HEVC代码学习38：decompressSlice函数

好久没看HEVC了，今天回归，来学习一下解码端的decompressSlice函数，有两个，一个是TDecGop类的成员函数，另一个是TDecSlice类的。

TDecGop::decompressSlice

该函数比较简单，是解码一帧图像的上层入口函数，在TDecGop::xDecodeSlice中被调用。主要功能是完成熵解码器初始化和计时工作，并会调用TDecSlice::decompressSlice解码一帧图像。

//解码slice
Void TDecGop::decompressSlice(TComInputBitstream* pcBitstream, TComPic* pcPic)
{
  //当前slice
  TComSlice*  pcSlice = pcPic->getSlice(pcPic->getCurrSliceIdx());
  // Table of extracted substreams.
  // These must be deallocated AND their internal fifos, too.
  TComInputBitstream **ppcSubstreams = NULL;

  //-- For time output for each slice
  //计时器，统计每一个slice解码时间
  clock_t iBeforeTime = clock();

  //熵解码器初始化
  m_pcSbacDecoder->init( (TDecBinIf*)m_pcBinCABAC );
  m_pcEntropyDecoder->setEntropyDecoder (m_pcSbacDecoder);

  const UInt uiNumSubstreams = pcSlice->getNumberOfSubstreamSizes()+1;

  // init each couple {EntropyDecoder, Substream}
  ppcSubstreams    = new TComInputBitstream*[uiNumSubstreams];
  for ( UInt ui = 0 ; ui < uiNumSubstreams ; ui++ )
  {
    ppcSubstreams[ui] = pcBitstream->extractSubstream(ui+1 < uiNumSubstreams ? (pcSlice->getSubstreamSize(ui)<<3) : pcBitstream->getNumBitsLeft());
  }

  //调用TDecSlice::decompressSlice解码slice
  m_pcSliceDecoder->decompressSlice( ppcSubstreams, pcPic, m_pcSbacDecoder);
  // deallocate all created substreams, including internal buffers.
  //清楚buffer
  for (UInt ui = 0; ui < uiNumSubstreams; ui++)
  {
    delete ppcSubstreams[ui];
  }
  delete[] ppcSubstreams;

  //解码时间
  m_dDecTime += (Double)(clock()-iBeforeTime) / CLOCKS_PER_SEC;
}

TDecSlice::decompressSlice

该函数在TDecGop::decompressSlice中被调用，用于完成一帧图像的解码。其中涉及了大量熵编码的内容，个人不是很熟悉，可能存在错误，请指正。

主要流程如下：
1.初始化：CTU、熵解码器等。
2.如果是dependent slice，利用之前的slice优化上下文模型。
3.遍历所有CTU进行解码：
 3.1 初始化CTU。
 3.2 设置熵解码器，加载上下文模型。
 3.3 SAO。
 3.4 调用decodeCtu（详见HEVC代码学习39：decodeCtu和xDecodeCU函数）和decompressCtu解码CTU。
 3.5 存储第二个CTU的上下文模型。
 3.6 如果是最后一个CTU，不分析剩余比特；否则分析剩余比特。
4.如果是dependent slice，加载上一个slice结束时的上下文模型。

//解码slice
Void TDecSlice::decompressSlice(TComInputBitstream** ppcSubstreams, TComPic* pcPic, TDecSbac* pcSbacDecoder)
{
  //初始化
  TComSlice* pcSlice                 = pcPic->getSlice(pcPic->getCurrSliceIdx());

  const Int  startCtuTsAddr          = pcSlice->getSliceSegmentCurStartCtuTsAddr();
  const Int  startCtuRsAddr          = pcPic->getPicSym()->getCtuTsToRsAddrMap(startCtuTsAddr);
  const UInt numCtusInFrame          = pcPic->getNumberOfCtusInFrame();

  const UInt frameWidthInCtus        = pcPic->getPicSym()->getFrameWidthInCtus();
  const Bool depSliceSegmentsEnabled = pcSlice->getPPS()->getDependentSliceSegmentsEnabledFlag();
  const Bool wavefrontsEnabled       = pcSlice->getPPS()->getEntropyCodingSyncEnabledFlag();

  m_pcEntropyDecoder->setEntropyDecoder ( pcSbacDecoder  );
  m_pcEntropyDecoder->setBitstream      ( ppcSubstreams[0] );
  m_pcEntropyDecoder->resetEntropy      (pcSlice);

  // decoder doesn't need prediction & residual frame buffer
  pcPic->setPicYuvPred( 0 );
  pcPic->setPicYuvResi( 0 );

#if ENC_DEC_TRACE
  g_bJustDoIt = g_bEncDecTraceEnable;
#endif
  DTRACE_CABAC_VL( g_nSymbolCounter++ );
  DTRACE_CABAC_T( "\tPOC: " );
  DTRACE_CABAC_V( pcPic->getPOC() );
  DTRACE_CABAC_T( "\n" );

#if ENC_DEC_TRACE
  g_bJustDoIt = g_bEncDecTraceDisable;
#endif

  // The first CTU of the slice is the first coded substream, but the global substream number, as calculated by getSubstreamForCtuAddr may be higher.
  // This calculates the common offset for all substreams in this slice.
  //计算起始子码流偏移
  const UInt subStreamOffset=pcPic->getSubstreamForCtuAddr(startCtuRsAddr, true, pcSlice);

  //处理dependent slice
  if (depSliceSegmentsEnabled)
  {
    // modify initial contexts with previous slice segment if this is a dependent slice.
    const UInt startTileIdx=pcPic->getPicSym()->getTileIdxMap(startCtuRsAddr);
    const TComTile *pCurrentTile=pcPic->getPicSym()->getTComTile(startTileIdx);
    const UInt firstCtuRsAddrOfTile = pCurrentTile->getFirstCtuRsAddr();

    if( pcSlice->getDependentSliceSegmentFlag() && startCtuRsAddr != firstCtuRsAddrOfTile)
    {
      if ( pCurrentTile->getTileWidthInCtus() >= 2 || !wavefrontsEnabled)
      {
        pcSbacDecoder->loadContexts(&m_lastSliceSegmentEndContextState);
      }
    }
  }

  // for every CTU in the slice segment...
  //遍历所有CTU进行解码。
  Bool isLastCtuOfSliceSegment = false;
  for( UInt ctuTsAddr = startCtuTsAddr; !isLastCtuOfSliceSegment && ctuTsAddr < numCtusInFrame; ctuTsAddr++)
  {
    //初始化CTU
    const UInt ctuRsAddr = pcPic->getPicSym()->getCtuTsToRsAddrMap(ctuTsAddr);
    const TComTile ¤tTile = *(pcPic->getPicSym()->getTComTile(pcPic->getPicSym()->getTileIdxMap(ctuRsAddr)));
    const UInt firstCtuRsAddrOfTile = currentTile.getFirstCtuRsAddr();
    const UInt tileXPosInCtus = firstCtuRsAddrOfTile % frameWidthInCtus;
    const UInt tileYPosInCtus = firstCtuRsAddrOfTile / frameWidthInCtus;
    const UInt ctuXPosInCtus  = ctuRsAddr % frameWidthInCtus;
    const UInt ctuYPosInCtus  = ctuRsAddr / frameWidthInCtus;
    const UInt uiSubStrm=pcPic->getSubstreamForCtuAddr(ctuRsAddr, true, pcSlice)-subStreamOffset;
    TComDataCU* pCtu = pcPic->getCtu( ctuRsAddr );
    pCtu->initCtu( pcPic, ctuRsAddr );

    //设置熵解码器
    m_pcEntropyDecoder->setBitstream( ppcSubstreams[uiSubStrm] );

    // set up CABAC contexts' state for this CTU
    //为CTU设置上下文模型
    if (ctuRsAddr == firstCtuRsAddrOfTile)
    {
      if (ctuTsAddr != startCtuTsAddr) // if it is the first CTU, then the entropy coder has already been reset
      {
        m_pcEntropyDecoder->resetEntropy(pcSlice);
      }
    }
    else if (ctuXPosInCtus == tileXPosInCtus && wavefrontsEnabled)
    {
      // Synchronize cabac probabilities with upper-right CTU if it's available and at the start of a line.
      if (ctuTsAddr != startCtuTsAddr) // if it is the first CTU, then the entropy coder has already been reset
      {
        m_pcEntropyDecoder->resetEntropy(pcSlice);
      }
      TComDataCU *pCtuUp = pCtu->getCtuAbove();
      if ( pCtuUp && ((ctuRsAddr%frameWidthInCtus+1) < frameWidthInCtus)  )
      {
        TComDataCU *pCtuTR = pcPic->getCtu( ctuRsAddr - frameWidthInCtus + 1 );
        if ( pCtu->CUIsFromSameSliceAndTile(pCtuTR) )
        {
          // Top-right is available, so use it.
          pcSbacDecoder->loadContexts( &m_entropyCodingSyncContextState );
        }
      }
    }

#if ENC_DEC_TRACE
    g_bJustDoIt = g_bEncDecTraceEnable;
#endif
    //SAO
    if ( pcSlice->getSPS()->getUseSAO() )
    {
      SAOBlkParam& saoblkParam = (pcPic->getPicSym()->getSAOBlkParam())[ctuRsAddr];
      Bool bIsSAOSliceEnabled = false;
      Bool sliceEnabled[MAX_NUM_COMPONENT];
      for(Int comp=0; comp < MAX_NUM_COMPONENT; comp++)
      {
        ComponentID compId=ComponentID(comp);
        sliceEnabled[compId] = pcSlice->getSaoEnabledFlag(toChannelType(compId)) && (comp < pcPic->getNumberValidComponents());
        if (sliceEnabled[compId])
        {
          bIsSAOSliceEnabled=true;
        }
        saoblkParam[compId].modeIdc = SAO_MODE_OFF;
      }
      if (bIsSAOSliceEnabled)
      {
        Bool leftMergeAvail = false;
        Bool aboveMergeAvail= false;

        //merge left condition
        Int rx = (ctuRsAddr % frameWidthInCtus);
        if(rx > 0)
        {
          leftMergeAvail = pcPic->getSAOMergeAvailability(ctuRsAddr, ctuRsAddr-1);
        }
        //merge up condition
        Int ry = (ctuRsAddr / frameWidthInCtus);
        if(ry > 0)
        {
          aboveMergeAvail = pcPic->getSAOMergeAvailability(ctuRsAddr, ctuRsAddr-frameWidthInCtus);
        }

        pcSbacDecoder->parseSAOBlkParam( saoblkParam, sliceEnabled, leftMergeAvail, aboveMergeAvail, pcSlice->getSPS()->getBitDepths());
      }
    }

    //解码CTU
    m_pcCuDecoder->decodeCtu     ( pCtu, isLastCtuOfSliceSegment );
    m_pcCuDecoder->decompressCtu ( pCtu );

#if ENC_DEC_TRACE
    g_bJustDoIt = g_bEncDecTraceDisable;
#endif

    //Store probabilities of second CTU in line into buffer
    //存储上下文模型
    if ( ctuXPosInCtus == tileXPosInCtus+1 && wavefrontsEnabled)
    {
      m_entropyCodingSyncContextState.loadContexts( pcSbacDecoder );
    }

    //最后一个CTU熵解码器处理
    if (isLastCtuOfSliceSegment)
    {
#if DECODER_CHECK_SUBSTREAM_AND_SLICE_TRAILING_BYTES
      pcSbacDecoder->parseRemainingBytes(false);
#endif
      if(!pcSlice->getDependentSliceSegmentFlag())
      {
        pcSlice->setSliceCurEndCtuTsAddr( ctuTsAddr+1 );
      }
      pcSlice->setSliceSegmentCurEndCtuTsAddr( ctuTsAddr+1 );
    }
    else if (  ctuXPosInCtus + 1 == tileXPosInCtus + currentTile.getTileWidthInCtus() &&
             ( ctuYPosInCtus + 1 == tileYPosInCtus + currentTile.getTileHeightInCtus() || wavefrontsEnabled)
            )//非最后一个CTU熵解码器处理
    {
      // The sub-stream/stream should be terminated after this CTU.
      // (end of slice-segment, end of tile, end of wavefront-CTU-row)
      UInt binVal;
      pcSbacDecoder->parseTerminatingBit( binVal );
      assert( binVal );
#if DECODER_CHECK_SUBSTREAM_AND_SLICE_TRAILING_BYTES
      pcSbacDecoder->parseRemainingBytes(true);
#endif
    }

  }

  assert(isLastCtuOfSliceSegment == true);

  //dependent slice上下文模型
  if( depSliceSegmentsEnabled )
  {
    m_lastSliceSegmentEndContextState.loadContexts( pcSbacDecoder );//ctx end of dep.slice
  }

}