MATLAB: Strange behaviour of vrrotvec. What is [0 0 0] axis

Simulink 3D Animationvrrotvec rotation quaternion

I have such an example:

v0 = [0 0 -1]; v1 = [0 0 1];

q = vrrotvec(v0, v1);

So, as I understand, v1 is v0 rotated by 180 degrees in x-axis. But I get q = [0 0 0 pi]

Why?

Best Answer

Actually the rotation could be 180 degrees about any axis that is in the xy-plane, like the y-axis. So the x-axis is not the only correct answer.

To me it seems that the function is not working well. In this case there are infinite choices of correct axis, so the function should return at least one correct axis, for example the y-axis or the x-axis.

However, the axis matters if you want to do this rotation for a 3D body. For a 3D-body the result is different if you rotate 180 degrees about x-axis, or 180 degrees about y-axis. But from those two vectors, it is not possible to determine, whether you want a rotation about x-axis, or y-axis or some other axis in xy-plane.

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MATLAB indexing starts at 1.

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MATLAB: Does quatrotate() produce negative rotations

Hello Dereck, there is a difference between a) rotating a reference frame (e.g. relative to a "fixed" vector) and b) rotating a vector relative to a reference frame. Check out this answer for reference.

angle2quat converts rotation angles [to quaternions]. Rotation angles in turn are used to rotate reference frames a), not vectors b). To illustrate, if you do not convert to quaternions but keep working with rotation angles and matrices you could do the same by computing the direction cosine matrix, DCM:

 DCM = angle2dcm(yaw,pitch,roll)
 DCM =
   0.707106781186548   0.707106781186547                   0
  -0.707106781186547   0.707106781186548                   0
                   0                   0   1.000000000000000
 rotate_Frame = DCM*A'
 ans =
   0.707106781186548
  -0.707106781186547
                   0

which, as pointed out above, rotates the reference frame relative to the vector.

If you need to rotate the vector instead, use rotx and equivalent:

 rotate_Vector = rotz(yaw*180/pi)*roty(pitch*180/pi)*rotx(roll*180/pi)*A'
 ans =
   0.707106781186547
   0.707106781186547
                   0

Best Answer

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