Parameter derivation for intra prediction

An apparatus for processing video data, comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:

• determine, for a conversion between a current video block of a video and a bitstream of the video, parameters for a linear model prediction or cross-color component prediction based on refined neighboring luma samples and chroma samples of the current video block;
• derive prediction values of a chroma component of the current video block based on the parameters and refined internal luma samples of the current video block;
• and
• perform the conversion based on the prediction values;
• wherein the refined neighboring luma samples and the refined internal luma samples of the current video block are determined by down-sampling neighboring luma samples and internal luma samples followed by a non-linear process wherein the parameters for the linear model prediction are α and β, wherein α=(C1−C0)/(L1−L0) and β=C0−αL0, wherein C0 and C1 are derived from neighboring chroma samples, and wherein L0 and L1 are derived from neighboring luma samples;
• wherein C0 and L0 are based on S neighboring chroma and luma samples, denoted {Cx1, Cx2, . . . , CxS} and {Lx1, Lx2, . . . , LxS}, respectively, wherein C1 and L1 are based on T neighboring chroma and luma samples, denoted {Cy1, Cy2, . . . , CyT} and {Ly1, Ly2, . . . , LyT}, respectively, wherein S=T,
• wherein {Cx1, Cx2, . . . , CxS} are corresponding to {Lx1, Lx2, . . . , LxS},
• wherein {Cy1, Cy2, . . . , CyT} are corresponding to {Ly1, Ly2, . . . , LyT},
• wherein C0=f0(Cx1, Cx2, . . . , CxS), L0=f1(Lx1, Lx2, . . . , LxS), C1=f2(Cy1, Cy2, . . . , CyT) and L1=f3(Ly1, Ly2, . . . , LyT), and
• wherein f0, f1, f2 and f3 are functions.

A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises:

• determining, for a conversion between a current video block of the video and the bitstream of the video, parameters for a linear model prediction or cross-color component prediction based on refined neighboring luma samples and chroma samples of the current video block;
• deriving prediction values of a chroma component of the current video block based on the parameters and refined internal luma samples of the current video block;
• and
• generating the bitstream based on the prediction values;
• wherein the refined neighboring luma samples and the refined internal luma samples of the current video block are determined by down-sampling neighboring luma samples and internal luma samples followed by a non-linear process wherein the parameters for the linear model prediction are α and β, wherein α=(C1−C0)/(L1−L0) and β=C0−αL0, wherein C0 and C1 are derived from neighboring chroma samples, and wherein L0 and L1 are derived from neighboring luma samples;
• wherein C0 and L0 are based on S neighboring chroma and luma samples, denoted {Cx1, Cx2, . . . , CxS} and {Lx1, Lx2, . . . , LxS}, respectively, wherein C1 and L1 are based on T neighboring chroma and luma samples, denoted {Cy1, Cy2, . . . , CyT} and {Ly1, Ly2, . . . , LyT}, respectively, wherein S=T,
• wherein {Cx1, Cx2, . . . , CxS} are corresponding to {Lx1, Lx2, . . . , LxS},
• wherein {Cy1, Cy2, . . . , CyT} are corresponding to {Ly1, Ly2, . . . , LyT},
• wherein C0=f0(Cx1, Cx2, . . . , CxS), L0=f1(Lx1, Lx2, . . . , LxS), C1=f2(Cy1, Cy2, . . . , CyT) and L1=f3(Ly1, Ly2, . . . , LyT), and
• wherein f0, f1, f2 and f3 are functions.