
Re: Please help me! Helical coordinates problem.
Helical angles represent the angleaxis parameters for the rotation. The axis of rotation (in global "origin" coordinates) is a unit vector. Each component of this unit vector is multiplied by the amount of rotation. The result is the three helical angles.
To get the rotation matrix from femur to pelvis, do the following steps:
 convert the Femur helical angles to the Femur rotation matrix Rfemur
 convert the Pelvis helical angles to the Pelvis rotation matrix Rpelvis
 the Femur to Pelvis rotation matrix is now: inv(Rfemur)*Rpelvis
Watch out how the rotation matrices are defined. I always define Rfemur as the matrix that transforms local femur coordinates into global coordinates. This is the most common definition, but you sometimes see the opposite.
For the conversion from angleaxis into rotation matrix, see: http://www.euclideanspace.com/maths/...angleToMatrix/
This uses the 4 variables angle, x, y, z. From your 3 helical angles (theta), you can calculate these as follows:
angle = norm(theta) (or angle = sqrt(theta(1)^2 + theta(2)^2 + theta(3)^2)
x = theta(1)/angle
y = theta(2)/angle
z = theta(3)/angle
Ton van den Bogert
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