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[Václav Šmilauer <eu@xxxxxxxx>]

> If I'm the author of this comment in the sources (I am, probably), 
> "close to identity" means close to null rotation. In other words the 
> corresponding rotation matrix is very close to identity matrix.
> So, let M be a rotation matrix defining a rotation of 10^-8 degrees. 
> Theoretically, M*M^(-1)=Id.
> Now, do the same numerically with quaternions : let q be the quaternion 
> representing the same rotation as matrix M, and compute q*q.conjugate(). 
> It seems, sometimes in Eigen, the result will be NaN.
> As a workaround, I set angle=0 when angle=NaN.

Bruno, could you post that issue to eigen's forums
http://eigen.tuxfamily.org/index.php?title=Main_Page#Get_support? They
are very responsive as far as my experience goes and will give us proper
mathematical explanation about the cause.

v