How to calculate power of a 2D fourier transformed image?

Hi. We are given some training data and asked to find power spectral features.

f = imread('S1.GIF');
z = fft2(double(f));
q = fftshift(z);
Magq = abs(q);
Phaseq = angle(q);
 imagesc(log(abs(q)+1));
 colorbar;

I used the code above to fourier transform the image. The image before transformed is

And the image after is

I want to calculate the power from 200 - 400(x-axis) using this method

by formula

I don't know how to use the image to calculate the power. Could anyone give me some help? Thank you very much!

Answer 1

[Image Analyst]

I don't see anything wrong with abs() like you're using. It does the formula you gave. Then just sum Magq in the 200-400 column zone:

p = sum(Magq(:, 200:400));