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vectors. rotating. 3d. blah.

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if I take 2 3d vectors, such as:
1   2
 \  |
  \ |
how do I calculate a third that would look like:
1   2   3
 \  |  /
  \ | /
it's basically a 180 degree rotation of vector 1 using vector 2 as the axis of rotation, and all 3 will be lined up, but I'm an idiot and can't get it to work in 3d space. It's for making stuff bounce off of a surface. Edited by - smart_idiot on November 9, 2001 9:58:12 AM

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Try this:

Let your vector 1 be V1 = (x1, y1, z1)
and let your vector 2 be V2 = (x2, y2, z2)
V2, defining your axis of rotation, should be a unit vector for this procedure

Calculate the dot product between V1 and V2

v1dotv2 = x1*x2 + y1*y2 + z1*z1

Then, let a new vector be the component of V1 that is normal to V2,

VN = V1 - V2*v1dotv2

Now just subtract VN twice from V1 to get the reflection V3:

V3 = V1 - 2*VN = V1 - 2*V1 + 2*V2*v1dotv2

Or, simplifying just a bit,

V3 = 2*V2*v1dotv2 - V1

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

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