Get envelope of a cyclic curve

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Nikesh Maharjan 2022-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/1875597-get-envelope-of-a-cyclic-curve

评论： William Rose 2023-1-12

采纳的回答： William Rose

Dear All, I need to get an envelope of a cyclic curve as shown shown in the attach picture and excel file.

Thank you.

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William Rose 2022-12-12

envelope.xlsx

@Nikesh Maharjan,

You're welcome.

You say "for a value to X {data(:2)}, there are are multiple values of Y {data(:,2)}. I want to plot only maximum absolute value of Ys corresponding to X."

I assume this means you want to remove points when the X-value is duplicated, keeping only the point with the highest Y value in such cases.

The code below does what you requested, and is plots the points after each stage of processing, so you can see what has happened.

You can find the convex hull of pts4 with convhull(), as demonstrated in my previous answer.

data=xlsread('envelope.xlsx');

pts1=data(:,4:5);

pts2=[data(:,4),abs(data(:,5))]; %absolute value of the Ys

%Next 3 lines: Remove duplicate X-value points, keep point with highest Y.

pts3=sortrows(pts2); %sort points

[~,idx,~]=unique(pts3(:,1),'last');

pts4=pts3(idx,:);

%Display number of points on the console.

fprintf('%d points originally; %d points after removing duplicates.\n',length(pts3),length(pts4))

1010 points originally; 580 points after removing duplicates.

subplot(221); plot(pts1(:,1),pts1(:,2),'-r.'); title('pts1')

subplot(222); plot(pts2(:,1),pts2(:,2),'-r.'); title('pts2')

subplot(223); plot(pts3(:,1),pts3(:,2),'-r.'); title('pts3')

subplot(224); plot(pts4(:,1),pts4(:,2),'-r.'); title('pts4')

Try it.

Nikesh Maharjan 2023-1-12

＠William Rose　Thank you very much. I really appreciate your help !!

William Rose 2023-1-12

@Nikesh Maharjan, you're welcome. Good luck with your work.

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Answer 1

William Rose 2022-12-11

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1875597-get-envelope-of-a-cyclic-curve#answer_1125177

envelope.xlsx

@Nikesh Maharjan,

Use convhull() to find the smallest convex shape that wraps around the points.

See code below.

data=xlsread('envelope.xlsx');

points=data(:,4:5);

k=convhull(points);

plot(points(:,1),points(:,2),'b.',points(k,1),points(k,2),'-r*')

Good luck.

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Get envelope of a cyclic curve

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