FView plugin tutorial: periodic triggering of an external device

This tutorial illustrates the steps necessary to trigger an external device at a constant, relatively slow rate. Specifically, we will generate a pulse on the CamTrig device every N frames.

A working copy of this can be found at http://github.com/motmot/fview_periodic_trigger/

Prerequisites

• read and understand FView plugin tutorial: live histogram of image intensity values
• the CamTrig hardware device plugged in and functioning
• fview_ext_trig (software for the CamTrig) installed and functioning

Introduction

From FView plugin tutorial: live histogram of image intensity values, you should have a working knowledge of how to create a Python package directory structure and a setup.py file with the appropriate entry point to make an FView plugin. Therefore, this tutorial will focus only on the unique aspects of periodic triggering.

Create the plugin logic

Now we’re going to create the module motmot.fview_periodic_trigger.fview_periodic_trigger with our new class FviewPeriodicTrigger. Open a new file named:

base/motmot/fview_periodic_trigger/fview_periodic_trigger.py

The contents of this file:

from __future__ import with_statement, division

import pkg_resources
import enthought.traits.api as traits

import motmot.fview.traited_plugin as traited_plugin
import motmot.fview_ext_trig.ttrigger as ttrigger

from enthought.traits.ui.api import View, Item, Group

class FviewPeriodicTrigger(traited_plugin.HasTraits_FViewPlugin):
    plugin_name = 'periodic trigger'
    trigger_device = traits.Instance(ttrigger.DeviceModel)

    Nth_frame = traits.Int(100)

    traits_view = View(Group(Item(name='Nth_frame')))

    def set_all_fview_plugins(self,plugins):
        """Get reference to 'FView external trigger' plugin"""

        # This method is called by FView to let plugins know about
        # each other.

        for plugin in plugins:
            if plugin.get_plugin_name()=='FView external trigger':
                self.trigger_device = plugin.trigger_device
        if self.trigger_device is None:
            raise RuntimeError('this plugin requires "FView external trigger"')

    def process_frame(self,cam_id,buf,buf_offset,timestamp,framenumber):
        if framenumber%self.Nth_frame == 0:
            if self.trigger_device is not None:

                 # fire pulse on EXT_TRIG1
                self.trigger_device.ext_trig1 = True

                 # toggle LED
                self.trigger_device.led1 = not self.trigger_device.led1

        draw_points = []
        draw_linesegs = []
        return draw_points, draw_linesegs

The initial several lines are standard import statements. Note that this plugin uses Enthought’s Traits module to facilitate event handling and GUI elements.

Next, we define the class FviewPeriodicTrigger, which is derived from the traited_plugin.HasTraits_FViewPlugin base class. This base class implements most of the functionality required for FView plugins, so we just have to implement or override a few things.

First, in our class, we give our plugin a name, in this case periodic trigger. Next, we tell traits that we’re going to have a trigger_device variable and an Nth_frame variable. The trigger_device variable is an instance of the motmot.fview_ext_trig.ttrigger.DeviceModel, and is used to interact with the Motmot CamTrig hardware. The Nth_frame variable defines how frequently we will pulse the external trigger pin (EXT_TRIG1) and toggle the LED.

The set_all_fview_plugins method is required because we need to find the CamTrig trigger device. We do this by checking each of the plugins registered with FView to see if it is the ‘FView external trigger’ plugin . This plug will have an attribute called trigger_device, which we want to keep a reference to.

Finally, the business end of this plugin, like most FView plugins, is the process_frame method. This method gets called on every frame and can be used to do realtime image analysis. We’re keeping things simple in this tutorial, however, and only testing the framecount and pulsing the external trigger and toggling the LED if it’s a multiple of the Nth_frame variable. The return value of process_frame are any points and line segments that FView should draw over the main display. In our case, we don’t want to draw anything, so we return a couple of empty lists.