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

[John O'Hagan]

On Tue, 2023-03-14 at 16:22 -0400, aapost wrote:
> On 3/14/23 06:54, John O'Hagan wrote:

[...]
> > 
> > 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.

[...]

> > 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
> > 
> > 
> 
> 
> I don't get any of the zoom/panning behavior with waitKey(1). But a 
> couple notes from the web:
> 
> 
> https://docs.opencv.org/2.4/modules/highgui/doc/user_interface.html
> 
> Note
> 
> This function should be followed by waitKey function which displays
> the 
> image for specified milliseconds. Otherwise, it won’t display the
> image. 
> For example, waitKey(0) will display the window infinitely until any 
> keypress (it is suitable for image display). waitKey(25) will display
> a 
> frame for 25 ms, after which display will be automatically closed.
> (If 
> you put it in a loop to read videos, it will display the video 
> frame-by-frame)
> 
> https://pythonexamples.org/python-opencv-imshow/
> 
> cv2.waitKey(0) is important for holding the execution of the python 
> program at this statement, so that the image window stays visible. If
> you do not provide this statement, cv2.imshow() executes in fraction
> of 
> a second and the program closes all the windows it opened, which
> makes 
> it almost impossible to see the image on the window.
> 
> if I change waitKey to 0, I get the ability to zoom/pan, but it
> places 
> the tk window in a blocked state because it is waiting on cv2 to
> return. 
> If I hit the ESC key, it releases the wait and gives control back to
> the 
> tk window, allowing me to press show again to continue to the next
> image.
> 
> I can try to change waitKey to a high ms like 10000000 and have
> zoom/pan 
> for that amount of time before it gives control back to tk (not a 
> sensical approach).
> 
> The fact that you can still see the image after tk takes back control
> is 
> I believe just a matter of design, some examples show 
> cv2.destroyAllWindows() after waitKey(0) to clean that up, but of
> course 
> if you are reusing the same window, that destroys the target.
> 
> You can resolve that if you move
> cv2.namedWindow('W', cv2.WND_PROP_FULLSCREEN)
> cv2.setWindowProperty('W', cv2.WND_PROP_FULLSCREEN,
> cv2.WINDOW_FULLSCREEN)
> 
> to inside the show, and destroy it after the wait, and make waitKey
> 0, 
> this allows creation/cleanup of that window per image
> 
> Hitting ESC when done zooming/panning on each image to get back to
> tk.

Thanks for your reply.

I'm afraid in the real app it won't be practical to have no gui control
while viewing images.

But your suggestions have made me realise that my issue has nothing to
do with tkinter, it seems to be the way imshow is supposed to work.
This code gives the same behaviour described above:

cv2.imshow('W', array)
cv2.waitKey(100)
time.sleep(20)

I've seen the docs you mention that say the window will close if no
keypress happens within the waitKey time, but that doesn't seem to be
what happens as you say. After waitKey returns, the image remains and
everything works, but for some reason the desktop complains about an
unresponsive app.

You can use cv2 to display video with:

while 1:
 frame = camera.read() #pseudocode
 cv2.imshow('W', frame)
 cv2.waitKey(1)

(In fact this used to work without waitKey if you called
cv2.startWindowThread() after creating the window, but that stopped
working at some point.)

The fact that this works, presumably because the image is being
replaced often enough, suggested the following workaround to my
problem, using a call to after() to re-display the current image before
the desktop starts complaining:

images=[cv2.imread(i) for i in ('a.jpg', 'b.jpg', 'c.jpg')]

cv2.namedWindow('W', cv2.WND_PROP_FULLSCREEN)
cv2.setWindowProperty('W', cv2.WND_PROP_FULLSCREEN,
cv2.WINDOW_FULLSCREEN)

counter=[0]

image = [None]

def show():
   im = images[counter[0] % 3]
   cv2.imshow('W', im)
   cv2.waitKey(1)
   image[:] = [im] 
   counter[0] += 1

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

def update():
   if image[0] is not None:
      cv2.imshow('W', image[0])
      cv2.waitKey(1)
      root.after(1000, update)

update()

mainloop()


Not ideal, but I'm starting to think that despite the "highgui" label,
cv2.imshow is not really suited for integration into a gui. Which is a
shame because it's so damn fast! 

Thanks

John