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

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Wesley on 20 Jan 2021
Commented: Wesley on 20 Jan 2021
%x1=dataA(:,1);
%y1=dataA(:,2);
f1 = fit(xAbove,yAbove,'smoothingspline','SmoothingParam',0.0001);%smoothingspline
%plot(f1,xAbove,yAbove)
plot(f1,'-r')
hold on
%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

KSSV on 20 Jan 2021
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 on 20 Jan 2021
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.

Raghav Gnanasambandam on 20 Jan 2021
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().
x = [xAbove;xBelow];
y = [yAbove;yBelow];
k = boundary(x,y);
hold on;
plot(x(k),y(k));
or
x = [xAbove;xBelow];
y = [yAbove;yBelow];
k = convhull(x,y);
hold on;
plot(x(k),y(k));
Hope this helps.