Methods and Apparatuses of Syntax Signaling Constraint for Cross-Component Adaptive Loop Filter in Video Coding System

An apparatus of processing video data in a video coding system, the apparatus comprising one or more electronic circuits configured for:

• receiving input video data of a current block in a current picture;
• applying a Cross-Component Adaptive Loop Filter (CCALF) processing on the current block based on CCALF coefficients signaled in one or more Adaptation Parameter Sets (APSs), wherein luma sample values of the current block are processed by the CCALF processing to refine each chroma component of the current block;
• signaling or parsing two Adaptive Loop Filter (ALF) signal flags and two CCALF signal flags in an APS with an APS parameter type equal to ALF, wherein the two ALF signal flags specify whether luma and chroma filter coefficients are signaled in the APS respectively, and the two CCALF signal flags specify whether cross-component Cb and Cr filter coefficients are signaled in the APS respectively, wherein values of the two ALF signal flags and the two CCALF signal flags in each APS with the APS parameter type equal to ALF are not all equal to 0;
• and
• encoding or decoding the current block in the current picture.