I1=imread('C:\Documents and Settings\lishurui\桌面\DRAGON_256_1.bmp');
subplot(2,2,1);
imshow(I1);
title('原始圖像');
I2=imnoise(I1,'salt & pepper',0.02);
subplot(2,2,2);
imshow(I2);
title('噪聲圖像');
f=double(I2);
g=fft2(f);
g=fftshift(g);
[N1,N2]=size(g);
n=2;
d0=10;
n1=fix(N1/2);
n2=fix(N2/2);
for i=1:N1
    for j=2:N2
        d=sqrt((i-n1)^2+(j-n2)^2);
       
            h=1/(1+0.414*(d0/d)^(2*n));
     
        result1(i,j)=h*g(i,j);
        if(g(i,j)>10)
            result2(i,j)=0;
        else
            result2(i,j)=g(i,j);
        end
    end
end
result1=ifftshift(result1);
result2=ifftshift(result1);
X2=ifft2(result1);
X3=uint8(real(X2));
subplot(2,2,3)
imshow(X3);
title('Butterworth濾波圖像');
X4=ifft2(result2);
X5=uint8(real(X4));
subplot(2,2,4)
imshow(X5);
title('理想低通濾波圖像');