How to calculate amplitude of each frequency?

I have an image and I have a code which gives the 2d fft of each color of the image. Now I need to calculate the amplitude of each frequency but I don’t know how to do it. Help is appreciated. Here’s my code so far:

im=imread('image ')/255.;
    im1=im;
    im=im(:,:,1);
    imagefft2=fft2(im);
    a=im_fft2;
    a(angle(imagefft2)<pi/2)=1;
    figure();
    im1(:,:,1)=abs(ifft2(a));
    imshow(im,[]);

Answer 1

[AdamG2013468]

For a 1-D fft, the process is as follows:

fs = %arbitrary sample frequency
N = length(signal);
fbins = [(0:1/N:1-1/N)*fs];    %frequency bin vector for plotting (x axis)
calval = N/2;                  %for two-sided ffts, calval should just be N for one-sided
[fftdat] = fft(signal);
fftmag = abs(fftdat)/calval;

%to visualize that expected magnitude is correct
figure
hold on
plot(fbins, fftmag)

See if you have any luck with a similar approach for your 2-D fft.

[Desiree]

I don’t understand the second line. What should be in the signal part?

[AdamG2013468]

That would be a generic time history signal, i.e. sine wave.

[Desiree]

I need to check the values of the amplitudes. If it is less than 1/8, then I should replace it by 0. How can I do that? Using if loop? For example if fttmag<1\8 , fftmag=0?

[AdamG2013468]

One way,

for i = 1:length(fftmag)
    if fftmag(i) < 1/8
        fftmag(i) = 0
    end
end

[Desiree]

I’m really sorry, but still I cannot understand the relationship between your first answer for 1d fft and my question. In my case I don’t have a function, but an image. It has nothing similar to what I have