Mismatch between spec and VTM about context modeling for intra_luma_not_planar_flag

[Jun_bd]

Problem statement:
In spec S2001-v1 (Table 131 – Assignment of ctxInc to syntax elements with context coded bins), the ctxInc for syntax element intra_luma_not_planar_flag (only one bin) is specified as intra_subpartitions_mode_flag, which means if "intra_subpartitions_mode_flag" is 1 (ISP mode), ctxInc is 1 and if "intra_subpartitions_mode_flag" is 0 (non-ISP) ctxInc is 0.

However, in the latest VTM (commit id: cb7b243e), the ctxInc for intra_luma_not_planar_flag is computed in the other way around (shown below, 1384 line of CABACReader.cpp).

unsigned ctx = (pu->cu->ispMode == NOT_INTRA_SUBPARTITIONS ? 1 : 0);

It can be seen that if current CU uses ISP mode (pu->cu->ispMode == NOT_INTRA_SUBPARTITIONS is FALSE), the ctxInc is 0 and if current CU uses non-ISP mode (pu->cu->ispMode == NOT_INTRA_SUBPARTITIONS is TRUE), the ctxInc is 1.

Suggestion:
Aligning spec with SW, i.e., specify the ctxInc for intra_luma_not_planar_flag as !intra_subpartitions_mode_flag in spec.

[biaowang]

by checking the spec. provided in JVET-N0185-v7, the raised issue is confirmed.
In fact, in spec provided in JVET-N0185-v7, the ctxInc for intra_luma_not_planar_flag is already
!intra_subpartitions_mode_flag

[bbross]

Good catch, will be fixed in JVET-S2001-v7.