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mo ma
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how otsu multithresh works?

Asked by mo ma
on 20 Aug 2013
hello can anyone explain me how otsu multithresh works?

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2 Answers

Answer by Amith Kamath on 22 Aug 2013

From http://en.wikipedia.org/wiki/Otsu%27s_method, the algorithm assumes that the image to be thresholded contains two classes of pixels or bi-modal histogram (e.g. foreground and background) then calculates the optimum threshold separating those two classes so that their combined spread (intra-class variance) is minimal.
MULTITHRESH can accept another argument that decides how many such classes of pixels are needed, so that you can segment the image over multiple levels.
http://www.mathworks.com/help/images/ref/multithresh.html#btksdqr-1 shows some examples using MULTITHRESH and IMQUANTIZE which may be instructive.

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Anand
Answer by Anand
on 22 Aug 2013

Look at the paper referenced in the documentation page for multithresh. The link is below:
Specifically, look at section III.B for an explanation.
The basic idea is to find the two thresholds that maximize between class variance over the gray levels in the image. A search-based optimization is used (fminsearch) to determine the two thresholds. The best way to understand is by looking at lines 141-155 in multithresh.
>>edit multithresh

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