c++ - Is point A nearby point B in 3D - distance check -

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I'm looking for a skilled algorithm to check whether I have a point in 3D or not. 

  Sqrt ((x2-x1) ^ 2 + (y2-y1) ^ 2 + (z2-z1) ^ 2) & lt; Radius

It does not look very fast and in fact I do not need such a big accuracy. How can I do this?

drop the distance to the square, and sqrt () , it Very fast: 

  ((x2-x1) ^ 2 + (y2-y1) ^ 2 + (z2-z1) ^ 2 & ray; radius * radius

Of course, in most cases at least radius * radius can be calculated prematurely and such as square radius is stored.


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