HM代码阅读：帧内预测（一）

本文的代码均是TEncSearch::estIntraPredLumaQT函数或被它调用的函数中的代码。在TEncSearch::estIntraPredLumaQT函数中，首先确定RMD（Rough Mode Decision）候选。RMD的候选模式的个数由以下两个数组确定：

const UChar g_aucIntraModeNumFast_UseMPM[MAX_CU_DEPTH] =
{
  3,  //   2x2
  8,  //   4x4
  8,  //   8x8
  3,  //  16x16
  3,  //  32x32
  3   //  64x64
};
const UChar g_aucIntraModeNumFast_NotUseMPM[MAX_CU_DEPTH] =
{
  3,  //   2x2
  9,  //   4x4
  9,  //   8x8
  4,  //  16x16   33
  4,  //  32x32   33
  5   //  64x64   33
};

从上面两个数组可以看出，RMD模式的个数与PU的大小有关。PU较小（4x4或8x8）时，RMD个数为8；PU较大（16x16或32x32或64x64）时，RMD个数为3。选择RMD时，uiMode按代价uiCost在CandCostList中从小到大排列在CandModeList中。

UInt TEncSearch::xUpdateCandList( UInt uiMode, Double uiCost, UInt uiFastCandNum, UInt * CandModeList, Double * CandCostList )
{
  UInt i;
  UInt shift=0;
  // 确定插入位置shift（从右至左）
  while ( shift1-shift ] )
  {
    shift++;
  }

  if( shift!=0 )
  {
    // 插入点及之后的模式号外后挪
    for(i=1; i<shift; i++)
    {
      CandModeList[ uiFastCandNum-i ] = CandModeList[ uiFastCandNum-1-i ];
      CandCostList[ uiFastCandNum-i ] = CandCostList[ uiFastCandNum-1-i ];
    }
    // 在插入点插入相应的模式号
    CandModeList[ uiFastCandNum-shift ] = uiMode;
    CandCostList[ uiFastCandNum-shift ] = uiCost;
    return 1;
  }

  return 0;
}

上面函数的返回值用作TEncSearch::estIntraPredLumaQT中定义的变量CandNum的增量，然而CandNum的值没有被使用，因此上面函数的返回值是没有用处的。CandCostList以从小到大的顺序，往后挪时，会把cost最大的模式覆盖掉，这样做是合理。

RMO后，再加入MPM（most probable mode）模式。如果MPM模式不在RMO产生的集合里，则加入MPM模式。

      if (m_pcEncCfg->getFastUDIUseMPMEnabled())
      {
        Int uiPreds[NUM_MOST_PROBABLE_MODES] = {-1, -1, -1};

        Int iMode = -1;
        pcCU->getIntraDirPredictor( uiPartOffset, uiPreds, COMPONENT_Y, &iMode ); // 获取MPM模式
        // MPM模式的个数
        const Int numCand = ( iMode >= 0 ) ? iMode : Int(NUM_MOST_PROBABLE_MODES);

        for( Int j=0; j < numCand; j++)
        {
          Bool mostProbableModeIncluded = false;
          Int mostProbableMode = uiPreds[j];

          for( Int i=0; i < numModesForFullRD; i++)
          {
            mostProbableModeIncluded |= (mostProbableMode == uiRdModeList[i]);
          }
          if (!mostProbableModeIncluded)
          {
            uiRdModeList[numModesForFullRD++] = mostProbableMode;
          }
        }
      }

获取MPM模式的代码如下（在指针piMode指向的变量中赋MPM的个数）：

/** Get most probable intra modes
*\param   uiAbsPartIdx    partition index
*\param   uiIntraDirPred  pointer to the array for MPM storage
*\param   compID          colour component ID
*\param   piMode          it is set with MPM mode in case both MPM are equal. It is used to restrict RD search at encode side.
*\returns Number of MPM
*/
Void TComDataCU::getIntraDirPredictor( UInt uiAbsPartIdx, Int uiIntraDirPred[NUM_MOST_PROBABLE_MODES], const ComponentID compID, Int* piMode ) const
{
  UInt        LeftPartIdx  = MAX_UINT;
  UInt        AbovePartIdx = MAX_UINT;
  Int         iLeftIntraDir, iAboveIntraDir;
  const TComSPS *sps=getSlice()->getSPS();
  const UInt partsPerMinCU = 1<<(2*(sps->getMaxTotalCUDepth() - sps->getLog2DiffMaxMinCodingBlockSize()));

  const ChannelType chType = toChannelType(compID);
  const ChromaFormat chForm = getPic()->getChromaFormat();
  // Get intra direction of left PU
  const TComDataCU *pcCULeft = getPULeft( LeftPartIdx, m_absZIdxInCtu + uiAbsPartIdx );

  if (isChroma(compID))
  {
    LeftPartIdx = getChromasCorrespondingPULumaIdx(LeftPartIdx, chForm, partsPerMinCU);
  }
  // 若左边PU不是Intra块，则赋为DC模式：Any unavailable prediction mode is considered to be Intra_DC.
  iLeftIntraDir  = pcCULeft ? ( pcCULeft->isIntra( LeftPartIdx ) ? pcCULeft->getIntraDir( chType, LeftPartIdx ) : DC_IDX ) : DC_IDX;

  // Get intra direction of above PU
  const TComDataCU *pcCUAbove = getPUAbove( AbovePartIdx, m_absZIdxInCtu + uiAbsPartIdx, true, true );

  if (isChroma(compID))
  {
    AbovePartIdx = getChromasCorrespondingPULumaIdx(AbovePartIdx, chForm, partsPerMinCU);
  }
  // 若上边PU不是Intra块，则赋为DC模式：Any unavailable prediction mode is considered to be Intra_DC.
  iAboveIntraDir = pcCUAbove ? ( pcCUAbove->isIntra( AbovePartIdx ) ? pcCUAbove->getIntraDir( chType, AbovePartIdx ) : DC_IDX ) : DC_IDX;

  if (isChroma(chType))
  {
    if (iLeftIntraDir  == DM_CHROMA_IDX)
    {
      iLeftIntraDir  = pcCULeft-> getIntraDir( CHANNEL_TYPE_LUMA, LeftPartIdx  );
    }
    if (iAboveIntraDir == DM_CHROMA_IDX)
    {
      iAboveIntraDir = pcCUAbove->getIntraDir( CHANNEL_TYPE_LUMA, AbovePartIdx );
    }
  }

  assert (2if(iLeftIntraDir == iAboveIntraDir) // 若左边和上边PU的模式相同，MPM数量为1。
  {
    if( piMode )
    {
      *piMode = 1;
    }

    if (iLeftIntraDir > 1) // angular modes
    {
      uiIntraDirPred[0] = iLeftIntraDir;
      uiIntraDirPred[1] = ((iLeftIntraDir + 29) % 32) + 2; 
      uiIntraDirPred[2] = ((iLeftIntraDir - 1 ) % 32) + 2; 
    }
    else //non-angular
    {
      uiIntraDirPred[0] = PLANAR_IDX;
      uiIntraDirPred[1] = DC_IDX;
      uiIntraDirPred[2] = VER_IDX;
    }
  }
  else // 若左边和上边PU的模式不同，MPM数量为2。
  {
    if( piMode )
    {
      *piMode = 2;
    }
    uiIntraDirPred[0] = iLeftIntraDir;  // 左边PU的模式作为MPM模式
    uiIntraDirPred[1] = iAboveIntraDir; // 上边PU的模式作为MPM模式

    if (iLeftIntraDir && iAboveIntraDir ) //both modes are non-planar
    {
      uiIntraDirPred[2] = PLANAR_IDX;
    }
    else
    {
      uiIntraDirPred[2] =  (iLeftIntraDir+iAboveIntraDir)<2? VER_IDX : DC_IDX;
    }
  }
  for (UInt i=0; iassert(uiIntraDirPred[i] < 35);
  }
}

上面的代码中条件if (isChroma(chType))均不会满足，调用时给的chType均是COMPONENT_Y。