Tkinter and cv2: "not responding" popup when imshow launched from tk app

[John O'Hagan]

Hi list

I'm trying to use cv2 to display images created as numpy arrays, from
within a tkinter app (which does other things with the arrays before
they are displayed as images). The arrays are colour-coded
visualisations of genomes and can be over a billion elements in size,
and I've found the PIL methods to display images in tkinter are too
slow and memory-heavy.

Here is minimal code that demonstrates the problem in the subject line:

import cv2
from tkinter import *

images=['a.jpg', 'b.jpg', 'c.jpg'] #change to image paths

cv2.namedWindow('W', cv2.WND_PROP_FULLSCREEN)
cv2.setWindowProperty('W', cv2.WND_PROP_FULLSCREEN, 
cv2.WINDOW_FULLSCREEN)
counter=[0]
def show():
   cv2.imshow('W', cv2.imread(images[counter[0] % len(images)]))
   cv2.waitKey(1)
   counter[0] += 1

root=Tk()
root.wm_attributes("-topmost", 1)
Button(root, text=' Show ', command=show).pack()
mainloop()

It works up to a point - I can cycle through the images by clicking the
button - but if I mouse-click on the displayed image (e.g. to use the
zooming and panning features of cv2), nothing happens, and a few
seconds later the image greys out and a popup appears saying "'Unknown'
is not responding" and giving the option of waiting or forcing close
(but sometimes these options are greyed out too). Clicking "wait", if
available, closes the popup but it comes back a few seconds later. If I
then click on the tkinter window titlebar, the popup changes to "'Tk'
is not responding". Clicking on the button still works and after a few
clicks the popup closes.

This happens under both x11 and wayland, but under wayland, I  also get
this error:

"QSocketNotifier: Can only be used with threads started with QThread
qt.qpa.wayland: Wayland does not support QWindow::requestActivate()"

and only every second button press displays a new image ,with only
every second image displayed.

I think this particular popup is a Gnome thing, but AIUI most DEs have
something similar to detect stuck apps. But the app is not stuck.

I suspect this is some kind of interaction between the call to
cv2.waitKey (which is necessary but I've never understood why!) and the
tkinter event loop, but it's beyond my knowledge.

Any suggestions about causes or workarounds?

Thanks

--

John