Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding the Image output in Frequency Domain

  1. Sep 28, 2012 #1
    I need help understanding how frequencies are distributed when taking the FFT of an Image and how can i determine those frequencies, I really need a detailed and easy tutorial with practical example to understand it.

    When we take the FFT of a Sine/Cos wave we can easily see its frequencies in FFT, here is my example i added comments in detail to understand,

    Code (Text):
    %Matlab Code,
     
    f=1e3;   %Frequency of Wave -> 1KHz
    A=4;    %Amplitude
    Fs = 1e6;  %Sampling Frequency
    Ts = 1/Fs;   %Sampling Rate
    t = 10/f;       %Time period of 10 Oscillation
    n = 0:Ts:t;  %Generating Samples
    % (1/f)/T = 1000 %Length for 1 Oscillation after Sampling
    % So Length of the Signal for 10 Oscillations is 10,000
    x=A*sin(2*pi*f*n);
    subplot(2,1 ,1);
    plot(n,x);
    % 100,000/10,000 = 100Hz <- First point = 100Hz
    % 2nd Point = 200Hz
    % 3rd Point = 300Hz
    % 4th Point = 400Hz
    % .
    % .
    % 10th Point = 1KHz <- Original Signal Frequency
    subplot(2,1 ,2);
    F=fft(x);
    plot(2*abs(F)./(t/Ts));
    xlim ([0 100])    % 0 = 100 , 1 = 200......10=1000......100 = 10,000
    set(gca, 'XTickLabel', ((get(gca,'XTick')))*100)
    xlabel ('Frequency in Hz')
    and here is the output image,
    untitled.jpg

    See how easily I can see the frequency of actual signal in Frequency domain, this is because i knew that the original frequency was 1KHz.

    but in case of Images, what are the frequencies ?

    here is the simple code,

    Code (Text):
    image = [0 0 0 0 1 1 1 1]
    The above discrete signal has 8 points so N = 8 How can i determine the frequencies and amplitude of this signal ? just like i did above for Sine wave

    If i take its FFT and display it ,


    Code (Text):
    f = fft2(image)
    imshow(fftshift(f)); % plotting
    Untitled.png

    it is nothing but 3-colors the middle one is white. (as expected)
    I've no idea what does that output mean , how could i understand what are the frequencies in the image from this output ?

    Can somebody explain me just one example of image FFT and how to determine the frequency etc like i did above in case of Sine wave?
     
    Last edited: Sep 28, 2012
  2. jcsd
  3. Sep 28, 2012 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    your output image isnt showing


    Dave
     
  4. Sep 28, 2012 #3
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Understanding the Image output in Frequency Domain
  1. Frequency Domain (Replies: 5)

Loading...