[ITK-users] How can I convert a versor into the three rotation angles?

[Marina Bendersky]

 New to image registration and ITK, I’m following the ITK software guide
and trying to understand the different outputs. I’m focusing on the example
ImageRegistration8.cxx: a rigid transform in 3D, and the example is
detailed in the Software guide Book2, section 3.6.4.

The manual explains that the versor (page 211)

• Versor=(−6.03744e−05,5.91487e−06,−0.0871932)

is equivalent to a rotation of 9.98 degrees around the Z axis.

I understand a versor is a unit quarterion, and have been trying to convert
this versor to the ~10 degrees angle (in the z direction) using the
equations in wikipedia. I read the slides on Quarterions found at
http://www.itk.org/CourseWare/Training/QuaternionsI.pdf, among other
sources, but I still cannot resolve my question.

I appreciate any help and advice.

Thank you!
Marina
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