does node mimic really publish the topic /output subscribe the topic /input?

[ly11tea]

In the document here,node mimic's Subscribed Topic is /input,Published topic is /output , but when i read the source code of mimic ,i found that node mimic subscribes the topic input/pose, Publishes the topic output/cmd_vel. where:

  ros::NodeHandle input_nh("input");
  ros::NodeHandle output_nh("output");
  twist_pub_ = output_nh.advertise<geometry_msgs::Twist>("cmd_vel", 1);
  pose_sub_ = input_nh.subscribe<turtlesim::Pose>("pose", 1, &Mimic::poseCallback, this);

it could be confusing ,because if i remap the topic in the launch file i will Write:

    <remap from="input" to="turtlesim1/turtle1/pose"/>
    <remap from="output" to="turtlesim2/turtle1/cmd_vel"/>

but in the end,mimic code subscribes the topic turtlesim1/turtle1/pose/pose,Publishes the topic turtlesim2/turtle1/cmd_vel/cmd_vel obviously ,it doesn't work i found one tutorial here ,the corresponding part of launch file is:

    <remap from="input" to="turtlesim1/turtle1"/>
    <remap from="output" to="turtlesim2/turtle1"/>

of course it works,but it seems to be contradict to the document in the beginning of this issue ,where: Subscribed Topic is /input(message type is turtlesim/Pose),Published topic is /output(message type is geometry_msgs/Twist) .because turtlesim2/turtle1 and turtlesim1/turtle1 is node here. do i misunderstand something ?thanks a lot!

[dirk-thomas]

I am not sure I understand your point. For each turtle there are topics pose and cmd_vel which are in the namespace of the turtle (e.g. turtleX). The same for the mimic node - it has two topics within the node names input and output.

Therefore you can either:

• remap the node namespace: input -> turtlesim1/turtleN and output -> turtlesim1/turtleN
• or remap the specific topics: input/pose -> turtlesim1/turtleN/pose and output/cmd_vel -> turtlesim1/turtleN/cmd_vel

If you think the documentation can be improved please proposed a concrete diff to the existing wiki page. I can't propose any change since I don't understand your confusion I guess.

[dirk-thomas]

I will close this for now due to no response. Please feel free to comment here with additional information and the ticket can be reopened.

[weidongguo]

I had the same confusion when I read the tutorial http://wiki.ros.org/ROS/Tutorials/UsingRqtconsoleRoslaunch today.

It explicitly stated that input and outputare topics. "Here we start the mimic node with the topics input and output renamed to turtlesim1 and turtlesim2. "

This is what confuses me. I think they shouldn't be called topics. They should be called namspaces. Thinking of them as namespaces, everything make sense.

remap will map namspace inputto namespace turtlesim1/turtle1. Topic names within the namspace input remain the same for example, pose happens to be a topic within namespace input, after the mapping, it will have a namespace turtlesim1/turtle1, and the full topic name now is turtlesim1/turtle1/pose

The benefit of this is that you don't have to explicitly state all the topic names if the only change is just the namespace but not the local topic names.

On the other hand, the actual topics are input/pose, and output/cmd_vel, so it also make sense to remap input/pose -> tutorialsim1/turtle1/pose and output/cmd_vel -> tutorialsim2/turtle1/cmd_vel because we want mimic to listen to topic tutorialsim1/turtle1/pose and publish whatever it's received to tutorialsim2/turtle1/cmd_vel. This works (tested)

However, we have to explicitly write out the topic names that are the same. Imagine if you have 100 topics. you don't wanna type all that out. we are coder. we want to do things in an automatic fashion.

I may not be right in terms of the exact terminologies, but the idea should be correct.

[dirk-thomas]

@weidongguo Please consider to contribute to the wiki and improve the content you find confusing.

[RicardoDanielCPinto]

Hey, I found this issue because i got the same confusion and i didn't got the answer here but i did found it by my self. What he's trying to say is that the input topic which is turtlesim1/turtle1/pose seems to be a position... "Pose"... But its not, if we check the type, it's "turtlesim/Pose" and if we check the data structure of this type we get: float32 x float32 y float32 theta float32 linear_velocity <------------- float32 angular_velocity <-------------

So its confusing trying to mimic a velocity with a position, but what happens is that mimic is getting the velocity information from /Pose topic to publish on turtlesim2/turtle1/cmd_vel

Check the source of mimic here: http://docs.ros.org/diamondback/api/turtlesim/html/mimic_8cpp_source.html

If you think its worth a change on wiki, please do it for me. Sorry my english