[ros-users] sensor_msgs Low Cost/Android review

[Chad Rockey]

Rospy can also use fields that do not have to be the same in all types of
messages.

For instance: 'rostopic echo /odom/header/stamp'  or more extreme:
'rostopic echo /odom/pose/pose/position/x'
http://www.ros.org/doc/api/nav_msgs/html/msg/Odometry.html

The topic is /odom, the message type is nav_msgs/Odometry.  At run time,
rostopic uses this function (
https://github.com/ros/ros_comm/blob/master/tools/rostopic/src/rostopic/__init__.py#L544)
to determine which piece of the message is relevant and return that type.
 (This is where the /odom/header/stamp' gets converted into
msg.header.stamp:
https://github.com/ros/ros_comm/blob/master/tools/rostopic/src/rostopic/__init__.py#L276
)

This is best used for types that are not passed back over the wire.
 'rostopic echo' prints these values to console.  rqt's plot will plot
numerical values for immediate display.  It's likely possible to create a
publisher based on the received types and publish a more appropriate output
message.  However, you still encounter the difficulties I mentioned
previously.

On Tue, Nov 13, 2012 at 3:46 PM, Brian Gerkey <gerkey at osrfoundation.org>wrote:

> On Tue, Nov 13, 2012 at 2:55 PM, Edwards, Shaun M. <sedwards at swri.org>
> wrote:
> > I understand why you would want messages to have semantic meaning, but I
> > think the thin node wrapper method is problematic from a coding and
> > usability standpoint.  From a coding standpoint, each wrapper node would
> be
> > a near copy and paste.  This is almost universally bad coding practice.
> > From a usability standpoint, it would require someone to code *something*
> > anytime they wanted to take an average of a reading.  I had a recent
> > application where I wanted to plot averages and current readings at the
> same
> > time to determine options for reducing noise.  I’d hate to have to
> program a
> > special node every time I wanted to perform the experiment.
> >
> > Perhaps there is a way (probably in python) to use some message
> > introspection to achieve a generic node.  The node could simply look for
> > int/real types and then perform the filtering automatically.  Perhaps a
> > parameter list could be used to specify the message members (as strings)
> > that should be filtered, intead.  Such an approach may also work with
> > messages of multiple members (I’m thinking pose).   What do you think?
>
> Attached is a simple rospy script that uses some of rostopic.  It can
> subscribe to any topic that contains a field called 'data' that Python
> can do arithmetic with, and can handle mixing types.  It'll do a naive
> moving-window average on the received values and publish the result as
> a std_msgs/Float64.
>
> To try it, start some publishers:
>
> rostopic pub -r 10 in1 std_msgs/Float64 1.0 &
> rostopic pub -r 10 in2 std_msgs/Float32 2.0 &
> rostopic pub -r 10 in3 std_msgs/Int64 42 &
>
> Then start the averager:
>
> python avg.py
>
> That's just a quick and dirty demo; lots more can be done in that
> direction.
>
>     brian.
>
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