Tzu-Der Chuang

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.

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.

Palette mode — A new coding tool in screen content coding extensions of HEVC

Palette mode, as a new coding tool, is adopted in the Screen Content Coding Extensions of High Efficiency Video Coding (HEVC SCC) that is being defined by the Joint Collaborative Team on Video Coding (JCT-VC). In the palette mode, pixels in a coding unit (CU) are represented by selected representative colours according to the characteristics of screen contents in which pixel values usually concentrate on few colour values. This paper introduces the palette mode in HEVC SCC, and our contributions to the palette mode are also presented. Experimental results show that disabling the palette mode suffers 20.3%, 13.0%, and 7.5% BD-rate increases for “YUV, text & graphics with motion, 1080p & 720p sequences” under all intra (AI), random access (RA), and low-delay B (LB) common test conditions, respectively.

Block Vector Prediction for Intra Block Copying in HEVC Screen Content Coding

In screen content video, the spatial correlation among pixels shows different characteristics as compared to natural content video. Intra-picture motion compensation (or intra block copy in HEVC) plays a key role in reducing the bit rate of representing high resolution video with contents such as text and graphics. The efficiency of intra block copy is highly related to the accuracy of block vector prediction. In this paper, several block vector prediction methods are proposed to improve the performance of intra block copy technology in HEVC screen content coding extension. Simulation results show that an average bit rate reduction of 8.0% can be achieved for typical 1080p text and graphics sequences in all intra configuration, when compared to the standards test model SCM-1.0. As a result, some of the proposed methods have been adopted into the standard working draft and reference software.

Improved palette index map coding on HEVC SCC

Palette mode is the new coding tool that has been adopted in the Screen Content Coding Extensions of High Efficiency Video Coding (HEVC SCC). Palette mode can represent colour clusters for screen content efficiently and can be summarized into two parts: palette coding tools and colour index map coding tools. This paper proposes two techniques to improve colour index map coding: transition copy and prediction across coding unit boundary. The former is for exploring the correlation between an index and all of the previously coded indices; the latter for utilizing the correlation between the content along the coding unit boundary. Experimental results reportedly show that, compared with HEVC SCC, the proposed techniques can achieve 3.2%, 2.2%, and 1.7% BD-rate savings compared with HEVC SCC for “YUV, text & graphics with motion, 1080p & 720p sequences” under all intra, random access, and low-delay B common test conditions, respectively.