Johns Blog

ROS and Gtk for Laboratory Control

At the lab in which I work (Andrew Straw, strawlab) we study the visual flight behaviour of Drosophila using virtual reality. The implementation of this will be explained in future posts and papers however for this post I am going to describe how I used Gtk¹ and ROS to build an interface to control and monitor running experiments (called the 'Operator Console').

A future post will address and release all the ROS+GObject² glue that lets these interfaces scale dynamically as nodes (dis)appear. This just shows the relevant Gtk parts and has some comments on what I would like from Gtk to make these sort of interfaces easier.

The screenshow shows the first tab of the 'Operator Console'³.

Implementation Notes

• I use the secondary icon support of Gtk.Entry to show the contents contain sensible data. Maybe validation support in Gtk would be useful here bug.
• The 'Description' entry is a Gtk.TextView, not a Gtk.Entry. It was necessary to apply custom CSS to make it look reasonably similar. Sadly, it does not support the full/same set of CSS properties as Gtk.Entry, so it was impossible to show the same border radius and focus colors bug. Perhaps a multi-line Gtk.Entry would be better.
• The bottom half of the window shows the utilisation of all computers. I tried a few versions of this, and simple sensibly formatted monospaced text looked much better than anything else I tried. Any suggestions?

This screenshot shows an example screen where we mix the control and monitoring or many instances of the same ROS node.

Implementation Notes

• The Gtk.Switch simultainously displays the status of the projector, and also allows control of the node. The is a common use-case in the software, and due to the asynchronous nature of the ROS messages, I need to distinguish these from user-generated signals. I have wrappers such as the following for many widgets⁴. Advice on how to distinguish this use-case would be preferred.

class UpdateableGtkSwitch(Gtk.Switch):
    def __init__(self, *args, **kwargs):
        Gtk.Switch.__init__(self, *args, **kwargs)
        self._changing = False
        self.connect_after("notify::active", self._changed)

    def _changed(self, *args):
        if self._changing:
            self.stop_emission("notify::active")

    def set_active(self, is_active):
        self._changing = True
        Gtk.Switch.set_active(self, is_active)
        self._changing = False

    def connect(self, *args, **kwargs):
        self.connect_after(*args, **kwargs)

• The "Standby (Computer1)" is for display only and mirrors the status of a ROS topic. I would like some way visually to inicate that this widget is not actually an editable Gtk.Entry. Currently the Gtk.Entry is set editable = False, it looks to out of place with sensitive = False. Perhaps I should add some custom CSS to color it slightly different. Suggestions are appreciated.

Closing Remarks

I'm really happy with the status of the PyGObject bindings. We have a few quite large applications built using them (and ROS) and I have no complaints about performance⁵ or otherwise. The conventional wisdom was that PyGTK (and GTK) were not suitable for threaded workloads but the threading model of ROS guarentees that the 'operator-console' shown above manages upwards of 50 background threads asynchronously updating the GUI state.