A quaternion is defined as the quotient of two directed lines in a
three-dimensional space or equivalently as the quotient of two Euclidean
vectors (https://en.wikipedia.org/wiki/Quaternion).
Quaternions are often used in calculations involving three-dimensional
rotations (https://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation),
as they provide greater mathematical robustness by avoiding the gimbal lock
problems that can be encountered when using Euler angles
(https://en.wikipedia.org/wiki/Gimbal_lock).
Quaternions are generally represented in this form:
w + xi + yj + zk
where x, y, z, and w are real numbers, and i, j, and k are three imaginary
numbers.
Our naming choice (x, y, z, w) comes from the desire to avoid confusion for
those interested in the geometric properties of the quaternion in the 3D
Cartesian space. Other texts often use alternative names or subscripts, such
as (a, b, c, d), (1, i, j, k), or (0, 1, 2, 3), which are perhaps
better suited for mathematical interpretations.
To avoid any confusion, as well as to maintain compatibility with a large
number of software libraries, the quaternions represented using the protocol
buffer below must follow the Hamilton convention, which defines ij = k
(i.e. a right-handed algebra), and therefore:
i^2 = j^2 = k^2 = ijk = −1
ij = −ji = k
jk = −kj = i
ki = −ik = j
Please DO NOT use this to represent quaternions that follow the JPL
convention, or any of the other quaternion flavors out there.
Definitions:
Unit (or normalized) quaternion: a quaternion whose norm is 1.
Pure quaternion: a quaternion whose scalar component (w) is 0.
Rotation quaternion: a unit quaternion used to represent rotation.
Orientation quaternion: a unit quaternion used to represent orientation.
A quaternion can be normalized by dividing it by its norm. The resulting
quaternion maintains the same direction, but has a norm of 1, i.e. it moves
on the unit sphere. This is generally necessary for rotation and orientation
quaternions, to avoid rounding errors:
https://en.wikipedia.org/wiki/Rotation_formalisms_in_three_dimensions
Note that (x, y, z, w) and (-x, -y, -z, -w) represent the same rotation,
but normalization would be even more useful, e.g. for comparison purposes, if
it would produce a unique representation. It is thus recommended that w be
kept positive, which can be achieved by changing all the signs when w is
negative.
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