software/spec mismatch for APS chroma present flag from R0433 integration

[taoranlu]

There is a software/spec mismatch related to APS chroma present flag in current VTM master from the integration of R0433 (MR1767).
In VLCWriter.cpp (and corresponding VLCReader.cpp), when signal the LMCS APS, the condition if (pcAPS->chromaPresentFlag) should include the signalling both of the sign and abs of lmcs delta crs. In current VTM, only the abs is inside the if condition and sign is left out of the if condition.

Spec:

if( aps_chroma_present_flag ) {

lmcs_delta_abs_crs
if( lmcs_delta_abs_crs > 0 )

lmcs_delta_sign_crs_flag

}

[fbossen]

My understanding is that text and SW are equivalent.
When chromaPresentFlag is false, deltaCRS is set to 0 and absCRS is set to 0 and lmcs_delta_sign_crs_flag is not coded, as intended.

  int deltaCRS = pcAPS->chromaPresentFlag ? param.chrResScalingOffset : 0;
  int signCRS = (deltaCRS < 0) ? 1 : 0;
  int absCRS = (deltaCRS < 0) ? (-deltaCRS) : deltaCRS;
  if (pcAPS->chromaPresentFlag)
  {
    WRITE_CODE(absCRS, 3, "lmcs_delta_abs_crs");
  }
  if (absCRS > 0)
  {
    WRITE_FLAG(signCRS, "lmcs_delta_sign_crs_flag");
  }

[taoranlu]

I agree it is working equivalently from software point of view. Just when people read the code, it is logically more clear that the condition should apply for both abs and sign. It is a trivial issue and it's fine if we stay as-is.