A novel low delay in-loop filtering WPP process for parallel HEVC encoding

Abstract

Wavefront parallel processing (WPP) is a parallelization technique that enables processing of several rows of Largest Coding Units (LCUs) in parallel. It achieves a relatively high level of parallelism with reasonable loss in compression performance. At the same time, the HEVC standard specifies two in-loop filters, namely the deblocking filter and the sample adaptive offset (SAO), to improve subjective quality as well as coding efficiency. Because SAO parameters cannot be precisely determined until the lower right deblocked samples are available, a delay of several rows is introduced to implement the filtering process. Since the reconstruction of the LCUs will not start until finish filtering, the total delay of WPP and filtering is at least two rows, which obviously influences the performance of parallel processing. In this paper, we propose a novel In-Loop Filtering WPP method that reduces the row delay into four LCUs and significantly improves the parallelism with little rate-distortion (RD) performance loss. Experimental results show that the proposed algorithms can achieve up to the 2.89× speedup compared with the existing WPP method with 24-core server, where the speedup improves with the increase of core number.