Webcam capture with OpenCV, Tkinter and multiprocessing: __THE_PROCESS_HAS_FORKED

child process calling self.run()The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().Break on __THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__() to debug.

This is what you get when you try to put the three pieces together, OpenCV, Tkinter and multiprocessing.

Take a simple example:

import multiprocessing
import cv2

import Tkinter as tk    # HERE1

def cam_loop(the_q):
    while True:
        the_q.put('foo in the queue')

def show_loop(the_q):
    cv2.VideoCapture(0)  # HERE2
    while True:
        from_queue = the_q.get()
        print from_queue

if __name__ == '__main__':
    try:
        the_q = multiprocessing.Queue(1)

        cam_process = multiprocessing.Process(target=cam_loop,args=(the_q, ))
        cam_process.start()

        show_process = multiprocessing.Process(target=show_loop,args=(the_q, ))
        show_process.start()

        cam_process.join()
        show_loop.join()
    except KeyboardInterrupt:
        cam_process.terminate()
        show_process.terminate()

Odd, even though not really using Tkinter in any place of the code, just by importing it, if you try to run this, it will throw the error above.
Comment out the "import Tkinter", and code will work OK.

Oddx2, it will not break if the line HERE2 is commented out. That is, you can import OpenCV, but when trying to use it inside the process, it will snap.

Seems like cv2 and Tkinter are not really good friends when living in multiprocessing fields.

Found some references:
http://stackoverflow.com/a/19082049/1956309
http://bugs.python.org/issue5527#msg194848

The solution, or workaround to be more precise, is to change the Tikinter import to somewhere after the fork is done, like:

import multiprocessing
import cv2

def cam_loop(the_q):
    while True:
        the_q.put('foo in the queue')

def show_loop(the_q):
    cv2.VideoCapture(0)
    while True:
        from_queue = the_q.get()
        print from_queue

if __name__ == '__main__':

    try:
        the_q = multiprocessing.Queue(1)

        cam_process = multiprocessing.Process(target=cam_loop,args=(the_q, ))
        cam_process.start()

        show_process = multiprocessing.Process(target=show_loop,args=(the_q, ))
        show_process.start()

        import Tkinter as tk
        cam_process.join()
        show_loop.join()
    except KeyboardInterrupt:
        cam_process.terminate()
        show_process.terminate()

Next:
A. flee from Tkinter, explore other guis.
B. Keep trying to make any sense of this.