Why DFT_REAL_OUTPUT is different from the real part of complex result

Houber · May 23, 2023, 7:16am

I foward DFT a image and get a complex result, then i want inverse it to my image, when i use C2C, i get the real part of result and it just my image. but when i use C2R, the result is nothing about my image. i can not understand it, why it is different? any help is expected, please! version opencv4.5.5 cuda10.1 windows10

laurent.berger · May 23, 2023, 8:16am

A minimal example is welcome

acronym :
real input to complex-Hermitian output (r2c)
complex-Hermitian input to real output (c2r)
c2c ??

Houber · May 23, 2023, 9:40am

my code is this:
C2C:
Mat img=imread(… );
GpuMat src, dst;
src.upload(img);
src.convertTo(src, CV_32F);
GpuMat planes[2]={src, GpuMat(src.size(), CV_32F, Scalar(0)};
GpuMat complex;
cuda::merge(planes, 2, complex);
cuda::dft(complex, complex, complex.size(), DFT_SCALE);
cuda::dft(complex, complex, complex.size(), DFT_INVERSE);
cuda::split(complex, planes);
dst=planes[0];
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
the follow is C2R:
Mat img=imread(… );
GpuMat src, dst;
src.upload(img);
src.convertTo(src, CV_32F);
GpuMat planes[2]={src, GpuMat(src.size(), CV_32F, Scalar(0)};
GpuMat complex;
cuda::merge(planes, 2, complex);
cuda::dft(complex, complex, complex.size(), DFT_SCALE);
cuda::dft(complex, dst, complex.size(), DFT_INVERSE | DFT_REAL_OUTPUT);//and complex is changed by this step

cudawarped · May 23, 2023, 11:16am

I’m not sure exactly what you are trying to achieve but the docs specify, that the source matrix for your C2R should be the result of R2C

DFT_REAL_OUTPUT specifies the output as real. The source matrix is the result of
real-complex transform, so the destination matrix must be real.

Have you looked at the C2C

github.com

opencv/opencv_contrib/blob/deb225554f7dc371708c049b276bc351fcdb69cd/modules/cudaarithm/test/test_arithm.cpp#L201


      
              virtual void SetUp()
              {
                  devInfo = GetParam();
          
                  cv::cuda::setDevice(devInfo.deviceID());
              }
          };
          
          namespace
          {
              void testC2C(const std::string& hint, int cols, int rows, int flags, bool inplace)
              {
                  SCOPED_TRACE(hint);
          
                  cv::Mat a = randomMat(cv::Size(cols, rows), CV_32FC2, 0.0, 10.0);
          
                  cv::Mat b_gold;
                  cv::dft(a, b_gold, flags);
          
                  cv::cuda::GpuMat d_b;
                  cv::cuda::GpuMat d_b_data;

and R2C then C2R

github.com

opencv/opencv_contrib/blob/deb225554f7dc371708c049b276bc351fcdb69cd/modules/cudaarithm/test/test_arithm.cpp#L280


      
                  cv::dft(a, b_gold, flags);
          
                  ASSERT_EQ(CV_32F, d_b.depth());
                  ASSERT_EQ(2, d_b.channels());
                  EXPECT_MAT_NEAR(b_gold, cv::Mat(d_b), rows * cols * 1e-4);
              }
          }
          
          namespace
          {
              void testR2CThenC2R(const std::string& hint, int cols, int rows, bool inplace)
              {
                  SCOPED_TRACE(hint);
          
                  cv::Mat a = randomMat(cv::Size(cols, rows), CV_32FC1, 0.0, 10.0);
          
                  cv::cuda::GpuMat d_b, d_c;
                  cv::cuda::GpuMat d_b_data, d_c_data;
                  if (inplace)
                  {
                      if (a.cols == 1)

test cases?

laurent.berger · May 24, 2023, 7:15am

It’s a mathematical property of fourier transform.

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Why DFT_REAL_OUTPUT is different from the real part of complex result

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