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why am i getting NaN from my function

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tomer orenshtein
tomer orenshtein 2020-1-26
评论： tomer orenshtein 2020-1-26
i was asked to write a function that gets b&w image and returns its entropy. im not allowed to use im_hist or entrtopy (obviously).
this is the function i wrote:
function [Entropy] = EntropyCalc(image)
image_hist = histogram(image);
values = image_hist.Values;
[size_x, size_y] = size(image);
total = size_x * size_y;
p = values./total;
Entropy = - sum(p.*log2(p));
end
but i keep getting NaN.
any ideas??
thanks!

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采纳的回答

Thiago Henrique Gomes Lobato
As David said your log2(p) is not safe, you can solve this problem by using this line of code:
Entropy = - sum(p.*log2(p+eps));
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Thiago Henrique Gomes Lobato
Thiago Henrique Gomes Lobato 2020-1-26
编辑：Thiago Henrique Gomes Lobato 2020-1-26
The difference is in the histogram function, you need to explicitely define the number of bins to 256 (make sure image is uint8) so it has the same result.
image_hist = histogram(image,256);
Or faster:
values = imhist(image(:));
tomer orenshtein
tomer orenshtein 2020-1-26
that was the thing... didnt know this option. (cant ust imhist in this hw). thanks again! helped me a lot!

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