VVC学习之二:VTM中CU划分结构QTMTT(2)

上次说了VTM中QTMTT划分的理论部分,这次准备结合代码理解,VTM完全就是C++的代码风格,由于水平有限,应该会有很多理解不到位的地方,还请大佬批评指正,欢迎交流。不同于HEVC,VTM中将CU划分方式也当做编码模式进行处理。在编码每个CU时,首先将初始化所有的编码模式,包括所有的预测模式和划分模式。这部分由lib/EncoderLib/EncCU.cppxCompressCU()完成。xCompressCU()调用initCULevel()初始化所有编码模式,编码模式的控制管理定义在EncModeCtrl类中,首先看一下initCULevel()这个函数,代码版本是VTM-4.0。

initCULevel( Partitioner &partitioner, const CodingStructure& cs )
{
  // Min/max depth
  unsigned minDepth = 0;
  unsigned maxDepth = g_aucLog2[cs.sps->getSpsNext().getCTUSize()] - g_aucLog2[cs.sps->getSpsNext().getMinQTSize( m_slice->getSliceType(), partitioner.chType )];
  if( m_pcEncCfg->getUseFastLCTU() )
  {
    if( auto adPartitioner = dynamic_cast<AdaptiveDepthPartitioner*>( &partitioner ) )
    {
      // LARGE CTU
      adPartitioner->setMaxMinDepth( minDepth, maxDepth, cs );    // 根据上、左、左上、右上相邻CU深度信息设置最小最大QT深度
    }
  }

  m_ComprCUCtxList.push_back( ComprCUCtx( cs, minDepth, maxDepth, NUM_EXTRA_FEATURES ) );   // 模式控制对象容器,存储模式及相关信息
  
  const CodingUnit* cuLeft  = cs.getCU( cs.area.blocks[partitioner.chType].pos().offset( -1, 0 ), partitioner.chType );
  const CodingUnit* cuAbove = cs.getCU( cs.area.blocks[partitioner.chType].pos().offset( 0, -1 ), partitioner.chType );
  
  // 根据上下文深度和CU尺寸调整划分执行顺序--先做QT还是后做QT
  const bool qtBeforeBt = ( (  cuLeft  &&  cuAbove  && cuLeft ->qtDepth > partitioner.currQtDepth && cuAbove->qtDepth > partitioner.currQtDepth )
                         || (  cuLeft  && !cuAbove  && cuLeft ->qtDepth > partitioner.currQtDepth )
                         || ( !cuLeft  &&  cuAbove  && cuAbove->qtDepth > partitioner.currQtDepth )
                         || ( !cuAbove && !cuLeft   && cs.area.lwidth() >= ( 32 << cs.slice->getDepth() ) ) )
                         && ( cs.area.lwidth() > ( cs.pcv->getMinQtSize( *cs.slice, partitioner.chType ) << 1 ) );   


  // set features
  ComprCUCtx &cuECtx  = m_ComprCUCtxList.back();
  cuECtx.set( BEST_NON_SPLIT_COST,  MAX_DOUBLE );
  cuECtx.set( BEST_VERT_SPLIT_COST, MAX_DOUBLE );
  cuECtx.set( BEST_HORZ_SPLIT_COST, MAX_DOUBLE );
  cuECtx.set( BEST_TRIH_SPLIT_COST, MAX_DOUBLE );
  cuECtx.set( BEST_TRIV_SPLIT_COST, MAX_DOUBLE );
  cuECtx.set( DO_TRIH_SPLIT,        cs.sps->getSpsNext().getMTTMode() & 1 );
  cuECtx.set( DO_TRIV_SPLIT,        cs.sps->getSpsNext().getMTTMode() & 1 );
  cuECtx.set( BEST_IMV_COST,        MAX_DOUBLE * .5 );
  cuECtx.set( BEST_NO_IMV_COST,     MAX_DOUBLE * .5 );
  cuECtx.set( QT_BEFORE_BT,         qtBeforeBt );
  cuECtx.set( DID_QUAD_SPLIT,       false );
  cuECtx.set( IS_BEST_NOSPLIT_SKIP, false );
  cuECtx.set( MAX_QT_SUB_DEPTH,     0 );

  const bool isReusingCu = isValid( cs, partitioner );
  cuECtx.set( IS_REUSING_CU,        isReusingCu );

  // QP
  int baseQP = cs.baseQP;
  if( m_pcEncCfg->getUseAdaptiveQP() )
  {
    baseQP = Clip3( -cs.sps->getQpBDOffset( CHANNEL_TYPE_LUMA ), MAX_QP, baseQP + xComputeDQP( cs, partitioner ) );
  }
  int minQP = baseQP;
  int maxQP = baseQP;


  if( m_pcEncCfg->getLumaLevelToDeltaQPMapping().isEnabled() )
  {
    if( partitioner.currDepth <= cs.pps->getMaxCuDQPDepth() )
    {
      CompArea clipedArea = clipArea( cs.area.Y(), cs.picture->Y() );
      // keep using the same m_QP_LUMA_OFFSET in the same CTU
      m_lumaQPOffset = calculateLumaDQP( cs.getOrgBuf( clipedArea ) );
    }
  }
  
  xGetMinMaxQP( minQP, maxQP, cs, partitioner, baseQP, *cs.sps, *cs.pps, true );

  // Add coding modes here
  // NOTE: Working back to front, as a stack, which is more efficient with the container
  // NOTE: First added modes will be processed at the end.

  //////////////////////////////////////////////////////////////////////////
  // Add unit split modes

  if( !cuECtx.get<bool>( QT_BEFORE_BT ) )     // 如果BT before QT,先将QT放入
  {
    for( int qp = maxQP; qp >= minQP; qp-- )
    {
      m_ComprCUCtxList.back().testModes.push_back( { ETM_SPLIT_QT, SIZE_2Nx2N, ETO_STANDARD, qp, false } );
    }
  }


  if( partitioner.canSplit( CU_TRIV_SPLIT, cs ) )    // cansplit判断是够可进行特定类别划分
  {
    // add split modes
    for( int qp = maxQP; qp >= minQP; qp-- )
    {
      m_ComprCUCtxList.back().testModes.push_back( { ETM_SPLIT_TT_V, SIZE_2Nx2N, ETO_STANDARD, qp, false } );   // 三叉树垂直划分
    }
  }

  if( partitioner.canSplit( CU_TRIH_SPLIT, cs ) )
  {
    // add split modes
    for( int qp = maxQP; qp >= minQP; qp-- )
    {
      m_ComprCUCtxList.back().testModes.push_back( { ETM_SPLIT_TT_H, SIZE_2Nx2N, ETO_STANDARD, qp, false } ); // 三叉树水平划分
    }
  }

  if( partitioner.canSplit( CU_VERT_SPLIT, cs ) )
  {
    // add split modes
    for( int qp = maxQP; qp >= minQP; qp-- )
    {
      m_ComprCUCtxList.back().testModes.push_back( { ETM_SPLIT_BT_V, SIZE_2Nx2N, ETO_STANDARD, qp, false } ); // 二叉树垂直划分
    }
    m_ComprCUCtxList.back().set( DID_VERT_SPLIT, true );
  }
  else
  {
    m_ComprCUCtxList.back().set( DID_VERT_SPLIT, false );
  }

  if( partitioner.canSplit( CU_HORZ_SPLIT, cs ) )
  {
    // add split modes
    for( int qp = maxQP; qp >= minQP; qp-- )
    {
      m_ComprCUCtxList.back().testModes.push_back( { ETM_SPLIT_BT_H, SIZE_2Nx2N, ETO_STANDARD, qp, false } );   // 二叉树水平
    }
    m_ComprCUCtxList.back().set( DID_HORZ_SPLIT, true );
  }
  else
  {
    m_ComprCUCtxList.back().set( DID_HORZ_SPLIT, false );
  }

  if( cuECtx.get<bool>( QT_BEFORE_BT ) )    // 如果QT before BT,在MT后将QT放入
  {
    for( int qp = maxQP; qp >= minQP; qp-- )
    {
      m_ComprCUCtxList.back().testModes.push_back( { ETM_SPLIT_QT, SIZE_2Nx2N, ETO_STANDARD, qp, false } );
    }
  }

  m_ComprCUCtxList.back().testModes.push_back( { ETM_POST_DONT_SPLIT } );

  if( isReusingCu )
  {
    m_ComprCUCtxList.back().testModes.push_back( { ETM_RECO_CACHED } );
  }

  xGetMinMaxQP( minQP, maxQP, cs, partitioner, baseQP, *cs.sps, *cs.pps, false );

  bool useLossless = false;
  int  lowestQP = minQP;
  if( cs.pps->getTransquantBypassEnabledFlag() )
  {
    useLossless = true; // mark that the first iteration is to cost TQB mode.
    minQP = minQP - 1;  // increase loop variable range by 1, to allow testing of TQB mode along with other QPs

    if( m_pcEncCfg->getCUTransquantBypassFlagForceValue() )
    {
      maxQP = minQP;
    }
  }

  //////////////////////////////////////////////////////////////////////////
  // Add unit coding modes: Intra, InterME, InterMerge ...

  for( int qpLoop = maxQP; qpLoop >= minQP; qpLoop-- )   // 帧内预测模式
  {
    const int  qp       = std::max( qpLoop, lowestQP );
    const bool lossless = useLossless && qpLoop == minQP;
    // add intra modes
    m_ComprCUCtxList.back().testModes.push_back( { ETM_IPCM,  SIZE_2Nx2N, ETO_STANDARD, qp, lossless } );
    m_ComprCUCtxList.back().testModes.push_back( { ETM_INTRA, SIZE_2Nx2N, ETO_STANDARD, qp, lossless } );
  }

  // add first pass modes
  if( !m_slice->isIntra() )    // 帧间预测模式
  {
    for( int qpLoop = maxQP; qpLoop >= minQP; qpLoop-- )
    {
      const int  qp       = std::max( qpLoop, lowestQP );
      const bool lossless = useLossless && qpLoop == minQP;

      if( m_pcEncCfg->getIMV() )
      {
        if( m_pcEncCfg->getIMV() == IMV_4PEL )
        {
          int imv = m_pcEncCfg->getIMV4PelFast() ? 3 : 2;
          m_ComprCUCtxList.back().testModes.push_back( { ETM_INTER_ME, SIZE_2Nx2N, EncTestModeOpts( imv << ETO_IMV_SHIFT ), qp, lossless } );
        }
        m_ComprCUCtxList.back().testModes.push_back( { ETM_INTER_ME, SIZE_2Nx2N, EncTestModeOpts( 1 << ETO_IMV_SHIFT ), qp, lossless } );
      }

      // add inter modes
      if( m_pcEncCfg->getUseEarlySkipDetection() )
      {
        m_ComprCUCtxList.back().testModes.push_back( { ETM_MERGE_SKIP,  SIZE_2Nx2N, ETO_STANDARD, qp, lossless } );

        if( cs.sps->getSpsNext().getUseAffine() )
        {
          m_ComprCUCtxList.back().testModes.push_back( { ETM_AFFINE,      SIZE_2Nx2N, ETO_STANDARD, qp, lossless } );
        }

        m_ComprCUCtxList.back().testModes.push_back( { ETM_INTER_ME,    SIZE_2Nx2N, ETO_STANDARD, qp, lossless } );
      }
      else
      {
        m_ComprCUCtxList.back().testModes.push_back( { ETM_INTER_ME,    SIZE_2Nx2N, ETO_STANDARD, qp, lossless } );

        m_ComprCUCtxList.back().testModes.push_back( { ETM_MERGE_SKIP,  SIZE_2Nx2N, ETO_STANDARD, qp, lossless } );

        if( cs.sps->getSpsNext().getUseAffine() )
        {
          m_ComprCUCtxList.back().testModes.push_back( { ETM_AFFINE,      SIZE_2Nx2N, ETO_STANDARD, qp, lossless } );
        }
      }
    }
  }

  // ensure to skip unprobable modes
  if( !tryModeMaster( m_ComprCUCtxList.back().testModes.back(), cs, partitioner ) )     // 判断模式执行条件
  {
    nextMode( cs, partitioner );
  }

  m_ComprCUCtxList.back().lastTestMode = EncTestMode();
}

类似于HEVC,每层CU首先进行逐个模式预测(除非判定当前CU为SKIP,将跳过剩下所有预测,直接进行划分),然后进行各种类型的CU划分。在initCULevel()最后调用了tryModeMaster()模式判定,同样在xcompressCU()中进行编码模式遍历时,会调用EncModeCtrl::nextMode(),其核心也是调用tryModeMaster()对编码模式进行提前跳过或者终止判定。针对划分的提前终止和跳过,主要是基于SKIP模式和上下文深度,例如1. 如果编码树上层最优为SKIP模式连续超过三层,则终止划分;2. 基于梯度阈值终止划分;3. 当前最优CU是SKIP模式,且其mt深度大于阈值(根据当前帧参考帧距离决定,加入CPR模式后,阈值默认为2),划分跳过。

对CU区域的划分,VTM通过QTBTPartitioner类进行管理,划分由splitCurrArea ()实现

void QTBTPartitioner::splitCurrArea( const PartSplit split, const CodingStructure& cs )
{
  CHECKD( !canSplit( split, cs ), "Trying to apply a prohibited split!" );

  bool isImplicit = isSplitImplicit( split, cs );    // 盘算CU是否横跨边界,如果横跨边界:split 和 约定划分相同,返回true,否则返回false
  bool canQtSplit = canSplit( CU_QUAD_SPLIT, cs );

  switch( split )
  {
  case CU_QUAD_SPLIT:
    m_partStack.push_back( PartLevel( split, PartitionerImpl::getCUSubPartitions( currArea(), cs ) ) );
    break;
  case CU_HORZ_SPLIT:
  case CU_VERT_SPLIT:
    m_partStack.push_back( PartLevel( split, PartitionerImpl::getCUSubPartitions( currArea(), cs, split ) ) );
    break;
  case CU_TRIH_SPLIT:
  case CU_TRIV_SPLIT:
    m_partStack.push_back( PartLevel( split, PartitionerImpl::getCUSubPartitions( currArea(), cs, split ) ) );
    break;
  case TU_MAX_TR_SPLIT:
    m_partStack.push_back( PartLevel( split, PartitionerImpl::getMaxTuTiling( currArea(), cs ) ) );
    break;
#if JVET_M0140_SBT    // sub block transform
  case SBT_VER_HALF_POS0_SPLIT:
  case SBT_VER_HALF_POS1_SPLIT:
  case SBT_HOR_HALF_POS0_SPLIT:
  case SBT_HOR_HALF_POS1_SPLIT:
  case SBT_VER_QUAD_POS0_SPLIT:
  case SBT_VER_QUAD_POS1_SPLIT:
  case SBT_HOR_QUAD_POS0_SPLIT:
  case SBT_HOR_QUAD_POS1_SPLIT:
    m_partStack.push_back( PartLevel( split, PartitionerImpl::getSbtTuTiling( currArea(), cs, split ) ) );
    break;
#endif
  default:
    THROW( "Unknown split mode" );
    break;
  }

  currDepth++;     // 每次划分,深度加一
#if _DEBUG
  m_currArea = m_partStack.back().parts.front();
#endif

  if( split == TU_MAX_TR_SPLIT )
  {
    currTrDepth++;
  }
#if JVET_M0140_SBT
  else if( split >= SBT_VER_HALF_POS0_SPLIT && split <= SBT_HOR_QUAD_POS1_SPLIT )
  {
    currTrDepth++;
  }
#endif
  else
  {
    currTrDepth = 0;
  }

  if( split == CU_HORZ_SPLIT || split == CU_VERT_SPLIT || split == CU_TRIH_SPLIT || split == CU_TRIV_SPLIT )   // mt划分
  {
    currBtDepth++;    // Bt depth 加1
    if( isImplicit ) currImplicitBtDepth++;
    currMtDepth++;    // mt depth 加1

    if( split == CU_TRIH_SPLIT || split == CU_TRIV_SPLIT )    // 三叉树划分,两边CU的btdepth 加一
    {
      // first and last part of triple split are equivalent to double bt split
      currBtDepth++;
    }
    m_partStack.back().canQtSplit = canQtSplit;
  }
  else if( split == CU_QUAD_SPLIT )    // QT 划分,bt/mt 深度为0
  {
    CHECK( currBtDepth > 0, "Cannot split a non-square area other than with a binary split" );
    CHECK( currMtDepth > 0, "Cannot split a non-square area other than with a binary split" );
    currMtDepth = 0;
    currBtDepth = 0;
    currQtDepth++;
  }
}

getCUSubPartitions()划分逻辑的具体实现,计算每个SubCU的长、宽以及位置信息。
typedef std::vector Partitioning;

Partitioning PartitionerImpl::getCUSubPartitions( const UnitArea &cuArea, const CodingStructure &cs, const PartSplit _splitType /*= CU_QUAD_SPLIT*/ )
{
  const PartSplit splitType = _splitType;

  if( splitType == CU_QUAD_SPLIT )
  {
    if( !cs.pcv->noChroma2x2 )
    {
      Partitioning sub;

      sub.resize( 4, cuArea );

      for( uint32_t i = 0; i < 4; i++ )
      {
        for( auto &blk : sub[i].blocks )
        {
          blk.height >>= 1;
          blk.width  >>= 1;
          if( i >= 2 ) blk.y += blk.height;
          if( i &  1 ) blk.x += blk.width;
        }

        CHECK( sub[i].lumaSize().height < MIN_TU_SIZE, "the split causes the block to be smaller than the minimal TU size" );
      }

      return sub;
    }
    else
    {
      const uint32_t minCUSize = ( cs.sps->getMaxCUWidth() >> cs.sps->getMaxCodingDepth() );

      bool canSplit = cuArea.lumaSize().width > minCUSize && cuArea.lumaSize().height > minCUSize;

      Partitioning ret;

      if( cs.slice->getSliceType() == I_SLICE )
      {
        canSplit &= cuArea.lumaSize().width > cs.pcv->minCUWidth && cuArea.lumaSize().height > cs.pcv->minCUHeight;
      }

      if( canSplit )
      {
        ret.resize( 4 );

        if( cuArea.chromaFormat == CHROMA_400 )
        {
          CompArea  blkY = cuArea.Y();
          blkY.width >>= 1;
          blkY.height >>= 1;
          ret[0]  = UnitArea( cuArea.chromaFormat, blkY );
          blkY.x += blkY.width;
          ret[1]  = UnitArea( cuArea.chromaFormat, blkY );
          blkY.x -= blkY.width;
          blkY.y += blkY.height;
          ret[2]  = UnitArea( cuArea.chromaFormat, blkY );
          blkY.x += blkY.width;
          ret[3]  = UnitArea( cuArea.chromaFormat, blkY );
        }
        else
        {
          for( uint32_t i = 0; i < 4; i++ )
          {
            ret[i] = cuArea;

            CompArea &blkY  = ret[i].Y();
            CompArea &blkCb = ret[i].Cb();
            CompArea &blkCr = ret[i].Cr();

            blkY.width  /= 2;
            blkY.height /= 2;

            // TODO: get those params from SPS
            if( blkCb.width > 4 )
            {
              blkCb.width  /= 2;
              blkCb.height /= 2;
              blkCr.width  /= 2;
              blkCr.height /= 2;
            }
            else if( i > 0 )
            {
              blkCb = CompArea();
              blkCr = CompArea();
            }

            if( ( i & 1 ) == 1 )
            {
              blkY.x  += blkY .width;
              blkCb.x += blkCb.width;
              blkCr.x += blkCr.width;
            }

            if( i > 1 )
            {
              blkY.y  += blkY .height;
              blkCb.y += blkCb.height;
              blkCr.y += blkCr.height;
            }
          }
        }
      }

      return ret;
    }
  }
  else if( splitType == CU_HORZ_SPLIT )
  {
    Partitioning sub;

    sub.resize(2, cuArea);

    for (uint32_t i = 0; i < 2; i++)
    {
      for (auto &blk : sub[i].blocks)
      {
        blk.height >>= 1;
        if (i == 1) blk.y += blk.height;
      }

      CHECK(sub[i].lumaSize().height < MIN_TU_SIZE, "the cs split causes the block to be smaller than the minimal TU size");
    }

    return sub;
  }
  else if( splitType == CU_VERT_SPLIT )
  {
    Partitioning sub;

    sub.resize( 2, cuArea );

    for( uint32_t i = 0; i < 2; i++ )
    {
      for( auto &blk : sub[i].blocks )
      {
        blk.width >>= 1;
        if( i == 1 ) blk.x += blk.width;
      }

      CHECK( sub[i].lumaSize().width < MIN_TU_SIZE, "the split causes the block to be smaller than the minimal TU size" );
    }

    return sub;
  }
  else if( splitType == CU_TRIH_SPLIT )
  {
    Partitioning sub;

    sub.resize( 3, cuArea );

    for( int i = 0; i < 3; i++ )
    {
      for( auto &blk : sub[i].blocks )
      {
        blk.height >>= 1;
        if( ( i + 1 ) & 1 ) blk.height >>= 1;
        if( i == 1 )        blk.y       +=     blk.height / 2;
        if( i == 2 )        blk.y       += 3 * blk.height;
      }

      CHECK( sub[i].lumaSize().height < MIN_TU_SIZE, "the cs split causes the block to be smaller than the minimal TU size" );
    }

    return sub;
  }
  else if( splitType == CU_TRIV_SPLIT )
  {
    Partitioning sub;

    sub.resize( 3, cuArea );

    for( int i = 0; i < 3; i++ )
    {
      for( auto &blk : sub[i].blocks )
      {
        blk.width >>= 1;

        if( ( i + 1 ) & 1 ) blk.width >>= 1;
        if( i == 1 )        blk.x      +=     blk.width / 2;
        if( i == 2 )        blk.x      += 3 * blk.width;
      }

      CHECK( sub[i].lumaSize().width < MIN_TU_SIZE, "the cs split causes the block to be smaller than the minimal TU size" );
    }

    return sub;
  }
  else
  {
    THROW( "Unknown CU sub-partitioning" );
    return Partitioning();
  }
}

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