Incorrect corresponding luma posiction for chroma mode derivation with lfnst_idx > 0 in subcluase 8.7.4.1

[hmjang]

Intra prediction mode is derived in sub-clause 8.7.4.1(transformation process) in order to select the LFNST set.
in SW, for chroma component, corresponding luma location is referred to derive chroma intra prediction mode. the corresponding luma location is the center position where it is the same for the INTRA_DM derivation process.

but in Spec, the corresponding position is pointed incorrectly for 4:2:0 and 4:2:2 color format as below. Because the variable nTbW and nTbH specify the chroma transform block size.

– When predModeIntra is equal to either INTRA_LT_CCLM, INTRA_L_CCLM, or INTRA_T_CCLM, predModeIntra is derived as follows:
– If intra_mip_flag[ xTbY + nTbW/ 2 ][ yTbY + nTbH / 2 ] is equal to 1, predModeIntra is set equal to INTRA_PLANAR.
– Otherwise, if CuPredMode[ 0 ][ xTbY + nTbW/ 2 ][ yTbY + nTbH/ 2 ] is equal to MODE_IBC or MODE_PLT, predModeIntra is set equal to INTRA_DC.
– Otherwise, predModeIntra is set equal to IntraPredModeY[ xTbY + nTbW/ 2 ][ yTbY + nTbH/ 2 ].

to derive correct position, scaling offset should be considered. the suggest description is depicted as below:

– When predModeIntra is equal to either INTRA_LT_CCLM, INTRA_L_CCLM, or INTRA_T_CCLM, predModeIntra is derived as follows:
– If intra_mip_flag[ xTbY + ( nTbW * subWidthC )/ 2 ][ yTbY + ( nTbH * subHeightC ) / 2 ] is equal to 1, predModeIntra is set equal to INTRA_PLANAR.
– Otherwise, if CuPredMode[ 0 ][ xTbY + ( nTbW * subWidthC )/ 2 ][ yTbY + ( nTbH * subHeightC )/ 2 ] is equal to MODE_IBC or MODE_PLT, predModeIntra is set equal to INTRA_DC.
– Otherwise, predModeIntra is set equal to IntraPredModeY[ xTbY + ( nTbW * subWidthC )/ 2 ][ yTbY + ( nTbH * subHeightC )/ 2 ].

[bbross]

Thanks for reporting.

This will be fixed JVET-P2001-vC.