title: Power spectrum of an image
body: <p>I have begun (a small project) to calculate the power spectrum of an image in the frequency domain.</p>
  
  <p>So, what I have till now is the following:</p>
  
  <pre><code>%// close all; clear all; %// not generally appreciated
  img   = imread('ajw_pic.jpg','jpg'); % it is a color image
  img = rgb2gray(img); %// change to gray
  psd = 10*log10(abs(fftshift(fft2(img))).^2 );
  figure(2); clf
  mesh(psd)
  </code></pre>
  
  <p>So far it looks good; I get the mesh plot which resembles the spectra I see in various academic papers. </p>
  
  <p>However, what I am looking for is a graph plot of this power spectra versus the frequency, and I am not entirely sure how to get this frequency vector. I could do for instance:</p>
  
  <pre><code>N=400;        %// the image is 400 x 400
  f=-N/2:N/2-1; %// possible frequencies?
  </code></pre>
  
  <p>but I am not convinced this is entirely correct as this gives rise to negative frequencies.</p>
  
  <p>I'd really appreciate if someone could point me in the right direction to plot log frequency versus the power spectrum.</p>