The adaptive loop filtering techniques in the HEVC standard

This article introduces adaptive loop filtering (ALF) techniques being considered for the HEVC standard. The key idea of ALF is to minimize the mean square error between original pixels and decoded pixels using Wiener-based adaptive filter coefficients. ALF is located at the last processing stage of each picture and can be regarded as a tool trying to catch and fix artifacts from previous stages. The suitable filter coefficients are determined by the encoder and explicitly signaled to the decoder. In order to achieve better coding efficiency, especially for high resolution videos, local adaptation is used for luma signals by applying different filter to different region in a picture. In addition to filter adaptation, filter on/off control at largest coding unit (LCU) level is also helpful for improving coding efficiency. Syntax-wise, filter coefficients are sent in a picture level header called adaptation parameter set (APS), and filter on/off flags of LCUs are interleaved at LCU level in the slice data. Besides supporting picture-based optimization of ALF, the syntax design can support low delay applications as well. When the filter coefficients in APS are trained by using a previous picture, filter on/off decisions can be made on the fly during encoding of LCUs, so the encoding latency is only one LCU. Simulation results show that the ALF can achieve on average 5% bit rate reduction and up to 27% bit rate reduction for 25 HD sequences. The run time increases are 1% and 10% for encoders and decoders, respectively, with un-optimized C++ codes in software.