# Draw the sphere to find the radius of the sphere

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Sterne_17 il 3 Feb 2023
Commentato: DGM il 4 Feb 2023
I want to fit a 3D sphere, and my goal is to find the radius of the sphere. Knowing the coordinates of some points on the sphere (x,y,z,), which function should I use to quickly draw a sphere and know its radius?
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DGM il 4 Feb 2023

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### Risposta accettata

John D'Errico il 3 Feb 2023
Estimation of the sphere parameters is not too difficult. First, since you have shown no data, I'll make some up.
XYZ = randn(50,3); XYZ = XYZ./sqrt(sum(XYZ.^2,2));
XYZ = 2.5*XYZ + [1 3 5] + randn(size(XYZ))/20;
plot3(XYZ(:,1),XYZ(:,2),XYZ(:,3),'o')
axis equal
grid on
box on
xlabel 'X'
ylabel 'Y'
zlabel 'Z'
The result should be a sphere of radius 2.5, centered at the point [1 3 5].
You can use the tool I attached called spherefit.
[C,R] = spherefit(XYZ)
C = 1×3
0.9925 3.0094 5.0107
R = 2.5086
As you can see, the code recovered the original parameters well enough. Now, to plot the sphere...
fimplicit3(@(x,y,z) (x - C(1)).^2 + (y - C(2)).^2 + (z - C(3)).^2 - R.^2)
hold on
plot3(XYZ(:,1),XYZ(:,2),XYZ(:,3),'ro')
axis equal
grid on
box on
xlabel 'X'
ylabel 'Y'
zlabel 'Z'
hold off
I could have built a surface directly, perhaps using meshgrid. But fimplicit3 is just too easy.
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John D'Errico il 4 Feb 2023
That is a completely different question, significantly different from what you asked and I answered. You will first need to use image processing tools, identifying the pixels on that perimeter. But then you still cannot easily convert a picture of the outline of a sphere into a sphere, since the picture lacks depth information. Anyway, I don't do image processing.
Sterne_17 il 4 Feb 2023
Modificato: Sterne_17 il 4 Feb 2023
I already know the coordinates (x,y,z) of several scatter points on the sphere in space, and I think I just need to bring their coordinates into the code you showed. I was wondering how to bring in the coordinates of these points? @John D'Errico
XYZ = randn(50,3); XYZ = XYZ./sqrt(sum(XYZ.^2,2));
XYZ = 2.5*XYZ + [1 3 5] + randn(size(XYZ))/20;
plot3(XYZ(:,1),XYZ(:,2),XYZ(:,3),'o')
axis equal
grid on
box on
xlabel 'X'
ylabel 'Y'
zlabel 'Z'

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