Unified Secondary Transform for Intra Coding Beyond Av1

In AV1, only separable transforms are applied as primary transform. Separable transform can efficiently capture the statistical correlation of residual samples along horizontal and vertical directions. However, for typical natural image/video contents which usually present arbitrary directionality in the residual samples, the efficiency of separable transform is rather limited comparing to the high-complexity non-separable primary transform. To capture the directionality of residual samples with relatively lower complexity, in this paper, a non-separable unified secondary transform scheme, which is customized for AV1 intra coding scheme, is proposed. The proposed method categorizes the secondary transform kernels according to the nominal intra prediction angles defined in AV1 and share the bases among the recursive filtering modes with DC prediction mode. Experimental results show that, based on the libaom implementation of AV1, the proposed method provides average 2.5% luma BD-rate for intra coding using a set of well-known video content as test material, and up to 14% luma BD-rate saving for still image coding using the DIV2K data set as test material.