Hello everyone, how do you fit two curves into a closed curve?My code is as follows.

load dataA.mat
%x1=dataA(:,1);
%y1=dataA(:,2);
f1 = fit(xAbove,yAbove,'smoothingspline','SmoothingParam',0.0001);%smoothingspline
%plot(f1,xAbove,yAbove)
plot(f1,'-r')
hold on
load dataB.mat
%x2=dataB(:,1);
%y2=dataB(:,2);
[xx,ind] = sort(xBelow);
yy2 = smooth(xBelow,yBelow,0.5,'rloess');
plot(xx,yy2(ind),'r-')
%f2 = fit(xBelow,yBelow,'smoothingspline','SmoothingParam',1);%smoothingspline
%plot(xBelow,yBelow)
%xBelow(1)=xAbove(1);
%yBelow(end)=yAbove(end);
%hold on

Answer 1

[KSSV]

load dataA ;
load dataB ;

C1 = [xAbove yAbove] ;
C2 = [xBelow yBelow] ;
% MErge them 
C = [C1 ;C2] ;
idx = boundary(C(:,1),C(:,2)) ;
C = C(idx,:) ; 
plot(C(:,1),C(:,2))

[Wesley]

Thank you very much for your enthusiastic answers. But I don't think these two methods are a good way, because the curve fitted by this method is not smooth, and the versatility is not strong.

[Wesley]

The problem I would like to solve is to close the two fitted curves.

[KSSV]

Already you have a closed curve with code shown in the answer..why you want to do a fitting agin?

[Wesley]

Because the current curve is closed, but some pictures extracted from similar pictures are quite different from the original pictures.

[Wesley]

And the curve cannot be derivated and be searched for extreme points

[Wesley]

My idea is similar to the idea of piecewise function fitting, where the critical points are equal at the nodes to realize the fitting of a closed curve.

Answer 2

[Raghav Gnanasambandam]

I am not sure whether it is actually needed to combine the two curves. I am assuming you just need to find the closed curve for the whole data. Depending on how you want to fit a closed curve, you can use either boundary() or convhull().

load dataA.mat
load dataB.mat

x = [xAbove;xBelow];
y = [yAbove;yBelow];

k = boundary(x,y);
hold on;
plot(x(k),y(k));

You can also refer this https://www.mathworks.com/matlabcentral/answers/269081-closed-curve-fitting-of-scatter-plot-data

or

load dataA.mat
load dataB.mat

x = [xAbove;xBelow];
y = [yAbove;yBelow];

k = convhull(x,y);
hold on;
plot(x(k),y(k));

Hope this helps.