How to calculate local maximum point from a derivative of a function?
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Hi. I am working with border irregularity of lesion. So I have determined the derivative of the border irregularity function to get the local maximums.We know the local maximum is detected when the derivative of the function crosses the zero point and the slope changes sign from + to −. I want to divide the curve in 8 region and count the abrupt cut off in every region so that I can have the final decision.
I found out upto this:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/314042/image.bmp)
And what I wanted is to point out the local maximums like this and count the abrupt cut off in each region:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/313169/image.png)
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darova
le 12 Juin 2020
TRY THIS SIMPLE EXAMPLE
x = 1:50;
y = sin(x);
[xc,yc] = polyxpoly(x,y,[0 50],[0 0]);
plot(x,y)
hold on
for i = 1:3
ix = 15*(i-1) < xc & xc < 15*i;
plot(xc(ix),yc(ix),'*','color',rand(1,3))
end
hold off
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Image Analyst
le 7 Juil 2020
Why not simply call imregionalmax()?
You can smooth the data with a sliding quadratic if you want to before that with sgolayfilt().
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Image Analyst
le 7 Juil 2020
You wanted the zero crossings of the derivative, because you want to know where the maxima (peaks) are, and the derivative is zero when the signal is at a max and the slope is zero. But if you simply use imregionalmax() you don't need to even deal with the derivative at all. It's much simpler and more direct.
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