Region-adaptive quantisation for prevention of contouring in coded video

For some compressed UHD sequences, contouring artefacts appear in areas where smooth light variations occur, affecting the subjective quality of the decoded video sequences. This type of artefact appears even when light compression is applied. In this paper, an analysis on using fine quantisation in areas prone to contouring artefacts is presented, targeting a reduction in the visibility of contouring artefacts. A technique to further reduce the visibility of contouring artefacts by modifying the rate-distortion costs considered during the video encoding process is also proposed. Contouring artefacts can be prevented or significantly reduced with these techniques with a low impact in terms of rate-distortion performance for high target quality needed for UHD broadcasting.