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Image segmentation using Otsu thresholding

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OTSU(I,N) segments the image I into N classes by means of Otsu's N-thresholding method.

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IDX = OTSU(I,N) segments the image I into N classes by means of Otsu's N-thresholding method. OTSU returns an array IDX containing the cluster indices (from 1 to N) of each point.
IDX = OTSU(I) uses two classes (N=2, default value).
[IDX,sep] = OTSU(I,N) also returns the value (sep) of the separability criterion within the range [0 1]. Zero is obtained only with data having less than N values, whereas one (optimal value) is obtained only with N-valued arrays.

If I is an RGB image, a Karhunen-Loeve transform is first performed on the three R,G,B channels. The segmentation is then carried out on the image component that contains most of the energy.
    load clown
    X = ind2gray(X,map);
    for n = 2:4
      IDX = otsu(X,n);
      imagesc(IDX), axis image off
      title(['n = ' int2str(n)],'FontWeight','bold')

See also:

Comments and Ratings (12)

Rasika Devi

usha diggi

Thanks for the code and brief explaination

Luqman Subki

In the example provided, what is 'X', anyone?

matlab code for luminance level thresholding


Venkat (view profile)


Jing (view profile)

Thanks a lot for the code!
I just have a question: in the n=2 case, what's the meaning for the following code? I tried to derive the equation, but could not make it. Would you please explain a bit? Thanks!!

sigma2B =...

I am geting this error...How do I correct it?

Undefined function or method 'otsu' for input arguments of type 'uint8'.


works smoothly...thx ;)

Matteo Niccoli

It has worked for me very nicely. I like to run it in a while loop that breaks when sep starts decreasing.


Dalal (view profile)

i am trying to segment the image of signature
is it possible to apply the same code ?
thank you


Fu (view profile)

Lukas Lansky

Works nice!



Minor modifications


The segmentation for RGB image has been improved: a KLT is performed and we keep the component of highest energy.


RGB images are now analyzed in the gray, R, G and B scales.


New screenshot.

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