Multiprocessing queue: pygame surface died

Question

I want to use multiprocessing to separate two tasks in pygame: Pulling an image from a webcam and showing the image on a screen. The reason I want to do this, is because

• I want to have fancy image processing
• I want to separate the web cam polling from the screen and user input

and want to separate the time delay of both, ideally optmizing the load. Using threading I have no issues sending images from one thread to the other. However, I get a laggy image. I would like to test if using multiprocessing can reduce delay of the image.

Here's the snag: I get an error after sending the image from the camera process to the screen process. After pulling the image from the queue using

imgmsg = img_q.get() 

I check the image for size with

imgmsg.img.get_width()

As said, with threads this returns the correct image width. However with multiprocessing I get the following error

Process Process-1: Traceback (most recent call last):   File "/usr/lib/python3.2/multiprocessing/process.py", line 267, in
_bootstrap
    self.run()   File "/usr/lib/python3.2/multiprocessing/process.py", line 116, in run
    self._target(*self._args, **self._kwargs)   File "test_photoBoothMultiProc.py", line 21, in consumer
    photoBoothScreen.screenThread(in_q, img_q)   File "/home/pi/pyBooth/thread_photoBoothScreen.py", line 68, in screenThread
    print(imgmsg.img.get_width()) pygame.error: display Surface quit

So it seems the image is lost in the queue? I have tried to read into this, and there seem to problems when trans mitting larger objects through a queue. Is this correct? How would I circumvent or fix this?

Answer 1

Serialize the images first (with pygame.image.tostring/pygame.image.fromstring) before sending to another process.

That should work. This way, you send only the data of the image itself, not a Surface instance (so it's totally independent from pygame).

Note that you could also compress the string further with simply calling .encode("zlib")/.decode("zlib") on that string.