360Lib：CPP-PSNR

CPP-PSNR是360Lib采纳的一种360视频的客观质量评估标准，通过投影到CPP格式计算PSNR。

TCPPPSNRMetric

下面来看360Lib中定义的CPP-PSNR类TCPPPSNRMetric，其中定义了重要函数xCalculateCPPPSNR，用于计算CPP-PSNR。

class TCPPPSNRMetric        //CPP PSNR
{
private:
  Bool          m_bCPPPSNREnabled;  
  Double        m_dCPPPSNR[3];

  CPos2D*       m_pCart2D;
  IPos2D*       m_fpTable;
  Int           m_iSphNumPoints;
  Int           m_cppWidth;   //CPP宽度
  Int           m_cppHeight;     //CPP高度

  Int           m_outputBitDepth[MAX_NUM_CHANNEL_TYPE];         ///< bit-depth of output file 输出文件的bit深度
  Int           m_referenceBitDepth[MAX_NUM_CHANNEL_TYPE];      ///< bit-depth of reference file 参考文件的bit深度
  ChromaFormat  m_chromaFormatIDC;  //颜色格式
#if SVIDEO_CPP_FIX
  InputGeoParam m_refGeoParam;
  InputGeoParam m_outGeoParam;
#else
  InputGeoParam m_cppGeoParam; 
#endif
  SVideoInfo    m_cppCodingVideoInfo;
  SVideoInfo    m_cppRefVideoInfo;
  SVideoInfo    m_cppVideoInfo;

  Void          setDefaultFramePackingParam(SVideoInfo& sVideoInfo);        //设置默认参数
  Void          fillSourceSVideoInfo(SVideoInfo& sVidInfo, Int inputWidth, Int inputHeight);    //根据投影格式填充源视频信息

  TGeometry     *m_pcOutputGeomtry;
  TGeometry     *m_pcReferenceGeomtry;
  TGeometry     *m_pcOutputCPPGeomtry;
  TGeometry     *m_pcRefCPPGeomtry;

public:
  TCPPPSNRMetric();
  virtual ~TCPPPSNRMetric();
  Bool    getCPPPSNREnabled()  { return m_bCPPPSNREnabled; }        //获取m_bCPPPSNREnabled
  Void    setCPPPSNREnabledFlag(Bool bEnabledFlag)  { m_bCPPPSNREnabled = bEnabledFlag; }       //设置m_bCPPPSNREnabled
  Void    setOutputBitDepth(Int iOutputBitDepth[MAX_NUM_CHANNEL_TYPE]);     //设置输出bit深度
  Void    setReferenceBitDepth(Int iReferenceBitDepth[MAX_NUM_CHANNEL_TYPE]);   //设置参考图形bit深度
  Void    setCPPWidth(Int iCPPWidth);       //设置CPP宽度
  Void    setCPPHeight(Int iCPPheight);     //设置CPP高度
  Void    setChromaFormatIDC(ChromaFormat iChromaFormatIDC);        //设置颜色格式
#if SVIDEO_CPP_FIX
  Void    setGeoParam(InputGeoParam iCPPGeoParam);
#else
  Void    setCPPGeoParam(InputGeoParam iCPPGeoParam);
#endif
  Void    setCPPVideoInfo(SVideoInfo iCppCodingVdideoInfo, SVideoInfo iCppRefVdideoInfo);       //设置CPP视频的编码视频和参考视频信息
  Double* getCPPPSNR() {return m_dCPPPSNR;}     //获取CPP-PSNR
  //Void    sphSampoints(Char* cSphDataFile);
  Void    sphToCart(CPos2D*, CPos3D*);      //2D to 3D
  Void    xCalculateCPPPSNR( TComPicYuv* pcOrgPicYuv, TComPicYuv* pcPicD );     //计算CPP-PSNR
  Void    initCPPPSNR(InputGeoParam inputGeoParam, Int cppWidth, Int cppHeight, SVideoInfo codingvideoInfo, SVideoInfo referenceVideoInfo);     //初始化CPP-PSNR
};

xCalculateCPPPSNR

下面来看xCalculateCPPPSNR函数，可以分为两部分：
1.将参考图像和输出图像转换为了CPP格式。
2.计算CPP格式下的PSNR即为CPP-PSNR。
CPP下存在无效图像区域，因此在计算PSNR前先计算了有效区域坐标。

//计算CPP-PSNR
Void TCPPPSNRMetric::xCalculateCPPPSNR( TComPicYuv* pcOrgPicYuv, TComPicYuv* pcPicD)
{
  Int iBitDepthForPSNRCalc[MAX_NUM_CHANNEL_TYPE];       //计算时的bit深度
  Int iReferenceBitShift[MAX_NUM_CHANNEL_TYPE];         //参考图像bit偏移
  Int iOutputBitShift[MAX_NUM_CHANNEL_TYPE];        //输出bit偏移

  TComPicYuv *TPicYUVRefCPP;        //参考图像
  TComPicYuv *TPicYUVOutCPP;        //输出图像

  //计算时的bit深度为输出bit深度和输入bit深度的最大值
  iBitDepthForPSNRCalc[CHANNEL_TYPE_LUMA] = std::max(m_outputBitDepth[CHANNEL_TYPE_LUMA], m_referenceBitDepth[CHANNEL_TYPE_LUMA]);
  iBitDepthForPSNRCalc[CHANNEL_TYPE_CHROMA] = std::max(m_outputBitDepth[CHANNEL_TYPE_CHROMA], m_referenceBitDepth[CHANNEL_TYPE_CHROMA]);
  //参考图像bit偏移 = 计算时的bit深度 - 参考图像bit深度
  iReferenceBitShift[CHANNEL_TYPE_LUMA] = iBitDepthForPSNRCalc[CHANNEL_TYPE_LUMA] - m_referenceBitDepth[CHANNEL_TYPE_LUMA];
  iReferenceBitShift[CHANNEL_TYPE_CHROMA] = iBitDepthForPSNRCalc[CHANNEL_TYPE_CHROMA] - m_referenceBitDepth[CHANNEL_TYPE_CHROMA];
  //输出图像bit偏移 = 计算时的bit深度 - 输出图像bit深度
  iOutputBitShift[CHANNEL_TYPE_LUMA] = iBitDepthForPSNRCalc[CHANNEL_TYPE_LUMA] - m_outputBitDepth[CHANNEL_TYPE_LUMA];
  iOutputBitShift[CHANNEL_TYPE_CHROMA] = iBitDepthForPSNRCalc[CHANNEL_TYPE_CHROMA] - m_outputBitDepth[CHANNEL_TYPE_CHROMA];

  memset(m_dCPPPSNR, 0, sizeof(Double)*3);
  Double SCPPDspsnr[3]={0, 0 ,0};

  // Convert Output and Ref to CPP_Projection
  //初始化CPP格式参考图像
  TPicYUVRefCPP = new TComPicYuv;
  TPicYUVRefCPP->createWithoutCUInfo  ( m_cppWidth, m_cppHeight, m_chromaFormatIDC, true );

  //初始化CPP格式输出图像
  TPicYUVOutCPP = new TComPicYuv;
  TPicYUVOutCPP->createWithoutCUInfo  ( m_cppWidth, m_cppHeight, m_chromaFormatIDC, true );

  // Converting Reference to CPP
  //将参考图像转换为CPP格式
  if ((m_pcReferenceGeomtry->getSVideoInfo()->geoType == SVIDEO_OCTAHEDRON || m_pcReferenceGeomtry->getSVideoInfo()->geoType == SVIDEO_ICOSAHEDRON) && m_pcReferenceGeomtry->getSVideoInfo()->iCompactFPStructure)
  {
    m_pcReferenceGeomtry->compactFramePackConvertYuv(pcOrgPicYuv);
  }
  else
  {
    m_pcReferenceGeomtry->convertYuv(pcOrgPicYuv);
  }
  m_pcReferenceGeomtry->geoConvert(m_pcRefCPPGeomtry);      //投影格式转换
  m_pcRefCPPGeomtry->framePack(TPicYUVRefCPP);      

  // Converting Output to CPP
  //将输出图像转换为CPP格式
  if ((m_pcOutputGeomtry->getSVideoInfo()->geoType == SVIDEO_OCTAHEDRON || m_pcOutputGeomtry->getSVideoInfo()->geoType == SVIDEO_ICOSAHEDRON) && m_pcOutputGeomtry->getSVideoInfo()->iCompactFPStructure)
  {
    m_pcOutputGeomtry->compactFramePackConvertYuv(pcPicD);
  }
  else
  {
    m_pcOutputGeomtry->convertYuv(pcPicD);
  }
  m_pcOutputGeomtry->geoConvert(m_pcOutputCPPGeomtry);  //投影格式转换
  m_pcOutputCPPGeomtry->framePack(TPicYUVOutCPP);

 for(Int chan=0; chan<pcPicD->getNumberValidComponents(); chan++)
  {
    //初始化亮度、色度的信息
    const ComponentID ch=ComponentID(chan);
    const Pel*  pOrg       = TPicYUVOutCPP->getAddr(ch);        //原始图像首地址
    const Int   iOrgStride = TPicYUVOutCPP->getStride(ch);  //原始图像跨度
    const Pel*  pRec       = TPicYUVRefCPP->getAddr(ch);        //重构图像首地址
    const Int   iRecStride = TPicYUVRefCPP->getStride(ch);      //重构图像跨度
    const Int   iWidth     = TPicYUVRefCPP->getWidth (ch);  //图像宽度
    const Int   iHeight    = TPicYUVRefCPP->getHeight(ch);  //图像高度

    Int   iSize            = 0;
    double fPhi, fLambda;
    double fIdxX, fIdxY;
    double fLamdaX, fLamdaY;

    //计算CPP格式下有效图像区域的PSNR
    for(Int y=0;y<iHeight;y++)
    {
      for(Int x=0;x<iWidth;x++)
      {
        fLamdaX = ((double)x / (iWidth)) * (2 * S_PI) - S_PI;
        fLamdaY = ((double)y / (iHeight)) * S_PI - (S_PI_2);

        fPhi = 3 * sasin(fLamdaY / S_PI);
        fLambda = fLamdaX / (2 * scos(2 * fPhi / 3) - 1);

        fLamdaX = (fLambda + S_PI) / 2 / S_PI * (iWidth);
        fLamdaY = (fPhi + (S_PI / 2)) / S_PI *  (iHeight);

        fIdxX = (int)((fLamdaX < 0) ? fLamdaX - 0.5 : fLamdaX + 0.5);
        fIdxY = (int)((fLamdaY < 0) ? fLamdaY - 0.5 : fLamdaY + 0.5);

        if(fIdxY >= 0 && fIdxX >= 0 && fIdxX < iWidth && fIdxY < iHeight)       //判断是否越界
        {
#if SVIDEO_CPP_FIX
          Intermediate_Int iDifflp = (Intermediate_Int)((pOrg[x] << iOutputBitShift[toChannelType(ch)]) - (pRec[x] << iReferenceBitShift[toChannelType(ch)]));
#else
          Intermediate_Int iDifflp  = (Intermediate_Int)((pOrg[x]<<iReferenceBitShift[toChannelType(ch)]) - (pRec[x]<<iOutputBitShift[toChannelType(ch)]) );
#endif
          SCPPDspsnr[chan]         += iDifflp * iDifflp;        //平方差累加
          iSize++;      //记录累加点的数量
        }
      }
      pOrg += iOrgStride;
      pRec += iRecStride;
    }
    SCPPDspsnr[chan] /= iSize;      //求MSE
  }

  for (Int ch_indx = 0; ch_indx < pcPicD->getNumberValidComponents(); ch_indx++)
  {
    const ComponentID ch=ComponentID(ch_indx);
    const Int maxval = 255<<(iBitDepthForPSNRCalc[toChannelType(ch)]-8) ;

    Double fReflpsnr = maxval*maxval;
    //求CPP-PSNR
    m_dCPPPSNR[ch_indx] = ( SCPPDspsnr[ch_indx] ? 10.0 * log10( fReflpsnr / (Double)SCPPDspsnr[ch_indx] ) : 999.99 );   
  }

  //清理内存
  if(TPicYUVRefCPP)
  {
    TPicYUVRefCPP->destroy();
    delete TPicYUVRefCPP;
    TPicYUVRefCPP = NULL;
  }
  if(TPicYUVOutCPP)
  {
    TPicYUVOutCPP->destroy();
    delete TPicYUVOutCPP;
    TPicYUVOutCPP = NULL;
  }
}

从代码来看，CPP-PSNR计算过程中，计算坐标fIdxX和fIdxY时存在舍入误差，引入了失真。在格式转换中，还使用了插值滤波器，也会引入失真。

ERP到CPP格式的转换

在360Lib中，读入的360视频都是以2D形式表示，在xCalculateCPPPSNR中，CPP格式由原始视频2D格式转换得到。以参考图像为例，这里我只研究输入为ERP的情况，那执行的代码为：

  //投影格式转换
  m_pcReferenceGeomtry->convertYuv(pcOrgPicYuv);
  m_pcReferenceGeomtry->geoConvert(m_pcRefCPPGeomtry);      
  m_pcRefCPPGeomtry->framePack(TPicYUVRefCPP);      

转换过程分为3步：
1.扩展到360Lib 2D平面：通过convertYuv，将ERP图像填充扩展到360Lib 2D坐标平面上。
2.几何信息转换：通过geoConvert进行几何信息转换。
3.YUV信息转换：通过framePack对YUV信息的转换。

实际格式转换由geoConvert实现，格式转换中需要进行插值，因此会引入失真。