Chroma from Luma 预测方法总结

1.术语

• IPP：Inter plane prediction
• CCP：Cross component predition

• CCLM：Cross-component linear model prediction

• CfL：Chroma from luma

2.背景

因为原来发的paper是基于RGB编码，看过相关性的paper；也就在那段时间，range extension software引入了CCP。

最早是RGB编码中的inter plane prediction，由Woo-Shik Kim搞出来的。265时代，被引入HEVC range extension，技术命名为cross component prediction，至此主要还是支持444 format；到了266、av1时代，JEM和BMS，改名为cross-component linear model prediction，增加了下采样对YUV420 format的支持，而AV1中叫chroma from luma，由Mozilla贡献；后面的提案又引入的5种enhanced linear model，在最近的会议中，enhanced linear model并没有被接受，仅保留CCLM。
但换汤不换药，最根源的思想还是在去除分量间的相关性。

E0077：enhanced linear model
K0190：only keep CCLM

3.算法详解

4.代码

预测流程

//! 预测流程
//! 调用1：编码：模式预测
//! 调用2：编码：实际编码
//! 调用3：解码：重构
    ...
    //===== get prediction signal =====
#if COM16_C806_LMCHROMA
    if( uiChFinalMode == LM_CHROMA_IDX )///< 使用cclm mode
    {
            predLMIntraChroma(rTu, compID, piPred, uiStride, uiWidth, uiHeight);
    }
    else                                ///< 使用正常intra mode
    {
#endif
      predIntraAng( compID, uiChFinalMode, piOrg, uiStride, piPred, uiStride, rTu, bUseFilteredPredictions );

#if COM16_C806_LMCHROMA
      if( compID == COMPONENT_Cr && pcCU->getSlice()->getSPS()->getUseLMChroma() )
      {                                 ///< Cb-to-Cr
        addCrossColorResi( rTu, compID, piPred, uiStride, uiWidth, uiHeight, pcResiYuv->getAddr( COMPONENT_Cb, uiAbsPartIdx ), pcResiYuv->getStride(COMPONENT_Cb) );
      }
    }
#endif
    ...

预测流程之Luma-to-Chroma

Void TComPrediction::predLMIntraChroma( TComTU& rTu, const ComponentID compID, Pel* pPred, UInt uiPredStride, UInt uiCWidth, UInt uiCHeight )
{

  // LLS parameters estimation -->
  Int a, b, iShift;
  xGetLMParameters( rTu, compID, uiCWidth, uiCHeight, 0, a, b, iShift );

  // get prediction -->
  Int  iLumaStride = m_iLumaRecStride;

  Pel  *pLuma = m_pLumaRecBuffer + iLumaStride + 1;

  for( Int i = 0; i < uiCHeight; i++ )
  {
    for( Int j = 0; j < uiCWidth; j++ )
    {
      pPred[j] = Clip3(0, maxV, ( ( a * pLuma[j] ) >> iShift ) + b );
    }

    pPred += uiPredStride;
    pLuma += iLumaStride;
  }
  // <-- end of get prediction
}

预测流程之Cb-to-Cr

//! Cb residual predict Cr residual
Void TComPrediction::addCrossColorResi( TComTU& rTu, const ComponentID compID, Pel* piPred, UInt uiPredStride, UInt uiWidth, UInt uiHeight, Pel* piResi, UInt uiResiStride )
{
  Int a, b, iShift;

  xGetLMParameters( rTu, compID, uiWidth, uiHeight, 1, a, b, iShift );      ///< cb-to-cr lambda

  Int offset = 1 << (iShift - 1);

  if (a >= 0)
  {
    return;
  }

  Pel*  pPred   = piPred;           ///< [out]Cr resi pred
  Pel*  pResi   = piResi;           ///< [in]Cb resi

  for( UInt uiY = 0; uiY < uiHeight; uiY++ )
  {
    for( UInt uiX = 0; uiX < uiWidth; uiX++ )
    {
      pPred[ uiX ] = Clip3(0, maxV, pPred[ uiX ] + (( pResi[ uiX ] * a + offset) >> iShift  ) );
    }
    pPred += uiPredStride;
    pResi += uiResiStride;
  }
}

预测之线性模型参数推算

//! 生成线性模型的参数
Void TComPrediction::xCalcLMParameters( Int x, Int y, Int xx, Int xy, Int iCountShift, Int iPredType, Int bitDepth, Int &a, Int &b, Int &iShift );

• Chroma from Luma
  

• Cr from Cb
  
  where

  λ=∑(Cb(n)∗Cb(n))>>9 λ = ∑ ( C b ( n ) ∗ C b ( n ) ) >> 9

熵编码

Void TEncSbac::codeIntraDirChroma( TComDataCU* pcCU, UInt uiAbsPartIdx )
{
  UInt uiIntraDirChroma = pcCU->getIntraDir( CHANNEL_TYPE_CHROMA, uiAbsPartIdx );

  if( uiIntraDirChroma == DM_CHROMA_IDX )
  {
    m_pcBinIf->encodeBin( 0, m_cCUChromaPredSCModel.get( 0, 0, 0 ) );   ///< dm
  }

#if COM16_C806_LMCHROMA
  else if( uiIntraDirChroma == LM_CHROMA_IDX && pcCU->getSlice()->getSPS()->getUseLMChroma() )
  {
    m_pcBinIf->encodeBin( 1, m_cCUChromaPredSCModel.get( 0, 0, 0 ) );
    m_pcBinIf->encodeBin( 0, m_cCUChromaPredSCModel.get( 0, 0, 1 ) );   ///< cclm
  }

#endif
  else
  {
    m_pcBinIf->encodeBin( 1, m_cCUChromaPredSCModel.get( 0, 0, 0 ) );
#if COM16_C806_LMCHROMA
    if (pcCU->getSlice()->getSPS()->getUseLMChroma())
    {
      m_pcBinIf->encodeBin( 1, m_cCUChromaPredSCModel.get( 0, 0, 1 ));  ///< intra pred mode
    }
#endif
    UInt uiAllowedChromaDir[ NUM_CHROMA_MODE ];
    pcCU->getAllowedChromaDir( uiAbsPartIdx, uiAllowedChromaDir );

#if COM16_C806_LMCHROMA
    for( Int i = 0; i < NUM_CHROMA_MODE - 2; i++ )
#endif
    {
      if( uiIntraDirChroma == uiAllowedChromaDir[i] )
      {
        uiIntraDirChroma = i;
        break;
      }
    }

    m_pcBinIf->encodeBinsEP( uiIntraDirChroma, 2 );
  }
  return;
}

熵解码

Void TDecSbac::parseIntraDirChroma( TComDataCU* pcCU, UInt uiAbsPartIdx, UInt uiDepth )
{
  UInt uiSymbol;

  m_pcTDecBinIf->decodeBin( uiSymbol, m_cCUChromaPredSCModel.get( 0, 0, 0 ) RExt__DECODER_DEBUG_BIT_STATISTICS_PASS_OPT_ARG(ctype) );
  if( uiSymbol == 0 )                                       ///< dm
  {
    uiSymbol = DM_CHROMA_IDX;
  }
  else
  {
#if COM16_C806_LMCHROMA
    if( pcCU->getSlice()->getSPS()->getUseLMChroma() )
    {
      m_pcTDecBinIf->decodeBin( uiSymbol, m_cCUChromaPredSCModel.get( 0, 0, 1 ) RExt__DECODER_DEBUG_BIT_STATISTICS_PASS_OPT_ARG(ctype) );
    }
    else
    {
      uiSymbol = 1;
    }

    if( uiSymbol == 0 )                                     ///< cclm
    {
      uiSymbol = LM_CHROMA_IDX;
    }
    else                                                    ///< intra pred mode
    {
#endif
      UInt uiIPredMode;
      m_pcTDecBinIf->decodeBinsEP( uiIPredMode, 2 RExt__DECODER_DEBUG_BIT_STATISTICS_PASS_OPT_ARG(ctype) );
      UInt uiAllowedChromaDir[ NUM_CHROMA_MODE ];
      pcCU->getAllowedChromaDir( uiAbsPartIdx, uiAllowedChromaDir );
      uiSymbol = uiAllowedChromaDir[ uiIPredMode ];
#if COM16_C806_LMCHROMA
    }
#endif
  }
  pcCU->setIntraDirSubParts( CHANNEL_TYPE_CHROMA, uiSymbol, uiAbsPartIdx, uiDepth );
}