Get envelope of a cyclic curve

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

Thank you.

[Nikesh Maharjan]

Thank you very much I learnt a new thing today.

However, there is one more step to solving my problem.

In my case, for a value to X {data(:2)}, there are are multiple values of Y {data(:,2)}. I want to plot only maximum abosolute value of Ys corresponding to X.

[William Rose]

@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]

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

[William Rose]

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

Answer 1

[William Rose]

@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.