[jsk_topic_tools] Add filtered relay node which can filter a topic and transform it.

[iory]

What is this?

rostopic echo has a filter function, but it cannot publish a topic.
topic_tools/transform can format a topic and publish it, but it does not have the ability to filter.
Transforming while filtering a topic is simple and powerful.
This node provides a filter and publish (i.e. relay) functions.

Subscribing Topic

• ~input (rospy/AnyMsg)

  Input message.

Publishing Topic

• ~output (~output_type)

  Output topic. You can specify ~output_type param to publish topic.

Parameters

• ~output_type (String, required)

  Message type of output_topic like std_msgs/Float32. This is the input for the get_message_class function.

• ~filter (String, default: None)

  Condition of messages that match a specified Python expression.

  The Python expression can access any of the Python builtins plus: topic (the topic of the message), m (the message) and t (time of message).

  For example, ~input topic is std_msgs/String and if you want to check whether a sentence is a hello, you can do the following.

filter: m.data == 'hello'

Note that, use escape sequence when using the following symbols <(&lt;), >(>), &(&), '(') and "(") in launch file.

• ~transform (String, default: m)

  Python expression that transform the input messages, which are given in the variable m. The default expression is m, which results in forwarding input (which can be a topic field) into output_topic.

• ~import (List[String], default: [])

  List of Python modules to import and use in the expression.

Usage

$ rosrun jsk_topic_tools filtered_relay.py ~input:=/right_endeffector/wrench \
    ~output:=/right_endeffector/force_norm \
    _filter:='numpy.linalg.norm([m.wrench.force.x, m.wrench.force.y, m.wrench.force.z]) > 10.0' \
    _output_type:=geometry_msgs/WrenchStamped _import:="[geometry_msgs, numpy]"

Example

The following example subscribe to /right_endeffector/wrench and only those with a force norm greater than 10 are published as /right_endeffector/force_norm.

roslaunch jsk_topic_tools sample_filtered_relay.launch

Use case

jsk-ros-pkg/jsk_robot#1570

[nakane11]

I checked this node works well.
I cannot come up with real example like jsk-ros-pkg/jsk_robot#1570 right now because input and filter should be simple, but in such cases this node is useful.

[k-okada]

since jsk-ros-pkg/jsk_robot#1570 has been closed and currently we do not have usage for this node. So may be we can leave this PR and re-start discussion / review until we found another use cases.

[iory]

OK. We will wait until someone needs this node.