Generate Export Pictures Does not Work for First Loop Execution

[GR]

Hi,

I am using the following code to generate png files with PyQtGraph. 

The code works quite well, but unfortunately, the first loop / picture ("0exported_image.png", that is: runner = 0) does not have any data in the output picture (respectively: has only default values in the exported graph). 

I additionally set a manual breakpoint in "update_data" and looked at the attributes of the class instance for runner = 0 and runner >0. It strikes that at several parts of the object tree the "picture" is set for runner >0, but is not for runner = 0:

"picture": {

    "py/object": "PyQt5.QtGui.QPicture"

},

This is the code I use. Does anybody have an idea what could be the reason for this? 

Thanks in advance.

import numpy as np
from PyQt5 import QtWidgets
from PyQt5.QtCore import pyqtSignal, pyqtSlot
import pyqtgraph as pg
import pyqtgraph.exporters
from pyqtgraph.Qt import QtCore, QtGui
from pyqtgraph import GraphicsLayoutWidget
from threading import Thread, Event
import time


# Routine to acquire and serve data
# This might be a camera driver, notifying when a new frame is available
def generate_data(callback, threadkill):
    # while not threadkill.is_set():
    #     width = 1600
    #     data = np.zeros(width)
    #     runner = int(np.random.random()*5)
    #     data.fill(runner)
    #     callback(data, runner)
    #     time.sleep(0.01)
    for runner in range(5):
        width = 1600
        data = np.zeros(width)
        data.fill(runner)
        my_var = callback(data, runner)
        time.sleep(0.01)
        print("callback called")

    # threadkill.set()
    # # sys.exit(1)


class PyQtGraphTest(GraphicsLayoutWidget):
    # Signal to indicate new data acquisition
    # Note: signals need to be defined inside a QObject class/subclass
    data_acquired = pyqtSignal(np.ndarray, int)

    def __init__(self):
        super().__init__()
        self.exporter = None

        self.setWindowTitle('Test pyqtgraph paint signals')
        self.resize(640, 400)
        self.plot = self.addPlot()
        self.spectrum = self.plot.plot()
        self.plot.enableAutoRange(pg.ViewBox.XYAxes)

        # Connect the signal
        self.data_acquired.connect(self.update_data)

        # Make and start the background thread to acquire data
        # Pass it the signal.emit as the callback function
        self.threadkill = Event()
        self.thread = Thread(target=generate_data, args=(self.data_acquired.emit, self.threadkill))
        self.thread.start()

        self.thread.join()
        # my_var = self.thread.isAlive()
        # print("my_var:", my_var)
        # self.threadkill.set()
        # print("fin")

    # Kill our data acquisition thread when shutting down
    def closeEvent(self, close_event):
        self.threadkill.set()

    # Slot to receive acquired data and update plot
    @pyqtSlot(np.ndarray, int)
    def update_data(self, data, runner):
        if not self.exporter:
            # Here we are passing the exporter the GraphicsLayout object that is
            # the central item (ci) inside this GraphicsLayoutWidget. That in
            # turn contains the two PlotItem objects.
            self.exporter = pg.exporters.ImageExporter(self.ci)
            self.exporter.parameters()['width'] = 640

        self.spectrum.setData(data)
        self.exporter.export(str(runner) + 'exported_image.png')
        print("exported image")


if __name__ == '__main__':
    import sys

    app = QtWidgets.QApplication(sys.argv)
    window = PyQtGraphTest()
    window.show()
    if (sys.flags.interactive != 1) or not hasattr(QtCore, 'PYQT_VERSION'):
        sys.exit(app.exec_())

    print("fin2")

[Patrick]

Hi,

I think there is a bug somewhere, where the first call to setData() doesn't result in the view range being updated. Your code is working fine, the plot is being updated with the correct data, but the axes scales aren't correct on the first call.

Without spending time tracing through why the ranges aren't being updated, a simple workaround would be to manually trigger the update by inserting "self.plot.autoRange()" after your self.spectrum.setData(data) line. Not ideal, but seems to work.

Patrick

[GR]

Hi Patrick,

thanks a lot! I will use that.

Wonderful - again!