HM16.9代码阅读-帧内预测（）

这篇博客讲HM代码中xPredIntraPlanar、xPredIntraAng、xDCPredFiltering三个函数，有关帧内预测的理论知识看 https://blog.csdn.net/shayashi/article/details/82877875 。
xPredIntraPlanar函数是对Planar模式预测的函数，Planar模式对应0，该模式的预测采用同双线性差值方式，代码如下：

Void TComPrediction::xPredIntraPlanar( const Pel* pSrc, Int srcStride, Pel* rpDst, Int dstStride, UInt width, UInt height )
{
  assert(width <= height);

  Int leftColumn[MAX_CU_SIZE+1], topRow[MAX_CU_SIZE+1], bottomRow[MAX_CU_SIZE], rightColumn[MAX_CU_SIZE];//参考像素
  UInt shift1Dhor = g_aucConvertToBit[ width ] + 2;//水平移动位数
  UInt shift1Dver = g_aucConvertToBit[ height ] + 2;//竖直移动位数
  //g_aucConvertToBit是对应的bit数 [4]==0 [8]==1 [16]==2 [32]==3 [64]==4 other -1
  // Get left and above reference column and row获取参考像素
  for(Int k=0;k<width+1;k++)
  {
    topRow[k] = pSrc[k-srcStride];//srcStride == 2 * width + 1
  }

  for (Int k=0; k < height+1; k++)
  {
    leftColumn[k] = pSrc[k*srcStride-1];
  }

  // Prepare intermediate variables used in interpolation
  Int bottomLeft = leftColumn[height];
  Int topRight   = topRow[width];
//bottomRow[k]= leftColumn[height]-topRow[k]，且将topRow的数值乘height
  for(Int k=0;k<width;k++)
  {
    bottomRow[k]  = bottomLeft - topRow[k];
    topRow[k]     <<= shift1Dver;
  }
//rightColumn[k]=topRow[width]-leftColumn[k]，将leftColumn的数值乘width
  for(Int k=0;k<height;k++)
  {
    rightColumn[k]  = topRight - leftColumn[k];
    leftColumn[k]   <<= shift1Dhor;
  }

  const UInt topRowShift = 0;

  // 生成预测信号
  //双线性插值就是像素对应的上、下和左、右参考像素成比例的和。
  //horPred(x,y) = (width - x) * leftColumn[y] + x * topRight
  //vertPred(x,y) = (width - y) * topRight[y] + y * bottomLeft
  //rpDst(x,y) = (horPred(x,y) + vertPred(x,y)) >> (shift1Dhor+1)
  for (Int y=0;y<height;y++)
  {
    Int horPred = leftColumn[y] + width;
    for (Int x=0;x<width;x++)
    {
      horPred += rightColumn[y];
      topRow[x] += bottomRow[x];

      Int vertPred = ((topRow[x] + topRowShift)>>topRowShift);
      rpDst[y*dstStride+x] = ( horPred + vertPred ) >> (shift1Dhor+1);//dstStride == width
    }
  }
}

xPredIntraAng函数是对DC模式和角度模式进行预测的函数：

Void TComPrediction::xPredIntraAng(       Int bitDepth,
                                    const Pel* pSrc,     Int srcStride,
                                          Pel* pTrueDst, Int dstStrideTrue,
                                          UInt uiWidth, UInt uiHeight, ChannelType channelType,
                                          UInt dirMode, const Bool bEnableEdgeFilters
                                  )
{
  Int width=Int(uiWidth);
  Int height=Int(uiHeight);

  // Map the mode index to main prediction direction and angle
  assert( dirMode != PLANAR_IDX ); //no planar
  const Bool modeDC        = dirMode==DC_IDX;

  // Do the DC prediction DC模式就是对块中的每一个像素设置为平均值
  if (modeDC)
  {
    const Pel dcval = predIntraGetPredValDC(pSrc, srcStride, width, height);

    for (Int y=height;y>0;y--, pTrueDst+=dstStrideTrue)
    {
      for (Int x=0; x<width;) // width is always a multiple of 4.
      {
        pTrueDst[x++] = dcval;
      }
    }
  }
  else // Do angular predictions
  {
    const Bool       bIsModeVer         = (dirMode >= 18);//是否是竖直模式
    const Int        intraPredAngleMode = (bIsModeVer) ? (Int)dirMode - VER_IDX :  -((Int)dirMode - HOR_IDX);//竖直模式：mode - 26 ；水平模式：10-mode
    const Int        absAngMode         = abs(intraPredAngleMode);
    const Int        signAng            = intraPredAngleMode < 0 ? -1 : 1;//sin正负
    const Bool       edgeFilter         = bEnableEdgeFilters && isLuma(channelType) && (width <= MAXIMUM_INTRA_FILTERED_WIDTH) && (height <= MAXIMUM_INTRA_FILTERED_HEIGHT);//当块大小小于等于16x16且是亮度预测且bEnableEdgeFilters ==true进行边缘滤波

    // Set bitshifts and scale the angle parameter to block size
    static const Int angTable[9]    = {0,    2,    5,   9,  13,  17,  21,  26,  32};//mode偏移量绝对值
    static const Int invAngTable[9] = {0, 4096, 1638, 910, 630, 482, 390, 315, 256}; // (256 * 32) / Angle
    Int invAngle                    = invAngTable[absAngMode];
    Int absAng                      = angTable[absAngMode];
    Int intraPredAngle              = signAng * absAng;

    Pel* refMain;
    Pel* refSide;

    Pel  refAbove[2*MAX_CU_SIZE+1];
    Pel  refLeft[2*MAX_CU_SIZE+1];

    // Initialize the Main and Left reference array.映射
    if (intraPredAngle < 0)
    {
      const Int refMainOffsetPreScale = (bIsModeVer ? height : width ) - 1;
      const Int refMainOffset         = height - 1;
      for (Int x=0;x<width+1;x++)
      {
        refAbove[x+refMainOffset] = pSrc[x-srcStride-1];
      }
      for (Int y=0;y<height+1;y++)
      {
        refLeft[y+refMainOffset] = pSrc[(y-1)*srcStride-1];
      }
      refMain = (bIsModeVer ? refAbove : refLeft)  + refMainOffset;
      refSide = (bIsModeVer ? refLeft  : refAbove) + refMainOffset;

      // Extend the Main reference to the left.
      Int invAngleSum    = 128;       // rounding for (shift by 8)
      for (Int k=-1; k>(refMainOffsetPreScale+1)*intraPredAngle>>5; k--)
      {
        invAngleSum += invAngle;
        refMain[k] = refSide[invAngleSum>>8];
      }
    }
    else// (2 <= mode <= 10) || (26 <= mode <= 34)
    {
      for (Int x=0;x<2*width+1;x++)
      {
        refAbove[x] = pSrc[x-srcStride-1];//srcStride == 2*width+1
      }
      for (Int y=0;y<2*height+1;y++)
      {
        refLeft[y] = pSrc[(y-1)*srcStride-1];
      }
      refMain = bIsModeVer ? refAbove : refLeft ;//主参考像素
      refSide = bIsModeVer ? refLeft  : refAbove;
    }

    // swap width/height if we are doing a horizontal mode:
    Pel tempArray[MAX_CU_SIZE*MAX_CU_SIZE];
    const Int dstStride = bIsModeVer ? dstStrideTrue : MAX_CU_SIZE;
    Pel *pDst = bIsModeVer ? pTrueDst : tempArray;
    if (!bIsModeVer)
    {
      std::swap(width, height);
    }

    if (intraPredAngle == 0)  // pure vertical or pure horizontal
    {
      for (Int y=0;y<height;y++)
      {
        for (Int x=0;x<width;x++)
        {
          pDst[y*dstStride+x] = refMain[x+1];
        }
      }

      if (edgeFilter)
      {
        for (Int y=0;y<height;y++)
        {//边缘滤波 当前像素值+（对应参考像素值和左上角参考像素和的1/2）
          pDst[y*dstStride] = Clip3 (0, ((1 << bitDepth) - 1), pDst[y*dstStride] + (( refSide[y+1] - refSide[0] ) >> 1) );
        }
      }
    }
    else
    {
      Pel *pDsty=pDst;

      for (Int y=0, deltaPos=intraPredAngle; y<height; y++, deltaPos+=intraPredAngle, pDsty+=dstStride)
      {
        const Int deltaInt   = deltaPos >> 5;//坐标整数部分
        const Int deltaFract = deltaPos & (32 - 1);//坐标分数部分
//pDsty[x] = ((32 - deltaFract) *  refMain[x + deltaInt+1] + deltaFract *  refMain[x + deltaInt+2] + 16) >> 5
        if (deltaFract)
        {
          // Do linear filtering
          const Pel *pRM=refMain+deltaInt+1;
          Int lastRefMainPel=*pRM++;
          for (Int x=0;x<width;pRM++,x++)
          {
            Int thisRefMainPel=*pRM;
            pDsty[x+0] = (Pel) ( ((32-deltaFract)*lastRefMainPel + deltaFract*thisRefMainPel +16) >> 5 );
            lastRefMainPel=thisRefMainPel;
          }
        }
        else
        {
          // Just copy the integer samples
          for (Int x=0;x<width; x++)
          {
            pDsty[x] = refMain[x+deltaInt+1];
          }
        }
      }
    }

    // Flip the block if this is the horizontal mode水平模式翻转
    if (!bIsModeVer)
    {
      for (Int y=0; y<height; y++)
      {
        for (Int x=0; x<width; x++)
        {
          pTrueDst[x*dstStrideTrue] = pDst[x];
        }
        pTrueDst++;
        pDst+=dstStride;
      }
    }
  }
}

xDCPredFiltering函数是对DC模式进行边缘滤波的函数，对第一行和第一列进行滤波：

Void TComPrediction::xDCPredFiltering( const Pel* pSrc, Int iSrcStride, Pel* pDst, Int iDstStride, Int iWidth, Int iHeight, ChannelType channelType )
{
  Int x, y, iDstStride2, iSrcStride2;
//亮度模式且小于等于16x16才进行滤波
  if (isLuma(channelType) && (iWidth <= MAXIMUM_INTRA_FILTERED_WIDTH) && (iHeight <= MAXIMUM_INTRA_FILTERED_HEIGHT))
  {
    //top-left pDst[0] = 当前像素值和左边像素上边参考像素进行2 1 1滤波
    pDst[0] = (Pel)((pSrc[-iSrcStride] + pSrc[-1] + 2 * pDst[0] + 2) >> 2);

    //top row (vertical filter)第一行像素值等于当前像素和上边参考像素进行3 1滤波
    for ( x = 1; x < iWidth; x++ )
    {
      pDst[x] = (Pel)((pSrc[x - iSrcStride] +  3 * pDst[x] + 2) >> 2);
    }

    //left column (horizontal filter)第一列像素值等于当前像素和左边参考像素进行3 1滤波
    for ( y = 1, iDstStride2 = iDstStride, iSrcStride2 = iSrcStride-1; y < iHeight; y++, iDstStride2+=iDstStride, iSrcStride2+=iSrcStride )
    {
      pDst[iDstStride2] = (Pel)((pSrc[iSrcStride2] + 3 * pDst[iDstStride2] + 2) >> 2);
    }
  }

  return;
}