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.(adsbygoogle = window.adsbygoogle || []).push({});

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,

and here is the output image,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')

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,

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 waveCode (Text):image = [0 0 0 0 1 1 1 1]

If i take its FFT and display it ,

Code (Text):f = fft2(image)

imshow(fftshift(f)); % plotting

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?

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# Understanding the Image output in Frequency Domain

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