# creating a curve from XYZ points (centreline) and then split curve into new points

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HB on 16 Jun 2019
Commented: Image Analyst on 16 Jun 2019
Hi all,
I have X Y and Z coordinates which represent points along a centreline. There are for instance 60 but I need to be able to change the number of points along the centreline. Therefore I want to load the X Y Z coordinates as a text file, create a curve that precisley follows the points as much as possible and then split this curve into points (such as 100 points).
What is the best way of doing this in MATLAB?
Thanks.

John D'Errico on 16 Jun 2019
Edited: John D'Errico on 16 Jun 2019
This is a simple problem of interpolation, except that you don't know how to do the interpolation. Standard tools like interp1 don't do it. And interp2 or interp3 are not designed to solve the problem either.
The trick is to use a tool designed to solve that specific problem. (Or to know how to write such a tool. Easier to download the tool itself.) interparc does exactly what you want. Download it from the FEX, here:
interparc can take any set of points that represents a completely general path in 2 or 3 or more dimensions. You tell it how many points to interpolate along the path, and it does so.
x = sort(rand(50,1));
y = sin(3*x);
z = cos(5*x);
xyzi = interparc(100,x,y,z,'spline');
size(xyzi)
ans =
100 3
plot3(x,y,z,'ro')
hold on
plot3(xyzi(:,1),xyzi(:,2),xyzi(:,3),'b-')
box on
grid on So initially, 50 points spaced arbitrarily along a path. Now 100 points, spaced equally in arclength along that curve and smoothly interpolated.

#### 1 Comment

Image Analyst on 16 Jun 2019
Definitely an awesome answer John. +1 vote. Very useful function.
HB, attach your data if you still can't get interparc() to work.

Matt J on 16 Jun 2019
Edited: Matt J on 16 Jun 2019
Use a curve fitting function, like lsqcurvefit().

HB on 16 Jun 2019
Thanks. Any ideas about splitting the curve into points?
Matt J on 16 Jun 2019
That part, I didn't understand.