【VTM10.0】量化之一般量化技术

量化公式：

其中：

函数是quant函数，个人理解见注释：

void Quant::quant(TransformUnit &tu, const ComponentID &compID, const CCoeffBuf &pSrc, TCoeff &uiAbsSum, const QpParam &cQP, const Ctx& ctx)
{
  const SPS &sps            = *tu.cs->sps;
  const CompArea &rect      = tu.blocks[compID];
  const uint32_t uiWidth        = rect.width;
  const uint32_t uiHeight       = rect.height;
  const int channelBitDepth = sps.getBitDepth(toChannelType(compID));

  const CCoeffBuf &piCoef   = pSrc;
        CoeffBuf   piQCoef  = tu.getCoeffs(compID);

  const bool useTransformSkip      = (tu.mtsIdx[compID] == MTS_SKIP);
  const int  maxLog2TrDynamicRange = sps.getMaxLog2TrDynamicRange(toChannelType(compID));

  {
    CoeffCodingContext cctx(tu, compID, tu.cs->slice->getSignDataHidingEnabledFlag());
    const TCoeff entropyCodingMinimum = -(1 << maxLog2TrDynamicRange);
    const TCoeff entropyCodingMaximum =  (1 << maxLog2TrDynamicRange) - 1;

    TCoeff deltaU[MAX_TB_SIZEY * MAX_TB_SIZEY];
    int scalingListType = getScalingListType(tu.cu->predMode, compID);
    CHECK(scalingListType >= SCALING_LIST_NUM, "Invalid scaling list");
    const uint32_t uiLog2TrWidth = floorLog2(uiWidth);
    const uint32_t uiLog2TrHeight = floorLog2(uiHeight);
    int *piQuantCoeff = getQuantCoeff(scalingListType, cQP.rem(useTransformSkip), uiLog2TrWidth, uiLog2TrHeight);
    const bool disableSMForLFNST = tu.cs->slice->getExplicitScalingListUsed() ? tu.cs->slice->getSPS()->getDisableScalingMatrixForLfnstBlks() : false;
    const bool isLfnstApplied = tu.cu->lfnstIdx > 0 && (tu.cu->isSepTree() ? true : isLuma(compID));
    const bool disableSMForACT = tu.cs->slice->getSPS()->getScalingMatrixForAlternativeColourSpaceDisabledFlag() && (tu.cs->slice->getSPS()->getScalingMatrixDesignatedColourSpaceFlag() == tu.cu->colorTransform);
    const bool enableScalingLists = getUseScalingList(uiWidth, uiHeight, useTransformSkip, isLfnstApplied, disableSMForLFNST, disableSMForACT);

    // for blocks that where width*height != 4^N, the effective scaling applied during transformation cannot be
    // compensated by a bit-shift (the quantised result will be sqrt(2) * larger than required).
    // The quantScale table and shift is used to compensate for this.
    const bool needSqrtAdjustment= TU::needsBlockSizeTrafoScale( tu, compID );
    const int defaultQuantisationCoefficient    = g_quantScales[needSqrtAdjustment?1:0][cQP.rem(useTransformSkip)];
    int iTransformShift = getTransformShift(channelBitDepth, rect.size(), maxLog2TrDynamicRange) + ( needSqrtAdjustment?-1:0);

    if (useTransformSkip && sps.getSpsRangeExtension().getExtendedPrecisionProcessingFlag())
    {
      iTransformShift = std::max<int>(0, iTransformShift);
    }

    //获得iQBits，
    const int iQBits = QUANT_SHIFT + cQP.per(useTransformSkip) + (useTransformSkip ? 0 : iTransformShift);
    // QBits will be OK for any internal bit depth as the reduction in transform shift is balanced by an increase in Qp_per due to QpBDOffset

    //本来对与I帧，f取1/3，B帧或者P帧，f取1/6，这里把f左移了9位，变成171和85。所以后面要-9
    const int64_t iAdd = int64_t(tu.cs->slice->isIRAP() ? 171 : 85) << int64_t(iQBits - 9);
    const int qBits8 = iQBits - 8;

    const uint32_t lfnstIdx = tu.cu->lfnstIdx;
    //得到要量化的点的总个数
    const int maxNumberOfCoeffs = lfnstIdx > 0 ? ((( uiWidth == 4 && uiHeight == 4 ) || ( uiWidth == 8 && uiHeight == 8) ) ? 8 : 16) : piQCoef.area();
    memset( piQCoef.buf, 0, sizeof(TCoeff) * piQCoef.area() );//初始化piQCoef.buf

    const ScanElement* scan = g_scanOrder[SCAN_GROUPED_4x4][SCAN_DIAG][gp_sizeIdxInfo->idxFrom(uiWidth)][gp_sizeIdxInfo->idxFrom(uiHeight)];

    for (int uiScanPos = 0; uiScanPos < maxNumberOfCoeffs; uiScanPos++)//遍历每个点
    {
      const int uiBlockPos = scan[uiScanPos].idx;
      const TCoeff iLevel   = piCoef.buf[uiBlockPos];
      const TCoeff iSign    = (iLevel < 0 ? -1: 1);//记录符号

      //相当与d*MF
      const int64_t  tmpLevel = (int64_t)abs(iLevel) * (enableScalingLists ? piQuantCoeff[uiBlockPos] : defaultQuantisationCoefficient);

      //完成量化值的计算(d*MF+f')>>(qbits+T_shift)
      const TCoeff quantisedMagnitude = TCoeff((tmpLevel + iAdd ) >> iQBits);
      deltaU[uiBlockPos] = (TCoeff)((tmpLevel - ((int64_t)quantisedMagnitude<<iQBits) )>> qBits8);//为下面SDH技术做准备

      uiAbsSum += quantisedMagnitude;//为下面SDH技术做准备
      const TCoeff quantisedCoefficient = quantisedMagnitude * iSign;//加上符号

      //将量化结果放到piQCoef.buf中
      piQCoef.buf[uiBlockPos] = Clip3<TCoeff>( entropyCodingMinimum, entropyCodingMaximum, quantisedCoefficient );
    } // for n
    if ((tu.cu->bdpcmMode && isLuma(compID)) || (tu.cu->bdpcmModeChroma && isChroma(compID)) )
    {
      fwdResDPCM( tu, compID );
    }
    if( cctx.signHiding() )//是否使用SDH技术
    {
      if(uiAbsSum >= 2) //this prevents TUs with only one coefficient of value 1 from being tested
      {
        xSignBitHidingHDQ(piQCoef.buf, piCoef.buf, deltaU, cctx, maxLog2TrDynamicRange);
      }
    }
  } //if RDOQ
  //return;
}