HEVC代码学习6：filterHor和filterVer函数

帧间预测分为运动估计ME和运动补偿MV，其中用到了MV的亚像素搜索，需要使用filterHor和filterVer进行插值。
这里使用的是HM16，在之前版本中分为filterHorLuma、filterHorChroma和filterHorLuma、filterVerChroma。HM16中已经将亮度和色度插值滤波器整合。

我们来从最底层插值滤波器函数filter看起。filter函数代码如下，对同一亚像素位置（如1/4处）处进行插值，插值后的MV存在dst中。

/** 插值滤波器 * \brief Apply FIR filter to a block of samples   * * \tparam N          Number of taps  抽头数 * \tparam isVertical Flag indicating filtering along vertical direction * \tparam isFirst    Flag indicating whether it is the first filtering operation * \tparam isLast     Flag indicating whether it is the last filtering operation * \param  bitDepth   Bit depth of samples * \param  src        Pointer to source samples  源MV地址 * \param  srcStride  Stride of source samples * \param  dst        Pointer to destination samples   插值后的MV地址 * \param  dstStride  Stride of destination samples * \param  width      Width of block * \param  height     Height of block * \param  coeff      Pointer to filter taps  抽头系数 */template<Int N, Bool isVertical, Bool isFirst, Bool isLast>Void TComInterpolationFilter::filter(Int bitDepth, Pel const *src, Int srcStride, Pel *dst, Int dstStride, Int width, Int height, TFilterCoeff const *coeff){  Int row, col;  Pel c[8];   //系数数组  c[0] = coeff[0];   //抽头0系数  c[1] = coeff[1];   //抽头1系数  if ( N >= 4 )    //N≥4时，取抽头2和3系数（UV分量）  {    c[2] = coeff[2];    c[3] = coeff[3];  }  if ( N >= 6 )    //N≥6时，取抽头4和5系数  {    c[4] = coeff[4];    c[5] = coeff[5];  }  if ( N == 8 )    //N=8时，取抽头6和7系数（Y分量）  {    c[6] = coeff[6];    c[7] = coeff[7];  }  Int cStride = ( isVertical ) ? srcStride : 1;   //垂直情况下步长为srcStride，水平情况下为1  src -= ( N/2 - 1 ) * cStride;   //找到前面N/2-1个整数点  Int offset;  Pel maxVal;  Int headRoom = std::max<Int>(2, (IF_INTERNAL_PREC - bitDepth));  Int shift    = IF_FILTER_PREC;  // with the current settings (IF_INTERNAL_PREC = 14 and IF_FILTER_PREC = 6), though headroom can be  // negative for bit depths greater than 14, shift will remain non-negative for bit depths of 8->20  assert(shift >= 0);  if ( isLast )  {    shift += (isFirst) ? 0 : headRoom;    offset = 1 << (shift - 1);    offset += (isFirst) ? 0 : IF_INTERNAL_OFFS << IF_FILTER_PREC;    maxVal = (1 << bitDepth) - 1;  }  else  {    shift -= (isFirst) ? headRoom : 0;    offset = (isFirst) ? -IF_INTERNAL_OFFS << shift : 0;    maxVal = 0;  }  for (row = 0; row < height; row++)  {    for (col = 0; col < width; col++)    {      Int sum;      //从N/2-1个整数点开始，取N个整数点乘对应的抽头系数求和。      sum  = src[ col + 0 * cStride] * c[0];          sum += src[ col + 1 * cStride] * c[1];          if ( N >= 4 )      {        sum += src[ col + 2 * cStride] * c[2];        sum += src[ col + 3 * cStride] * c[3];      }      if ( N >= 6 )      {        sum += src[ col + 4 * cStride] * c[4];        sum += src[ col + 5 * cStride] * c[5];      }      if ( N == 8 )      {        sum += src[ col + 6 * cStride] * c[6];        sum += src[ col + 7 * cStride] * c[7];      }      Pel val = ( sum + offset ) >> shift;        if ( isLast )      {        val = ( val < 0 ) ? 0 : val;    //保证不小于0        val = ( val > maxVal ) ? maxVal : val;   //保证不越界      }      dst[col] = val;   //存储插值后的MV    }    src += srcStride;       dst += dstStride;  }}

再来看上一层的函数，函数代码如下。是一个分类处理，分为普通操作、第一次操作和最后一次操作，分别传入不同参数调用底层filter函数进行插值。这里直接通过N来区分是亮度插值和色度插值。

/** * \brief Filter a block of samples (horizontal) * * \tparam N          Number of taps 抽头数：Y-8，UV-4 * \param  bitDepth   Bit depth of samples * \param  src        Pointer to source samples * \param  srcStride  Stride of source samples * \param  dst        Pointer to destination samples * \param  dstStride  Stride of destination samples * \param  width      Width of block * \param  height     Height of block * \param  isLast     Flag indicating whether it is the last filtering operation * \param  coeff      Pointer to filter taps 抽头系数 */template<Int N>     //插值Void TComInterpolationFilter::filterHor(Int bitDepth, Pel *src, Int srcStride, Pel *dst, Int dstStride, Int width, Int height, Bool isLast, TFilterCoeff const *coeff){  if ( isLast )  {    filter<N, false, true, true>(bitDepth, src, srcStride, dst, dstStride, width, height, coeff);  }  else  {    filter<N, false, true, false>(bitDepth, src, srcStride, dst, dstStride, width, height, coeff);  }}template<Int N>Void TComInterpolationFilter::filterVer(Int bitDepth, Pel *src, Int srcStride, Pel *dst, Int dstStride, Int width, Int height, Bool isFirst, Bool isLast, TFilterCoeff const *coeff){  if ( isFirst && isLast )  {    filter<N, true, true, true>(bitDepth, src, srcStride, dst, dstStride, width, height, coeff);  }  else if ( isFirst && !isLast )  {    filter<N, true, true, false>(bitDepth, src, srcStride, dst, dstStride, width, height, coeff);  }  else if ( !isFirst && isLast )  {    filter<N, true, false, true>(bitDepth, src, srcStride, dst, dstStride, width, height, coeff);  }  else  {    filter<N, true, false, false>(bitDepth, src, srcStride, dst, dstStride, width, height, coeff);  }}

最后来看最上层的filterHor和filterVer，代码如下，也是一个分类处理，调用下层函数处理。分了三种情况：frac = 0、亮度和色度。frac = 0，即整数，不需要插值。

/** * \brief Filter a block of Luma/Chroma samples (horizontal)     * * \param  compID     Chroma component ID * \param  src        Pointer to source samples * \param  srcStride  Stride of source samples * \param  dst        Pointer to destination samples * \param  dstStride  Stride of destination samples * \param  width      Width of block * \param  height     Height of block * \param  frac       Fractional sample offset  分数偏移 * \param  isLast     Flag indicating whether it is the last filtering operation * \param  fmt        Chroma format * \param  bitDepth   Bit depth */Void TComInterpolationFilter::filterHor(const ComponentID compID, Pel *src, Int srcStride, Pel *dst, Int dstStride, Int width, Int height, Int frac, Bool isLast, const ChromaFormat fmt, const Int bitDepth ){  if ( frac == 0 )   //整数直接复制  {    filterCopy(bitDepth, src, srcStride, dst, dstStride, width, height, true, isLast );  }  else if (isLuma(compID))   //Y分量小数插值  {    assert(frac >= 0 && frac < LUMA_INTERPOLATION_FILTER_SUB_SAMPLE_POSITIONS);    filterHor<NTAPS_LUMA>(bitDepth, src, srcStride, dst, dstStride, width, height, isLast, m_lumaFilter[frac]);  }  else  {    const UInt csx = getComponentScaleX(compID, fmt);    assert(frac >=0 && csx<2 && (frac<<(1-csx)) < CHROMA_INTERPOLATION_FILTER_SUB_SAMPLE_POSITIONS);    filterHor<NTAPS_CHROMA>(bitDepth, src, srcStride, dst, dstStride, width, height, isLast, m_chromaFilter[frac<<(1-csx)]);  }}Void TComInterpolationFilter::filterVer(const ComponentID compID, Pel *src, Int srcStride, Pel *dst, Int dstStride, Int width, Int height, Int frac, Bool isFirst, Bool isLast, const ChromaFormat fmt, const Int bitDepth ){  if ( frac == 0 )  {    filterCopy(bitDepth, src, srcStride, dst, dstStride, width, height, isFirst, isLast );  }  else if (isLuma(compID))  {    assert(frac >= 0 && frac < LUMA_INTERPOLATION_FILTER_SUB_SAMPLE_POSITIONS);    filterVer<NTAPS_LUMA>(bitDepth, src, srcStride, dst, dstStride, width, height, isFirst, isLast, m_lumaFilter[frac]);  }  else  {    const UInt csy = getComponentScaleY(compID, fmt);    assert(frac >=0 && csy<2 && (frac<<(1-csy)) < CHROMA_INTERPOLATION_FILTER_SUB_SAMPLE_POSITIONS);    filterVer<NTAPS_CHROMA>(bitDepth, src, srcStride, dst, dstStride, width, height, isFirst, isLast, m_chromaFilter[frac<<(1-csy)]);  }}