id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
1076	Mismatch between spec and VTM on assignment of ctxInc to amvr_precision_idx	wangyang.cs		"There is a mismatch between spec and VTM on assignment of ctxInc to amvr_precision_idx.
In JVET-R2001-vA, according to Table 126, 1 or 2 bins are used when amvr_precision_idx is context coded:
'''Affine: 1 bin'''
'''IBC: 1 bin'''
'''Non-affine, non-IBC: 2 bins'''

Table 126 – Syntax elements and associated binarizations
||= Syntax structure =||= Syntax element =||||= Binarization ||
|| || ||=Process=||=Input parameters =||
||=coding_unit()=||=amvr_precision_idx[ ][ ]=||=RT=||=cMax = ( inter_affine_flag  = =  0  &&  CuPredMode[ 0 ][ x0 ][ y0 ]  !=  MODE_IBC ) ? 2 : 1, cRiceParam = 0=||

Two ctxIncs are assigned to the bins of amvr_precision_idx. One is assigned to the first bin, the other is assigned to the second bin (if any).

Table 131 – Assignment of ctxInc to syntax elements with context coded bins
||=Syntax element=||||||||||||=binIdx=||
|| ||='''0'''=||='''1'''=||='''2'''=||='''3'''=||='''4'''=||='''>=5'''=||
||=amvr_precision_idx[ ][ ]=||0||1||na||na||na||na||

'''However, in VTM-8.0, three ctxIncs are assigned to the bins of amvr_precision_idx depending on whether current block is affine or IBC mode.'''

In {{{void CABACReader::imv_mode()}}}
{{{
if ( cu.affine )
{
  return;
}
const SPS *sps = cu.cs->sps;
unsigned value = 0;
if (CU::isIBC(cu))
  value = 1;
else
  value = m_BinDecoder.decodeBin( Ctx::ImvFlag( 0 ) );
DTRACE( g_trace_ctx, D_SYNTAX, ""imv_mode() value=%d ctx=%d\n"", value, 0 );
cu.imv = value;
if( sps->getAMVREnabledFlag() && value )
{
  if (!CU::isIBC(cu))
  {
    value = m_BinDecoder.decodeBin(Ctx::ImvFlag(4));
    DTRACE(g_trace_ctx, D_SYNTAX, ""imv_mode() value=%d ctx=%d\n"", value, 4);
    cu.imv = value ? 1 : IMV_HPEL;   
  }
  if (value)
  {
    value = m_BinDecoder.decodeBin( Ctx::ImvFlag( 1 ) );
    DTRACE( g_trace_ctx, D_SYNTAX, ""imv_mode() value=%d ctx=%d\n"", value, 1 );
    value++;
    cu.imv = value;
  }
}
}}}

For non-affine and non-IBC, 2 bins are used to code amvr_precision_idx. Ctx::ImvFlag(4) and Ctx::ImvFlag( 1 ) assigned to the first bin and the second bin.
For IBC, 1 bin is used to code amvr_precision_idx. Ctx::ImvFlag( 1 ) is also assigned to the first bin of amvr_precision_idx. The same ctxInc is also assigned to the second bin for non-affine and non-IBC.

In {{{void CABACReader::affine_amvr_mode()}}}
{{{
unsigned value = 0;
value = m_BinDecoder.decodeBin( Ctx::ImvFlag( 2 ) );
DTRACE( g_trace_ctx, D_SYNTAX, ""affine_amvr_mode() value=%d ctx=%d\n"", value, 2 );
if( value )
{
  value = m_BinDecoder.decodeBin( Ctx::ImvFlag( 3 ) );
  DTRACE( g_trace_ctx, D_SYNTAX, ""affine_amvr_mode() value=%d ctx=%d\n"", value, 3 );
  value++;
}
}}}
For affine, 1 bin is used to code amvr_precision_idx. Ctx::ImvFlag( 3 ) is assigned to the first bin.

Suggest changes in JVET-R2001-vA as follows:

'''9.3.2.2 Initialization process for context variables'''
'''Table 51 – Association of ctxIdx and syntax elements for each initializationType
in the initialization process'''
||='''Syntax structure'''=||='''Syntax element'''=||='''ctxTable'''=||||||='''initType'''=||
||=coding_unit( )=||=amvr_precision_idx[ ][ ]=||=Table 89=||=0..[[ticket:1|1]][[span(style=color:#0000FF, 2)]]=||=[[ticket:1|2]][[span(style=color:#0000FF,3)]]..[[ticket:1|3]][[span(style=color:#0000FF,5)]]=||=[[ticket:1|4]][[span(style=color:#0000FF,6)]]..[[ticket:1|5]][[span(style=color:#0000FF,8)]]=||

'''Table 89 – Specification of initValue and shiftIdx for ctxIdx of amvr_precision_idx'''
||='''Initialization variable'''=||||||||||||||||||='''ctxIdx of amvr_precision_idx'''=||
|| ||='''0'''=||='''1'''=||='''2'''=||='''3'''=||='''4'''=||='''5'''=||=[[span(style=color:#0000FF,'''6''')]]=||=[[span(style=color:#0000FF,'''7''')]]=||=[[span(style=color:#0000FF,'''8''')]]=||
||='''initValue'''=||=EP=||=EP=||=EP=||=EP=||=EP=||=EP=||=[[span(style=color:#0000FF,EP)]]=||=[[span(style=color:#0000FF,EP)]]=||=[[span(style=color:#0000FF,EP)]]=||
||='''shiftIdx'''=||=0=||=0=||=0=||=0=||=0=||=0=||=[[span(style=color:#0000FF,0)]]=||=[[span(style=color:#0000FF,0)]]=||=[[span(style=color:#0000FF,0)]]=||

'''9.3.4.2 Derivation process for ctxTable, ctxIdx and bypassFlag'''
'''9.3.4.2.1 General'''
'''Table 131 – Assignment of ctxInc to syntax elements with context coded bins'''
||='''Syntax element'''=||||||||||||='''binIdx'''=||
|| ||='''0'''=||='''1'''=||='''2'''=||='''3'''=||='''4'''=||='''>=5'''=||
||=amvr_precision_idx[ ][ ]=||[[ticket:1|0]][[span(style=color:#0000FF,(CuPredMode[ 0 ][ x0 ][ y0 ]  ==  MODE_IBC)  ? 1 : (inter_affine_flag  = =  0 ? 0 : 2))]]||1||na||na||na||na||"	defect	closed	minor	VVC D7 vC	spec	VVC D8 vB	fixed		ksuehring bbross XiangLi fbossen jvet@… jvet@…