Recently, I am doing fourier transform on gray bmp files. I first import the bmp file to image, then use data=image[[1,1]] to get the pixel value. Then I do fd=Fourier[data], ListDensityPlot[fd, Mesh->False]. But it seems the result is been swamped. The center is divided into four parts and showed at the four corners.I don't know why. I hope I can get help from warmhearted people.