HEVC学习（二十） —— 熵编码之一

从本篇开始，接下来的若干篇会逐步分析HM 9.1中有关熵编码的过程。在此以及以后的几篇，我都默认大家对熵编码（针对CABAC，因为HEVC只有这一种编码方式，而H.264有CAVLC和CABAC两种）的基本概念和流程有了一定的基础，同时，由于HEVC中的CABAC基本流程与H.264中的基本一致，因此，这里暂时不会介绍熵编码框架和流程，有兴趣的可以参考H.264中的相关资料。我会按照编码的执行顺序逐步地分析代码，到最后会再给出总结。

 

本文首先介绍编码器的初始化过程，即draft 9.3.4.1的部分。

 

在Void TEncSlice::compressSlice( TComPic*& rpcPic )有这么一段：

  // set entropy coder
  if( m_pcCfg->getUseSBACRD() ) //!< 默认条件下m_bUseSBACRD为true //!< draft 9.3.4.1
  {
    m_pcSbacCoder->init( m_pcBinCABAC );
    m_pcEntropyCoder->setEntropyCoder   ( m_pcSbacCoder, pcSlice );
    m_pcEntropyCoder->resetEntropy      (); //!< 主要进行上下文模型的初始化，codILow和codIRange的初始化等
    m_pppcRDSbacCoder[0][CI_CURR_BEST]->load(m_pcSbacCoder);
    pppcRDSbacCoder = (TEncBinCABAC *) m_pppcRDSbacCoder[0][CI_CURR_BEST]->getEncBinIf();
    pppcRDSbacCoder->setBinCountingEnableFlag( false );
    pppcRDSbacCoder->setBinsCoded( 0 );
  }

重点看m_pcEntropyCoder->resetEntropy ();

该函数中调用的initBuffer功能即是对相应的context变量进行初始化，这个可参考draft 9.3.1.1的相关内容。

Void TEncSbac::resetEntropy           ()
{
  Int  iQp              = m_pcSlice->getSliceQp();
  SliceType eSliceType  = m_pcSlice->getSliceType();
  
  Int  encCABACTableIdx = m_pcSlice->getPPS()->getEncCABACTableIdx();
  if (!m_pcSlice->isIntra() && (encCABACTableIdx==B_SLICE || encCABACTableIdx==P_SLICE) && m_pcSlice->getPPS()->getCabacInitPresentFlag())
  {
    eSliceType = (SliceType) encCABACTableIdx;
  }
  //! draft 9.3.1.1 Initialization process for context variables
  m_cCUSplitFlagSCModel.initBuffer       ( eSliceType, iQp, (UChar*)INIT_SPLIT_FLAG );  
  m_cCUSkipFlagSCModel.initBuffer        ( eSliceType, iQp, (UChar*)INIT_SKIP_FLAG );
  m_cCUMergeFlagExtSCModel.initBuffer    ( eSliceType, iQp, (UChar*)INIT_MERGE_FLAG_EXT);
  m_cCUMergeIdxExtSCModel.initBuffer     ( eSliceType, iQp, (UChar*)INIT_MERGE_IDX_EXT);
  m_cCUPartSizeSCModel.initBuffer        ( eSliceType, iQp, (UChar*)INIT_PART_SIZE );
  m_cCUAMPSCModel.initBuffer             ( eSliceType, iQp, (UChar*)INIT_CU_AMP_POS );
  m_cCUPredModeSCModel.initBuffer        ( eSliceType, iQp, (UChar*)INIT_PRED_MODE );
  m_cCUIntraPredSCModel.initBuffer       ( eSliceType, iQp, (UChar*)INIT_INTRA_PRED_MODE );
  m_cCUChromaPredSCModel.initBuffer      ( eSliceType, iQp, (UChar*)INIT_CHROMA_PRED_MODE );
  m_cCUInterDirSCModel.initBuffer        ( eSliceType, iQp, (UChar*)INIT_INTER_DIR );
  m_cCUMvdSCModel.initBuffer             ( eSliceType, iQp, (UChar*)INIT_MVD );
  m_cCURefPicSCModel.initBuffer          ( eSliceType, iQp, (UChar*)INIT_REF_PIC );
  m_cCUDeltaQpSCModel.initBuffer         ( eSliceType, iQp, (UChar*)INIT_DQP );
  m_cCUQtCbfSCModel.initBuffer           ( eSliceType, iQp, (UChar*)INIT_QT_CBF );
  m_cCUQtRootCbfSCModel.initBuffer       ( eSliceType, iQp, (UChar*)INIT_QT_ROOT_CBF );
  m_cCUSigCoeffGroupSCModel.initBuffer   ( eSliceType, iQp, (UChar*)INIT_SIG_CG_FLAG );
  m_cCUSigSCModel.initBuffer             ( eSliceType, iQp, (UChar*)INIT_SIG_FLAG );
  m_cCuCtxLastX.initBuffer               ( eSliceType, iQp, (UChar*)INIT_LAST );
  m_cCuCtxLastY.initBuffer               ( eSliceType, iQp, (UChar*)INIT_LAST );
  m_cCUOneSCModel.initBuffer             ( eSliceType, iQp, (UChar*)INIT_ONE_FLAG );
  m_cCUAbsSCModel.initBuffer             ( eSliceType, iQp, (UChar*)INIT_ABS_FLAG );
  m_cMVPIdxSCModel.initBuffer            ( eSliceType, iQp, (UChar*)INIT_MVP_IDX );
  m_cCUTransSubdivFlagSCModel.initBuffer ( eSliceType, iQp, (UChar*)INIT_TRANS_SUBDIV_FLAG );
  m_cSaoMergeSCModel.initBuffer      ( eSliceType, iQp, (UChar*)INIT_SAO_MERGE_FLAG );
  m_cSaoTypeIdxSCModel.initBuffer        ( eSliceType, iQp, (UChar*)INIT_SAO_TYPE_IDX );
  m_cTransformSkipSCModel.initBuffer     ( eSliceType, iQp, (UChar*)INIT_TRANSFORMSKIP_FLAG );
  m_CUTransquantBypassFlagSCModel.initBuffer( eSliceType, iQp, (UChar*)INIT_CU_TRANSQUANT_BYPASS_FLAG );
  // new structure
  m_uiLastQp = iQp;
  
  m_pcBinIf->start();
  
  return;
}

这些initBuffer初始化的context的变量值可以参考draft中Table 9-5 至Table 9-31，代码中的数组及对应的表格如下所列：

// initial probability for cu_transquant_bypass flag
static const UChar
INIT_CU_TRANSQUANT_BYPASS_FLAG[3][NUM_CU_TRANSQUANT_BYPASS_FLAG_CTX] =
{
  { 154 }, 
  { 154 }, 
  { 154 }, 
}; //!< Table 9-8

// initial probability for split flag
static const UChar 
INIT_SPLIT_FLAG[3][NUM_SPLIT_FLAG_CTX] =  
{
  { 107,  139,  126, },
  { 107,  139,  126, }, 
  { 139,  141,  157, }, 
}; //!< Table 9-7

static const UChar 
INIT_SKIP_FLAG[3][NUM_SKIP_FLAG_CTX] =  
{
  { 197,  185,  201, }, 
  { 197,  185,  201, }, 
  { CNU,  CNU,  CNU, }, 
}; //!< Table 9-9

static const UChar
INIT_MERGE_FLAG_EXT[3][NUM_MERGE_FLAG_EXT_CTX] = 
{
  { 154, }, 
  { 110, }, 
  { CNU, }, 
}; //!< Table 9-15

static const UChar 
INIT_MERGE_IDX_EXT[3][NUM_MERGE_IDX_EXT_CTX] =  
{
  { 137, }, 
  { 122, }, 
  { CNU, }, 
}; //!< Table 9-16

static const UChar 
INIT_PART_SIZE[3][NUM_PART_SIZE_CTX] =  
{
  { 154,  139,  CNU,  CNU, }, 
  { 154,  139,  CNU,  CNU, }, 
  { 184,  CNU,  CNU,  CNU, }, 
}; //!< Table 9-12

static const UChar 
INIT_CU_AMP_POS[3][NUM_CU_AMP_CTX] =  
{
  { 154, }, 
  { 154, }, 
  { CNU, }, 
};

static const UChar 
INIT_PRED_MODE[3][NUM_PRED_MODE_CTX] = 
{
  { 134, }, 
  { 149, }, 
  { CNU, }, 
}; //!< Table 9-11

static const UChar 
INIT_INTRA_PRED_MODE[3][NUM_ADI_CTX] = 
{
  { 183, }, 
  { 154, }, 
  { 184, }, 
}; //!< Table 9-13

static const UChar 
INIT_CHROMA_PRED_MODE[3][NUM_CHROMA_PRED_CTX] = 
{
  { 152,  139, }, 
  { 152,  139, }, 
  {  63,  139, }, 
}; //!< Table 9-14

static const UChar 
INIT_INTER_DIR[3][NUM_INTER_DIR_CTX] = 
{
  {  95,   79,   63,   31,  31, }, 
  {  95,   79,   63,   31,  31, }, 
  { CNU,  CNU,  CNU,  CNU, CNU, }, 
}; //!< Table 9-17

static const UChar 
INIT_MVD[3][NUM_MV_RES_CTX] =  
{
  { 169,  198, }, 
  { 140,  198, }, 
  { CNU,  CNU, }, 
}; //!< Table 9-19

static const UChar 
INIT_REF_PIC[3][NUM_REF_NO_CTX] =  
{
  { 153,  153 }, 
  { 153,  153 }, 
  { CNU,  CNU }, 
};

static const UChar 
INIT_DQP[3][NUM_DELTA_QP_CTX] = 
{
  { 154,  154,  154, }, 
  { 154,  154,  154, }, 
  { 154,  154,  154, }, 
}; //!< Table 9-10

static const UChar 
INIT_QT_CBF[3][2*NUM_QT_CBF_CTX] =  
{
  { 153,  111,  CNU,  CNU,  CNU,  149,   92,  167,  CNU,  CNU, }, 
  { 153,  111,  CNU,  CNU,  CNU,  149,  107,  167,  CNU,  CNU, }, 
  { 111,  141,  CNU,  CNU,  CNU,   94,  138,  182,  CNU,  CNU, }, 
}; //!< Table 9-23、Table 9-24

static const UChar 
INIT_QT_ROOT_CBF[3][NUM_QT_ROOT_CBF_CTX] = 
{
  {  79, }, 
  {  79, }, 
  { CNU, }, 
}; //!< Table 9-21

static const UChar 
INIT_LAST[3][2*NUM_CTX_LAST_FLAG_XY] =  
{
  { 125,  110,  124,  110,   95,   94,  125,  111,  111,   79,  125,  126,  111,  111,   79,
    108,  123,   93,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU, 
  }, 
  { 125,  110,   94,  110,   95,   79,  125,  111,  110,   78,  110,  111,  111,   95,   94,
    108,  123,  108,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,
  }, 
  { 110,  110,  124,  125,  140,  153,  125,  127,  140,  109,  111,  143,  127,  111,   79, 
    108,  123,   63,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU,  CNU, 
  }, 
}; //!< Table 9-26、Table 9-27

static const UChar 
INIT_SIG_CG_FLAG[3][2 * NUM_SIG_CG_FLAG_CTX] =  
{
  { 121,  140,  
    61,  154, 
  }, 
  { 121,  140, 
    61,  154, 
  }, 
  {  91,  171,  
    134,  141, 
  }, 
}; //!< Table 9-28

static const UChar 
INIT_SIG_FLAG[3][NUM_SIG_FLAG_CTX] = 
{
  { 170,  154,  139,  153,  139,  123,  123,   63,  124,  166,  183,  140,  136,  153,  154,  166,  183,  140,  136,  153,  154,  166,  183,  140,  136,  153,  154,  170,  153,  138,  138,  122,  121,  122,  121,  167,  151,  183,  140,  151,  183,  140,  }, 
  { 155,  154,  139,  153,  139,  123,  123,   63,  153,  166,  183,  140,  136,  153,  154,  166,  183,  140,  136,  153,  154,  166,  183,  140,  136,  153,  154,  170,  153,  123,  123,  107,  121,  107,  121,  167,  151,  183,  140,  151,  183,  140,  }, 
  { 111,  111,  125,  110,  110,   94,  124,  108,  124,  107,  125,  141,  179,  153,  125,  107,  125,  141,  179,  153,  125,  107,  125,  141,  179,  153,  125,  140,  139,  182,  182,  152,  136,  152,  136,  153,  136,  139,  111,  136,  139,  111,  }, 
}; //!< Table 9-29

static const UChar 
INIT_ONE_FLAG[3][NUM_ONE_FLAG_CTX] = 
{
  { 154,  196,  167,  167,  154,  152,  167,  182,  182,  134,  149,  136,  153,  121,  136,  122,  169,  208,  166,  167,  154,  152,  167,  182, }, 
  { 154,  196,  196,  167,  154,  152,  167,  182,  182,  134,  149,  136,  153,  121,  136,  137,  169,  194,  166,  167,  154,  167,  137,  182, }, 
  { 140,   92,  137,  138,  140,  152,  138,  139,  153,   74,  149,   92,  139,  107,  122,  152,  140,  179,  166,  182,  140,  227,  122,  197, }, 
}; //!< Table 9-30

static const UChar 
INIT_ABS_FLAG[3][NUM_ABS_FLAG_CTX] =  
{
  { 107,  167,   91,  107,  107,  167, }, 
  { 107,  167,   91,  122,  107,  167, }, 
  { 138,  153,  136,  167,  152,  152, }, 
}; //!< Table 9-31

static const UChar 
INIT_MVP_IDX[3][NUM_MVP_IDX_CTX] =  
{
  { 168,  CNU, }, 
  { 168,  CNU, }, 
  { CNU,  CNU, }, 
}; //!< Table 9-20

static const UChar 
INIT_SAO_MERGE_FLAG[3][NUM_SAO_MERGE_FLAG_CTX] = 
{
  { 153,  }, 
  { 153,  }, 
  { 153,  }, 
}; //!< Table 9-5

static const UChar 
INIT_SAO_TYPE_IDX[3][NUM_SAO_TYPE_IDX_CTX] = 
{
  { 200, }, 
  { 185, }, 
  { 160, }, 
}; //!< Table 9-6

static const UChar
INIT_TRANS_SUBDIV_FLAG[3][NUM_TRANS_SUBDIV_FLAG_CTX] =
{
  { 153,  138,  138, },
  { 124,  138,   94, },
  { 224,  167,  122, },
}; //!< Table 9-22

static const UChar
INIT_TRANSFORMSKIP_FLAG[3][2*NUM_TRANSFORMSKIP_FLAG_CTX] = 
{
  { 139,  139}, 
  { 139,  139}, 
  { 139,  139}, 
}; //!< Table 9-25

需要注意的是，HM和draft不是每一个地方都是一一对应的，可能有若干变量的值或者命名有所差异，具体情况还是等具体进行编码时再来讨论吧。

TEncSbac::resetEntropy()在最后还调用了m_pcBinIf->start();它完成如下工作：

Void TEncBinCABAC::start()
{
  m_uiLow            = 0;   //!< codILow
  m_uiRange          = 510; //!< codIRange
  m_bitsLeft         = 23;
  m_numBufferedBytes = 0;
  m_bufferedByte     = 0xff;
}