Hi,

I have no experience in using quaternions in blender but one thing I do notice is that one can manipulate all the parameters (WXYZ) of the rotation quaternion. As far a I remember, if you want a quaternion to rotate some object, the quaternion needs to be normalized. If you mess with the rotation quaternion parameters you could end up with a quaternion that is not normalized. In fact in most cases you would need to renormalize it. Which is done automatically by blender when switching between Euler and Quaternion rotation type.

No back to your problem. I can confirm the behavior you described in a recent SVN build. And what is even more dramatic is this:

A quaternion rotation around 45 degrees is actually displayed as a rotation around 90 degrees. In fact a quaternion rotation around an arbitrary axis is applied with a double of the angle.

Here's how you can try it:

1) Open Blender with default cube scene.
2) In the object properties change the rotation type from "XYZ Euler" to "Quaternion (WXYZ)".
3) Switch to orthograpic side view (Hit NUMPAD3 and then NUMPAD5).
4) Rotate around the x axis with the mouse (Hit RKEY + XKEY and then move your mouse).
5) Watch the angle in the lower left side of the 3D view window as you move your mouse. And notice that if you roughly rotate around angle "alpha", "alpha/2" is displayed. You can also hit CTRLKEY to use stepwise rotation.

If you want you can repeat the same in "XYZ Euler" rotation mode and see that the indicated rotation is correct. Meanwhile I try to find out what's wrong with the quaternion rotation display and from where the angle number comes. In other words, which part of the code is responsible for displaying this number and where is it calculated it. My first searches brought me to "source/blender/blenlib/intern/math_rotation.c" but we'll see.

One last note: Since objects and armatures share the same rotation code you can easily repeat the behavior with a single armature bode. It'll also rotate twice the magnitude of the angle you've entered.

In other words: Sebastian, what you've found is a general error with quaternion calculation. Since a 45 degree rotation in quaternion mode currently is displayed as a 90 degree rotation, it looks like your cube's rotation is reset to zero rotation. But in fact it has rotated!!!

I hope this helps!

-Konrad-

Sorry, I've made a few typos but here is an essential one. It had to be "2*alpha" instead of "alpha/2":

5) Watch the angle in the lower left side of the 3D view window as you move your mouse. And notice that if you roughly rotate around angle "alpha", "2*alpha" is displayed. You can also hit CTRLKEY to use stepwise rotation.

I have created a patch that fixes this issue (http://projects.blender.org/tracker/index.php?func=detail&aid=25701&group_id=9&atid=127). There was a little typo error within the code (just one line).

I've committed the patch to SVN and from revision 34390 on your bug should be fixed.

Your reported issue has been fixed in SVN. Thanks for taking the
time to report!.

Attached patch number to provide more info on the fix.

hi Konrad,
Thx for the fast patch.
everything seems to work fine now.

Cheers

Sebastian