In Suk Chong

A Hybrid Video Coder Based on Extended Macroblock Sizes, Improved Interpolation, and Flexible Motion Representation

This paper describes a video coding technology proposal submitted by Qualcomm in response to a joint call for proposals (CfP) issued by ITU-T SG16 Q.6 (VCEG) and ISO/IEC JTC1/SC29/WG11 (MPEG) in January 2010. The proposed video codec follows a hybrid coding approach based on temporal prediction, followed by transform, quantization, and entropy coding of the residual. Some of its key features are extended block sizes (up to 64 × 64), single pass switched interpolation filters with offsets, mode-dependent directional transforms for intra-coding, luma and chroma high precision filtering, geometric motion partitions, adaptive motion vector resolution and efficient 16-point transforms. It also incorporates internal bit-depth increase and modified quadtree-based adaptive loop filtering. Simulation results are presented to demonstrate the high compression efficiency achieved by the proposed video codec at the expense of moderate increase in encoding and decoding complexity compared to the advanced video coding standard (AVC/H.264). For the random access and low delay configurations, it achieved average bit rate reductions of 30.9% and 33.0% for equivalent peak signal-to-noise ratio, respectively, compared to the corresponding AVC anchors. The proposed codec scored highly in both subjective evaluations and objective metrics and was among the best-performing CfP proposals.

Adaptive Loop Filtering for Video Coding

Adaptive loop filtering for video coding is to minimize the mean square error between original samples and decoded samples by using Wiener-based adaptive filter. The proposed ALF is located at the last processing stage for each picture and can be regarded as a tool to catch and fix artifacts from previous stages. The suitable filter coefficients are determined by the encoder and explicitly signaled to the decoder. In order to achieve better coding efficiency, especially for high resolution videos, local adaptation is used for luma signals by applying different filters to different regions or blocks in a picture. In addition to filter adaptation, filter on/off control at coding tree unit (CTU) level is also helpful for improving coding efficiency. Syntax-wise, filter coefficients are sent in a picture level header called adaptation parameter set, and filter on/off flags of CTUs are interleaved at CTU level in the slice data. This syntax design not only supports picture level optimization but also achieves a low encoding latency. Simulation results show that the ALF can achieve on average 7% bit rate reduction for 25 HD sequences. The run time increases are 1% and 10% for encoders and decoders, respectively, without special attention to optimization in C++ code.

Adaptive overlapped block motion compensation

An adaptive overlapped block motion compensation (OBMC) scheme, which takes the rate, distortion and complexity factors into account, is proposed in this research. With this scheme, the encoder uses the classification information to determine which blocks could be distorted after OBMC. Then, the BMC (block motion compensation) search rather than the OBMC search is applied to these blocks. Depending on the allowed computational complexity of the decoder, the encoder chooses the best set of OBMC macroblocks which requires a higher complexity. Other macroblocks simply adopt BMC. Furthermore, the motion bit rate can be reduced by smoothing the motion field of BMC blocks during motion search in the encoder. An effective group OBMC motion search scheme is applied to this new adaptive OBMC motion scheme. Experimental results show that the complexity of both the encoder and the decoder is reduced while the visual quality remains the same due to the improvement in the oversmooth region.

The adaptive loop filtering techniques in the HEVC standard

This article introduces adaptive loop filtering (ALF) techniques being considered for the HEVC standard. The key idea of ALF is to minimize the mean square error between original pixels and decoded pixels using Wiener-based adaptive filter coefficients. ALF is located at the last processing stage of each picture and can be regarded as a tool trying to catch and fix artifacts from previous stages. The suitable filter coefficients are determined by the encoder and explicitly signaled to the decoder. In order to achieve better coding efficiency, especially for high resolution videos, local adaptation is used for luma signals by applying different filter to different region in a picture. In addition to filter adaptation, filter on/off control at largest coding unit (LCU) level is also helpful for improving coding efficiency. Syntax-wise, filter coefficients are sent in a picture level header called adaptation parameter set (APS), and filter on/off flags of LCUs are interleaved at LCU level in the slice data. Besides supporting picture-based optimization of ALF, the syntax design can support low delay applications as well. When the filter coefficients in APS are trained by using a previous picture, filter on/off decisions can be made on the fly during encoding of LCUs, so the encoding latency is only one LCU. Simulation results show that the ALF can achieve on average 5% bit rate reduction and up to 27% bit rate reduction for 25 HD sequences. The run time increases are 1% and 10% for encoders and decoders, respectively, with un-optimized C++ codes in software.

A hybrid video codec based on extended block sizes, recursive integer transforms, improved interpolation, and flexible motion representation

This paper describes video coding technology proposal submitted by Qualcomm Inc. in response to a joint call for proposal (CfP) issued by ITU-T SG16 Q.6 (VCEG) and ISO/IEC JTC1/SC29/WG11 (MPEG) in January 2010. Proposed video codec follows a hybrid coding approach based on temporal prediction, followed by transform, quantization, and entropy coding of the residual. Some of its key features are extended block sizes (up to 64x64), recursive integer transforms, single pass switched interpolation filters with offsets (single pass SIFO), mode dependent directional transform (MDDT) for intra-coding, luma and chroma high precision filtering, geometry motion partitioning, adaptive motion vector resolution. It also incorporates internal bit-depth increase (IBDI), and modified quadtree based adaptive loop filtering (QALF). Simulation results are presented for a variety of bit rates, resolutions and coding configurations to demonstrate the high compression efficiency achieved by the proposed video codec at moderate level of encoding and decoding complexity. For random access hierarchical B configuration (HierB), the proposed video codec achieves an average BD-rate reduction of 30.88c/o compared to the H.264/AVC alpha anchor. For low delay hierarchical P (HierP) configuration, the proposed video codec achieves an average BD-rate reduction of 32.96c/o and 48.57c/o, compared to the H.264/AVC beta and gamma anchors, respectively.