Kivy image does not reload and app stops responding - python-3.x

Question

I am trying to build an application with Python using Kivy. This is what I am trying to achieve:
Use input from a video or camera to detect images using a detector that (to simplify) finds all circles in an image, and highlights them somehow, producing a new image as a result. This is done by a class which I call detector. Detector makes use of OpenCV for image processing.
The detector's output image is written into display.jpg
A Kivy app has an image field of which source is display.jpg
(This doesn't work) I reload the image source using self.image.reload() so that my application refreshes the output for the user
Here is my code
class GUI(App):
def build(self):
self.capture = cv2.VideoCapture(VIDEO_SOURCE)
self.detector = detector(self.capture)
layout = GridLayout(cols=2)
self.InfoStream = Label(text = 'Info', size_hint=(20,80))
StartButton = Button(text = 'Start', size_hint=(80,20))
StartButton.bind(on_press=lambda x:self.start_program())
self.image = Image(source = 'display.jpg')
layout.add_widget(self.image)
layout.add_widget(self.InfoStream)
layout.add_widget(StartButton)
layout.add_widget(Label(text = 'Test'), size_hint=(20,20))
return layout
def start_program(self):
while True:
self.detector.detect_frame()
self.update_image() # After detecting each frame, I run this
def update_image(self, *args):
if self.detector.output_image is not None:
cv2.imwrite('display.jpg', self.detector.output_image)
self.image.reload() # I use reload() to refresh the image on screen
def exit(self):
self.stop()
def on_stop(self):
self.capture.release()
if __name__ == '__main__':
GUI().run()
What happens is that I can successfully start the application by pressing my StartButton. I see output on console proving it's cyclically running through frames from my video stream, and I also see the source image display.jpg being updated in real time.
However, after I start, the app window seems to simply freeze. It goes "Not responding" and greys out, never showing any refreshed image.
Following some existing code from other sources, I also attempted to refresh the image using a scheduled task, with Clock.schedule_interval(self.update_image, dt=1), but the result was the same.
Is my way of refreshing the image correct? Are there better ways to do so?

Answer 1

The while True loop on the main thread will cause your App to be non-responsive. You can eliminate that loop by using Clock.schedule_interval as below:
class GUI(App):
def build(self):
self.capture = cv2.VideoCapture(VIDEO_SOURCE)
self.detector = detector(self.capture)
layout = GridLayout(cols=2)
self.InfoStream = Label(text = 'Info', size_hint=(20,80))
StartButton = Button(text = 'Start', size_hint=(80,20))
StartButton.bind(on_press=lambda x:self.start_program())
self.image = Image(source = 'display.jpg')
layout.add_widget(self.image)
layout.add_widget(self.InfoStream)
layout.add_widget(StartButton)
layout.add_widget(Label(text = 'Test'), size_hint=(20,20))
return layout
def start_program(self):
Clock.schedule_interval(self.update_image, 0.05)
def update_image(self, *args):
self.detector.detect_frame()
if self.detector.output_image is not None:
cv2.imwrite('display.jpg', self.detector.output_image)
self.image.reload() # I use reload() to refresh the image on screen
def exit(self):
self.stop()
def on_stop(self):
self.capture.release()
if __name__ == '__main__':
GUI().run()
Here is another question where the poster is using a Texture instead of writing the captured image to a file. Might be a more efficient approach.