roipoly command in matlab

a=imread('data1.jpg');
[m n]=ginput(1);
bw=roipoly(a,m,n);
figure,imshow(bw);
[M N]=size(bw);
for i=1:m
 for j=1:n
    if bw(i,j)==1
       b(i,j)=a(i,j);
   else
      b(i,j)=0;
   end
end
end
figure,imshow(b,[]);title('output image');