HEVC码率控制（二）：从compressGOP()到compressSlice()

compressGOP()函数要点：

• 根据iGOPid逐次递增，对一个GOP内的picture依次编码
• 根据picture level对当前picture进行码率分配
• 实际中一个Pic就是一个Slice，因此关于slice的循环只执行一次（默认）
• 根据已经编过的picture，来得到当前picture的lambda
• 得到当前picture的lambda后，对该picture下每一个CTU进行码率分配
• 一个picture编完,码流写好了，根据实际编码所用的比特，更新对应level下的lambda参数

compressGOP(){    ...    for(Int iGOPid=0;iGOPid<m_iGopSize;iGOPid++) //GOP内picture循环    {        m_pcSliceEncoder->initEncSlice();        //当前Slice编码参数QP、lambda的默认设置，会在码率控制中进一步被修改        ...        if(m_pcCfg->getUseRateCtrl())        {            m_pcRateCtrl->initRCPic(frameLevel);//根据frame level进行比特分配            ...            lambda=m_pcRateCtrl->getRCPic()->estimatePicLambda();//确定当前picture的lambda            sliceQP = m_pcRateCtrl->getRCPic()->estimatePicQP( lambda,.. );//QP由lambda计算            ...         }        for(UInt nextCtuTsAddr = 0; nextCtuTsAddr < numberOfCtusInFrame; ) //这个循环只执行了一次        {            ...            m_pcSliceEncoder->compressSlice();            ...        }        ...        //编完一帧，开始熵编码，写码流等等        ...        if(m_pcCfg->getUseRateCtrl())        {            ...            updataAfterPicture(); //编完一帧，更新当前picture level对应的lambda参数值（alpha和beta）            ...        }    }//iGOPid loop}

• estimatePicLambda()函数中，在得到lambda之后，对一个picture内所有LCU进行比特分配：

Double TEncRCPic::estimatePicLambda( ){  Double alpha         = m_encRCSeq->getPicPara( m_frameLevel ).m_alpha;  Double beta          = m_encRCSeq->getPicPara( m_frameLevel ).m_beta;  //alpha和beta从数组中对应level处取，这两个值在编完前一个相同level的帧后，被设置  Double bpp       = (Double)m_targetBits/(Double)m_numberOfPixel;  Double estLambda;  if (eSliceType == I_SLICE)  {    estLambda = calculateLambdaIntra(alpha, beta, pow(m_totalCostIntra/(Double)m_numberOfPixel, BETA1), bpp);  }  else  {    estLambda = alpha * pow( bpp, beta );  }  Double lastLevelLambda = -1.0;  Double lastPicLambda   = -1.0;  Double lastValidLambda = -1.0;  //对estLambda进行Clip操作  m_estPicLambda = estLambda; //TEncRCPic::m_estPicLambda  Double totalWeight = 0.0;  // initial BU bit allocation weight  for ( Int i=0; i<m_numberOfLCU; i++ )  {    Double alphaLCU, betaLCU;    if ( m_encRCSeq->getUseLCUSeparateModel() )    {      alphaLCU = m_encRCSeq->getLCUPara( m_frameLevel, i ).m_alpha;      betaLCU  = m_encRCSeq->getLCUPara( m_frameLevel, i ).m_beta;    }    else    {      alphaLCU = m_encRCSeq->getPicPara( m_frameLevel ).m_alpha;      betaLCU  = m_encRCSeq->getPicPara( m_frameLevel ).m_beta;    }    m_LCUs[i].m_bitWeight =  m_LCUs[i].m_numberOfPixel * pow( estLambda/alphaLCU, 1.0/betaLCU );    if ( m_LCUs[i].m_bitWeight < 0.01 )    {      m_LCUs[i].m_bitWeight = 0.01;    }    totalWeight += m_LCUs[i].m_bitWeight;  }  for ( Int i=0; i<m_numberOfLCU; i++ )  {    Double BUTargetBits = m_targetBits * m_LCUs[i].m_bitWeight / totalWeight;    m_LCUs[i].m_bitWeight = BUTargetBits;  }  return estLambda;}

m_bitWeight即使每一个CTU对应的比特分配权重

• 跟新picture level 的alpha和beta函数updateAfterPicture():

void TEncRCPic::updateAfterPicture(){    //计算inputLambda    //计算calLambda        alpha += m_encRCSeq->getAlphaUpdate() * ( log( inputLambda ) - log( calLambda ) ) * alpha;    Double lnbpp = log( picActualBpp );    lnbpp = Clip3( -5.0, -0.1, lnbpp );    //按照码率控制算法，跟新alpha和beta    TRCParameter rcPara;    rcPara.m_alpha = alpha;    rcPara.m_beta  = beta;    m_encRCSeq->setPicPara( m_frameLevel, rcPara );    //将更新后的alpha和beta保存在数组中}

compressSlice()函数

• 按照CTU的先后顺序循环遍历编码
• 编完一个CTU之后，要更新参数

void TEncSlice::compressSlice(){    ...     xDetermineStartAndBoundingCtuTsAddr ( startCtuTsAddr, boundingCtuTsAddr, pcPic );     ...     for( UInt ctuTsAddr = startCtuTsAddr; ctuTsAddr < boundingCtuTsAddr; ++ctuTsAddr )      //对一个Slice内所有CTU开始循环编码     {         ...          if ( m_pcCfg->getUseRateCtrl() )          {              Int estQP        = pcSlice->getSliceQp();              Double estLambda = -1.0;              Double bpp       = -1.0;          //根据LCU的m_bitWeight这个成员计算当前LCU的比特，再除以LCU内Pixel总数，得到bpp。而m_bitWeight这个成员是得到Pic的lambda之后被赋值。              estLambda = m_pcRateCtrl->getRCPic()->getLCUEstLambdaAndQP(bpp, pcSlice->getSliceQp(), &estQP);          ...            m_pcRdCost->setLambda(estLambda, pcSlice->getSPS()->getBitDepths());         }         m_pcCuEncoder->compressCtu(pCtu);         ...         if(m_pcCfg->getUseRateCtrl())         {             ...             updateAfterCTU();             ...         }         //将CTU的编码结果累加到Picture的统计量之中         m_uiPicTotalBits+=pCtu->getTotalBits();         m_dPicRdcost+=PCtu->getTotalCost();         m_uiPicDist+=PCtu->getTotalDistortion;    }}