Chris,

In animation, the problems with Euler angles mainly arise when doing

motion editing. As long as you leave them alone they are fine.

Motion editing could be: resampling (interpolation), amplification,

motion blending etc. If you perform these operations on Euler angles

directly, you can get strange results, especially near gimbal lock.

The quaternion representation seems to behave better.

It seems that quaternions are the same as the "euler parameters" which

are often used in computational kinematics:

http://www.cs.berkeley.edu/~laura/cs184/quat/quaternion.html

I have also heard the term "angle-axis representation", i.e. the rotation

is represented as a rotation of magnitude A about an axis (Ux,Uy,Uz).

Euler parameters are defined as follows

e0 = cos(A/2)

e1 = Ux*sin(A/2)

e2 = Uy*sin(A/2)

e3 = Uz*sin(A/2)

The sum of squares of these parameters is exactly one.

See also "Computer Aided Kinematics and Dynamics of Mechanical Systems",

by E.J. Haug.

Note that this representation is closely related to the three "helical

angles" proposed by Herman Woltring. The helical angle representation is:

h1 = A*Ux

h2 = A*Uy

h3 = A*Uz

Why quaternions are not used more in biomechanics? This probably has

something to do with interpretation. Euler angles can be associated

with the rotations in a mechanical linkage or 3-D goniometer (Grood

and Suntay, J Biomech Eng, 1983). The other representations work well

for computation but are not so easily interpreted.

On the other hand, Woltring makes some good points on error propagation

in his 1994 paper (J Biomech 27:1399-1414). Near gimbal lock, Euler angles

become increasingly sensitive to measuring errors.

For the newcomers on Biomch-L, I also recommend reading the debate between

Grood and Woltring, about 10 years ago on Biomch-L:

http://isb.ri.ccf.org/biomch-l/files/angles3d.topic

Ton van den Bogert

--

A.J. (Ton) van den Bogert, PhD

Department of Biomedical Engineering

Cleveland Clinic Foundation

9500 Euclid Avenue (ND-20)

Cleveland, OH 44195, USA

Phone/Fax: (216) 444-5566/9198

---------------------------------------------------------------

To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl

For information and archives: http://isb.ri.ccf.org/biomch-l

---------------------------------------------------------------

In animation, the problems with Euler angles mainly arise when doing

motion editing. As long as you leave them alone they are fine.

Motion editing could be: resampling (interpolation), amplification,

motion blending etc. If you perform these operations on Euler angles

directly, you can get strange results, especially near gimbal lock.

The quaternion representation seems to behave better.

It seems that quaternions are the same as the "euler parameters" which

are often used in computational kinematics:

http://www.cs.berkeley.edu/~laura/cs184/quat/quaternion.html

I have also heard the term "angle-axis representation", i.e. the rotation

is represented as a rotation of magnitude A about an axis (Ux,Uy,Uz).

Euler parameters are defined as follows

e0 = cos(A/2)

e1 = Ux*sin(A/2)

e2 = Uy*sin(A/2)

e3 = Uz*sin(A/2)

The sum of squares of these parameters is exactly one.

See also "Computer Aided Kinematics and Dynamics of Mechanical Systems",

by E.J. Haug.

Note that this representation is closely related to the three "helical

angles" proposed by Herman Woltring. The helical angle representation is:

h1 = A*Ux

h2 = A*Uy

h3 = A*Uz

Why quaternions are not used more in biomechanics? This probably has

something to do with interpretation. Euler angles can be associated

with the rotations in a mechanical linkage or 3-D goniometer (Grood

and Suntay, J Biomech Eng, 1983). The other representations work well

for computation but are not so easily interpreted.

On the other hand, Woltring makes some good points on error propagation

in his 1994 paper (J Biomech 27:1399-1414). Near gimbal lock, Euler angles

become increasingly sensitive to measuring errors.

For the newcomers on Biomch-L, I also recommend reading the debate between

Grood and Woltring, about 10 years ago on Biomch-L:

http://isb.ri.ccf.org/biomch-l/files/angles3d.topic

Ton van den Bogert

--

A.J. (Ton) van den Bogert, PhD

Department of Biomedical Engineering

Cleveland Clinic Foundation

9500 Euclid Avenue (ND-20)

Cleveland, OH 44195, USA

Phone/Fax: (216) 444-5566/9198

---------------------------------------------------------------

To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl

For information and archives: http://isb.ri.ccf.org/biomch-l

---------------------------------------------------------------