Radar images (e.g., those publicly available from the Sentinel 1 satellite) typically come as complex-valued images. You look at the norm of the complex numbers to understand the structures, but for interferometry you need the phase also.

If you compute the Fourier transform of a scalar-valued image, you obtain a complex-valued image. It would be nice to be able to process these transforms with gimp (e.g., to "paint" a few parts in black to set those frequencies to zero, thereby defining a band-pass filter).