Research On Sample Adaptive Offset For Screen Content Coding

To meet the diversification and high-definition of video application, Video Coding Experts Group (VCEG) and Moving Picture Experts Group (MPEG) jointly formulate High Efficiency Video Coding (HEVC). After the first edition of HEVC was finalized, many research works were extended to Scalable HEVC (SHVC), three-dimensional (3D) HEVC (3D-HEVC), Screen Content Coding (SCC) and so on. Screen content videos are widely used in many applications such as remote desktop, video conference, network education. Unlike natural videos, screen content videos are generated by computer, mobile phones, or other electronic devices.HEVC follows the conventional hybrid coding structure, while it also intuduces some coding tools. For in-loop filters, HEVC improves the deblocking filter, and SAO is newly introduced. The main research achievement include:1. An improved SAO scheme is proposed for screen content coding. Through analyzing the effectiveness of the in-loop filters, it is noted that band offset (BO) process achieves much more coding gains for text region which mostly employ intra block copy (IntraBC) prediction mode. The intraBC prediction process in HEVC is performed by using the already reconstructed region for block matching, which is similar to motion compensation. SAO can abtain more coding gains under Low Delay B and Random Access coding structure. If BO process is applied after one coding tree unit (CTU) encoded, the distortion between original and reconstructed samples copied by the IntraBC prediction will be further reduced, which is simple to operate and can obtain good coding efficiency. Experimental results show that the proposed scheme achieves up to 3.4% BD-rate reduction in All-intra (AI) for screen content sequences with encoding and decoding time no increase.2. We propose to modify the deblocking filter with paying attention to IntraBC prediction mode. For the deblocking filtering of IntraBC coded blocks, the SCM1.0 regards the IntraBC block as similar to the convertional intra-coded block. It means that SCM-1.0 sets the boundary strength to 2 on the block boundary of two adjacent blocks when one of the blocks is encoded with IntraBC prediction mode. However, when the two blocks are encoded with IBC having the same block vector without residual coefficients, there is no discontinuity between two blocks. For this case, we propose to modify the deblocking filter process with a special attention to IntraBC treated as inter-coded blocks. The experimental results show that the scheme can improved the subjective quality of images.