Xiaozhong Xu

Screen content coding using non-square intra block copy for HEVC

To achieve high coding performance for screen content, the intra block copy (IntraBC) performs block matching within a limited area of the reconstructed samples inside the current picture. We further extend its notion from the unit of square coding unit (CU) to non-square prediction unit (PU) partitions. This design is then justified by theoretical and empirical analyses which reveal the same fact that blocks coded by IntraBC mode tend to enable more at smaller partition levels. Besides, the syntax design of the proposed method is fully aligned with that of inter partition modes. Therefore the architecture-wise change in video codec design can be minimized. The experimental results justify the effectiveness of the proposed mode for coding of screen content video. In particular, up to 19.5% rate reduction (with an average of 12.7%) relative to the HM-12.0+RExt-4.1 anchor can be achieved on top of the usage of CU-based IntraBC prediction.

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.

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.

Overview of HEVC extensions on screen content coding

Technologies for coding non-camera-captured video contents have received great interests lately due to the rapid growth of application areas such as wireless display and screen sharing, etc. In response to the market demands, the ITU-T Video Coding Expert Group and ISO/IEC Motion Picture Expert Group have jointly launched a new standardization project, i.e. the High-Efficiency Video Coding (HEVC) extensions on screen content coding (HEVC SCC). Several new video coding tools, including intra block copy, palette coding, adaptive color transform, and adaptive motion resolution, have been developed and adopted into HEVC SCC draft standard. This paper reviews the main features and coding technologies in the current HEVC SCC draft standard, with discussions about the performance and complexity aspects compared with prior arts.

PU level intra block copying with flipping mode

With the development of HEVC Screen Content Coding (SCC) extension, intra block copying (IntraBC) mode have been proven to be a very efficient tool in coding of typical screen content video materials. Currently, the decision of IntraBC usage is on coding unit (or CU) level. The coding modes for all the possible partitions inside a CU should be the same. If a CU contains pixels of different objects, the whole block may not be coded efficiently in either inter mode or IntraBC mode. Also, sometimes there are some structural similarity between the current block and the one in the reconstructed area of the same picture, but the textual orientation is just in opposite direction. In this paper, two new methods are proposed to improve the IntraBC mode. The first one is PU level IntraBC decision, which allows a CU to be partitioned into combination of IntraBC coded PU and inter coded PU. The other one is the flipping mode for IntraBC, where the reference block is actually mirrored before doing compensation. Simulation results show that the proposed methods can bring noticeable coding gains on top of the current IntraBC design.