Method and apparatus for controlling coding tools

A video decoding apparatus, comprising:

• an entropy decoder configured to:
• decode, from a sequence parameter set (SPS) level of a bitstream, a first enable flag indicating whether one or more coding tools are enabled, the one or more coding tools including a first coding tool configured to encode a sample value using luma component mapping based on a piecewise linear model, and
• decode, from a picture level of the bitstream, a second enable flag indicating whether the first coding tool is enabled for a current picture, when the first enable flag indicates that the first coding tool is enabled;
• an acquisition unit configured to acquire a value of an application flag indicating whether to apply the first coding tool for a current slice partitioned from the current picture, by, according to a value of the second enable flag, either setting the application flag to a predetermined value or decoding the application flag from a slice level of the bitstream;
• and
• a processor configured to execute the first coding tool depending on the value of the application flag,
• wherein, when the first coding tool is executed, the processor is configured to:
• generate a mapped luma prediction sample from a luma prediction sample which is predicted using an inter prediction, based on a piecewise linear model corresponding to the luma prediction sample;
• generate a luma reconstructed sample by adding a luma residual sample reconstructed from the bitstream and the mapped luma prediction sample;
• and
• reversely map the luma reconstructed sample using an inverse piecewise linear model having an inverse relationship with the piecewise linear model,
• wherein the processor is configured to generate the mapped luma prediction sample by performing a process comprising:
• deriving piecewise linear models respectively corresponding to first sections to which the luma prediction sample is allowed to belong, based on a bit depth and information on second sections onto which the mapped luma prediction sample is allowed to belong, the information on the second sections decoded from the bitstream, and
• deriving the mapped luma prediction sample, by determining a first target section to which the luma prediction sample belongs among the first sections and mapping the luma prediction sample to a mapping section using a piecewise linear model corresponding to the first target section, the mapping section being a section among the second sections which corresponds to the first target section,
• wherein the processor is further configured to perform a chroma scaling process comprising:
• deriving chroma scaling relationships respectively corresponding to the second sections, based on the information on the second sections and chroma scaling information decoded from the bitstream,
• determining, among the second sections, a section to which an average of luma reconstructed samples positioned on above and left sides of a current block belong, and
• scaling chroma residual samples using a chroma scaling relationship corresponding to the section to which the average belongs, to thereby reconstruct chroma samples using the scaled chroma residual samples and chroma prediction samples.