# How to calculate a symmetrical vector

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Hi Im doing this project now and I ran into some problem with the maths. What I have is two vertices in 3D. Given a normal of one of the vertices how do I calculate a normal that is symmetrical to it with respect to the vector between the two vertices? What I would like is for the normals to align themselves like so: \_________/ <-normals, i got the first one, how to calculate the second? o-------->o <-vertices and vector between them or like so: |_________| o-------->o or like so: -o------->o- I hope I make sense :-) Thanks for any help Radek

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I'm not sure I understand the question. Could the second normal be nothing more than a reflection across an axis? Sorry I can't help more.

\ / Reflection across "y" axis (2D)
Normal 1 = < x, y >
Normal 2 = < -x, y >

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I think it would be like the reflection of the normal across the plane of which the line between the vertices is the normal. And the plane is halfway between the two vertices. I hope that's clear - I'm going to bed now! Nearly midnight!

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I think this is similar to DrGUI's answer, but anyway, given two points p0 and p1, and a normal n0, you should be able to get n1 as you describe like this:

Vector3 n = Normalize(p0-p1);
n1 = n0-2*n0.Dot(n)*n;

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