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Rigid body Constraint: Generic are crazy for rotation along Y axis (Bullet issue?)
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Description

Windows 7 64

Blender 2.71 9337574

Every time when I use Generic Constraint and Object can rotate along Y axis, animation are crazy. Only Y axis is broken.

  1. Open file.
  2. Press Alt + A and see what will happen with green cube.

Not only I have this problem.

Details

Type
Bug

Event Timeline

Piotr Załóg (piotr) raised the priority of this task from to Needs Triage by Developer.
Piotr Załóg (piotr) updated the task description. (Show Details)
Piotr Załóg (piotr) set Type to Bug.
Bastien Montagne (mont29) renamed this task from Rigid body Constraint: Generic are crazy to Rigid body Constraint: Generic are crazy for rotation along Y axis (Bullet issue?).Sep 15 2014, 12:43 PM
Bastien Montagne (mont29) lowered the priority of this task from Needs Triage by Developer to Normal.

Confirmed.
From a quick print session, issue seems to happen in Bullet (at least, values gotten from RB_body_get_orientation() are first stable, with rotation along Y axis, and it soon starts to diverge to random axes...).

Sergej, you might know better! ;)

The most plausible explanation i could find on this issue is that the 6dof constraint ("generic" in Blender terms) is based on Euler angles, which are prone to gimbal lock type issues. The ultimate solution would have to happen on the Bullet side as a quaternion-based 6dof constraint, but in the meantime you can use a true hinge constraint (which is more stable anyway) or just rotate the cube and use the x or z axis instead of y. See this thread for discussion:

http://bulletphysics.org/Bullet/phpBB3/viewtopic.php?t=8153

We can't really fix this on our side, so i'm archiving this report.