BMS/VTM代码学习：decompressSlice函数

今天正式转入BMS代码的学习，最近在干解码端的东西，就从解码这边开始看了。先来看decompressSlice函数。在之前的 HEVC代码学习38：decompressSlice函数中已经讲过HM中的两个decompressSlice函数，BMS中整合为一个decompressSlice函数，变化不大。

DecSlice::decompressSlice

该函数比较简单，是解码一帧图像的上层入口函数，在DecLib::xDecodeSlice中被调用。主要功能是完成编码结构CodingStructure、熵解码器等信息的初始化，对遍历每个CTU调用DecCu::decompressCtu解码一帧图像。

对比和HM、JEM的函数调用结构可以看出，BMS做了很多简化。

在宏定义中可以看到有两类：HEVC开头和JEM开头的，这些是原HM和JEM中的代码，现在HEVC开头的基本都是关闭的，从宏定义可以很清楚的看出变化。

流程如下：
1.初始化编码结构、熵编码器等。
2.遍历全部CTU调用DecCu::decompressCtu进行解码，更新上细纹模型。
3.如果是最后一个CTU，不分析剩余比特；否则分析剩余比特。

代码分析：

Void DecSlice::decompressSlice(Slice* slice, InputBitstream* bitstream)
{
    //-- For time output for each slice
    slice->startProcessingTimer();

    const SPS*     sps = slice->getSPS();       //SPS
    Picture*       pic = slice->getPic();       //当期图像
#if HEVC_TILES_WPP
    const TileMap& tileMap = *pic->tileMap;
#endif
#if JEM_TOOLS
    CABACReader&   cabacReader = *m_CABACDecoder->getCABACReader(sps->getSpsNext().getCABACEngineMode());       //加载CABAC
#if JEM_TOOLS 
    m_CABACDataStore->updateBufferState(slice, false);      //更新slice
#else
    m_CABACDataStore->updateBufferState(slice);
#endif
#else
    CABACReader&   cabacReader = *m_CABACDecoder->getCABACReader(0);
#endif

    // setup coding structure       加载编码结构
    CodingStructure& cs = *pic->cs;     //获取当前CU
    cs.slice = slice;           //slice
    cs.sps = sps;           //sps
    cs.pps = slice->getPPS();       //pps
#if HEVC_VPS        
    cs.vps = slice->getVPS();       //vps，VVC中好像不用了
#endif
    cs.pcv = slice->getPPS()->pcv;      //pcv，不清楚这个量
    cs.chromaQpAdj = 0;     //色度QP自适应

    cs.picture->resizeSAO(cs.pcv->sizeInCtus, 0);

    const unsigned numSubstreams = slice->getNumberOfSubstreamSizes() + 1;

    // init each couple {EntropyDecoder, Substream}
    // Table of extracted substreams.
    std::vector<InputBitstream*> ppcSubstreams(numSubstreams);      //输入的bit流
    for (unsigned idx = 0; idx < numSubstreams; idx++)      //分成子bit流
    {
        ppcSubstreams[idx] = bitstream->extractSubstream(idx + 1 < numSubstreams ? (slice->getSubstreamSize(idx) << 3) : bitstream->getNumBitsLeft());
    }

#if HEVC_DEPENDENT_SLICES
    const int       startCtuTsAddr = slice->getSliceSegmentCurStartCtuTsAddr();
#else
    const int       startCtuTsAddr = slice->getSliceCurStartCtuTsAddr();        //起始CTU地址
#endif
#if HEVC_TILES_WPP
  const int       startCtuRsAddr          = tileMap.getCtuTsToRsAddrMap(startCtuTsAddr);
#else
#if HEVC_DEPENDENT_SLICES
  const int       startCtuRsAddr          = startCtuTsAddr;
#endif
#endif
  const unsigned  numCtusInFrame          = cs.pcv->sizeInCtus;     //一帧CTU数量
  const unsigned  widthInCtus             = cs.pcv->widthInCtus;    //一行CTU数量
#if HEVC_DEPENDENT_SLICES
  const bool      depSliceSegmentsEnabled = cs.pps->getDependentSliceSegmentsEnabledFlag();
#endif
#if HEVC_TILES_WPP
  const bool      wavefrontsEnabled       = cs.pps->getEntropyCodingSyncEnabledFlag();
#endif

  cabacReader.initBitstream( ppcSubstreams[0] );        //初始化bit流
#if JEM_TOOLS
  cabacReader.initCtxModels( *slice, m_CABACDataStore );        //初始化上个文模型
#else
  cabacReader.initCtxModels( *slice );
#endif

  // Quantization parameter 设置QP
#if HEVC_DEPENDENT_SLICES
  if(!slice->getDependentSliceSegmentFlag())
  {
#endif
    pic->m_prevQP[0] = pic->m_prevQP[1] = slice->getSliceQp();
#if HEVC_DEPENDENT_SLICES
  }
#endif
  CHECK( pic->m_prevQP[0] == std::numeric_limits<Int>::max(), "Invalid previous QP" );

  DTRACE( g_trace_ctx, D_HEADER, "=========== POC: %d ===========\n", slice->getPOC() );

  // 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.
#if HEVC_DEPENDENT_SLICES
  const unsigned subStreamOffset = tileMap.getSubstreamForCtuAddr( startCtuRsAddr, true, slice );
#elif HEVC_TILES_WPP
  const unsigned subStreamOffset = 0;
#endif

#if HEVC_DEPENDENT_SLICES
  if( depSliceSegmentsEnabled )
  {
    // modify initial contexts with previous slice segment if this is a dependent slice.
    const unsigned  startTileIdx          = tileMap.getTileIdxMap(startCtuRsAddr);
    const Tile&     currentTile           = tileMap.tiles[startTileIdx];
    const unsigned  firstCtuRsAddrOfTile  = currentTile.getFirstCtuRsAddr();
    if( slice->getDependentSliceSegmentFlag() && startCtuRsAddr != firstCtuRsAddrOfTile )
    {
      if( currentTile.getTileWidthInCtus() >= 2 || !wavefrontsEnabled )
      {
        cabacReader.getCtx() = m_lastSliceSegmentEndContextState;
      }
    }
  }
#endif
  // for every CTU in the slice segment...
  bool isLastCtuOfSliceSegment = false;     //是否是最后一个CTU、
  //遍历全部CTU进行解码
  for( unsigned ctuTsAddr = startCtuTsAddr; !isLastCtuOfSliceSegment && ctuTsAddr < numCtusInFrame; ctuTsAddr++ )
  {
#if HEVC_TILES_WPP
    const unsigned  ctuRsAddr             = tileMap.getCtuTsToRsAddrMap(ctuTsAddr);
    const Tile&     currentTile           = tileMap.tiles[ tileMap.getTileIdxMap(ctuRsAddr) ];
    const unsigned  firstCtuRsAddrOfTile  = currentTile.getFirstCtuRsAddr();
    const unsigned  tileXPosInCtus        = firstCtuRsAddrOfTile % widthInCtus;
    const unsigned  tileYPosInCtus        = firstCtuRsAddrOfTile / widthInCtus;
#else
    const unsigned  ctuRsAddr             = ctuTsAddr;  //CTU索引
#endif
    const unsigned  ctuXPosInCtus         = ctuRsAddr % widthInCtus;    //x坐标
    const unsigned  ctuYPosInCtus         = ctuRsAddr / widthInCtus;    //y坐标
#if HEVC_TILES_WPP
    const unsigned  subStrmId             = tileMap.getSubstreamForCtuAddr( ctuRsAddr, true, slice ) - subStreamOffset;
#else
    const unsigned  subStrmId             = 0;
#endif
    const unsigned  maxCUSize             = sps->getMaxCUWidth();   //CU最大尺寸
#if JEM_TOOLS
    const CIPFSpec  cipf                  = getCIPFSpec( slice, ctuXPosInCtus, ctuYPosInCtus );
#endif
    Position pos( ctuXPosInCtus*maxCUSize, ctuYPosInCtus*maxCUSize) ;   //二维坐标(x,y)
    UnitArea ctuArea(cs.area.chromaFormat, Area( pos.x, pos.y, maxCUSize, maxCUSize ) );

    DTRACE_UPDATE( g_trace_ctx, std::make_pair( "ctu", ctuRsAddr ) );

    cabacReader.initBitstream( ppcSubstreams[subStrmId] );      //初始化bit流

#if HEVC_TILES_WPP
    // set up CABAC contexts' state for this CTU
    if( ctuRsAddr == firstCtuRsAddrOfTile )
    {
      if( ctuTsAddr != startCtuTsAddr ) // if it is the first CTU, then the entropy coder has already been reset
      {
#if JEM_TOOLS
        cabacReader.initCtxModels( *slice, m_CABACDataStore );
#else
        cabacReader.initCtxModels( *slice );
#endif
      }
      pic->m_prevQP[0] = pic->m_prevQP[1] = slice->getSliceQp();
    }
    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
      {
#if JEM_TOOLS
        cabacReader.initCtxModels( *slice, m_CABACDataStore );
#else
        cabacReader.initCtxModels( *slice );
#endif
      }
      if( cs.getCURestricted( pos.offset(maxCUSize, -1), slice->getIndependentSliceIdx(), tileMap.getTileIdxMap( pos ), CH_L ) )
      {
        // Top-right is available, so use it.
        cabacReader.getCtx() = m_entropyCodingSyncContextState;
      }
      pic->m_prevQP[0] = pic->m_prevQP[1] = slice->getSliceQp();
    }
#endif

#if JEM_TOOLS
    // load ctx from previous frame
    if( cipf.loadCtx )
    {
      m_CABACDataStore->loadCtxStates( slice, cabacReader.getCtx(), cipf.ctxId );       //加载上下文模型
    }
#endif
#if JEM_TOOLS

    if( ctuRsAddr == 0 )
    {
      cabacReader.alf( cs );
    }
#endif

    isLastCtuOfSliceSegment = cabacReader.coding_tree_unit(cs, ctuArea, pic->m_prevQP, ctuRsAddr);          //是否是最后一个CTU

    m_pcCuDecoder->decompressCtu( cs, ctuArea );        //decompressCtu

#if HEVC_TILES_WPP
    if( ctuXPosInCtus == tileXPosInCtus+1 && wavefrontsEnabled )
    {
      m_entropyCodingSyncContextState = cabacReader.getCtx();
    }
#endif

#if JEM_TOOLS
    // store CABAC context to be used in next frames
    if( cipf.storeCtx )
    {
      m_CABACDataStore->storeCtxStates( slice, cabacReader.getCtx(), cipf.ctxId );      //存储上下文模型
    }
#endif

    if( isLastCtuOfSliceSegment )       //最后一个CTU
    {
#if DECODER_CHECK_SUBSTREAM_AND_SLICE_TRAILING_BYTES
      cabacReader.remaining_bytes( false );
#endif
#if HEVC_DEPENDENT_SLICES
      if( !slice->getDependentSliceSegmentFlag() )
      {
#endif
        slice->setSliceCurEndCtuTsAddr( ctuTsAddr+1 );
#if HEVC_DEPENDENT_SLICES
      }
      slice->setSliceSegmentCurEndCtuTsAddr( ctuTsAddr+1 );
#endif
    }
#if HEVC_TILES_WPP
    else if( ( ctuXPosInCtus + 1 == tileXPosInCtus + currentTile.getTileWidthInCtus () ) &&
             ( ctuYPosInCtus + 1 == tileYPosInCtus + currentTile.getTileHeightInCtus() || wavefrontsEnabled ) )
    {
      // The sub-stream/stream should be terminated after this CTU.
      // (end of slice-segment, end of tile, end of wavefront-CTU-row)
      unsigned binVal = cabacReader.terminating_bit();
      CHECK( !binVal, "Expecting a terminating bit" );
#if DECODER_CHECK_SUBSTREAM_AND_SLICE_TRAILING_BYTES
      cabacReader.remaining_bytes( true );
#endif
    }
#endif
  }
  CHECK( !isLastCtuOfSliceSegment, "Last CTU of slice segment not signalled as such" );

#if HEVC_DEPENDENT_SLICES
  if( depSliceSegmentsEnabled )
  {
    m_lastSliceSegmentEndContextState = cabacReader.getCtx();  //ctx end of dep.slice
  }
#endif
  // deallocate all created substreams, including internal buffers.
  for( auto substr: ppcSubstreams )
  {
    delete substr;
  }
  slice->stopProcessingTimer();
}

BMS/VTM代码学习：decompressSlice函数

DecSlice::decompressSlice

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