Low-complexity separable multiplier-less loop filter for video coding

In this paper, we present a low-complexity loop filter for video coding. We begin by presenting a set of non-Wiener based loop filters that can complement the Wiener-based adaptive loop filter which was considered as a tool for possible adoption in the HEVC standard. We devise filters which can be adaptively operated on different regions in an image. We devise a quad-tree based signaling for the filters, and present various loop filters: such as bilateral and Gaussian, as well as a separable 3-tap filter which can be implemented by just shifts and adds. The proposed 3-tap filter is thus hardware friendly with minimal complexity. In terms of compression performance, the proposed 3-tap filter can approach other sophisticated filters, albeit at a substantially reduced complexity; and can provide compression gains of 2.3% on average; and upto 7.0% for Low-Delay-P configuration over a data-set of 19 diverse HD, and UHD sequences of upto 8K resolution when implemented on top of the HM14.0 software for HEVC. Finally, we also present results for combining our proposed filters with the Wiener-based adaptive loop filter considered in HEVC, and illustrate that there is a significant amount of compression gain that can be achieved by loop filters for the next generation of video coding standard beyond HEVC.