HEVC参考软件(HM)代码阅读之帧内预测:Void TComPrediction::xPredIntraAng

前言

该系列文章记录自己阅读HM代码的理解，防止自己以后遗忘，同时可以交流学习，可能会有错误，欢迎批评指正。

帧内预测

HEVC的帧内预测(intra)有以下特点：
1. 使用四叉树的编码块划分框架。
2. 使用细致的预测角度划分方式，共33个方向（如下图所示），加上planar模式，DC模式，一共35个方向模式。
3. 使用planar模式预测平滑区域。
4. 在特定情况下对块边缘进行滤波。
5. 对参考边缘进行自适应滤波。

还有其它一些特定，涉及到之后再加上。本文函数主要涉及特性2和特性4.

源代码及注释

 //Function for deriving the angular Intra predictions

/** Function for deriving the simplified angular intra predictions.
* \param bitDepth           bit depth
 * \param pSrc               pointer to reconstructed sample array，指向当前块的(0,0)位置
 * \param srcStride          the stride of the reconstructed sample array，相当于图像的宽
 * \param pTrueDst           reference to pointer for the prediction sample array
 * \param dstStrideTrue      the stride of the prediction sample array
 * \param uiWidth            the width of the block
 * \param uiHeight           the height of the block
 * \param channelType        type of pel array (luma/chroma)
 * \param format             chroma format
 * \param dirMode            the intra prediction mode index
 * \param blkAboveAvailable  boolean indication if the block above is available
 * \param blkLeftAvailable   boolean indication if the block to the left is available
 * \param bEnableEdgeFilters indication whether to enable edge filters：针对垂直和水平模式使用，boundray smoothing
 *
 * This function derives the prediction samples for the angular mode based on the prediction direction indicated by
 * the prediction mode index. The prediction direction is given by the displacement of the bottom row of the block and
 * the reference row above the block in the case of vertical prediction or displacement of the rightmost column
 * of the block and reference column left from the block in the case of the horizontal prediction. The displacement
 * is signalled at 1/32 pixel accuracy. When projection of the predicted pixel falls inbetween reference samples,
 * the predicted value for the pixel is linearly interpolated from the reference samples. All reference samples are taken
 * from the extended main reference.
 */
//NOTE: Bit-Limit - 25-bit source
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
  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);//2-17, 18-35
    const Int        intraPredAngleMode = (bIsModeVer) ? (Int)dirMode - VER_IDX :  -((Int)dirMode - HOR_IDX);
    const Int        absAngMode         = abs(intraPredAngleMode);
    const Int        signAng            = intraPredAngleMode < 0 ? -1 : 1;
    const Bool       edgeFilter         = bEnableEdgeFilters && isLuma(channelType) && (width <= MAXIMUM_INTRA_FILTERED_WIDTH) && (height <= MAXIMUM_INTRA_FILTERED_HEIGHT);

    // Set bitshifts and scale the angle parameter to block size
    static const Int angTable[9]    = {0,    2,    5,   9,  13,  17,  21,  26,  32};
    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.
    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
    {
      for (Int x=0;x<2*width+1;x++)
      {
        refAbove[x] = pSrc[x-srcStride-1];
      }
      for (Int y=0;y<2*height+1;y++)
      {
        refLeft[y] = pSrc[(y-1)*srcStride-1];
      }
      refMain = bIsModeVer ? refAbove : refLeft ;
      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)//boundary smoothing
      {
        for (Int y=0;y<height;y++)
        {
          pDst[y*dstStride] = Clip3 (0, ((1 << bitDepth) - 1), pDst[y*dstStride] + (( refSide[y+1] - refSide[0] ) >> 1) );//Boundary Smoothing
        }
      }
    }
    else
    {
      Pel *pDsty=pDst;

      for (Int y=0, deltaPos=intraPredAngle; y<height; y++, deltaPos+=intraPredAngle, pDsty+=dstStride)
      {
        const Int deltaInt   = deltaPos >> 5;
        const Int deltaFract = deltaPos & (32 - 1);

        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 );//1/32 pixel accuracy interpolation
            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
    if (!bIsModeVer)
    {
      for (Int y=0; y<height; y++)
      {
        for (Int x=0; x<width; x++)
        {
          pTrueDst[x*dstStrideTrue] = pDst[x];
        }
        pTrueDst++;
        pDst+=dstStride;
      }
    }
  }
}

代码解读

这里以垂直偏左方向预测为例（预测的方向模式在18-25）：代码中主要的数据结构的含义如上图。
代码主要实现了两部分功能：
1. 先计算refMain和refSide，将refMain扩展
2. 插值计算当前块的每一行
补：对于水平竖直预测，且预测块不大于16x16，进行边缘滤波

reference

1. Jani Lainema, Frank Bossen, Woo-Jin Han, Junghye Min, and Kemal Ugur. Intra Coding of the HEVC Standard. CSVT.