Raph,


> I am also fighting hard to grasp the concepts of quaternions.
> Could you explain how to get back the Pich, Roll and Yaw values from a
> quaternion?
> I understand I should come back to the axis-angle representation, but how to
> extract the roll value in radians for example?

Wikipedia is a good reference:
<http://en.wikipedia.org/wiki/Conversion_between_quaternions_and_Euler_angles>

You also don't have to implement these conversions yourself, there are
a number of great libraries around that can have well tested
implementations. The KDL library is a good example.
<http://www.orocos.org/kdl/geometry>

Hope this helps,

Wim




-- 
--
Wim Meeussen
Willow Garage Inc.
<http://www.willowgarage.com)