Rotation Matrix - in Three Dimensions - Conversion From and To Axis-angle - Determining The Axis

Determining The Axis

Given a rotation matrix R, a vector u parallel to the rotation axis must satisfy

since the rotation of around the rotation axis must result in . The equation above may be solved for which is unique up to a scalar factor.

Further, the equation may be rewritten

which shows that is the null space of . Viewed another way, is an eigenvector of R corresponding to the eigenvalue (every rotation matrix must have this eigenvalue).

Read more about this topic:  Rotation Matrix, In Three Dimensions, Conversion From and To Axis-angle

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