Why can not show full image when i use fastatan2?

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Hi all , when i try to write gaussian gradient diection.I find some problem in the loop to calcuate dst.The variable dst1 & dst2 can show full image ,but the variable dst can not show full image.I don't know what happens. I am very confused.Could you tell mell what's wrong.Thanks your advance.

     int KernelSize=5;
    int n=int(KernelSize/2);
     vector<float>  dx;
    vector<float> dy;

    for (int i = -n; i <= n; ++i)
    {
        dx.push_back(g1(i, sigma));
        dy.push_back(g1(i, sigma));
    }

    normalize(dx);
    normalize(dy);
    Mat kx=Mat(dx);
    Mat ky=Mat(dy);
    vector<float> kc;
    kc.push_back(0.0);
    kc.push_back(0.0);
    kc.push_back(1.0);
    kc.push_back(0.0);
    kc.push_back(0.0);
    Mat k=Mat(kc);

     Mat temp1;
     Mat temp2;
     Mat dst1,dst2;
     cv::sepFilter2D(src,temp1,CV_32FC1,kx,kc);
     cv::sepFilter2D(src,temp2,CV_32FC1,kc,ky);
       cv::normalize(temp1,dst1, 0x00, 0xFF, cv::NORM_MINMAX, CV_8U);
      imshow("temp1",dst1);
      cv::normalize(temp2,dst2, 0x00, 0xFF, cv::NORM_MINMAX, CV_8U);
      imshow("temp2",dst2);
      Mat temp=cv::Mat::zeros(src.rows, src.cols, CV_32FC1);

      for(int y=0;y<temp1.rows;y++)
      {
           for(int x=0;x<temp1.cols;x++)
           {
               float a=(float)temp2.at<float>(y,x);
               float b=(float)temp1.at<float>(y,x);
              temp.at<float>(y,x)=cv::fastAtan2(a,b);
           }
      }

    cv::normalize(temp,dst, 0x00, 0xFF, cv::NORM_MINMAX, CV_8U);

    where g1 is:
    float g1(const int &i, const float &sigma)
{
  return (float)(-(float)i / (sigma * sigma)
    * exp(-(float)(i * i) / (2.0 * sigma * sigma)));
}

& normalize:

    void normalize(vector<float> &kernel)
    {
      float f=0.0;
      for(int i=0;i<kernel.size();i++)
      {
         f+=fabs(kernel[i]);
      }
       if(f>0)
      {
         for(int i=0;i<kernel.size();i++)
         {
           kernel[i]/=f;
          }
      }

   }

NG.png