Finding the x value where there area under the curve (from the centre) is 80% of total area

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Jason
Jason 2016-11-21
编辑： Guillaume 2016-11-21
Hi. I have a set of data points that I perform a spline to to fill in the gaps. I have split the spline into two splines - one each side of the max (black dotted line).
I want to find the area for each half, starting with the blue spline and at the peak x value (i.e.black dotted line) and find the x coordinate that equates to 80% of the total area under the curve. I have tried to use cumsum, but am not getting sensible answer.
cdf = cumsum(horizontalProfile1(1:indexOfMax));
sum(cdf)
% Normalize
cdf = cdf / sum(cdf)
width80 = 2 * find(cdf >= 0.8, 1, 'first')
title(['E80 ',num2str(width80)],'fontsize',10,'FontWeight','normal','Color','b')
It might be because Im summing from 1 to index of max, but even when I switch it it gives an empty matrix for width50?

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采纳的回答

Guillaume
Guillaume 2016-11-21
编辑：Guillaume 2016-11-21
Your normalisation is completely wrong. You need to divide by the total area which is sum(horizontalProfile1(1:indexOfMax)), or cdf(end), not the sum of the cumsum.
cdf = cdf / cdf(end); %or cdf = cdf / sum(horizontalProfile1(1:indexOfMax)) %same number
You were dividing the cumsum by such a huge number that the normalised cdf never went above 0.8, thus find returned empty.

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KSSV
KSSV 2016-11-21
doc trapz.

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