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How to calculate local maximum point from a derivative of a function?

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joynob ahmed
joynob ahmed 2020-6-9
编辑： joynob ahmed 2020-7-9
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:
And what I wanted is to point out the local maximums like this and count the abrupt cut off in each region:

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darova
darova 2020-6-12
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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joynob ahmed
joynob ahmed 2020-7-9
darova
Earlier you have helped me to find out the intersection point of the curve with the positive x axis. Then I had to divide the curve in 8 region and find if minimum one intersection point is there and if so then I would count the region of value 1 otherwise 0. But after finding the intersection points,I couldn't divide it. So I am trying after interchanging the steps. That I have divided the points and now I want the intersection point.
As in my image you can see that there are 2 regions(1 and 8) where no zero crossing is happened.So its value will be 6:

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Image Analyst
Image Analyst 2020-7-7
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
Image Analyst 2020-7-7
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.
joynob ahmed
joynob ahmed 2020-7-9
编辑：joynob ahmed 2020-7-9
Image Analyst
I understood your point. I have tried it but failed. See I have got this:
form this after dividing the points into 8 region:
Now if I want the local maxima from the first image what should I do?
My code is given below:
figure
plot(distance)
I=imgaussfilt(distance,20);
figure
plot(I)
n = 8 ;
d=I';
a=length(I);
% check whether data needed to be appended to reshape into n
b = n * ceil(a / n)-a ;
%
d = [d NaN(1,b)] ;
iwant = reshape(d,[],n);
% plot
figure
plot(iwant','b*')

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