Optimised selection of structure of pictures for video coding

Encoders based on the High Efficiency Video Coding (HEVC) standard consider an input sequence as a succession of slices grouped in Structures of Pictures (SOP). The SOP used while encoding specifies many parameters, such as the coding order of frames, or the reference frames used during inter-prediction. Reference encoders typically make use of a fixed SOP structure of a given size, which is periodically repeated throughout the whole sequence. In this paper, the usage of unconventional SOP structures is first analysed, showing that most sequences benefit from usage of larger SOPs, and that the selection of the optimal SOP is highly content dependent. As a result, an algorithm is proposed to automatically select the optimal SOP size based on a low-complexity texture analysis of neighbouring frames. The algorithm is capable of adaptively changing SOP size during the encoding. Extensive evaluation shows that consistent bit-rate reductions are reported at the same objective quality as an effect of using the proposed algorithm.