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

[Thomas Passin]

On 3/14/2023 6:54 AM, John O'Hagan wrote:
> 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?
> 

I don't know anything about the specifics here, but with billions of 
elements you will not be able to show more than a small fraction on the 
screen.  So I think that a progressive disclosure approach would pay 
off.  Sample the arrays down to a more workable size before creating the 
screen images.  If you want to zoom in, resample them and recreate new 
images that only cover the zoomed in region in more detail.

It would also be useful to cache the generated images so they can be 
re-displayed without needing to be regenerated each time.