【VTM10.0】反量化之DQ技术

DQ反量化时，会把初始状态设置为0，然后根据level（也就是k）来进行下个点的state判断，如下图所示：

反DQ的函数是dequantBlock函数，个人理解见注释：

void Quantizer::dequantBlock( const TransformUnit& tu, const ComponentID compID, const QpParam& cQP, CoeffBuf& recCoeff, bool enableScalingLists, int* piDequantCoef) const
  {

    //----- set basic parameters -----
    const CompArea&     area      = tu.blocks[ compID ];
    const int           numCoeff  = area.area();//指area内的像素有多少
    const SizeType      hsId      = gp_sizeIdxInfo->idxFrom( area.width  );
    const SizeType      vsId      = gp_sizeIdxInfo->idxFrom( area.height );
    const CoeffScanType scanType  = SCAN_DIAG;
    const ScanElement *scan       = g_scanOrder[SCAN_GROUPED_4x4][scanType][hsId][vsId];
    const TCoeff*       qCoeff    = tu.getCoeffs( compID ).buf;
          TCoeff*       tCoeff    = recCoeff.buf;

    //----- reset coefficients and get last scan index -----
    ::memset( tCoeff, 0, numCoeff * sizeof(TCoeff) );
    int lastScanIdx = -1;
    for( int scanIdx = numCoeff - 1; scanIdx >= 0; scanIdx-- )
    {
      if (qCoeff[scan[scanIdx].idx])
      {
        lastScanIdx = scanIdx;//获得最后一个记录的系数的位置
        break;
      }
    }
    if( lastScanIdx < 0 )
    {
      return;
    }

    //----- set dequant parameters -----
    const int         qpDQ                  = cQP.Qp(tu.mtsIdx[compID] == MTS_SKIP) + 1;
    const int         qpPer                 = qpDQ / 6;
    const int         qpRem                 = qpDQ - 6 * qpPer;
    const SPS&        sps                   = *tu.cs->sps;
    const ChannelType chType                = toChannelType( compID );
    const int         channelBitDepth       = sps.getBitDepth( chType );
    const int         maxLog2TrDynamicRange = sps.getMaxLog2TrDynamicRange( chType );
    const TCoeff      minTCoeff             = -( 1 << maxLog2TrDynamicRange );
    const TCoeff      maxTCoeff             =  ( 1 << maxLog2TrDynamicRange ) - 1;
    const int         nomTransformShift     = getTransformShift( channelBitDepth, area.size(), maxLog2TrDynamicRange );
    const bool        clipTransformShift    = ( tu.mtsIdx[compID] == MTS_SKIP && sps.getSpsRangeExtension().getExtendedPrecisionProcessingFlag());
    const bool    needsSqrt2ScaleAdjustment = TU::needsSqrt2Scale(tu, compID);
    const int         transformShift        = ( clipTransformShift ? std::max<int>( 0, nomTransformShift ) : nomTransformShift ) + (needsSqrt2ScaleAdjustment?-1:0);
    Intermediate_Int  shift                 = IQUANT_SHIFT + 1 - qpPer - transformShift + (enableScalingLists ? LOG2_SCALING_LIST_NEUTRAL_VALUE : 0);
    Intermediate_Int  invQScale             = g_invQuantScales[needsSqrt2ScaleAdjustment?1:0][ qpRem ];
    Intermediate_Int  add = (shift < 0) ? 0 : ((1 << shift) >> 1);
    //----- dequant coefficients -----DQ解码，首先设置state为0
    for( int state = 0, scanIdx = lastScanIdx; scanIdx >= 0; scanIdx-- )
    {
      const unsigned  rasterPos = scan[scanIdx].idx;
      const TCoeff&   level     = qCoeff[ rasterPos ];
      if( level )//量化值为0就直接跳过了，因为量化为0的，反量化的值也为0，tCoeff[rasterPos]初始化的时候就全部设置的为0
      {
        if (enableScalingLists)//是否使用了量化矩阵
          invQScale = piDequantCoef[rasterPos];//scalingfactor*levelScale
        if (shift < 0 && (enableScalingLists || scanIdx == lastScanIdx))
        {
          invQScale <<= -shift;
        }
        //如果state是1或0，(state>>1)为0；如果state是2或3，(state>>1)为1。这是因为0，1的量化步长是偶数倍，2，3的量化步长是奇数倍
        Intermediate_Int  qIdx      = ( level << 1 ) + ( level > 0 ? -(state>>1) : (state>>1) );
        int64_t  nomTCoeff          = ((int64_t)qIdx * (int64_t)invQScale + add) >> ((shift < 0) ? 0 : shift);
        tCoeff[rasterPos]           = (TCoeff)Clip3<int64_t>(minTCoeff, maxTCoeff, nomTCoeff);//得到反量化后的值
      }
      
      //state状态改变
      //32040 : 0111 1101 0010 1000，32040中每4位分别对应3，2，1，0的下一个候选state。
      //如果本身是state1，那么下一个状态可能是3或1，那么(state<<2)就表示30240先右移8位，这时的低4位为1101
      //然后根据level的奇偶判断是否还要右移两位，如果时level是奇数，再右移2位，低2位变成11,&3后结果为3，表示下一个状态是3。
      state = ( 32040 >> ((state<<2)+((level&1)<<1)) ) & 3;   // the 16-bit value "32040" represent the state transition table
    }
  }