文章和代码以及样例图片等相关资源，已经归档至【Github仓库：digital-image-processing-matlab】或者公众号【AIShareLab】回复 数字图像处理 也可获取。
26.edge
功能：识别强度图像中的边界。
语法：
BW = edge(I,'sobel')
BW = edge(I,'sobel',thresh)
BW = edge(I,'sobel',thresh,direction)
[BW,thresh] = edge(I,'sobel',...)
BW = edge(I,'prewitt')
BW = edge(I,'prewitt',thresh)
BW = edge(I,'prewitt',thresh,direction)
[BW,thresh] = edge(I,'prewitt',...)
BW = edge(I,'roberts')
BW = edge(I,'roberts',thresh)
[BW,thresh] = edge(I,'roberts',...)
BW = edge(I,'log')
BW = edge(I,'log',thresh)
BW = edge(I,'log',thresh,sigma)
[BW,threshold] = edge(I,'log',...)
BW = edge(I,'zerocross',thresh,h)
[BW,thresh] = edge(I,'zerocross',...)
BW = edge(I,'canny')
BW = edge(I,'canny',thresh)
BW = edge(I,'canny',thresh,sigma)
[BW,threshold] = edge(I,'canny',...) 举例
I = imread('rice.tif');
BW1 = edge(I,'prewitt');
BW2 = edge(I,'canny');
imshow(BW1);
figure, imshow(BW2)27.erode
功能：弱化二进制图像的边界。
语法：
BW2 = erode(BW1,SE)
BW2 = erode(BW1,SE,alg)
BW2 = erode(BW1,SE,...,n)举例
BW1 = imread('text.tif');
SE = ones(3,1);
BW2 = erode(BW1,SE);
imshow(BW1)
figure, imshow(BW2) 相关命令： bwmorph, dilate
28.fft2
功能：进行二维快速傅里叶变换。
语法：
B = fft2(A)
B = fft2(A,m,n)举例
load imdemos saturn2
imshow(saturn2)B = fftshift(fft2(saturn2));
imshow(log(abs(B)),[]), colormap(jet(64)), colorbar相关命令： dct2, fftshift, idct2, ifft2
29.fftn
功能：进行n 维快速傅里叶变换。语法：
B = fftn(A)
B = fftn(A,siz) 相关命令： fft2, ifftn30.fftshift
功能：把快速傅里叶变换的DC 组件移到光谱中心。语法：
B = fftshift(A)举例
B = fftn(A);
C = fftshift(B);相关命令： fft2, fftn, ifftshift
31.filter2
功能：进行二维线性过滤操作。语法：
B = filter2(h,A)
B = filter2(h,A,shape)举例
A = magic(6)
A =
35 1 6 26 19 24
3 32 7 21 23 25
31 9 2 22 27 20
8 28 33 17 10 15
30 5 34 12 14 16
4 36 29 13 18 11
h = fspecial('sobel')
h =
1 2 1
0 0 0
–1 –2 –1
B = filter2(h,A,'valid')
B =
–8 4 4 –8
–23 –44 –5 40
–23 –50 1 40
–8 4 4 –8相关命令： conv2, roifilt2
32.freqspace
功能：确定二维频率响应的频率空间。语法：
[f1,f2] = freqspace(n)
[f1,f2] = freqspace([m n])
[x1,y1] = freqspace(...,'meshgrid')
f = freqspace(N)
f = freqspace(N,'whole')相关命令： fsamp2, fwind1, fwind2
33.freqz2
功能：计算二维频率响应。语法：
[H,f1,f2] = freqz2(h,n1,n2)
[H,f1,f2] = freqz2(h,[n2 n1])
[H,f1,f2] = freqz2(h,f1,f2)
[H,f1,f2] = freqz2(h)
[...] = freqz2(h,...,[dx dy])
[...] = freqz2(h,...,dx)
freqz2(...)举例
Hd = zeros(16,16);
Hd(5:12,5:12) = 1;
Hd(7:10,7:10) = 0;
h = fwind1(Hd,bartlett(16));
colormap(jet(64))
freqz2(h,[32 32]);
axis ([–1 1 –1 1 0 1])34.fsamp2
功能：用频率采样法设计二维FIR 过滤器。语法：
h = fsamp2(Hd)
h = fsamp2(f1,f2,Hd,[m n])举例
[f1,f2] = freqspace(21,'meshgrid');
Hd = ones(21);
r = sqrt(f1.^2 + f2.^2);
Hd((r<0.1)|(r>0.5)) = 0;
colormap(jet(64))
mesh(f1,f2,Hd)相关命令： conv2, filter2, freqspace, ftrans2, fwind1, fwind2
35.fspecial
功能：创建预定义过滤器。语法：
h = fspecial(type)
h = fspecial(type,parameters)举例
I = imread('saturn.tif');
h = fspecial('unsharp',0.5);
I2 = filter2(h,I)/255;
imshow(I)
figure, imshow(I2)相关命令： conv2, edge, filter2, fsamp2, fwind1, fwind2
36.ftrans2
功能：通过频率转换设计二维FIR 过滤器。
语法：
h = ftrans2(b,t) h = ftrans2(b) 举例
colormap(jet(64))
b = remez(10,[0 0.05 0.15 0.55 0.65 1],[0 0 1 1 0 0]);
[H,w] = freqz(b,1,128,'whole');
plot(w/pi–1,fftshift(abs(H)))相关命令：
conv2, filter2, fsamp2, fwind1, fwind2
参考文献：
[2] [阮秋琦. 数字图像处理（MATLAB版）[M]. 北京：电子工业出版社, 2014.](https://github.com/timerring/digital-image-processing-matlab/blob/main/reference/Digital_Image_Processing_(MATLAB_version).pdf)
[3] [冈萨雷斯. 数字图像处理（第三版）[M]. 北京：电子工业出版社, 2011.](https://github.com/timerring/digital-image-processing-matlab/blob/main/reference/Digital_Image_Processing_(Third_Edition).pdf)