Filtering in the frequency domain is a common image and signal processing technique. It can smooth, sharpen, de-blur, and restore some images. The filtered image must be transformed back to the spatial domain. These steps can be implemented in IDL using the Fast Fourier transform function FFT . A -1 argument to the FFT function indicates a transformation from the spatial to the frequency domain, and a 1 indicates the reverse transformation.
Many common filters take advantage of what is called a frequency image or Euclidean distance map.
An Euclidean distance map for a two-dimensional image is an array of the same size as the image. Each pixel in the distance map is given a value that is equal to its distance from the nearest corner of the two-dimensional array. The DIST command in IDL is used to create the Euclidean distance map or frequency image. A common type of filter to use in frequency domain filtering is a Butterworth frequency filter. To see how frequency domain filtering works, open the image file convec.dat , which is among the files Research Systems provides with its demonstation directories.