Yu-Wen Huang

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.

Sample adaptive offset for HEVC

A new video coding tool, sample adaptive offset (SAO), is introduced in this paper. SAO has been adopted into the Working Draft of the new video coding standard, High-Efficiency Video Coding (HEVC). The SAO is located after deblocking in the video coding loop. The concept of SAO is to classify reconstructed pixels into different categories and then reduce the distortion by simply adding an offset for each category of pixels. The pixel intensity and edge properties are used for pixel classification. To further improve the coding efficiency, a picture can be divided into regions for localization of offset parameters. Simulation results show that SAO can achieve on average 2% bit rate reduction and up to 6% bit rate reduction. The run time increases for encoders and decoders are only 2%.

Motion vector coding techniques for HEVC

High Efficiency Video Coding (HEVC) is a new international video coding standard that has been developed by the Joint Collaborative Team on Video Coding (JCT-VC). In this paper, an overview of the motion vector coding techniques for HEVC is presented. Our three proposed coding tools for the motion vector predictor (MVP) in the Inter, Skip and Merge modes of HEVC are also presented, which includes a new location of the temporal MVP, a priority-based derivation method of spatial MVPs, and a derivation method of temporal MVPs. A combination of these three tools can achieve on average 1.3%, 1.8%, 1.2% and 2.2% bit rate reductions for high efficiency random access, low complexity random access, high efficiency low delay, and low complexity low delay, respectively.

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.

Motion Vector Coding in the HEVC Standard

High Efficiency Video Coding (HEVC) is an emerging international video coding standard developed by the Joint Collaborative Team on Video Coding (JCT-VC). Compared to H.264/AVC, HEVC has achieved substantial compression performance improvement. During the HEVC standardization, we proposed several motion vector coding techniques, which were crosschecked by other experts and then adopted into the standard. In this paper, an overview of the motion vector coding techniques in HEVC is firstly provided. Next, the proposed motion vector coding techniques including a priority-based derivation algorithm for spatial motion candidates, a priority-based derivation algorithm for temporal motion candidates, a surrounding-based candidate list, and a parallel derivation of the candidate list, are also presented. Based on HEVC test model 9 (HM9), experimental results show that the combination of the proposed techniques achieves on average 3.1% bit-rate saving under the common test conditions used for HEVC development.

One-pass encoding algorithm for adaptive loop filter in high-efficiency video coding

In this paper, a one-pass encoding algorithm is proposed for adaptive loop filter (ALF) in high-efficiency video coding (HEVC). ALF can improve both subjective and objective video quality, but it also requires a lot of encoding passes (i.e. picture buffer accesses) that will significantly increase external memory access, encoding latency, and power consumption. Therefore, we propose a method to estimate filtering distortion without performing real filter operation. The number of encoding passes can be effectively reduced from 16 to 1. Combined with an initial guess of filter-on/off blocks by using time-delayed filters, the proposed one-pass algorithm only induces average 0.17% BD-rate increase.

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.

Palette Mode Coding in HEVC Screen Content Coding Extension

Palette mode is a new coding tool included in the HEVC screen content coding extension (SCC) to improve the coding efficiency for screen contents such as computer generated video with substantial amount of text and graphics. It is observed that a local area in screen content typically has a few colors separated by sharp edges. To exploit such characteristics, palette mode represents samples in a block with indexes pointing to the color entries in a palette table. This paper provides a detailed overview of the palette mode in HEVC SCC in terms of palette generation, coding of the palette, and coding of the palette indexes for the samples in the palette block. Several improvements to palette mode coding, which have been proposed but not included in HEVC SCC, are also described. Simulation results are presented to quantify the bitrate savings provided by the palette mode for equal distortions.

Localized multiple adaptive interpolation filters with single-pass encoding

Adaptive interpolation filtering (AIF) algorithms have been proposed to enhance the hybrid video coding scheme recently. Although these algorithms can improve coding efficiency significantly, their encoders suffer huge increase in complexity in terms of latency and memory access due to its inherent multi-pass encoding procedure. In this paper, we present a novel single-pass solution for these algorithms, which allows optimal selection among different interpolation filters. In this solution, time-delayed interpolation filters are used to achieve single-pass encoding, and localized ratedistortion (RD) selection is used to compensate the possible coding loss from time-delayed filters. Experimental results show that the proposed method is efficient for All AIF techniques in current ITU-T/SG16 reference software. By using the proposed method, single-pass encoding with multiple AIF filters can be achieved while maintaining similar coding efficiency as multi-pass AIF.

Intra Block Copy in HEVC Screen Content Coding Extensions

With the emerging applications such as online gaming and Wi-Fi display, screen content video, including computer generated text, graphics and animations, becomes more popular than ever. Traditional video coding technologies typically were developed based on models that fit into natural, camera-captured video. The distinct characteristics exhibited between these two types of contents necessitate the exploration of coding efficiency improvement given that new tools can be developed specially for screen content video. The HEVC Screen Content Coding Extensions (HEVC SCC) have been developed to incorporate such new coding tools in order to achieve better compression efficiency. In this paper, intra block copy (IBC, or intra picture block compensation), also named current picture referencing (CPR) in HEVC SCC, is introduced and discussed. This tool is very efficient for coding of screen content video in that repeated patterns in text and graphics rich content occur frequently within the same picture. Having a previously reconstructed block with equal or similar pattern as a predictor can effectively reduce the prediction error and therefore improve coding efficiency. Simulation results show that up to 50% BD rate reduction in all intra coding can be achieved with intra block copy enabled, compared to the HEVC reference encoder without this tool. Significant BD rate reductions for other coding configurations are also observed.