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quaternions with python

asked 2012-02-06 00:43:04 -0500

joan gravatar image

I have recently started using rospy after being working with roscpp. I wanted to reproduce this piece of C++ code:

geometry_msgs::Quaternion quat = tf::createQuaternionMsgFromYaw(th);
geometry_msgs::PoseStamped goal;
goal.pose.orientation = quat;

For what I have found, the closest form would be something like:

quat = quaternion_to_msg(tf.transformations.quaternion_from_euler(0, 0, th))
goal = PoseStamped()
goal.pose.orientation = quat

But this actually does not work because 'quaternion_from_euler' returns a four float array, instead of an object with {x, y, z, w} fields.

I have searched for some common function to do this easy transform, but I have not found it within the ros libraries. What I have seen is that many people make his own implementation:

def quaternion_to_msg(q):
  msg = Quaternion()
  msg.x = q[0]
  msg.y = q[1]
  msg.z = q[2]
  msg.w = q[3]
  return msg

goal.pose.orientation = quaternion_to_msg(quat)

Does ROS provide a common solution for this situation?

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2 Answers

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answered 2012-02-06 02:15:24 -0500

Python has a nifty feature which allows you to expand a tuple into an argument list. Rather than setting all of the quaternion's fields individually, you can use the constructor as follows

q = tf.transformations.quaternion_from_euler(0, 0, th)
msg = Quaternion(*q)

This is equivalent to doing

q = tf.transformations.quaternion_from_euler(0, 0, th)
msg = Quaternion(q[0], q[1], q[2], q[3])
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Where can one find documentation on these mysterious functions ? is quite... primitive.

victorp gravatar image victorp  ( 2013-03-18 22:52:47 -0500 )edit

This is a python feature, not ROS. As such, it is documented in the python docs:

Dan Lazewatsky gravatar image Dan Lazewatsky  ( 2013-03-19 04:50:46 -0500 )edit

Thanks, but I mean about the tf python api.

victorp gravatar image victorp  ( 2013-03-19 05:26:10 -0500 )edit

What do you mean? The tf api docs look pretty complete to me.

Dan Lazewatsky gravatar image Dan Lazewatsky  ( 2013-03-19 05:49:34 -0500 )edit

Is there documentation for the tf.transformation module?

bit-pirate gravatar image bit-pirate  ( 2014-01-05 03:23:24 -0500 )edit

answered 2012-02-07 05:53:29 -0500

Wim gravatar image

updated 2012-02-07 05:54:08 -0500

I'd highly recommend taking a look at PyKDL, which is part of the KDL library here or here. It has well tested support for working with quaternions, Euler angles, axis-angle, etc.

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Asked: 2012-02-06 00:43:04 -0500

Seen: 10,838 times

Last updated: Feb 07 '12