# Quaternion rotation problem

Hi,

I am trying to determine Eular angles [roll, yaw, pitch] from Quaternions [w, x, y, z] which are being streamed to me from a PS3 Move controller. If I don't rotate the controller and just move it then the eular angles I derive from the quaternions match the controller position. e.g If the quaternions I get back are [w = 0.89, x = 0.37, y = 0.23, z = -0.09] the Eular angles I derive are [roll = 0, yaw = 28, pitch = 46], which is what I expect. Now if I dont move the controller just rotate it around it's own axis then the Eular's I derive from the quaternions suggest that my pitch and yaw have changed. e.g. If I dont move the controller from the earlier mentioned position and just rotate it 35 degress then the quaternions I get back are [w = 0.83, x = 0.29, y = 0.33, z = -0.34] which map out to Eular angles [roll = -20, yaw = 50, pitch = 50]. The answer I am looking for is [roll = 35, yaw = 28, pitch = 46].

The formulas I have tried for conversion from Quaternion to Eular are

yaw = atan2(2*qy*qw-2*qx*qz , 1 - 2*qy2 - 2*qz2)
roll = asin(2*qx*qy + 2*qz*qw)
pitch = atan2(2*qx*qw-2*qy*qz , 1 - 2*qx2 - 2*qz2)

and the one's mentioned on wikipedia
http://en.wikipedia.org/wiki/Conversion_between_quaternions_and_Euler_angles

but I have not had any luck.

Any ideas, suggestions or formula's will be highly appreciated.

Thanks
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Software DeveloperCommented:
I don't think there's anything wrong with the formula.  My guess is that the values are returned in the order, x-y-z-w rather than w-x-y-z as you have described them.
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Software DeveloperCommented:
Notice that what you have as the z-value changes most as you change the angle of the controller.
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Author Commented:
I think the values are fine because the yaw pitch numbers come back correct as long as I dont rotate the controller. I found a solution that works for me at http://noelhughes.net/uploads/quat_2_euler_paper_ver3.pdf. The guy takes the quternion and applies a transformation to a unit vector. The dot product between the transformed vector and the original give me angles I need.

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Software DeveloperCommented:
The yaw and pitch might be OK because the middle two of the values in x-y-z-w and w-x-y-z would be used correctly by the algorithm.  They may end up in the wrong axis, but that depends on the coordinate system in use.  Still, you have an answer that works, problem solved.
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Author Commented:
The link works for me, but might not work for everyone. It was not easy to find the solution to this problem.
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