HEVC代码学习40：decodePredInfo和decodePUWise函数

在之前的 HEVC代码学习39：decodeCtu和xDecodeCU函数 中提到，xDecodeCU通过迭代完成每个CU的解码，其中skip模式会直接解码merge列表和索引，取出对应MV；PCM模式也会直接进行解码；其他模式会调用decodePredInfo解码预测模式信息（帧内预测、帧间预测）。下面就来学习decodePredInfo及其调用的重要函数decodePUWise。

decodePredInfo

decodePredInfo实际是一个入口函数，其功能是输入当前待解码CU及其对应的绝对偏移索引、深度和子CU，进行非skip模式和非PCM模式解码，根据预测模式调用对应函数来进行解码。如果是帧内模式，调用decodeIntraDirModeLuma解码帧内预测模式；否则会调用decodePUWise解码帧间预测模式。

decodeIntraDirModeLuma解码帧内预测模式时，先构建MPM，取出选中的帧内预测模式序号。decodePUWise解码帧间预测模式，要分merge和AMVP处理解码运动信息。下一章将具体来看decodePUWise函数。

decodeIntraDirModeLuma代码比较简单，分析如下：

//解码预测模式
Void TDecEntropy::decodePredInfo    ( TComDataCU* pcCU, UInt uiAbsPartIdx, UInt uiDepth, TComDataCU* pcSubCU )
{
  //帧内预测
  if( pcCU->isIntra( uiAbsPartIdx ) )                                 // If it is Intra mode, encode intra prediction mode.
  {
    //解码亮度帧内角度预测模式
    decodeIntraDirModeLuma  ( pcCU, uiAbsPartIdx, uiDepth );
    if (pcCU->getPic()->getChromaFormat()!=CHROMA_400)      //非YUV400，解码色度角度预测模式
    {
      decodeIntraDirModeChroma( pcCU, uiAbsPartIdx, uiDepth );
      if (enable4ChromaPUsInIntraNxNCU(pcCU->getPic()->getChromaFormat()) && pcCU->getPartitionSize( uiAbsPartIdx )==SIZE_NxN)
      {
        UInt uiPartOffset = ( pcCU->getPic()->getNumPartitionsInCtu() >> ( pcCU->getDepth(uiAbsPartIdx) << 1 ) ) >> 2;
        decodeIntraDirModeChroma( pcCU, uiAbsPartIdx + uiPartOffset,   uiDepth+1 );
        decodeIntraDirModeChroma( pcCU, uiAbsPartIdx + uiPartOffset*2, uiDepth+1 );
        decodeIntraDirModeChroma( pcCU, uiAbsPartIdx + uiPartOffset*3, uiDepth+1 );
      }
    }
  }
  else        //帧间预测                                                        // if it is Inter mode, encode motion vector and reference index
  {
    //解码PU运动信息
    decodePUWise( pcCU, uiAbsPartIdx, uiDepth, pcSubCU );
  }
}

decodePUWise

从名字就可以看出，解码PU级的信息，实际功能是解码PU的运动信息。

主要流程如下：
一.初始化分块信息、候选列表。
二.遍历待解码CU，对其中所有PU分块进行帧间预测解码。
1.如果是merge模式，解码Merge Index，并调用getInterMergeCandidates构造merge候选列表，根据m索引取出选中的MV候选。
2.如果是AMVP模式，调用decodeMvdPU解码MVD，调用decodeMVPIdxPU解码MVP索引，计算MV=MVP+MVD。

注意这里只是在解码运动信息，没有进行预测图像的重构。

代码分析：

/** decode motion information for every PU block.
 * \param pcCU
 * \param uiAbsPartIdx
 * \param uiDepth
 * \param pcSubCU
 * \returns Void
 */
//解码PU运动信息
Void TDecEntropy::decodePUWise( TComDataCU* pcCU, UInt uiAbsPartIdx, UInt uiDepth, TComDataCU* pcSubCU )
{
  PartSize ePartSize = pcCU->getPartitionSize( uiAbsPartIdx );      //PU划分情况
  UInt uiNumPU = ( ePartSize == SIZE_2Nx2N ? 1 : ( ePartSize == SIZE_NxN ? 4 : 2 ) );       //分块数量
  UInt uiPUOffset = ( g_auiPUOffset[UInt( ePartSize )] << ( ( pcCU->getSlice()->getSPS()->getMaxTotalCUDepth() - uiDepth ) << 1 ) ) >> 4;

  TComMvField cMvFieldNeighbours[MRG_MAX_NUM_CANDS << 1]; // double length for mv of both lists //候选MV列表
  UChar uhInterDirNeighbours[MRG_MAX_NUM_CANDS];        //候选索引列表

  //初始化
  for ( UInt ui = 0; ui < pcCU->getSlice()->getMaxNumMergeCand(); ui++ )
  {
    uhInterDirNeighbours[ui] = 0;       
  }
  Int numValidMergeCand = 0;        
  Bool hasMergedCandList = false;

  //从CTU复制当前PU的帧间信息
  pcSubCU->copyInterPredInfoFrom( pcCU, uiAbsPartIdx, REF_PIC_LIST_0 );
  pcSubCU->copyInterPredInfoFrom( pcCU, uiAbsPartIdx, REF_PIC_LIST_1 );

  //遍历PU的每一个分块
  for ( UInt uiPartIdx = 0, uiSubPartIdx = uiAbsPartIdx; uiPartIdx < uiNumPU; uiPartIdx++, uiSubPartIdx += uiPUOffset )
  {
    //解码merge flag
    decodeMergeFlag( pcCU, uiSubPartIdx, uiDepth, uiPartIdx );
    //merge flag为真
    if ( pcCU->getMergeFlag( uiSubPartIdx ) )
    {
      //解码merge index
      decodeMergeIndex( pcCU, uiPartIdx, uiSubPartIdx, uiDepth );
#if ENVIRONMENT_VARIABLE_DEBUG_AND_TEST
      if (bDebugPredEnabled)
      {
        std::cout << "Coded merge flag, CU absPartIdx: " << uiAbsPartIdx << " PU(" << uiPartIdx << ") absPartIdx: " << uiSubPartIdx;
        std::cout << " merge index: " << (UInt)pcCU->getMergeIndex(uiSubPartIdx) << std::endl;
      }
#endif

      //获取merge index
      UInt uiMergeIndex = pcCU->getMergeIndex(uiSubPartIdx);
      if ( pcCU->getSlice()->getPPS()->getLog2ParallelMergeLevelMinus2() && ePartSize != SIZE_2Nx2N && pcSubCU->getWidth( 0 ) <= 8 )
      {
        //第一个分块时执行，初始化分块
        if ( !hasMergedCandList )
        {
          pcSubCU->setPartSizeSubParts( SIZE_2Nx2N, 0, uiDepth ); // temporarily set.
          pcSubCU->getInterMergeCandidates( 0, 0, cMvFieldNeighbours, uhInterDirNeighbours, numValidMergeCand );
          pcSubCU->setPartSizeSubParts( ePartSize, 0, uiDepth ); // restore.
          hasMergedCandList = true;
        }
      }
      else  //其他分块获取merge索引和候选
      {
        uiMergeIndex = pcCU->getMergeIndex(uiSubPartIdx);
        pcSubCU->getInterMergeCandidates( uiSubPartIdx-uiAbsPartIdx, uiPartIdx, cMvFieldNeighbours, uhInterDirNeighbours, numValidMergeCand, uiMergeIndex );
      }

      pcCU->setInterDirSubParts( uhInterDirNeighbours[uiMergeIndex], uiSubPartIdx, uiPartIdx, uiDepth );

      TComMv cTmpMv( 0, 0 );
      //遍历两个list0和list1
      for ( UInt uiRefListIdx = 0; uiRefListIdx < 2; uiRefListIdx++ )
      {
        //当参考索引数大于0时，取出merge参考列表
        if ( pcCU->getSlice()->getNumRefIdx( RefPicList( uiRefListIdx ) ) > 0 )
        {
          pcCU->setMVPIdxSubParts( 0, RefPicList( uiRefListIdx ), uiSubPartIdx, uiPartIdx, uiDepth);
          pcCU->setMVPNumSubParts( 0, RefPicList( uiRefListIdx ), uiSubPartIdx, uiPartIdx, uiDepth);
          pcCU->getCUMvField( RefPicList( uiRefListIdx ) )->setAllMvd( cTmpMv, ePartSize, uiSubPartIdx, uiDepth, uiPartIdx );
          pcCU->getCUMvField( RefPicList( uiRefListIdx ) )->setAllMvField( cMvFieldNeighbours[ 2*uiMergeIndex + uiRefListIdx ], ePartSize, uiSubPartIdx, uiDepth, uiPartIdx );

        }
      }
    }
    else    //非merge
    {
      //解码帧间预测方向
      decodeInterDirPU( pcCU, uiSubPartIdx, uiDepth, uiPartIdx );
      //遍历list0和list1
      for ( UInt uiRefListIdx = 0; uiRefListIdx < 2; uiRefListIdx++ )
      {
        //当参考索引数大于0时，取出AMVP参考列表
        if ( pcCU->getSlice()->getNumRefIdx( RefPicList( uiRefListIdx ) ) > 0 )
        {
          decodeRefFrmIdxPU( pcCU,    uiSubPartIdx,              uiDepth, uiPartIdx, RefPicList( uiRefListIdx ) );
          decodeMvdPU      ( pcCU,    uiSubPartIdx,              uiDepth, uiPartIdx, RefPicList( uiRefListIdx ) );
          decodeMVPIdxPU   ( pcSubCU, uiSubPartIdx-uiAbsPartIdx, uiDepth, uiPartIdx, RefPicList( uiRefListIdx ) );
#if ENVIRONMENT_VARIABLE_DEBUG_AND_TEST
          if (bDebugPredEnabled)
          {
            std::cout << "refListIdx: " << uiRefListIdx << std::endl;
            std::cout << "MVD horizontal: " << pcCU->getCUMvField(RefPicList(uiRefListIdx))->getMvd( uiAbsPartIdx ).getHor() << std::endl;
            std::cout << "MVD vertical:   " << pcCU->getCUMvField(RefPicList(uiRefListIdx))->getMvd( uiAbsPartIdx ).getVer() << std::endl;
            std::cout << "MVPIdxPU: " << pcCU->getMVPIdx(RefPicList( uiRefListIdx ), uiSubPartIdx) << std::endl;
            std::cout << "InterDir: " << (UInt)pcCU->getInterDir(uiSubPartIdx) << std::endl;
          }
#endif
        }
      }
    }

    //不确认这一块是做的什么，好像是list1不可用
    if ( (pcCU->getInterDir(uiSubPartIdx) == 3) && pcSubCU->isBipredRestriction(uiPartIdx) )
    {
      pcCU->getCUMvField( REF_PIC_LIST_1 )->setAllMv( TComMv(0,0), ePartSize, uiSubPartIdx, uiDepth, uiPartIdx);
      pcCU->getCUMvField( REF_PIC_LIST_1 )->setAllRefIdx( -1, ePartSize, uiSubPartIdx, uiDepth, uiPartIdx);
      pcCU->setInterDirSubParts( 1, uiSubPartIdx, uiPartIdx, uiDepth);
    }
  }
  return;
}