Matlab - How to fix : Warning: Displaying real part of complex input ?

Question

Given the code

function [nImg,mask] = myFunc(img,rl,rh)


    [n m] = size(img);
    mask = ones(n, m);

    % do some stuff
    %   more 
    %   and more 
    % 

    fourierImg = fft2(img); % take the fourier transform 2d for the given image
    fourierImg = fftshift(fourierImg); %  shift the fourier transform 
    output = mask.*fourierImg; % calc with the mask  % THAT LINE CAUSES 
    %  Warning: Displaying real part of complex input ? 
    ishifting = ifftshift(output); % grab the DC element 
    nImg = ifft2(ishifting); % inverse back to the image dimension

end

I keep getting : Warning: Displaying real part of complex input when I execute the line output = mask.*fourierImg; % calc with the mask .

How can I fix that ?

Regards

Answer 1

The warning means that you're trying to plot complex values on real axes.

I believe that it's not that line that triggers that warning, but rather a plot command (or its similar) somewhere else in your code.

The result of FFT transforms is usually complex, so if you want to plot these values, use two plots: one for the magnitude and one for the phase (or one plot for the real part and one for the imaginary part, but this is far less common to do so). To obtain the magnitude use the abs command.

Following your comment, instead of imshow(X) try imshow(abs(X)) or imshow(real(X)).