On residual quad-tree coding in HEVC

In the current working draft of HEVC, residual quad-tree (RQT) coding is used to encode prediction residuals in both Intra and Inter coding units (CU). However, the rationale for using RQT as a coding tool is different in the two cases. For Intra prediction units, RQT provides an efficient syntax for coding a number of sub-blocks with the same intra prediction mode. For Inter CUs, RQT adapts to the spatial-frequency variations of the CU, using as large a transform size as possible while catering to local variations in residual statistics. While providing coding gains, effective use of RQT currently requires an exhaustive search of all possible combinations of transform sizes within a block. In this paper, we exploit our insights to develop two fast RQT algorithms, each designed to meet the needs of Intra and Inter prediction residual coding.