[置顶] HEVC帧间预测之二——TComDataCU::xGetColMVP的分析

在上一篇介绍的函数中，在求TMVP时调用了一个比较重要的函数，xGetColMVP。本文对该函数进行较为详细的分析：

Bool TComDataCU::xGetColMVP( RefPicList eRefPicList, Int uiCUAddr, Int uiPartUnitIdx, TComMv& rcMv, Int& riRefIdx )
{//! 这部分的内容可以参考draft 8.5.2.1.7, Derivation process for temporal luma motion vector prediction
  //! 这个函数的返回条件可以参考该部分的第3小点，The variables mvLXCol and availableFlagLXCol are derived as follows ...
  UInt uiAbsPartAddr = uiPartUnitIdx;

  RefPicList  eColRefPicList;
  Int iColPOC, iColRefPOC, iCurrPOC, iCurrRefPOC, iScale;
  TComMv cColMv;

  // use coldir. 
  //! Colocated Picture
  TComPic *pColPic = getSlice()->getRefPic( RefPicList(getSlice()->isInterB() ? 1-getSlice()->getColFromL0Flag() : 0), getSlice()->getColRefIdx());
  //! Colocated CU
  TComDataCU *pColCU = pColPic->getCU( uiCUAddr );
  if(pColCU->getPic()==0||pColCU->getPartitionSize(uiPartUnitIdx)==SIZE_NONE) //!< Colocated Picture or colocated CU不存在
  {
    return false;
  }
  iCurrPOC = m_pcSlice->getPOC(); //!< 当前slice的POC   
  iCurrRefPOC = m_pcSlice->getRefPic(eRefPicList, riRefIdx)->getPOC(); //!< 当前slice参考帧的POC
  iColPOC = pColCU->getSlice()->getPOC();  //!< 当前slice的colocated picuture的POC

  if (pColCU->isIntra(uiAbsPartAddr)) //!< colocated CU为帧内预测模式，即没有MV，故直接返回false
  {
    return false;
  }
  eColRefPicList = getSlice()->getCheckLDC() ? eRefPicList : RefPicList(getSlice()->getColFromL0Flag());

  Int iColRefIdx = pColCU->getCUMvField(RefPicList(eColRefPicList))->getRefIdx(uiAbsPartAddr); //!< colocated CU对应的参考帧的序号

  if (iColRefIdx < 0 )
  {
    eColRefPicList = RefPicList(1 - eColRefPicList);
    iColRefIdx = pColCU->getCUMvField(RefPicList(eColRefPicList))->getRefIdx(uiAbsPartAddr);

    if (iColRefIdx < 0 )
    {
      return false;
    }
  }

  // Scale the vector.
  iColRefPOC = pColCU->getSlice()->getRefPOC(eColRefPicList, iColRefIdx);
  cColMv = pColCU->getCUMvField(eColRefPicList)->getMv(uiAbsPartAddr);

  iCurrRefPOC = m_pcSlice->getRefPic(eRefPicList, riRefIdx)->getPOC();
  Bool bIsCurrRefLongTerm = m_pcSlice->getRefPic(eRefPicList, riRefIdx)->getIsLongTerm();
  Bool bIsColRefLongTerm = pColCU->getSlice()->getRefPic(eColRefPicList, iColRefIdx)->getIsUsedAsLongTerm();

  if ( bIsCurrRefLongTerm != bIsColRefLongTerm ) 
  {
    return false;
  }

  if ( bIsCurrRefLongTerm || bIsColRefLongTerm )
  {
    rcMv = cColMv; //!< draft 式子(8-155)
  }
  else
  {
    iScale = xGetDistScaleFactor(iCurrPOC, iCurrRefPOC, iColPOC, iColRefPOC); //!< 接下来分析
    if ( iScale == 4096 )
    {
      rcMv = cColMv;
    }
    else
    {
      rcMv = cColMv.scaleMv( iScale ); //!< draft 式子(8-158)
    }
  }
  return true;
}

Int TComDataCU::xGetDistScaleFactor(Int iCurrPOC, Int iCurrRefPOC, Int iColPOC, Int iColRefPOC)
{
  Int iDiffPocD = iColPOC - iColRefPOC;		//!< draft 式子(8-159)
  Int iDiffPocB = iCurrPOC - iCurrRefPOC;	//!< draft 式子(8-160)
  
  if( iDiffPocD == iDiffPocB )
  {
    return 4096;
  }
  else
  {	
    Int iTDB      = Clip3( -128, 127, iDiffPocB );
    Int iTDD      = Clip3( -128, 127, iDiffPocD );
    Int iX        = (0x4000 + abs(iTDD/2)) / iTDD; //!< draft 式子(8-156)
    Int iScale    = Clip3( -4096, 4095, (iTDB * iX + 32) >> 6 ); //!< draft 式子(8-157)
    return iScale;
  }
}

 

  const TComMv scaleMv( Int iScale ) const
  {//! draft 式子(8-158)
    Int mvx = Clip3( -32768, 32767, (iScale * getHor() + 127 + (iScale * getHor() < 0)) >> 8 );
    Int mvy = Clip3( -32768, 32767, (iScale * getVer() + 127 + (iScale * getVer() < 0)) >> 8 );
    return TComMv( mvx, mvy );
  }