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I am trying to write a small script to play video from files using opencv and pygame. The videos should play whenever a key is being pressed and keep playing until the end. When the key is released, the background color changes and I record the time between the keypress and the keyrelease.

So far so good, it seems to be working as expected. My only issue is with the fact that I cannot set the frame rate as I want to. All the videos have the same length (exactly 2500 ms, 20 fps) and when I run the script the videos are way too fast. I tried multiple things (video.set(cv2.cv.CV_CAP_PROP_FPS, ), ...) but it didn't work. The closest I get is by adding a self.sleep(50) somewhere in the loop. It seems to adjust the frame rate but it doens't seem to be very reliable as the length of each video is not consistent. Any advice would be greatly appreciated ! Thanks a lot !

import cv2
import numpy
import pygame

red = (255,0,0)

background_color = (0,0,0,)

#check whether the correct backend is being used (for OpenSesame)
if self.get('canvas_backend') == 'legacy':
    surface = exp.surface
elif self.get('canvas_backend') == 'xpyriment':
    surface = exp.window
else:
    raise Exception('This script requires legacy or xpyriment (no opengl)')


# Full path to the video file
path = pool[self.get('video')]
# Open the video and determine the video dimensions
video = cv2.VideoCapture(path)

#initiate the video with a keypress and start the clock
key = None
while key == None:
   for event in pygame.event.get():
       if event.type == pygame.KEYDOWN:
           start_time = self.time()
           key = event.key
           go = True

while go:

    # Get a frame and break the loop if we failed, usually because the video
    # ended.
    success, frame = video.read()
    if not success:
        break

    #for some reasons the frames appeared inverted
    frame = numpy.fliplr(frame)
    frame = numpy.rot90(frame)

        # The video uses BGR colors and PyGame needs RGB
    frame = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)

        # Create a PyGame surface
    surf = pygame.surfarray.make_surface(frame)

    #set the frame rate to 20 fps ?? To be verified.
    self.sleep(50)

    # Fill the background color of the PyGame surface whenever a key is released
    for event in pygame.event.get():
        if event.type == pygame.KEYUP:
            background_color = red
            surface.fill(background_color)
            pygame.display.update
            end_time = self.time()  

    # Show the PyGame surface!
    surface.blit(surf, ((surface.get_rect().centerx-150),(surface.get_rect().centery-100)))
    pygame.display.flip()



#define the experimental variable
rt = end_time- start_time
exp.set('keyreleaseRT', rt)
print '%s was released after %d ms' % (key, rt)
user1969717
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  • "adding a self.sleep(50) somewhere in the loop" -- at 20 FPS, the whole iteration would need to take 50ms. In your iteration you sleep 50ms and also read and decode the frame, flip, rotate, convert color, blit, swap buffers, etc... As you can imagine, all this other work also takes some time, so when you add it up, your iterations are longer than 50 ms. | Either measure the actual elapsed time and sleep for the remainder, or perhaps use a pygame timer periodically fires an event? Or if possible, use some library that's actually meant for playback of videos. – Dan Mašek Feb 09 '17 at 13:39
  • Hmm, makes perfect sense. Thanks for spotting the obvious (not so obvious for me apparently) and for suggesting to use a pygame timer. It seems to work as expected when using tick_busy_loop(20) – user1969717 Feb 10 '17 at 14:01

0 Answers0