shannon entropy and entropy of grayscale

How can i find the entropy of a gray image without using the "entropy(I)" , "entropy" command? with just codes and arrays?

[the cyclist]

Sounds like homework.

Can you use imhist?

[elTurco]

Yes i can

Answer 1

[the cyclist]

I = imread('image.png');

figure
imagesc(I)

% Using histogram from imhist
p = imhist(I);
p = p/numel(I);
p(p==0) = [];

entropyFromImhist = -sum(p.*log2(p))

entropyFromImhist = 4.3978

% Built-in entropy function
entropy(I)

ans = 4.3978

Answer 2

[埃博拉酱]

Entropy.m is open source, why don't you just check out its source code?

edit entropy