HEVC编码之precompressSlice函数

HEVC中每一个slice的QP值并不一定像设定的QP值一样，之所以会这样都是precompressSlice函数的“功劳”

Void TEncSlice::precompressSlice( TComPic* pcPic )
{
  // if deltaQP RD is not used, simply return
  if ( m_pcCfg->getDeltaQpRD() == 0 )
  {
    return;
  }

  if ( m_pcCfg->getUseRateCtrl() )
  {
    printf( "\nMultiple QP optimization is not allowed when rate control is enabled." );
    assert(0);
    return;
  }

  TComSlice* pcSlice        = pcPic->getSlice(getSliceIdx());

  if (pcSlice->getDependentSliceSegmentFlag())
  {
    // if this is a dependent slice segment, then it was optimised
    // when analysing the entire slice.
    return;
  }

  if (pcSlice->getSliceMode()==FIXED_NUMBER_OF_BYTES)
  {
    // TODO: investigate use of average cost per CTU so that this Slice Mode can be used.
    printf( "\nUnable to optimise Slice-level QP if Slice Mode is set to FIXED_NUMBER_OF_BYTES\n" );
    assert(0);
    return;
  }

  Double     dPicRdCostBest = MAX_DOUBLE;
  UInt       uiQpIdxBest = 0;

  Double dFrameLambda;
#if FULL_NBIT
  Int    SHIFT_QP = 12 + 6 * (pcSlice->getSPS()->getBitDepth(CHANNEL_TYPE_LUMA) - 8);
#else
  Int    SHIFT_QP = 12;
#endif

  // set frame lambda
  if (m_pcCfg->getGOPSize() > 1)
  {
    dFrameLambda = 0.68 * pow (2, (m_viRdPicQp[0]  - SHIFT_QP) / 3.0) * (pcSlice->isInterB()? 2 : 1);
  }
  else
  {
    dFrameLambda = 0.68 * pow (2, (m_viRdPicQp[0] - SHIFT_QP) / 3.0);
  }
  m_pcRdCost      ->setFrameLambda(dFrameLambda);

  // for each QP candidate
  //以QP值作为候选
  for ( UInt uiQpIdx = 0; uiQpIdx < 2 * m_pcCfg->getDeltaQpRD() + 1; uiQpIdx++ )
  {
    pcSlice       ->setSliceQp             ( m_viRdPicQp    [uiQpIdx] );
#if ADAPTIVE_QP_SELECTION
    pcSlice       ->setSliceQpBase         ( m_viRdPicQp    [uiQpIdx] );
#endif
    setUpLambda(pcSlice, m_vdRdPicLambda[uiQpIdx], m_viRdPicQp    [uiQpIdx]);

    // try compress
    compressSlice   ( pcPic, true, m_pcCfg->getFastDeltaQp());

    UInt64 uiPicDist        = m_uiPicDist; // Distortion, as calculated by compressSlice.
    // NOTE: This distortion is the chroma-weighted SSE distortion for the slice.
    //       Previously a standard SSE distortion was calculated (for the entire frame).
    //       Which is correct?

    // TODO: Update loop filter, SAO and distortion calculation to work on one slice only.
    // m_pcGOPEncoder->preLoopFilterPicAll( pcPic, uiPicDist );

    // compute RD cost and choose the best
    Double dPicRdCost = m_pcRdCost->calcRdCost( (Double)m_uiPicTotalBits, (Double)uiPicDist, DF_SSE_FRAME);

    if ( dPicRdCost < dPicRdCostBest )
    {
      uiQpIdxBest    = uiQpIdx;
      dPicRdCostBest = dPicRdCost;
    }
  }

  // set best values
  pcSlice       ->setSliceQp             ( m_viRdPicQp    [uiQpIdxBest] );
#if ADAPTIVE_QP_SELECTION
  pcSlice       ->setSliceQpBase         ( m_viRdPicQp    [uiQpIdxBest] );
#endif
  setUpLambda(pcSlice, m_vdRdPicLambda[uiQpIdxBest], m_viRdPicQp    [uiQpIdxBest]);
}

比较precompressSlice和compressSlice 这两个函数会大有不同。