Methods and apparatus for restricted secondary transform and signaling thereof in image coding

An apparatus for encoding or decoding a video sequence, wherein a Low-Frequency Non-Separable Transform (LFNST) mode and Transform Skip (TS) mode are supported, the apparatus comprising one or more electronic circuits or processors arranged to:

• receive input data related to a current coding unit (CU) in a current picture, wherein the CU is partitioned into one or more transform blocks (TBs) and the input data correspond to residual data at a video encoder side or a coded data of the current CU at a video decoder side;
• determining a syntax at an encoder side or at a decoder side, wherein said determining the syntax is performed by signaling the syntax at the encoder side or by parsing the syntax at the decoder side if one or more conditions are satisfied, wherein the syntax indicates whether the LFNST mode is applied to the current CU and/or which LFNST kernel is applied if the LFNST mode is applied, and said one or more conditions comprise a target condition corresponding to that all target TBs in a target TB set have a TS mode indication among more than one color components as false, and wherein the target TB set is selected from said one or more TBs;
• and
• encode the current CU at the encoder side or decode the current CU at the decoder side according to the LFNST mode as indicated by the syntax.