Complex numbers in a frequency domain of a 2D image
The real and imaginary plane are also the symmetric and anti-symmetric planes.  Image content that is symmetric around the center lines will show up in the real plane.  Image content that is opposite in brightness across the center lines will appear in the imaginary plane.  Data that is neither or some combination of both will end up in both, with the phase angle indicating the mix or a shift (or rotation wrapping around the edges) from symmetric to anti-symmetric.