function beta=frazionePotenza(im,R)

%filtro passa bassa ideale
%im=imread('cameraman.tif','tif'); R=70;

F_im=fft2(im);
F_im=fftshift(F_im);
R_im=real(F_im); I_im=imag(F_im);
%imshow(log(abs(R_im))/max(max(log(abs(R_im)))));
P_im=R_im.*R_im+I_im.*I_im;
%imshow(log(P_im)/max(max(log(P_im))));
[m,n]=size(P_im);
mask=zeros(m,n);
mask(round(m/2)-(R+1):round(m/2)+(R+1),round(n/2)-(R+1):round(n/2)+(R+1))=1.0;
for i=round(m/2)-(R+1):round(m/2)+(R+1),
    for j=round(n/2)-(R+1):round(n/2)+(R+1),
        if(sqrt((i-1-m/2)^2+(j-1-n/2)^2)>R), mask(i,j)=0.0; end;
    end;
end;
%imshow(mask);
P_im_tot=sum(P_im(:));
P_im_new=P_im.*mask;
P_im_tot_new=sum(P_im_new(:));
beta=100*P_im_tot_new/P_im_tot;

F_im_new=F_im.*mask;
%imshow(log(abs(real(F_im_new)))/max(max(log(abs(real(F_im_new))))));
im_out=ifft2(fftshift(F_im_new));
imshow(uint8(round(im_out)));