function SHAN = entropy(x, n)
% 計算熵，n是圖像 x的灰度級，如果 n是默認則 n = 256
error(nargchk(1, 2, nargin));
if nargin<2
    n = 256;
end
x = double(x);
xh = hist(x(:), n);
xh = xh/sum(xh(:));
i = find(xh);
SHAN = -sum(xh(i).*log2(xh(i)));