HEVC/H265 HM10 0 分析（三）TAppDecTop cpp

在TAppDecTop.cpp  ,最重要的是decode 函数，下面将对其进行分析，是解码上层的一个重要函数。

代码如下，代码后将进行分析。

Void TAppDecTop::decode()
{
  Int                 poc;
  TComList<TComPic*>* pcListPic = NULL;

  ifstream bitstreamFile(m_pchBitstreamFile, ifstream::in | ifstream::binary);
  if (!bitstreamFile)
  {
    fprintf(stderr, "\nfailed to open bitstream file `%s‘ for reading\n", m_pchBitstreamFile);
    exit(EXIT_FAILURE);
  }

  InputByteStream bytestream(bitstreamFile);

  // create & initialize internal classes
  xCreateDecLib();
  xInitDecLib  ();
  m_iPOCLastDisplay += m_iSkipFrame;      // set the last displayed POC correctly for skip forward.

  // main decoder loop
  Bool recon_opened = false; // reconstruction file not yet opened. (must be performed after SPS is seen)

  while (!!bitstreamFile)
  {
    /* location serves to work around a design fault in the decoder, whereby
     * the process of reading a new slice that is the first slice of a new frame
     * requires the TDecTop::decode() method to be called again with the same
     * nal unit. */
    streampos location = bitstreamFile.tellg();
    AnnexBStats stats = AnnexBStats();
    Bool bPreviousPictureDecoded = false;

    vector<uint8_t> nalUnit;
    InputNALUnit nalu;
    byteStreamNALUnit(bytestream, nalUnit, stats);

    // call actual decoding function
    Bool bNewPicture = false;
    if (nalUnit.empty())
    {
      /* this can happen if the following occur:
       *  - empty input file
       *  - two back-to-back start_code_prefixes
       *  - start_code_prefix immediately followed by EOF
       */
      fprintf(stderr, "Warning: Attempt to decode an empty NAL unit\n");
    }
    else
    {
      read(nalu, nalUnit);
      if( (m_iMaxTemporalLayer >= 0 && nalu.m_temporalId > m_iMaxTemporalLayer) || !isNaluWithinTargetDecLayerIdSet(&nalu)  )
      {
        if(bPreviousPictureDecoded)
        {
          bNewPicture = true;
          bPreviousPictureDecoded = false;
        }
        else
        {
          bNewPicture = false;
        }
      }
      else
      {
        bNewPicture = m_cTDecTop.decode(nalu, m_iSkipFrame, m_iPOCLastDisplay);
		
        if (bNewPicture)
        {
          bitstreamFile.clear();
          /* location points to the current nalunit payload[1] due to the
           * need for the annexB parser to read three extra bytes.
           * [1] except for the first NAL unit in the file
           *     (but bNewPicture doesn‘t happen then) */
          bitstreamFile.seekg(location-streamoff(3));
          bytestream.reset();
        }
        bPreviousPictureDecoded = true; 
      }
    }
    if (bNewPicture || !bitstreamFile)
    {
		
      m_cTDecTop.executeLoopFilters(poc, pcListPic);
	  printf("\npoc =%d\n",poc);
    }

    if( pcListPic )
    {
		printf("\nnaluType =%d\n",nalu.m_nalUnitType);
      if ( m_pchReconFile && !recon_opened )
      {
        if (!m_outputBitDepthY) { m_outputBitDepthY = g_bitDepthY; }
        if (!m_outputBitDepthC) { m_outputBitDepthC = g_bitDepthC; }

        m_cTVideoIOYuvReconFile.open( m_pchReconFile, true, m_outputBitDepthY, m_outputBitDepthC, g_bitDepthY, g_bitDepthC ); // write mode
        recon_opened = true;
      }
      if ( bNewPicture && 
           (   nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_IDR
            || nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_IDR_N_LP
            || nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_BLA_N_LP
            || nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_BLANT
            || nalu.m_nalUnitType == NAL_UNIT_CODED_SLICE_BLA ) )
      {
        xFlushOutput( pcListPic );
      }
      // write reconstruction to file
      if(bNewPicture)
      {
        xWriteOutput( pcListPic, nalu.m_temporalId );
      }
    }
  }
  
  xFlushOutput( pcListPic );
  // delete buffers
  m_cTDecTop.deletePicBuffer();
  
  // destroy internal classes
  xDestroyDecLib();
}

代码xCreateDecLib 和 xInitDecLib 重要是初始化四叉树和解码需要的全局变量和申请内存。

当提供一个码流文件的文件名后，进行ifstream bitstreamFile(m_pchBitstreamFile, ifstream::in | ifstream::binary); 以二进制方式打开文件名，码流以字节方式InputByteStream bytestream(bitstreamFile);进行读操作。

我们都知道，HEVC/H265 是以NAL方式组织数据的，解析VPS,SPS,PPS,SEI,SEI_SUFFIX 后，其他的是一个个slice的NAL数据，而Deblocking & SAO Filters 等滤波是对整个picuture进行滤波操作，出现从第二帧开始，每帧的第一个slice两次进行解析，这是参考软件的一个bug或不好的地方，其实完全可以知道是否是最后一个slice，不必进行两次打开。

所以出现：

bitstreamFile.clear();

bitstreamFile.seekg(location-streamoff(3));
bytestream.reset();

大家有兴趣，可以先增加一个变量，判断是否是最后一个slice，就不需要执行上面代码了。

如下代码是从来不会执行，因为HEVC/H265没有cavlc，也就不会有slice part A ，slice part B，slice part C ，是现实的编解码告诉了设计者，slice part A ，slice part B，slice part C 没有人使用，就被抛弃了，实际的编解码从来没有实现slice part A ，slice part B，slice part C 等编解码的。

 if( (m_iMaxTemporalLayer >= 0 && nalu.m_temporalId > m_iMaxTemporalLayer) || !isNaluWithinTargetDecLayerIdSet(&nalu)  )
      {
        if(bPreviousPictureDecoded)
        {
          bNewPicture = true;
          bPreviousPictureDecoded = false;
        }
        else
        {
          bNewPicture = false;
        }
      }

解码和滤波后当然就是输出重构数据，xWriteOutput( pcListPic, nalu.m_temporalId )， 如果slice是NAL_UNIT_CODED_SLICE_IDR，NAL_UNIT_CODED_SLICE_IDR_N_LP，NAL_UNIT_CODED_SLICE_BLA_N_LP，NAL_UNIT_CODED_SLICE_BLANT，NAL_UNIT_CODED_SLICE_BLA中的一种，将解码产生的picture全部清空，没有任何参考帧，相当于一个新的sequence进行解码了。

其他的代码很简单，请自己分析。