[MATLAB] Low pass filter using Fourier transformation

B=imread('lenna.jpg');

A=double(rgb2gray(B));

subplot(1,2,1);

imshow(A,[]);

title('Original Image');

%imshow(C,[]);

[m n]=size(A);

F=zeros(m,n,'double');

F=A;

C=fft2(A);

D=fftshift(C);

%imshow(uint8(C));

%[th r]=cart2pol(real(C),imag(C));

limit=15;

for u=1:m

    for v=1:n

        D1(u,v)=sqrt(((u-(m/2))^2)+((v-(n/2))^2));

        if D1(u,v)<=limit

            H=1;

        else

            H=0;

        end

        I1(u,v)=H*D(u,v);

    end

end

% F1=ifft2(I1);

F1=ifftshift(I1);

F2=ifft2(F1);

subplot(1,2,2);

imshow(F2,[]);

title('Ideal LowPass');

%Y=fftshift();

%imshow(uint8(Y));