HM速率控制部分代码学习

1. 分配GOP级的目标比特数
2. 分配Slice级的目标比特数
3. 根据Slice级的目标比特数，确定Slice级的量化参数；根据Slice级的目标比特数，和已经编码的CTU，分配下一个CTU的比特数。
4. 根据CTU的目标比特数，确定CTU的量化参数。
5. 编码完一个CTU，更新CTU的量化参数。
6. 编码完一个Slice，更新Slice的量化参数。

首先说明速率控制重要的类及功能：

还有一个类 TEncRateCtrl管理上面三个类的对象，通过都名字能看明白。

码率控制在编码过程中哪些部分起作用。
下面这段代码是在compressGOP函数里面对帧级别的λ和QP的控制

   if ( m_pcCfg->getUseRateCtrl() ) // TODO: does this work with multiple slices and slice-segments?
    {
      Int frameLevel = m_pcRateCtrl->getRCSeq()->getGOPID2Level( iGOPid );
      if ( pcPic->getSlice(0)->getSliceType() == I_SLICE )
      {
        frameLevel = 0;
      }
      m_pcRateCtrl->initRCPic( frameLevel );
      estimatedBits = m_pcRateCtrl->getRCPic()->getTargetBits()

      Int sliceQP = m_pcCfg->getInitialQP();
      if ( ( pcSlice->getPOC() == 0 && m_pcCfg->getInitialQP() > 0 ) || ( frameLevel == 0 && m_pcCfg->getForceIntraQP() ) ) // QP is specified
      {
        Int    NumberBFrames = ( m_pcCfg->getGOPSize() - 1 );
        Double dLambda_scale = 1.0 - Clip3( 0.0, 0.5, 0.05*(Double)NumberBFrames );
        Double dQPFactor     = 0.57*dLambda_scale;
        Int    SHIFT_QP      = 12;
        Int    bitdepth_luma_qp_scale = 0;
        Double qp_temp = (Double) sliceQP + bitdepth_luma_qp_scale - SHIFT_QP;
        lambda = dQPFactor*pow( 2.0, qp_temp/3.0 );
      }
      else if ( frameLevel == 0 )   // intra case, but use the model
      {
        m_pcSliceEncoder->calCostSliceI(pcPic); // TODO: This only analyses the first slice segment - what about the others?

        if ( m_pcCfg->getIntraPeriod() != 1 )   // do not refine allocated bits for all intra case
        {
          Int bits = m_pcRateCtrl->getRCSeq()->getLeftAverageBits();
          bits = m_pcRateCtrl->getRCPic()->getRefineBitsForIntra( bits )

          if ( bits < 200 )
          {
            bits = 200;
          }
          m_pcRateCtrl->getRCPic()->setTargetBits( bits );
        }

        list listPreviousPicture = m_pcRateCtrl->getPicList();
        m_pcRateCtrl->getRCPic()->getLCUInitTargetBits();
        lambda  = m_pcRateCtrl->getRCPic()->estimatePicLambda( listPreviousPicture, pcSlice->getSliceType());
        sliceQP = m_pcRateCtrl->getRCPic()->estimatePicQP( lambda, listPreviousPicture );
      }
      else    // normal case
      {
        list listPreviousPicture = m_pcRateCtrl->getPicList();
        lambda  = m_pcRateCtrl->getRCPic()->estimatePicLambda( listPreviousPicture, pcSlice->getSliceType());
        sliceQP = m_pcRateCtrl->getRCPic()->estimatePicQP( lambda, listPreviousPicture );
      }

      sliceQP = Clip3( -pcSlice->getSPS()->getQpBDOffset(CHANNEL_TYPE_LUMA), MAX_QP, sliceQP );
      m_pcRateCtrl->getRCPic()->setPicEstQP( sliceQP );

      m_pcSliceEncoder->resetQP( pcPic, sliceQP, lambda );
    }

下面这段代码是CompressSlice函数里面对CTU的QP和λ的控制

 if ( m_pcCfg->getUseRateCtrl() )
    {
      Int estQP        = pcSlice->getSliceQp();
      Double estLambda = -1.0;
      Double bpp       = -1.0;

      if ( ( pcPic->getSlice( 0 )->getSliceType() == I_SLICE && m_pcCfg->getForceIntraQP() ) || !m_pcCfg->getLCULevelRC() )
      {
        estQP = pcSlice->getSliceQp();
      }
      else
      {
        bpp = m_pcRateCtrl->getRCPic()->getLCUTargetBpp(pcSlice->getSliceType());
        if ( pcPic->getSlice( 0 )->getSliceType() == I_SLICE)
        {
          estLambda = m_pcRateCtrl->getRCPic()->getLCUEstLambdaAndQP(bpp, pcSlice->getSliceQp(), &estQP);
        }
        else
        {
          estLambda = m_pcRateCtrl->getRCPic()->getLCUEstLambda( bpp );
          estQP     = m_pcRateCtrl->getRCPic()->getLCUEstQP    ( estLambda, pcSlice->getSliceQp() );
        }

        estQP     = Clip3( -pcSlice->getSPS()->getQpBDOffset(CHANNEL_TYPE_LUMA), MAX_QP, estQP );

        m_pcRdCost->setLambda(estLambda, pcSlice->getSPS()->getBitDepths());
      }

      m_pcRateCtrl->setRCQP( estQP );
    }