Adding Code into the Main Display



For this particular application it would be of interest to not only see the beam image on the screen, but to also calculate the maximum point on the image and display the coordinates in a label.

To do so, we will need to add some Python code to our main screen developed at the Main Screen section.

Since we already have the screen designed in the UI file, we can re-use it in our Python-based display, and hook up code to interact with widgets.

This is accomplished by subclassing pydm.Display (See Subclassing Display for more details).

  • Step 1.

    Open a new text file. The first thing that we will do is add the imports needed for the code that will follow.

    import time
    from os import path
    from pydm import Display
    from scipy.ndimage.measurements import maximum_position
  • Step 2.

    Let’s create our Python class that will inherit from Display (See Subclassing Display).

    class BeamPositioning(Display):
        def __init__(self, parent=None, args=None, macros=None):
            super(BeamPositioning, self).__init__(parent=parent, args=args, macros=None)
            # Attach our custom process_image method
            self.ui.imageView.process_image = self.process_image
            # Hook up to the newImageSignal so we can update
            # our widgets after the new image is done
            # Store blob coordinate
            self.blob = (0, 0)
        def ui_filename(self):
            # Point to our UI file
            return 'main.ui'
        def ui_filepath(self):
            # Return the full path to the UI file
            return path.join(path.dirname(path.realpath(__file__)), self.ui_filename())

    Breaking the class constructor code into pieces, we have:

    1. Replaced the default PyDMImageView method process_image with our own custom method.

    2. Hooked up our show_blob method to the newImageSignal that is emitted by the PyDMImageView every time a new image is displayed.

    3. Initialized the self.blob variable with (0, 0).

    4. Implemented a ui_filename method returning the name of the UI file to be used and compose the screen.

    5. Implemented a ui_filepath method returning the full path to the ui_filename so PyDM can properly load it.

    • Step 2.1.

      Add code to the process_image callback method so we can calculate the blob position.


      The process_image method is defined in the PyDMImageView widget and more information about it can be found at the PyDMImage widget documentation page.

      Since this method runs in a separated QThread, we shouldn’t manipulate widgets in this method, since this code runs outside of the Qt Main Thread.

      def process_image(self, new_image):
          # Consider the maximum as the Blob since we have only
          # one.
          self.blob = maximum_position(new_image)
          # Send the original image data to the image widget
          return new_image

      In process_image we call the scipy method maximum_position to calculate the coordinates for the maximum spot and save it to self.blob. At the end, this method returns the unmodified image, which the ImageView will display. If you’d like to manipulate the image before displaying it, you can do so in this method, and return the manipulated version.

    • Step 2.2.

      Add code to the show_blob method so we update the QLabel with the new blob position calculated in process_image.

      def show_blob(self, *args, **kwargs):
          # If we have a blob, present the coordinates in label
          if self.blob != (0, 0):
              blob_txt = "Blob Found:"
              blob_txt += " ({}, {})".format(self.blob[1], self.blob[0])
              # If no blob was found, present the "Not Found" message
              blob_txt = "Blob Not Found"
          # Update the label text
  • Step 3.

    Save this file as


    For this tutorial it is important to use this file name as it will be referenced at the other sections. If you change it please remember to also change in the other steps when referenced.

  • Step 4.

    Test the Main Screen:



You can download this file using this link.