%两个程序，亲测可用

clear all

a=imread('moon.tif');

figure,imshow(a)

count=imhist(a);

[m,n]=size(a);

N=m*n;

L=256;

count=count/N;%%每一个像素的分布概率

count

for i=1:L

if count(i)~=0

st=i-1;

break;

end

end

st

for i=L:-1:1

if count(i)~=0

nd=i-1;

break;

end

end

nd

f=count(st+1:nd+1); %f是每个灰度出现的概率

size(f)

E=[];

for Th=st:nd-1 %%%设定初始分割阈值为Th

av1=0;

av2=0;

Pth=sum(count(1:Th+1));

%%%第一类的平均相对熵为

for i=0:Th

av1=av1-count(i+1)/Pth*log(count(i+1)/Pth+0.00001);

end

%%%第二类的平均相对熵为

for i=Th+1:L-1

av2=av2-count(i+1)/(1-Pth)*log(count(i+1)/(1-Pth)+0.00001); end

E(Th-st+1)=av1+av2;

end

position=find(E==(max(E)));

th=st+position-1