HEVC码率控制（三）：picture level的比特分配

每个picture的比特分配权重，跟GOP级别的Lagrangian multiplier lambda有关。已知GOP内每个picture的alpha和beta，在保证所有Picture的target bit加起来等于GOP的target bit的前提下，通过二分法，迭代求出GOP的拉格朗日乘子。

在 TEncTop:encode()函数中，执行compress GOP( )之前，还执行了一句对每个GOP的初始化：

Void TEncTop:encode){    ...  if ( m_RCEnableRateControl )  {    m_cRateCtrl.initRCGOP( m_iNumPicRcvd );  }  // compress GOP  m_cGOPEncoder.compressGOP(m_iPOCLast, m_iNumPicRcvd, m_cListPic, rcListPicYuvRecOut, accessUnitsOut, false, false, snrCSC, m_printFrameMSE);}
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• 在initRCGOP()函数中，完成了对每个picture的比特分配权重的确定

Void TEncRateCtrl::initRCGOP( Int numberOfPictures ){  m_encRCGOP = new TEncRCGOP;  m_encRCGOP->create( m_encRCSeq, numberOfPictures );}
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• 在create()函数中，通过二分法完成了对GOP level 的lambda求解

Void TEncRCGOP::create( TEncRCSeq* encRCSeq, Int numPic ){  destroy();  Int targetBits = xEstGOPTargetBits( encRCSeq, numPic );//GOP level bit allocation  if ( encRCSeq->getAdaptiveBits() > 0 && encRCSeq->getLastLambda() > 0.1 )  {    Double targetBpp = (Double)targetBits / encRCSeq->getNumPixel();    Double basicLambda = 0.0;    Double* lambdaRatio = new Double[encRCSeq->getGOPSize()];    Double* equaCoeffA = new Double[encRCSeq->getGOPSize()];    Double* equaCoeffB = new Double[encRCSeq->getGOPSize()];    if ( encRCSeq->getAdaptiveBits() == 1 )   // for GOP size =4, low delay case    {      if ( encRCSeq->getLastLambda() < 120.0 )      {        lambdaRatio[1] = 0.725 * log( encRCSeq->getLastLambda() ) + 0.5793;        lambdaRatio[0] = 1.3 * lambdaRatio[1];        lambdaRatio[2] = 1.3 * lambdaRatio[1];        lambdaRatio[3] = 1.0;      }      else      {        lambdaRatio[0] = 5.0;        lambdaRatio[1] = 4.0;        lambdaRatio[2] = 5.0;        lambdaRatio[3] = 1.0;      }    }    else if ( encRCSeq->getAdaptiveBits() == 2 )  // for GOP size = 8, random access case    {      if ( encRCSeq->getLastLambda() < 90.0 )      {        lambdaRatio[0] = 1.0;        lambdaRatio[1] = 0.725 * log( encRCSeq->getLastLambda() ) + 0.7963;        lambdaRatio[2] = 1.3 * lambdaRatio[1];        lambdaRatio[3] = 3.25 * lambdaRatio[1];        lambdaRatio[4] = 3.25 * lambdaRatio[1];        lambdaRatio[5] = 1.3  * lambdaRatio[1];        lambdaRatio[6] = 3.25 * lambdaRatio[1];        lambdaRatio[7] = 3.25 * lambdaRatio[1];      }      else      {        lambdaRatio[0] = 1.0;        lambdaRatio[1] = 4.0;        lambdaRatio[2] = 5.0;        lambdaRatio[3] = 12.3;        lambdaRatio[4] = 12.3;        lambdaRatio[5] = 5.0;        lambdaRatio[6] = 12.3;        lambdaRatio[7] = 12.3;      }    }    xCalEquaCoeff( encRCSeq, lambdaRatio, equaCoeffA, equaCoeffB, encRCSeq->getGOPSize() );//根据每个picture的alpha和beta，初始化系数数组A和B    basicLambda = xSolveEqua( targetBpp, equaCoeffA, equaCoeffB, encRCSeq->getGOPSize() );//二分法解等式方程    encRCSeq->setAllBitRatio( basicLambda, equaCoeffA, equaCoeffB );    delete []lambdaRatio;    delete []equaCoeffA;    delete []equaCoeffB;  }  m_picTargetBitInGOP = new Int[numPic];  Int i;   Int totalPicRatio = 0;//一个GOP内不同PIC的权重  Int currPicRatio = 0;  for ( i=0; i<numPic; i++ )  {    totalPicRatio += encRCSeq->getBitRatio( i );  }  for ( i=0; i<numPic; i++ )  {    currPicRatio = encRCSeq->getBitRatio( i );    m_picTargetBitInGOP[i] = (Int)( ((Double)targetBits) * currPicRatio / totalPicRatio );  }  m_encRCSeq    = encRCSeq;  m_numPic       = numPic;  m_targetBits   = targetBits;  m_picLeft      = m_numPic;  m_bitsLeft     = m_targetBits;}
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二分法实现lambda求解的代码：

Double TEncRCGOP::xSolveEqua( Double targetBpp, Double* equaCoeffA, Double* equaCoeffB, Int GOPSize ){  Double solution = 100.0;  Double minNumber = 0.1;  Double maxNumber = 10000.0;  for ( Int i=0; i<g_RCIterationNum; i++ )  {    Double fx = 0.0;    for ( Int j=0; j<GOPSize; j++ )    {      fx += equaCoeffA[j] * pow( solution, equaCoeffB[j] );    }    if ( fabs( fx - targetBpp ) < 0.000001 )    {      break;    }    if ( fx > targetBpp )    {      minNumber = solution;      solution = ( solution + maxNumber ) / 2.0;    }    else    {      maxNumber = solution;      solution = ( solution + minNumber ) / 2.0;    }  }  solution = Clip3( 0.1, 10000.0, solution );  return solution;}