# Parametrization of O(3)

Is there a simple way to parametrize the orthogonal group O(3) of 3 by 3 orthogonal matrices?

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The question needs more detail. What does it mean to "parametrize" a matrix group like this? (And what is the motivation?) – Jim Humphreys Nov 18 '10 at 15:05
Though this appeared in Neil Strickland's answer below, I mention for emphasis that the most useful answer is the quaternion parametrization of SO(3), a.k.a. Euler-Rodrigues formula: en.wikipedia.org/wiki/Euler%E2%80%93Rodrigues_formula – Jairo Bochi May 2 at 7:46

The general element is $\pm\exp(A)$ where $A$ is skew-symmetric. (This gives each element infinitely often). This trick essentially works for all compact Lie groups.

There is also the Cayley parameterization: $(I+A)(I-A)^{-1}$ for skew-symmetric $A$ is the general element of $SO(3)$ which lacks an eigenvalue $-1$ (so isn't a half-turn.) This parameterizes all such matrices once each.

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First, if $A\in O(3)$ then either $A$ or $-A$ is in $SO(3)$ so you can just restrict attention to $SO(3)$. For that, one answer is the Cayley transform: let $so(3)$ denote the set of $3\times 3$ antisymmetric matrices (which is homeomorphic to $\mathbb{R}^3$) and put

$U=\{R\in SO(3) : \det(R+I)\neq 0\}$

Then $U$ is a dense open subset of $SO(3)$ and there is a homeomorphism $f:so(3)\to U$ given by $f(A)=(I+A)(I-A)^{-1}$.

Another answer is to use the double cover $g:S^3\to SO(3)$ given by the conjugation action unit quaternions on purely imaginary quaternions. There is an explicit formula here: http://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation#Conversion_to_and_from_the_matrix_representation

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+1 for mentioning the quaternion connection, which I believe is used frequently in computer vision, and computer graphics. – Suvrit Nov 18 '10 at 15:45
I'd also like to support Suvrit's view that quaternions are a good way to parameterize rotations. Very clean and simple to implement in software. – Deane Yang Nov 18 '10 at 19:19
BTW, an interesting (though not very well written) Wikipedia article on the quaternion parametrization is this one: en.wikipedia.org/wiki/Euler%E2%80%93Rodrigues_formula – Jairo Bochi May 2 at 7:48

I assume that you are interested in the case of matrices with real entries. Perhaps one should reduce the problem to $SO(3)$, since $O(3)$ has two connected components, or we may use the determinant $\pm1$ as one parameter. A natural parametrization is unlikely, or will not be injective, because $SO(3)$ has a non-trivial topology: its fundamental group is $\mathbb Z/2\mathbb Z$. One possibility is to give a direction $\vec u$ (a unitary vector) of the axis of rotation, and the angle $\theta\in\mathbb R/2\pi\mathbb Z$ of this rotation. But then$(-\vec u,\theta+\pi)\sim(\vec u,\theta)$, and all pairs $(\vec u,0)$ yield the same element $I_3$.

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This is a way to construct a diffeomorphism with $\mathbf R P^3$. – Claudio Gorodski Jun 3 '11 at 12:59

If there were a really simple way we wouldn't need the concept of "gimbal lock" (http://en.wikipedia.org/wiki/Gimbal_lock). In other words the manifold in question is compact but isn't the 3-torus, and pretending it is has to break down somewhere. The unit quaternions are the "slickest" surrogate, but "parametrize" implies charts and global charts aren't going to work out "simply".

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