关于#matlab#的问题，如何解决？

img = imread('asquares.tif');
figure(1); imshow(img); title('Original Image');

% 设置截止频率
d1 = 15;
d2 = 80;
d3 = 300;

% 图像傅里叶变换
img_fft = fft2(img);

% 构造理想低通滤波器
h1 = zeros(size(img));
h2 = zeros(size(img));
h3 = zeros(size(img));
[x, y] = meshgrid(1:size(img,2), 1:size(img,1));
dist = sqrt((x-round(size(img,2)/2)).^2 + (y-round(size(img,1)/2)).^2);
h1(dist<=d1) = 1;
h2(dist<=d2) = 1;
h3(dist<=d3) = 1;

% 应用低通滤波器
img_filter1 = real(ifft2(img_fft.*h1));
img_filter2 = real(ifft2(img_fft.*h2));
img_filter3 = real(ifft2(img_fft.*h3));

% 显示
figure(2);
subplot(3, 4, 1); imshow(img); title('Original Image');
subplot(3, 4, 2); imshow(h1); title(['Ideal Lowpass, D=' num2str(d1)]);
subplot(3, 4, 3); imshow(img_filter1); title(['Filtered Image, D=' num2str(d1)]);
subplot(3, 4, 6); imshow(h2); title(['Ideal Lowpass, D=' num2str(d2)]);
subplot(3, 4, 7); imshow(img_filter2); title(['Filtered Image, D=' num2str(d2)]);
subplot(3, 4, 10); imshow(h3); title(['Ideal Lowpass, D=' num2str(d3)]);
subplot(3, 4, 11); imshow(img_filter3); title(['Filtered Image, D=' num2str(d3)]);

% 构造高斯低通滤波器
h1 = fspecial('gaussian', size(img), d1);
h2 = fspecial('gaussian', size(img), d2);
h3 = fspecial('gaussian', size(img), d3);

% 应用低通滤波器
img_filter1 = imfilter(img, h1);
img_filter2 = imfilter(img, h2);
img_filter3 = imfilter(img, h3);

% 显示
subplot(3, 4, 4); imshow(h1); title(['Gaussian Lowpass, D=' num2str(d1)]);
subplot(3, 4, 5); imshow(img_filter1); title(['Filtered Image, D=' num2str(d1)]);
subplot(3, 4, 8); imshow(h2); title(['Gaussian Lowpass, D=' num2str(d2)]);
subplot(3, 4, 9); imshow(img_filter2); title(['Filtered Image, D=' num2str(d2)]);
subplot(3, 4, 12); imshow(h3); title(['Gaussian Lowpass, D=' num2str(d3)]);
subplot(3, 4, 12); imshow(img_filter3); title(['Filtered Image, D=' num2str(d3)]);

% 构造巴特沃斯低通滤波器
n = 2;
[b1, a1] = butter(n, d1/(size(img,1)/2));
[b2, a2] = butter(n, d2/(size(img,1)/2));
[b3, a3] = butter(n, d3/(size(img,1)/2));

% 应用滤波器
img_filter1 = imfilter(img, b1, a1);
img_filter2 = imfilter(img, b2, a2);
img_filter3 = imfilter(img, b3, a3);

% 显示
figure(3);
subplot(3, 4, 1); imshow(img); title('Original Image');
subplot(3, 4, 2); freqz(b1, a1); title(['Butterworth Lowpass, D=' num2str(d1)]);
subplot(3, 4, 3); imshow(img_filter1); title(['Filtered Image, D=' num2str(d1)]);
subplot(3, 4, 6); freqz(b2, a2); title(['Butterworth Lowpass, D=' num2str(d2)]);
subplot(3, 4, 7); imshow(img_filter2); title(['Filtered Image, D=' num2str(d2)]);
subplot(3, 4, 10); freqz(b3, a3); title(['Butterworth Lowpass, D=' num2str(d3)]);
subplot(3, 4, 11); imshow(img_filter3); title(['Filtered Image, D=' num2str(d3)]);