# Convert quaternion to a different coordinate system

OSVR has a right-handed system: x is right, y is up, z is near.

I need to convert orientation data from a sensor in OSVR to a different right-handed system in which x is forward, y is left, and z is up.

I need to calculate the transformation that will convert a quaternion from one system to the other.

I naively tried:

void osvrPoseCallback(const geometry_msgs::PoseStampedConstPtr &msg) { // osvr to ros
geometry_msgs::PoseStamped outputMsg;

outputMsg.pose.orientation.x = msg->pose.orientation.y;
outputMsg.pose.orientation.y = msg->pose.orientation.z;
outputMsg.pose.orientation.z = msg->pose.orientation.x;
outputMsg.pose.orientation.w = msg->pose.orientation.w;

osvrPosePub.publish(outputMsg);
ros::spinOnce();
}


But that made things really weird. Facing north pitch is pitch, facing west, pitch is yaw, facing south, pitch is negative pitch...

How can I convert my OSVR quaternion to the a corresponding quaternion in the new coordinate system?

• Shoot, you're right! I got thrown off. Yes, both systems are right handed. – Nate Gardner Apr 28 '17 at 21:18

Summarizing your two coordinate spaces:

direction   OSVR    Output
------------------------------
right         x      -y
up            y       z
backward      z      -x


And since they're both right-handed, the sign of the angles does not change.

So to map from OSVR to the output coordinate system, we just map the letters as above:

outputMsg.pose.orientation.x = -1 * msg->pose.orientation.z;
outputMsg.pose.orientation.y = -1 * msg->pose.orientation.x;
outputMsg.pose.orientation.z = msg->pose.orientation.y;
outputMsg.pose.orientation.w = msg->pose.orientation.w;


Or, to go the other way and convert into OSVR's system, we flip it:

outputMsg.pose.orientation.x = -1 * msg->pose.orientation.y;
outputMsg.pose.orientation.y = msg->pose.orientation.z;
outputMsg.pose.orientation.z = -1 * msg->pose.orientation.x;
outputMsg.pose.orientation.w = msg->pose.orientation.w;

• I did the math too and this answer seems correct to me. – sam hocevar Sep 1 '17 at 23:01