Mismatch on ScalingMatrixDCRec derivation in eq.(104) between Spec and SW

[tsukuba.takeshi]

In the spec (JVET-S2001vH), ScalingMatrixDCRec derivation is defined as in eq. (104):

ScalingMatrixDCRec[ id − 14 ] = ( ScalingMatrixDCPred + scaling_list_dc_coef[ id − 14 ] ) & 255 (104)

However, in VTM-10.2, ScalingMatrixDCRec derivation is implemented as below:

VLCReader.cpp void HLSSyntaxReader::decodeScalingList(...)
{
... 
    READ_SVLC(scalingListDcCoefMinus8, "scaling_list_dc_coef_minus8");
    nextCoef += scalingListDcCoefMinus8;
    if (isPredictor)
    {
      predCoef = (PredListId >= SCALING_LIST_1D_START_16x16) ? scalingList->getScalingListDC(PredListId) : srcPred[0];
    }
    scalingList->setScalingListDC(scalingListId, (nextCoef + predCoef + 256) & 255);
...
}

As can be seen, there is "+256" in SW but not in Spec.

Looking at the adopt text JVET-P1034, "+256" (and also "&255") is not defined in the syntax table.

      ...
	scaling_list_dc_coef [ scalingListId ]	se(v)
	nextCoef = scaling_list_dc_coef [ scalingListId ] + nextCoef	
      ...

Looking at the text in JVET-P2001vC, "+256" and "%256 (or &255)" are found in ScalingMatrixDCRec derivation as below:

ScalingMatrixDCRec[ id − 14 ] = ( ScalingMatrixDCPred + scaling_list_dc_coef[ id − 14 ] + 256 ) % 256 ) (7 71)

But after the text cleanup of JVET-P2001vD, "+256" was deleted.

I don't know which one is correct.

[fbossen]

A & 255 is the same as (A + 256) & 255
A % 256 is not the same as (A + 256) % 256

When using % 256 one needs to be careful to work with positive numbers.
However, there is no such issue with & 255.

I don't see an issue here.

[tsukuba.takeshi]

I agree that there is no issue.