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# Re: Rotate3D bug solution
*To*: mathgroup@smc.vnet.net
*Subject*: [mg10382] Re: Rotate3D bug solution
*From*: sidles@u.washington.edu (John Sidles)
*Date*: Mon, 12 Jan 1998 04:10:12 -0500
*Organization*: University of Washington, Seattle
*References*: <199712250838.DAA15159@smc.vnet.net.> <34A499DD.5E16@physics.uwa.edu.au> <684v6o$q8r@smc.vnet.net> <68csvb$aa4@smc.vnet.net>
In article <68csvb$aa4@smc.vnet.net>,
Selwyn Hollis <shollis@peachnet.campus.mci.net> wrote:
>Mark Evans wrote:
>
>> Paul Abbott wrote:
>> >
>> > The use of Eulerian angles for specifying rotations in 3D IS
>> standard
>> > (at least in maths and physics -- especially in quantum mechanics,
>> > crystallography, and angular momemntum theory). Note that the
>> eulerian
>> > angle parametrization avoids the singularities that arise in other
>> > parametrizations.
>> >
>>
>> In the same sense, you could say that sea shells are standard legal
>> tender if you live in a certain part of the world.
>>
>> Paul is right that there is nothing technically wrong with this kind
>> of
>> rotation. My point was that Mathematica packages should be written
>> for
>> a wider audience. It seems intuitive that the most common
>> understanding of a rotation matrix is one that rotates sequentially
>> about each of the three coordinate axes. The fact that Mathematica
>> does not offer this rotation by default is a slip-up in my mind.
>
>Right on, Mark!
>
>I've never heard of Eulerian angles before encountering them in
>Mathematica. Maybe they're the usual tricks-of-the-trade to a few
>quantum physicists, but you'll be hard-pressed to find a reference to
>them in any but the most esoteric mathematics literature.
>
Well, the latitude and longitude coordinates on the globe are
(essentially) Eulerian coordinates -- so they're not *too* esoteric.
Here's a very important and useful, yet simple, theorem which anyone
working with rotations needs to know -- it explains why seemingly
inequivalent conventions are actually precisely equivalent.
Let $R(v)$ be a function which computes the three-by-three matrix
associated with a three-vector $v$, where the direction of $v$ gives
the axis of rotation, and the magnitude of $v$ gives the angle of
rotation. (Exercise: program this useful utility function in
Mathematica, together with its inverse function v(R) -- answer given at
end!).
Let $U$ be an arbitrary rotation matrix, with $U^{t}$ the matrix
transpose of $U$. Since $U$ is a rotation, $U^{t}$ is the matrix
inverse of $U$, i.e., $U^{t} U = I$.
Then for any $U$ and $v$, here's the key theorem!
U R(v) U^{t} = R(U v)
To see how this clarifies the literature on rotations, suppose Textbook
A defines Euler matrices in terms of rotation angles ${\theta, \phi,
\psi}$ about fixed unit axes ${\hat{n}_1,\hat{n}_2,\hat{n}_3}$ as
follows
R(\theta, \phi, \psi)
\edef R(\theta \hat{n}_1) R(\phi \hat{n}_2) R(\psi \hat{n}_3)
\edef R_1 R_2 R_3
Now we use the above theorem to rewrite this in terms of *moving* axes.
R(\theta,\phi,\psi) = R(\psi R1 R2 \hat{n_3})
R(\phi R1 \hat{n_2}) R(\theta \hat{n_1})
Cool! Its the same three angles, but now applied in the *opposite*
order, and about moving instead of fixed axes. Yet the final matrix
is the same. And it is perfectly reasonable for Textbook B to adopt
this moving-axis convention to define Euler angles.
Given all this ambiguity, I no longer use Euler angles when doing
calculations involving rotations. It is just too easy to confuse the
various signs and conventions!
A much safer strategy, which I recommend, is to simply code the
function R(v) and its inverse v(R) as utility routines in whatever
language you prefer. You will find that these two functions, plus
ordinary matrix multiplication, suffice for *any* calculation
involving rotations. No more Euler angle torture! No more sine and
cosine functions with obscure arguments!
Here's some Mathematica code for R(v):
rotationMatrix[x_] := Block[
{angle,xHat,pPar,pPerp,pOrthog},
angle = Sqrt[x.x]//N;
If[angle<10^-6,Return[id]];
xHat = x/angle;
pPar = Outer[Times,xHat,xHat];
pPerp =DiagonalMatrix[{1.0,1.0,1.0}] - pPar;
pOrthog = {
{0.0,xHat[[3]],-xHat[[2]]},
{-xHat[[3]],0.0,xHat[[1]]},
{xHat[[2]],-xHat[[1]],0.0}
};
pPar + Cos[angle]* pPerp + Sin[angle]*pOrthog
]
It is left as an exercise to (a) figure out how the above works, and (b)
code the inverse function! Coding the inverse function is quite a
nontrivial exercise. Hint: (a) determine the cosine() from the trace
of R, then determine the sine() and the the axis of rotation from the
antisymmetric part of R.
This works for all rotations *except* for angles near Pi, for which the
axis of rotation should be set to the (unique) eigenvector of the
symmetric part of R that has unit eigenvalue (this is because sin(Pi)
= 0).
A final refinement: the above algorithm assumes that R is orthogonal.
But what if R is just *close* to orthogonal, but has some accumulated
numerical imprecision? Even worse, you can bet that some future user
of your v(R) routine will hand it an R matrix that is grossly
non-orthonormal! The right thing to do, therefore, is to condition R
before calculating v, by (a) calculating the singular value
decomposition for R, then (b) adjusting all the singular values to
have unit magnitude. This will yield a cleaned-up exactly orthogonal
R which (formally) is the orthogonal matrix that is closest to the
input matrix in the least-mean squares sense. Better issue an error
message if the singular values are far from unity -- because this
indicates abuse of the inverse routine!
The Mathematica routines SingularValue[] and Eigensystem[] are well
suited to the above tasks -- it's tougher in "C".
I wrote this up at length because I have a student who is generating
animations in Mathematica and POV-Ray -- he might as well learn to do
things the easy way!
Happy rotating ... JAS
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