关于MATLAB图像灰度处理的问题

a=imread('a.bmp');
c=zeros(size(a));
pos=find(a<20);
c(pos)=a(pos);
b(:,:,3)=c;
c=zeros(size(a));
pos=find((a>=20)&(a<40));
c(pos)=a(pos);
b(:,:,2)=c;
c=zeros(size(a));
pos=find(a>=40);
c(pos)=a(pos);
b(:,:,1)=c;
b=uint8(b);

a=imread('a.bmp');
c=zeros(size(a));
pos=find(a<20);
c(pos)=a(pos);
b(:,:,3)=c;
c=zeros(size(a));
pos=find((a>=20)&(a<40));
c(pos)=a(pos);
b(:,:,2)=c;
c=zeros(size(a));
pos=find(a>=40);
c(pos)=a(pos);
b(:,:,1)=c;
b=uint8(b);

a=imread('a.bmp');
c=zeros(size(a));
pos=find(a<20);
c(pos)=a(pos);
b(:,:,3)=c;
c=zeros(size(a));
pos=find((a>=20)&(a<40));
c(pos)=a(pos);
b(:,:,2)=c;
c=zeros(size(a));
pos=find(a>=40);
c(pos)=a(pos);
b(:,:,1)=c;
b=uint8(b);
这还好!!!!!恩!!