coders to vary the position of the transform block, unlike state-of-art video codecs where the position of the transform block is fixed. In addition to changing the position of the transform block, the size of the transform can also be varied within the SVT framework, to better localize the prediction error so that the underlying correlations are better exploited. It is shown in this paper that by varying the position of the transform block and its size, characteristics of prediction error are better localized, and the coding efficiency is thus improved. The proposed algorithm is implemented and studied in the H.264/AVC framework. We show that the proposed algorithm achieves 5.85% bitrate reduction compared to H.264/AVC on average over a wide range of test set. Gains become more significant at medium to high bitrates for most tested sequences and the bitrate reduction may reach 13.50%, which makes the proposed algorithm very suitable for future video coding solutions focusing on high fidelity video applications. The gain in coding efficiency is achieved with a similar decoding complexity which makes the proposed algorithm easy to be incorporated in video codecs. However, the encoding complexity of SVT can be relatively high because of the need to perform a number of rate distortion optimization (RDO) steps to select the best location parameter (LP), which indicates the position of the transform. In this paper, a novel low complexity algorithm is also proposed, operating on a macroblock and a block level, to reduce the encoding complexity of SVT. Experimental results show that the proposed low complexity algorithm can reduce the number of LPs to be tested in RDO by about 80% with only a marginal penalty in the coding efficiency.

Video coding using pruned transforms and interleaving of multiple block

Technologies used in todays video coding standards have been designed and optimized mainly for standard definition (SD) resolutions and below. When moving to higher resolutions, data in video frames tend to become more correlated spatially. In this paper, we study how to take advantage of this phenomenon to lower computational requirements for high definition (HD) video coding. A coding method based on low complexity pruned transforms and interleaving of multiple transform coefficient blocks is proposed. An example implementation of this method in the context of H.264/AVC is also presented. Experimental results show that the proposed method maintains the high compression efficiency of H.264/AVC while significantly lowering the coding complexity of the codec.