Multiple transform set selection

A novel multiple transform set selection method (MTSS) is introduced in this paper. With the MTSS method, the coding units (CUs) coded with intra predictions generated from decoder-side intra mode derivation mode (DIMD), template-based intra mode derivation mode (TIMD), spatial geometric partitioning mode (SGPM), matrix-based intra prediction (MIP), EIP extrapolation filter based intra prediction (EIP), and intra template matching prediction (ITMP) can be coded with an alternative low frequency non-separable transform (LFNST)/non-separable primary transform (NSPT) set. The derivation of original transform set remains unchanged. The conventional intra prediction modes (IPM) used in generating the prediction for the current CU and the conventional IPMs derived by the histogram of gradients (HoG) of neighboring reconstructed pixels are employed to derive the alternative transform set. The selection of the original or the alternative transform set for the current CU is signaled in the bit-stream.