def _start(queue, stop_event):
    """
    Thread target function. Starts the ImageQueue._populate function which runs
    indefinitely until stop_event is set.

    Args:
        queue:      A reference to the ImageQueue object onto which the threads
                    apply.
        stop_event: An even that can be set in the main thread to stop
                    population of the ImageQueue
    """
    while not stop_event.is_set():
        queue._populate()
