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I always find topic_tools/transform very useful, but somewhat difficult to use. I always helps me to write the expressions in a text editor, so that you can experiment and jump around a little bit more. I think this is the transform you are after: I am not able to test it right now, but you'll get the idea.

rosrun topic_tools transform /ts /tsnew geometry_msgs/TwistStamped 
  'geometry_msgs.msg.TwistStamped(
    header=m.header, 
    twist=geometry_msgs.msg.Twist(
      linear=geometry_msgs.msg.Vector3(
        numpy.linalg.norm([m.twist.linear.x, m.twist.linear.y, m.twist.linear.z]), 
        0.0, 
        0.0)))' 
   --import geometry_msgs

I have split it into several lines for clarity. Few notes, the imports can be a bit finicky and it's useful to test with the python interpreter.

I always find topic_tools/transform very useful, but somewhat difficult to use. I always helps me to write the expressions in a text editor, so that you can experiment and jump around a little bit more. I think this is the transform you are after: I am not able to test it right now, but you'll get the idea.

rosrun topic_tools transform /ts /tsnew geometry_msgs/TwistStamped  \\
 'geometry_msgs.msg.TwistStamped(
'geometry_msgs.msg.TwistStamped( \\
    header=m.header,   \\
 twist=geometry_msgs.msg.Twist(
twist=geometry_msgs.msg.Twist( \\
 linear=geometry_msgs.msg.Vector3(
linear=geometry_msgs.msg.Vector3( \\
        numpy.linalg.norm([m.twist.linear.x, m.twist.linear.y, m.twist.linear.z]),   \\
        0.0,   \\
        0.0)))'   \\
   --import geometry_msgs

I have split it into several lines for clarity. Few notes, the imports can be a bit finicky and it's useful to test with the python interpreter.

I always find topic_tools/transform very useful, but somewhat difficult to use. I always helps me to write the expressions in a text editor, so that you can experiment and jump around a little bit more. I think this is the transform you are after: I am not able to test it right now, but you'll get the idea.

rosrun topic_tools transform /ts /tsnew geometry_msgs/TwistStamped \\
\
  'geometry_msgs.msg.TwistStamped( \\
\
    header=m.header,  \\
\
    twist=geometry_msgs.msg.Twist( \\
\
      linear=geometry_msgs.msg.Vector3( \\
\
        numpy.linalg.norm([m.twist.linear.x, m.twist.linear.y, m.twist.linear.z]),  \\
\
        0.0,  \\
\
        0.0)))'  \\
\
   --import geometry_msgs

I have split it into several lines for clarity. Few notes, the imports can be a bit finicky and it's useful to test with the python interpreter.