HEVC熵解码代码分析—代码解码流程（2）

首页 > 代码库 > HEVC熵解码代码分析—代码解码流程（2）

HEVC熵解码代码分析—代码解码流程（2）

2024-08-09 14:04:03 218人阅读

虽然熵解码的过程并不是很复杂，但是因为涉及太多的文档和函数比较让人心烦，所以稍微进行一下整理

熵解码一般一开始就是需要对上下文模式进行一定得初始化

在HEVC中

因为是类结构，所以在声明对象的时候，基本就完成了一般的初始化工作，下面详细介绍一下：

在类TDecSbac当中包含所有的上下文模式，但是在HM中有两个对应的结构

 ContextModel         m_contextModels[MAX_NUM_CTX_MOD]; //<一共512个 元素
  Int                  m_numContextModels;
  ContextModel3DBuffer m_cCUSplitFlagSCModel;
  ContextModel3DBuffer m_cCUSkipFlagSCModel;
  ContextModel3DBuffer m_cCUMergeFlagExtSCModel;
  ContextModel3DBuffer m_cCUMergeIdxExtSCModel;
  ContextModel3DBuffer m_cCUPartSizeSCModel;
  ContextModel3DBuffer m_cCUPredModeSCModel;
  ContextModel3DBuffer m_cCUIntraPredSCModel;

ContextModel m_contextModels[MAX_NUM_CTX_MOD]; HEVC一共有512个上下文模式

然后m_numContextModels 这个类成员变量用于记录每一个语法元素有多少的上下文模式

ContextModel3DBuffer类型对应的是每一个语法元素的上下文模式

m_numContextModels中比较重要的两个变量

UChar         m_ucState;        //<当前slice中对应的状态 state
UInt          m_binsCoded;      //<当前对应的bin的数量，具体还不是很清楚，后续继续解释

ContextModel3DBuffer

ContextModel* m_contextModel; ///< array of context models
  const UInt    m_sizeX;        ///< X size of 3D buffer
  const UInt    m_sizeXY;       ///< X times Y size of 3D buffer
  const UInt    m_sizeXYZ;      ///< total size of 3D buffer

一共有512个上下文模式，在每一个3D模式中，ContextModel指向512中的模式，三个Size变量是对应在一个语法元素中模式有多少，

再通过具体偏移量得到对应的ContextModel

初始化：

在创建对象的时候就进行初始化

<span style="color:#ff0000;">ContextModel3DBuffer::ContextModel3DBuffer</span>( UInt uiSizeZ, UInt uiSizeY, UInt uiSizeX, ContextModel *basePtr, Int &count )
: m_sizeX  ( uiSizeX )
, m_sizeXY ( uiSizeX * uiSizeY )
, m_sizeXYZ( uiSizeX * uiSizeY * uiSizeZ )
{
  // allocate 3D buffer
  m_contextModel = basePtr;
  count += m_sizeXYZ;
}

<span style="color:#ff0000;">TDecSbac::TDecSbac()</span>
// new structure here
: m_pcBitstream                              ( 0 )
, m_pcTDecBinIf                              ( NULL )
, m_numContextModels                         ( 0 )
, m_cCUSplitFlagSCModel                      ( 1,             1,                      NUM_SPLIT_FLAG_CTX                   , m_contextModels + m_numContextModels, m_numContextModels)
, m_cCUSkipFlagSCModel                       ( 1,             1,                      NUM_SKIP_FLAG_CTX                    , m_contextModels + m_numContextModels, m_numContextModels)
, m_cCUMergeFlagExtSCModel                   ( 1,             1,                      NUM_MERGE_FLAG_EXT_CTX               , m_contextModels + m_numContextModels, m_numContextModels)
, m_cCUMergeIdxExtSCModel                    ( 1,             1,                      NUM_MERGE_IDX_EXT_CTX                , m_contextModels + m_numContextModels, m_numContextModels)
, m_cCUPartSizeSCModel                       ( 1,             1,                      NUM_PART_SIZE_CTX                    , m_contextModels + m_numContextModels, m_numContextModels)
, m_cCUPredModeSCModel                       ( 1,             1,                      NUM_PRED_MODE_CTX                    , m_contextModels + m_numContextModels, m_numContextModels)
, m_cCUIntraPredSCModel                      ( 1,             1,                      NUM_ADI_CTX                          , m_contextModels + m_numContextModels, m_numContextModels)
, m_cCUChromaPredSCModel                     ( 1,             1,                      NUM_CHROMA_PRED_CTX                  , m_contextModels + m_numContextModels, m_numContextModels)

完成ContextModel3DBuffer模式中对应的ContextModel和每一个语法元素中包含的上下文模式的大小

下一步初始化是在一个Slice开始的时候进行初始化，这里的初始化，主要是初始化解码环境

主要是包括算数编码中的range,MPS等信息

函数入口：

Void TDecSlice::decompressSlice(TComInputBitstream** ppcSubstreams, TComPic* pcPic, TDecSbac* pcSbacDecoder)
中的
  m_pcEntropyDecoder->setEntropyDecoder ( pcSbacDecoder  );  //<多态信息中设置编码方式为CABAC
  m_pcEntropyDecoder->setBitstream      ( ppcSubstreams[0] ); //<设置对应的码流信息
  m_pcEntropyDecoder->resetEntropy      (pcSlice); //<初始化解码环境

在函数

<pre name="code" class="cpp">Void TDecSbac::resetEntropy(TComSlice* pSlice)
{
  SliceType sliceType  = pSlice->getSliceType();
  Int       qp         = pSlice->getSliceQp();

  if (pSlice->getPPS()->getCabacInitPresentFlag() && pSlice->getCabacInitFlag())  //XT?
  {
    switch (sliceType)
    {
    case P_SLICE:           // change initialization table to B_SLICE initialization
      sliceType = B_SLICE;
      break;
    case B_SLICE:           // change initialization table to P_SLICE initialization
      sliceType = P_SLICE;
      break;
    default     :           // should not occur
      assert(0);
      break;
    }
  }

  m_cCUSplitFlagSCModel.initBuffer                ( sliceType, qp, (UChar*)INIT_SPLIT_FLAG );
  m_cCUSkipFlagSCModel.initBuffer                 ( sliceType, qp, (UChar*)INIT_SKIP_FLAG );
  m_cCUMergeFlagExtSCModel.initBuffer             ( sliceType, qp, (UChar*)INIT_MERGE_FLAG_EXT );
  m_cCUMergeIdxExtSCModel.initBuffer              ( sliceType, qp, (UChar*)INIT_MERGE_IDX_EXT );
  m_cCUPartSizeSCModel.initBuffer                 ( sliceType, qp, (UChar*)INIT_PART_SIZE );
  m_cCUPredModeSCModel.initBuffer                 ( sliceType, qp, (UChar*)INIT_PRED_MODE );
  m_cCUIntraPredSCModel.initBuffer                ( sliceType, qp, (UChar*)INIT_INTRA_PRED_MODE );
  m_cCUChromaPredSCModel.initBuffer               ( sliceType, qp, (UChar*)INIT_CHROMA_PRED_MODE );
  m_cCUInterDirSCModel.initBuffer                 ( sliceType, qp, (UChar*)INIT_INTER_DIR );
  m_cCUMvdSCModel.initBuffer                      ( sliceType, qp, (UChar*)INIT_MVD );
  m_cCURefPicSCModel.initBuffer                   ( sliceType, qp, (UChar*)INIT_REF_PIC );
  m_cCUDeltaQpSCModel.initBuffer                  ( sliceType, qp, (UChar*)INIT_DQP );
  m_cCUQtCbfSCModel.initBuffer                    ( sliceType, qp, (UChar*)INIT_QT_CBF );
  m_cCUQtRootCbfSCModel.initBuffer                ( sliceType, qp, (UChar*)INIT_QT_ROOT_CBF );
  m_cCUSigCoeffGroupSCModel.initBuffer            ( sliceType, qp, (UChar*)INIT_SIG_CG_FLAG );
  m_cCUSigSCModel.initBuffer                      ( sliceType, qp, (UChar*)INIT_SIG_FLAG );
  m_cCuCtxLastX.initBuffer                        ( sliceType, qp, (UChar*)INIT_LAST );
  m_cCuCtxLastY.initBuffer                        ( sliceType, qp, (UChar*)INIT_LAST );
  m_cCUOneSCModel.initBuffer                      ( sliceType, qp, (UChar*)INIT_ONE_FLAG );
  m_cCUAbsSCModel.initBuffer                      ( sliceType, qp, (UChar*)INIT_ABS_FLAG );
  m_cMVPIdxSCModel.initBuffer                     ( sliceType, qp, (UChar*)INIT_MVP_IDX );
  m_cSaoMergeSCModel.initBuffer                   ( sliceType, qp, (UChar*)INIT_SAO_MERGE_FLAG );
  m_cSaoTypeIdxSCModel.initBuffer                 ( sliceType, qp, (UChar*)INIT_SAO_TYPE_IDX );
  m_cCUTransSubdivFlagSCModel.initBuffer          ( sliceType, qp, (UChar*)INIT_TRANS_SUBDIV_FLAG );
  m_cTransformSkipSCModel.initBuffer              ( sliceType, qp, (UChar*)INIT_TRANSFORMSKIP_FLAG );
  m_CUTransquantBypassFlagSCModel.initBuffer      ( sliceType, qp, (UChar*)INIT_CU_TRANSQUANT_BYPASS_FLAG );
  m_explicitRdpcmFlagSCModel.initBuffer           ( sliceType, qp, (UChar*)INIT_EXPLICIT_RDPCM_FLAG);
  m_explicitRdpcmDirSCModel.initBuffer            ( sliceType, qp, (UChar*)INIT_EXPLICIT_RDPCM_DIR);
  m_cIntraBCPredFlagSCModel.initBuffer            ( sliceType, qp, (UChar*)INIT_INTRABC_PRED_FLAG );
  m_cCrossComponentPredictionSCModel.initBuffer   ( sliceType, qp, (UChar*)INIT_CROSS_COMPONENT_PREDICTION );
  m_ChromaQpAdjFlagSCModel.initBuffer             ( sliceType, qp, (UChar*)INIT_CHROMA_QP_ADJ_FLAG );
  m_ChromaQpAdjIdcSCModel.initBuffer              ( sliceType, qp, (UChar*)INIT_CHROMA_QP_ADJ_IDC );
  m_cCUColourTransformFlagSCModel.initBuffer      ( sliceType, qp, (UChar*)INIT_COLOUR_TRANS);
  m_cIntraBCBVDSCModel.initBuffer                 ( sliceType, qp, (UChar*)INIT_INTRABC_BVD );
  m_PLTModeFlagSCModel.initBuffer                 ( sliceType, qp, (UChar*)INIT_PLTMODE_FLAG );
  m_SPointSCModel.initBuffer                      ( sliceType, qp, (UChar*)INIT_SPOINT );
  m_cCopyTopRunSCModel.initBuffer                 ( sliceType, qp, (UChar*)INIT_TOP_RUN);
  m_cRunSCModel.initBuffer                        ( sliceType, qp, (UChar*)INIT_RUN);
  m_PLTSharingModeFlagSCModel.initBuffer          ( sliceType, qp, (UChar*)INIT_PLT_REUSE_FLAG );
  m_PLTScanRotationModeFlagSCModel.initBuffer     ( sliceType, qp, (UChar*)INIT_SCAN_ROTATION_FLAG );
  for (UInt statisticIndex = 0; statisticIndex < RExt__GOLOMB_RICE_ADAPTATION_STATISTICS_SETS ; statisticIndex++)
  {
    m_golombRiceAdaptationStatistics[statisticIndex] = 0;
  }

  m_pcTDecBinIf->start();
}

其中初始化buffur

Void ContextModel3DBuffer::initBuffer( SliceType sliceType, Int qp, UChar* ctxModel )
{
  ctxModel += sliceType * m_sizeXYZ;

  for ( Int n = 0; n < m_sizeXYZ; n++ )
  {
    m_contextModel[ n ].init( qp, ctxModel[ n ] );
    m_contextModel[ n ].setBinsCoded( 0 );
  }
}

传入的ctxModel是预先知道的表格，通过init进行初始化各个 contextModel 的state 和bin变量

在start()函数当中初始化算数解码中解码环境

TDecBinCABAC::start()
{
  assert( m_pcTComBitstream->getNumBitsUntilByteAligned() == 0 );
#if RExt__DECODER_DEBUG_BIT_STATISTICS
  TComCodingStatistics::UpdateCABACStat(STATS__CABAC_INITIALISATION, 512, 510, 0);
#endif
  m_uiRange    = 510;
  m_bitsNeeded = -8;
  m_uiValue    = http://www.mamicode.com/(m_pcTComBitstream->readByte() << 8);>






HEVC熵解码代码分析—代码解码流程（2）

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HEVC熵解码代码分析—代码解码流程（2）

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