Resetting only one rotation axis

Question

I would like to find a solution for taking a rotation represented as a matrix and then resetting one of it's components. Basically I want to be able to multiply a vector by this matrix and get a direction that is rotation around x and z axis and be constant along the y axis (up). I want to take object rotation and get the vector that represents gravity but in object local space and disregarding the yaw. So I want to reset the yaw.

I don't want to convert this to euler angles. I would prefer using a quaternion or doing some sequence of operations on the rotation matrix directly in order to avoid possible bugs with certain angles.

Ok, so I have the follwoing:

btTransform t; 
mBody->getMotionState()->getWorldTransform(t); 
btMatrix3x3 trans = t.getBasis().inverse(); 
btVector3 up = (trans * btVector3(0, 1, 0));

I realized that if I used quaternion then I got completely wrong results (why?). Now I'm getting a vector in object space that represents up vector in world space. BUT I want to rotate this vector so that it represents global up vector in object space WHEN MODEL HAS ZERO rotation around Y axis. So I have to somehow rotate this vector back. How?

Community · Accepted Answer · 2017-05-23 12:05:35Z

1

You can use quaternion swing twist decomposition with passed "Y" axis. It will decompose quaternion to rotation around Y axis and rotation around axis that is perpendicular to Y.

It is described here, in my answer. Component of a quaternion rotation around an axis

edited May 23, 2017 at 12:05

CommunityBot

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answered Dec 3, 2014 at 16:39

minorlogic

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