Using DCT filter as processing filter

Hello, I'm trying to implement this simple denoising filter (attached also): http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6877832

 function output = dct_filter(I_bw)  % import a double grayscale image

    X = dct(I_bw);  % apply DCT
    Xs = sort(abs(X(:)),'ascend'); 

    % Determine the 30% of the DCT coefficients are significant
    low_band = Xs((1:ceil(length(Xs)*0.3)));  
    max_low = max(low_band);
    sigcoeff = (abs(X) <= max_low);

    % Reconstruct the signal using only the significant components.
    X(~sigcoeff) = 0;  
    output = idct(X);      
 end

Has anyone ane idea why doesn't it work?