[ros-users] rxplot artifacts

Ken Conley kwc at willowgarage.com
Mon May 10 17:43:01 UTC 2010

Looking at the code, my hunch is performance-related jitter inside of
rxplot. If you have timestamped data, this isn't an issue, but if you don't
have timestamped data, the best rxplot can do is look at the clock when it
receives the data. Looking at the code, rxplot is taking the timestamp later
than it should (and inside of a lock). If this lock is causing the jitter,
it should be easy to test by changing _ros_cb() in
rxtools/src/rxtools/rosplot.py to:

    def _ros_cb(self, msg):
        t = rospy.get_time()
                # #944: use message header time if present
                if msg.__class__._has_header:
                    self.buff_x.append(msg.header.stamp.to_sec() -
                    self.buff_x.append(t -
                #self.axes[index].plot(datax, buff_y)
            except AttributeError, e:
                self.error = RosPlotException("Invalid topic spec [%s]:
%s"%(self.name, str(e)))

It's possible that this change won't be enough to eliminate the jitter, but
I'm interested if it is. If this change works, we can patch ROS
appropriately. I'll note that for non-timestamped data, jitter is always
possible as the receipt latency can cause these artifacts.

 - Ken

On Mon, May 10, 2010 at 5:25 AM, Ruben Smits
<ruben.smits at mech.kuleuven.be>wrote:

> On Monday 10 May 2010 14:15:14 Stefan Kohlbrecher wrote:
> > Hi everyone,
> >
> > I am currently trying out the Orocos-ROS integration. In the process
> > of doing so, I added a WritePort to my testing TaskContext that
> > oscillates a floating point variable between values 0.0f and 1.0f. The
> > task context runs at a update rate of 200 Hz. When I plot the values
> > with rxplot, I get a large amount of artifacts, as can be see the
> > attached file 'rxplot_jitter.png'. When pausing rxplot for a while and
> > resuming, the data plotted for the paused period looks correct as
> > expected (see ' rxplot_pause.png'). Is this normal behavior and can it
> > be improved somehow?
> I suspect this is a rxplot issue ..., but I don't know the internals of
> rxplot.
> If you look at the values using rostopic echo, do you see the same
> artifacts?
> Ruben
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