# How can obtain the relative orientation between two quaternions?

How do I obtain the relative orientation given two orientations (represented by quaternions q0 and q1)?

• not sure what you are asking here, since you are not telling how did you calculate the quaternion in the first place. but my answer here might be related gamedev.stackexchange.com/questions/67199/… and btw you are mixing euler angles with quaternions which is sth I also addressed in the answer. – concept3d Jan 1 '14 at 23:09
• which API are you using? it is supposed to be stated in the documentation of how do you interpret orientation. – concept3d Jan 1 '14 at 23:37
• from what I understand you can't extract euler angles in the traditional sense. Try converting the quaternion to a matrix and there is a way of analyzing that matrix to extract the euler angles which may not work. But TBH I feel you are asking the wrong question. why do you want to extract pitch in the first place ? – concept3d Jan 1 '14 at 23:48

Quaternion is another representation of axis angle. The solution is to create a new quaternion from the original quaternion that only has the needed components.

An axis angle representation can be converted to a quaternion using the following formula

q = cos(R/2);
q = sin(R/2)*x;
q = sin(R/2)*y;
q = sin(R/2)*z;


Where R is the angle in radians, and (x,y,z) represents the axis, and quaternion is (R,x,y,z).

So in order to create a new quaternion with only the pitch component you just zero out the other components and normalize the quaternion:

Quaternion q; // this is your original quaternion
q.x = 0.0;
q.z = 0.0;
q.Normalize();


Sensors AFAIK calculate orientation relative to gravity, in other words Y (or Z) is the direction of the gravity. You need take that into consideration. And I think you don't need to multiply it with the inverse initial orientation.

• This works great if I know that the motion is pitch. In general, that is unknown. – ilp Jan 2 '14 at 0:32
• @ilp well, what you can do, is extract the component with the highest contribution to the rotation and use that. So instead of taking a fixed axis. Use the one with the highest contribution and zero out the others. – concept3d Jan 2 '14 at 0:35
• @ilp at this point I can't answer your question, because I don't know the neccessary details :-) maybe you should edit the question and give it a context so we are able to help. – concept3d Jan 2 '14 at 0:43
• @ilp edited the answer. – concept3d Jan 2 '14 at 1:12

The relative orientation is obtained simply by division:

q = q0 / q1


Or, if division is not available:

q = q0 * inverse(q1)


Note that since the quaternions used to represent rotations are unit quaternions, the inverse of q1 is simply its conjugate q1*, and is obtained by flipping the sign of x, y, z but not w.

• if you checked the edits, he is already doing that q = q0 * inverse(q1), so am not particularly sure what he is trying to do. – concept3d Jan 2 '14 at 10:54