Why 2D sinusoidal grating Phase spectrum looks horizontal line?

Hi Everyone, Good day! Can anybody explain clearly what is FFT phase spectrum of 2D images? When I plotted phase spectrum of 2D sinusoidal grating image (0 orientation)I get as below, though I am not sure it is correct or not.

Appreciated if someone can help me in this.Thank you.

Answer 1

[Massimo Zanetti]

Basically Matlab function FFT2 computes the discrete 1D FFT on image columns first, and then again it applies 1D FFT to the rows of the result.

Therefore, the result represents the 2D spectral behaviour of the image as if by moving from one column to another (and from one row to another) you are moving forward in time.. So, your spectral components (phase) are two-dimensional and the ones with highest magnitude (magnitude) are dominant.

[Divya Krishnakumar]

Thank you so much for your answer. First part is clear for me, but about phase spectrum why it shows dotted horizontal line ?

[Massimo Zanetti]

Because your image is basically the same row repeated many times, so any row has the same spectral components. :)

Moreover, given the magnitude image, you can see this row is basically a composition of few frequencies (the few pixels in magnitude image), therefore something like a sin or cos. And this fits with your image, which seems to be similar to a 2D sin wave that develops only in one direction.

Is it more clear now? If this answer helped you, please accept it.

[Divya Krishnakumar]

Thanks Mr.Massimo Zanetti !