HEVC码率控制浅析——HM代码阅读之四

继续分析第一篇提到的compressSlice中对LCU的RC参数初始化：

#if RATE_CONTROL_LAMBDA_DOMAIN
      Double oldLambda = m_pcRdCost->getLambda();
      if ( m_pcCfg->getUseRateCtrl() )
      {
        Int estQP        = pcSlice->getSliceQp();
        Double estLambda = -1.0;
        Double bpp       = -1.0;

#if M0036_RC_IMPROVEMENT
        if ( ( rpcPic->getSlice( 0 )->getSliceType() == I_SLICE && m_pcCfg->getForceIntraQP() ) || !m_pcCfg->getLCULevelRC() )
#else
        if ( rpcPic->getSlice( 0 )->getSliceType() == I_SLICE || !m_pcCfg->getLCULevelRC() )
#endif
        { //!< 如果当前slice为I slice或者不进行LCU level RC，则LCU直接使用当前slice的QP
          estQP = pcSlice->getSliceQp();
        }
        else
        {
#if RATE_CONTROL_INTRA
          bpp = m_pcRateCtrl->getRCPic()->getLCUTargetBpp(pcSlice->getSliceType());
          if ( rpcPic->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() );
          }
#else
          bpp       = m_pcRateCtrl->getRCPic()->getLCUTargetBpp();
          estLambda = m_pcRateCtrl->getRCPic()->getLCUEstLambda( bpp );
          estQP     = m_pcRateCtrl->getRCPic()->getLCUEstQP    ( estLambda, pcSlice->getSliceQp() );
#endif

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

          m_pcRdCost->setLambda(estLambda);
#if M0036_RC_IMPROVEMENT
#if RDOQ_CHROMA_LAMBDA
          // set lambda for RDOQ
          Double weight=m_pcRdCost->getChromaWeight();
          m_pcTrQuant->setLambda( estLambda, estLambda / weight );
#else
          m_pcTrQuant->setLambda( estLambda );
#endif
#endif
        }

        m_pcRateCtrl->setRCQP( estQP );
        pcCU->getSlice()->setSliceQpBase( estQP ); //!< 设置编码时使用的QP值
      }
#endif

#if RATE_CONTROL_INTRA
Double TEncRCPic::getLCUTargetBpp(SliceType eSliceType)
#else
Double TEncRCPic::getLCUTargetBpp() //!< LCU level bit allocation
#endif
{
  Int   LCUIdx    = getLCUCoded(); //!< 未编码LCU数
  Double bpp      = -1.0;
  Int avgBits     = 0;
#if !M0036_RC_IMPROVEMENT
  Double totalMAD = -1.0;
  Double MAD      = -1.0;
#endif

#if RATE_CONTROL_INTRA
  if (eSliceType == I_SLICE){
    Int noOfLCUsLeft = m_numberOfLCU - LCUIdx + 1;
    Int bitrateWindow = min(4,noOfLCUsLeft);
    Double MAD      = getLCU(LCUIdx).m_costIntra;

    if (m_remainingCostIntra > 0.1 )
    {
      Double weightedBitsLeft = (m_bitsLeft*bitrateWindow+(m_bitsLeft-getLCU(LCUIdx).m_targetBitsLeft)*noOfLCUsLeft)/(Double)bitrateWindow;
      avgBits = Int( MAD*weightedBitsLeft/m_remainingCostIntra );
    }
    else
    {
      avgBits = Int( m_bitsLeft / m_LCULeft );
    }
    m_remainingCostIntra -= MAD;
  }
  else
  {
#endif
#if M0036_RC_IMPROVEMENT
  Double totalWeight = 0;
  for ( Int i=LCUIdx; i<m_numberOfLCU; i++ )
  {
    totalWeight += m_LCUs[i].m_bitWeight;
  }
  Int realInfluenceLCU = min( g_RCLCUSmoothWindowSize, getLCULeft() );
  avgBits = (Int)( m_LCUs[LCUIdx].m_bitWeight - ( totalWeight - m_bitsLeft ) / realInfluenceLCU + 0.5 );
#else
  if ( m_lastPicture == NULL ) //!< 如果前一帧图像为空，则直接求平均比特数
  {
    avgBits = Int( m_bitsLeft / m_LCULeft );
  }
  else //!< 利用前一帧保存下来的MAD求出当前LCU对应的权重，注意：此处的MAD已经进行过K0103中公式的两个处理了。
  {
    MAD = m_lastPicture->getLCU(LCUIdx).m_MAD;
    totalMAD = m_lastPicture->getTotalMAD();
    for ( Int i=0; i<LCUIdx; i++ )
    {
      totalMAD -= m_lastPicture->getLCU(i).m_MAD;
    }

    if ( totalMAD > 0.1 )
    {
      avgBits = Int( m_bitsLeft * MAD / totalMAD );
    }
    else
    {
      avgBits = Int( m_bitsLeft / m_LCULeft );
    }
  }
#endif
#if RATE_CONTROL_INTRA
  }
#endif

  if ( avgBits < 1 )
  {
    avgBits = 1;
  }

  bpp = ( Double )avgBits/( Double )m_LCUs[ LCUIdx ].m_numberOfPixel;
  m_LCUs[ LCUIdx ].m_targetBits = avgBits;

  return bpp;
}

Double TEncRCPic::getLCUEstLambda( Double bpp )
{
  Int   LCUIdx = getLCUCoded();
  Double alpha;
  Double beta;
  if ( m_encRCSeq->getUseLCUSeparateModel() ) //!< enable LCU level RC
  {
    alpha = m_encRCSeq->getLCUPara( m_frameLevel, LCUIdx ).m_alpha;
    beta  = m_encRCSeq->getLCUPara( m_frameLevel, LCUIdx ).m_beta;
  }
  else //!< 只进行picture level 的 RC，故alpha，beta使用的是picture level的值
  {
    alpha = m_encRCSeq->getPicPara( m_frameLevel ).m_alpha;
    beta  = m_encRCSeq->getPicPara( m_frameLevel ).m_beta;
  }

  Double estLambda = alpha * pow( bpp, beta );
  //for Lambda clip, picture level clip
  Double clipPicLambda = m_estPicLambda;

  //for Lambda clip, LCU level clip
  Double clipNeighbourLambda = -1.0;
  for ( int i=LCUIdx - 1; i>=0; i-- )
  {
    if ( m_LCUs[i].m_lambda > 0 )
    {
      clipNeighbourLambda = m_LCUs[i].m_lambda;
      break;
    }
  }
  //!< 在K0103的section 3.2中进行了定义
  if ( clipNeighbourLambda > 0.0 )
  {
    estLambda = Clip3( clipNeighbourLambda * pow( 2.0, -1.0/3.0 ), clipNeighbourLambda * pow( 2.0, 1.0/3.0 ), estLambda );
  }  

  if ( clipPicLambda > 0.0 )
  {
    estLambda = Clip3( clipPicLambda * pow( 2.0, -2.0/3.0 ), clipPicLambda * pow( 2.0, 2.0/3.0 ), estLambda );
  }
  else
  {
    estLambda = Clip3( 10.0, 1000.0, estLambda );
  }

  if ( estLambda < 0.1 )
  {
    estLambda = 0.1;
  }

  return estLambda;
}

Int TEncRCPic::getLCUEstQP( Double lambda, Int clipPicQP )
{
  Int LCUIdx = getLCUCoded();
  Int estQP = Int( 4.2005 * log( lambda ) + 13.7122 + 0.5 );

  //for Lambda clip, LCU level clip
  Int clipNeighbourQP = g_RCInvalidQPValue;
  for ( int i=LCUIdx - 1; i>=0; i-- )
  {
    if ( (getLCU(i)).m_QP > g_RCInvalidQPValue )
    {
      clipNeighbourQP = getLCU(i).m_QP;
      break;
    }
  } 

  //!< 在K0103的section 3.2中进行了定义
  if ( clipNeighbourQP > g_RCInvalidQPValue )
  {
    estQP = Clip3( clipNeighbourQP - 1, clipNeighbourQP + 1, estQP );
  }

  estQP = Clip3( clipPicQP - 2, clipPicQP + 2, estQP );

  return estQP;
}

至此，RC的初始化过程分析完毕，从之前几篇也能发现，其实看懂代码并不难，只要把提案看明白了，对整个流程熟悉，再把HM的框架搞清楚了，基本上就是照着提案找对应代码的事情了。但是，应该指出的是，这里仅仅只是浅显分析代码的含义，有关算法的深层内涵，比如为什么要这么做，这个值为什么要设置成这样等问题，需要在了解代码的基础上，进一步地仔细研究才行。