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。