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Determining if points lie within a sphere

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Vinit Nagda
Vinit Nagda on 29 Sep 2021
Edited: Matt J on 29 Sep 2021
I have a set of points P(x,y,z) and a sphere defined by its centre (x0,y0,z0) and radius r. I want to find the points that lie within or on the edge of sphere. I can run a loop for all points and check the condition:
(x-x0)^2+(y-y0)^2+(z-z0)^2<=r^2
But, Is there a direct function or any cheap method that would directly give me the indices of all points that lie within the sphere? Something I can easily use when there are large number of points and different sphere radius to test?
Thank you.

Accepted Answer

Matt J
Matt J on 29 Sep 2021
Edited: Matt J on 29 Sep 2021
Do you really need the indices? You can readily obtain a logical mask of the points in the sphere with,
mask = (x-x0).^2+(y-y0).^2+(z-z0).^2<=r^2;
If you really do need the indices, then,
find(mask)
  3 Comments
Matt J
Matt J on 29 Sep 2021
Yes, I need the indices. Each point carries some weight. I have to evaluate the mean of the weights of points that lie within the sphere.
Incidentally, that doesn't explain why you need the indices. You could instead just do,
mean(weightVector(mask))

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