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)
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
<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 = (m_pcTComBitstream->readByte() << 8);
m_uiValue |= m_pcTComBitstream->readByte();
}
原文地址：http://blog.csdn.net/xietingcandice/article/details/42105663
