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Re: Re: Solve & RotationMatrix

There MUST be a built-in solver I'm missing, but here's a version 6.0  
solver, anyway. For version 5, you'd have to replace RandomReal with  
RandomArray or a Table of Random[] entries. I'm not sure if there are  
other issues or not; I've moved on!

randomRotationMatrix creates problems, and angleVector solves them.

angleVector uses Eigenvectors to find the axis of rotation, NullSpace to
find two perpendiculars to it, uses FindRoot to find the angle that allows  
m[t].perp == target.m for both perpendiculars, and uses Mod to scale the  
answer to the range -Pi to Pi. The code assumes the matrix is real, has a  
real eigenvector, and IS a rotation matrix. If not, FindRoot will fail  

I haven't allowed for difficulties due to imprecision in the input, which 
could also cause FindRoot to fail. To take care of that, you could replace  
FindRoot with FindMinimum applied to a sum of squared errors.

Of course, the axis is unique only up to a sign change.

(* code *)
Clear[realQ, realSelect, error, randomRotationMatrix, angleVector]
realQ[z_?NumericQ] := Re[z] == z
realQ[v_?VectorQ] := VectorQ[v, realQ]
realQ[v_?MatrixQ] := MatrixQ[v, realQ]
realSelect[m_?MatrixQ] := First@Select[m, realQ]
error[target_?MatrixQ, perp_?VectorQ, m_?MatrixQ] := #.# &[
   target.perp - m.perp]
error[target_?MatrixQ, perps_?MatrixQ, m_?MatrixQ] :=
  Chop@Total[error[target, #, m] & /@ perps]
randomRotationMatrix[] := Module[{t, v}, t = RandomReal[{-Pi, Pi}];
   v = Normalize@Table[RandomReal[], {i, 3}];
   {"angle" -> t, "axis" -> v,
    "rotation matrix" -> RotationMatrix[t, v]}]
angleVector[target_?MatrixQ] /; realQ[target] :=
  Module[{v, t, m, perps, root},
   v = realSelect@Eigenvectors[target];
   m = RotationMatrix[t, v];
   perps = Normalize /@ NullSpace[{v}];
   root = FindRoot[error[target, perps, m] == 0, {t, 0}];
   {"angle" -> Mod[t /. root, 2 \[Pi], -\[Pi]], "axis" -> v}

(* create a test problem *)
randomRotationMatrix[] // TableForm
target = "rotation matrix" /. %;

rotation  =


(* solve for the angle and vector *)



On Mon, 14 May 2007 04:49:33 -0500, Mathieu G <ellocomateo at> wrote:

> CKWong.P at a =E9crit :
>> Obviously, you need to provide the function RotationMatrix[ angle,
>> axisVector ] for the algorithm to work.
>> On the other hand, matrices for rotations about the Cartesian axes,
>> i.e., RotX,RotY, & RotZ, can be written done directly.  Why not do so?
> Hi,
> Thank you for your reply.
> I am not interested in a simple rotation matrix around an axis, but in
> finding the angles corresponding to a 3D rotation matrix.
> I compute VectN which is the vector normal to my surface, and I am
> interested in getting the rotation parameters that bring VectY to VectN.
> Regards,
> Mathieu


DrMajorBob at

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