Improve the Efficiency of Low Frequency Non-Separable Secondary Transform Based on Implicit Multiple Transform Selection

In order to effectively adapt to the dynamic transformation characteristics of the residual, multiple transform selection (MTS) was accepted in next generation video coding standard. The MTS can be divided into explicit MTS and implicit MTS, the difference between them was that the primary transform pairs are different, and the primary transform pair of implicit MTS can be determined uniquely by the size of the input block. However, the MTS is a separable transform, the compression capability is limited. So the Low Frequency Non-Separable Secondary Transform (LFNST) was adopted to perform the second transform in primary transform coefficients. But LFNST does not separate the difference between explicit MTS and implicit MTS, which would have effect on performance of LFNST. In this paper, when implicit MTS is enabled, we proposed that if primary transform pair is a combination of DCT2 (Discrete Cosine Transform type II) and DST7 (Discrete Sine Transform type VII), replacing DST7 by DCT2. Two schemes were designed and tested. The experimental results show that the proposed two schemes achieved 0.09% and 0.16% luma BD-rate reduction for all intra configurations respectively, and coding running time complexity nearly constant.