function dark = darkChannel(imRGB)
r=imRGB(:,:,1);
g=imRGB(:,:,2);
b=imRGB(:,:,3);
[m n] = size (r);
a = zeros(m,n);
for i = 1:m
for j = 1:n
a(i,j)=min(r(i,j),g(i,j));
a(i,j)=min(a(i,j),b(i,j));
end
end
d = ones(15,15);
fun = @(block_struct)min(min(block_struct.data))*d;
dark = blockproc(a,[15 15],fun);
dark = dark(1:m,1:n);