7. In-Loop Filters in HEVC

The HEVC standard specifies two in-loop filters, a deblocking filter and a sample adaptive offset (SAO). The in-loop filters are applied in the encoding and decoding loops, after the inverse quantization and before saving the picture in the decoded picture buffer. The deblocking filter is applied first. It attenuates discontinuities at the prediction and transform block boundaries. The second in-loop filter, SAO, is applied to the output of the deblocking filter and further improves the quality of the decoded picture by attenuating ringing artifacts and changes in sample intensity of some areas of a picture. The most important advantage of the in-loop filters is improved subjective quality of reconstructed pictures. In addition, using the filters in the decoding loop also increases the quality of the reference pictures and hence also the compression efficiency.