[vtkusers] rotating vtkProp3D using quaternions

[Sergio Aguirre]

Hi everyone.

I am trying to rotate a vtkProp3D by using quaternions.

Basically I have a line (defined by vtkLineSource, mapper & actor) that when
its end point is inside the bounds of an object (vtkProp3D) I would like to
use to rotate + translate that object following the line.

What I am doing now is:

Using
double qs[4], qf[4], qt[4], styQS[3][3], styQF[3][3];

1 - Save the initial matrix4x4 (styPoseE) of the line's end point and
generate a start-quaternion "qs" a 3x3 matrix using vtkMath tools

for (i=0; i<=2; i++)
for (j=0; j<=2; j++)
styQS[j][i] = styPoseE->GetElement(j,i);
vtkMath::Matrix3x3ToQuaternion(styQS, qi);

2 - I sample the line's movement and save the end matrix4x4 of the line's
end point in an final-quaternion "qf"

for (i=0; i<=2; i++)
 for (j=0; j<=2; j++)
styQF[j][i] = styPoseE->GetElement(j,i);
vtkMath::Matrix3x3ToQuaternion(styQF, qf);

3 - I calculate the conjugate of qs (qs')

qs[1] = -qi[1];
qs[2] = -qi[2];
qs[3] = -qi[3];

4 - I calculate a quaternion "qt" that describes the rotation that describes
the difference between qs and qt by qt = qf * qs'

qt[0] = (qf[0]*qs[0] - qf[1]*qs[1] - qf[2]*qs[2] - qf[3]*qs[3]);
qt[1] = (qf[0]*qs[1] + qf[1]*qs[0] + qf[2]*qs[3] - qf[3]*qs[2]);
qt[2] = (qf[0]*qs[2] - qf[1]*qs[3] + qf[2]*qs[0] + qf[3]*qs[1]);
qt[3] = (qf[0]*qs[3] + qf[1]*qs[2] - qf[2]*qs[1] + qf[3]*qs[0]);

5 - Also, I change the first element of qt (W) to degrees

qt[0] = vtkMath::DegreesFromRadians(qt[0]);

6 - I apply the qt quaternion to the vtkProp3D object using ->RotateWXYZ();

picked_object->RotateWXYZ(qt[0], qt[1], qt[2], qt[3]);

However, the end result is not what I had expected. My questions are:

- Does qt = qf * qs' describe the differential rotation between qf and qs? I
what to apply only the differential so as to do a "soft pull" rotation of
the vtkProp3D object and not have a new orientation defined by the line.
- I had initially passed the vtkMatrix4x4 of the line (styPoseE) to the
object (vtkProp3D) either using pokeMatrix or SetUserMatrix and do control
the rotation of the object BUT only after its orientation is reset to that
of the line and I want to only move it following the line.
- Is there a different approach I should consider? This one just makes the
object jitter uncontrollably

Thank you for any suggestions.

Sergio
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