Method and apparatus of cross-component linear model prediction in video coding system

A method of encoding colour pictures using coding tools including one or more CCLM (Cross Colour Linear Model) related modes, the method comprising:

• receiving pixel data associated with a current block comprising a first-colour block and a second-colour block;
• signalling a first syntax in a bitstream comprising encoded data for the current block, wherein the first syntax is related to whether the current block is coded in said one or more CCLM related modes;
• determining one or more model parameters for the second-colour block if the first syntax indicates the current block being coded in said one or more CCLM related modes, wherein a cross-colour predictor for the second-colour block is generated by applying one or more cross-colour models to reconstructed or predicted first-colour block using said one or more model parameters;
• signalling a second syntax in the bitstream if the first syntax indicates the current block being coded in said one or more CCLM related modes, wherein the second syntax is related to whether a multiple model CCLM mode is used or whether said one or more model parameters are explicitly signalled or implicitly derived;
• and
• signalling a feasible range and/or a step number related to scaling parameters from the bitstream, wherein a flag is derived to indicate if the feasible range and the step number related to the scaling parameters in a SPS (Sequence Parameter Set) or a PPS (Picture Parameter Set) is overwritten or not.

An apparatus for decoding colour pictures using coding tools including one or more CCLM (Cross Colour Linear Model) related modes, the apparatus comprising one or more electronics or processors arranged to:

• receive encoded data associated with a current block comprising a first-colour block and a second-colour block;
• parse a first syntax from a bitstream comprising the encoded data for the current block, wherein the first syntax is related to whether the current block is coded in said one or more CCLM related modes;
• parse a second syntax from the bitstream if the first syntax indicates the current block being coded in said one or more CCLM related modes, wherein the second syntax is related to whether a multiple model CCLM mode is used or whether one or more model parameters are explicitly signalled or implicitly derived;
• determine said one or more model parameters for the second-colour block if the first syntax indicates the current block being coded in said one or more CCLM related modes, wherein a cross-colour predictor for the second-colour block is generated by applying one or more cross-colour models to reconstructed or predicted first-colour block using said one or more model parameters;
• parse, from the bitstream, a feasible range and/or a step number related to scaling parameters, wherein a flag is derived to indicate whether the feasible range and the step number related to the scaling parameters in a SPS (Sequence Parameter Set) or a PPS (Picture Parameter Set) is overwritten or not;
• and
• decode the encoded data associated with the second-colour block using prediction data comprising the cross-colour predictor for the second-colour block.

An apparatus for encoding colour pictures using coding tools including one or more CCLM (Cross Colour Linear Model) related modes, the apparatus comprising one or more electronics or processors arranged to:

• receive pixel data associated with a current block comprising a first-colour block and a second-colour block;
• signal a first syntax in a bitstream comprising encoded data for the current block, wherein the first syntax is related to whether the current block is coded in said one or more CCLM related modes;
• determine one or more model parameters for the second-colour block if the first syntax indicates the current block being coded in said one or more CCLM related modes, wherein a cross-colour predictor for the second-colour block is generated by applying one or more cross-colour models to reconstructed or predicted first-colour block using said one or more model parameters;
• signal a second syntax in the bitstream if the first syntax indicates the current block being coded in said one or more CCLM related modes, wherein the second syntax is related to whether a multiple model CCLM mode is used or whether said one or more model parameters are explicitly signalled or implicitly derived;
• and
• signal a feasible range and/or a step number related to scaling parameters from the bitstream, wherein a flag is derived to indicate if the feasible range and the step number related to the scaling parameters in a SPS (Sequence Parameter Set) or a PPS Picture Parameter Set) is overwritten or not.