

Euler AnglesAlthough our emphasis in this article is on quaternions, there are of course other ways to approach the subject of rotations
and we would like to briefly mention rotations via Euler angles. The standard addon package Geometry`Rotations` provides us with the function Rotate3D . More specifically, Rotate3D[ vec, phi, theta, psi, ( pt: {a, b, c})] rotates the vector vec by specified Euler angles about point pt. The rotation given by the Euler angles phi, theta, and psi can be decomposed into a sequence of three successive rotations:
This version of Euler angles is not unique and definitions vary.
For an interesting indepth article on rotation techniques in the context of commercial computer game software confer [10]. Bobick's paper provides numerous technical reasons why quaternions are a must in many threedimensional applications as
he compares and contrasts various techniques.
In our article we shall show a simple way to make an Eulertoquaternion conversion and produce some examples. First load
some useful standard addon packages.
Noting the definition of Euler angles used in the Rotations package, and using some of the theory of quaternions previously discussed, we can construct a single quaternion that is equivalent
to a given Euler angles rotation. The negative signs make the rotation choices (CW, CC) about the axes consistent with the
choices made in the addon package. The function etoq[ phi, theta, psi ] creates the desired quaternion.
The first three basic equivalences are obviously correct, while the fourth is more complicated.
Next we can use the quaternion created by etoq to carry out a rotation of vector vec about point pt that is equivalent to the Euler rotation. The function is etoqrot[ vec, phi, theta, psi, pt] .
Now compare etoqrot and Rotate3D by rotating the vector (1, 1, 5) about the point (2, 1, 3) with Euler triple (, 10° , ). We see that the rotation, as applied to vecpt, involves an angle of about 60.7° around the axis 0.166486 +0.0446097 J0.985034 K and the final position (after translating) is (3.13382,0.499433,5.33798).
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