Missing a default/inferred value for alf_cc_cb/cr_filters_signalled_minus1

[swong10]

The value of alf_cc_cb/cr_filters_signalled_minus1 Syntax Element shall be in the range of 0 to 3. It is used to parse alf_ctb_cc_cb/cr_idc[ ][ ].

If alf_cc_cb/cr_filter_signal_flag is set to 0, alf_cc_cb/cr_filters_signalled_minus1 is not present in the bitstream.

The VVC spec semantic does not provide an inferred or default value for alf_cc_cb/cr_filters_signalled_minus1. But in the VTM, it is effectively assumed a value of 3 when it is not present (i.e. alf_cc_cb/cr_filters_signalled = 3+1 = 4). Because there is a line in the following code segment (
ccAlfFilterCount[0] = ccAlfFilterCount[1] = MAX_NUM_CC_ALF_FILTERS; which is equal to 4). These 2 VTM variables are not inferred or defaulted to 0 as we would expect.

In AlfParameter.h:

struct CcAlfFilterParam
{
  bool    ccAlfFilterEnabled[2];
  bool    ccAlfFilterIdxEnabled[2][MAX_NUM_CC_ALF_FILTERS];
  uint8_t ccAlfFilterCount[2];
  short   ccAlfCoeff[2][MAX_NUM_CC_ALF_FILTERS][MAX_NUM_CC_ALF_CHROMA_COEFF];
  int     newCcAlfFilter[2];
  int     numberValidComponents;
  CcAlfFilterParam()
  {
    reset();
  }
  void reset()
  {
    std::memset( ccAlfFilterEnabled, false, sizeof( ccAlfFilterEnabled ) );
    std::memset( ccAlfFilterIdxEnabled, false, sizeof( ccAlfFilterIdxEnabled ) );
    std::memset( ccAlfCoeff, 0, sizeof( ccAlfCoeff ) );
    ccAlfFilterCount[0] = ccAlfFilterCount[1] = MAX_NUM_CC_ALF_FILTERS;
    numberValidComponents = 3;
    newCcAlfFilter[0] = newCcAlfFilter[1] = 0;
  }

The choice of this inferred/default value does affect the CABAC decoding of a bitstream that has alf_cc_cb/cr_filter_signal_flag set to 0.

Please confirm if this is a conformance case, and if a default/inferred value should be specified in the VVC spec to match that of the VTM?

[swong10]

There are couple related settings that have similar issues, which may be addressed together in this ticket as well.

There is also no default/inferred value for alf_chroma_num_alt_filters_minus1 and alf_luma_num_filters_signalled_minus1 when they are not present in the bitstream. The syntax element alf_chroma_num_alt_filters_minus is also used in CABAC decoding of alf_ctb_filter_alt_idx[ ][ ][ ].

But in the VTM (code segment pasted below), these two SE are effectively defaulted to 0 (i.e. alf_chroma_num_alt_filters = 0 + 1 = 1), when they are not present in the bitstream.

In AlfParameters.h:
struct AlfParam
{

void reset()
{

…
numAlternativesChroma = 1;
…
numLumaFilters = 1;
…

}

Thanks
Sam.