a=imread('b.bmp');[r,c,d]=size(a);blue=zeros(r,c);blue(:,:,1)=zeros(r,c);blue(:,:,2)=zeros(r,c);blue(:,:,3)=a(:,:,3);blue=uint8(blue);imshow(blue);这个程序用来提取图像蓝色区域,请问unit2是干啥用的是uint8