Screen tearing using pygame on Raspberry Pi

I am stuck on a screen tearing problem for about a week now.

Here is my problem: I would like to make a program that can display a sequence of PNG pictures really fast (at a rate of 30 fps or more). To do this I use the pygame library, especially pygame.display.blit and pygame.display.flip.

Here is an example of the code (with a homemade delay function):

import time
import pygame

screen=pygame.display.set_mode(size, pygame.FULLSCREEN)
nbPicturesToLoad=12
pictures=range(nbPicturesToLoad)
for i in range(nbPicturesToLoad):
    pictures[i]=pygame.image.load(pictureName).convert() //Here pictureName depends on the value of i

(...)

for i in range(nbPicturesToLoad):
    timer1=time.time()
    screen.blit(pictures[i], (0,0))
    pygame.display.flip()
    timer2=time.time()
    timer=timer2-timer1
    while(timer<0.03333):
        timer2=time.time()
        timer=timer2-timer1

The timer allows me to have the same refresh rate on the screen. My problem is that one can see with their own eyes that there is a screen tearing problem (https://en.wikipedia.org/wiki/Screen_tearing). On the pygame documentation they recommend to use other flags for the screen init (pygame.HWSURFACE and pygame.DOUBLEBUF, http://www.pygame.org/docs/ref/display, y.set_mode). Apparently these could help to correct the screen tearing problems. But when I use these flags I see no screen tearing correction.

I've seen somewhere that pygame can't handle vsync on X11, which would explain why I see no difference with the HWSURFACE and DOUBLEBUF flags.

Has anyone ever encountered this vsync issue on the Raspberry Pi? Is there a parameter to put in the "config.txt" file? Does anyone have a way to get real vsync information to avoid these screen tearing effects?

I've seen that pyglet could handle vsync in the created windows, but for unknown reasons I can't display images in the pyglet window and I'm really not sure if it would fix the problem, especially if a Raspberry can't handle vsync right now.

Solution

Try using the Pygame clock to handle timing, it is much easier to implement and understand and it may solve the issue.

shown below:

import pygame as py
py.init()

WINDOW = (400,400)
screen = py.display.set_mode(WINDOW)

nbPicturesToLoad=12
pictures = range(nbPicturesToLoad)
for i in range(nbPicturesToLoad):
    pictures[i]=py.image.load(pictureName).convert()

FPS = 30
clock = py.time.Clock() # Create a clock object

done = False
while not done:
    for event in py.event.get():
        if event.type == py.QUIT:
            done = True
    for i in range(nbPicturesToLoad):
        screen.fill((255,255,255))
        screen.blit(pictures[i],(0,0))
        py.display.flip()
        clock.tick(FPS) # use pygame.time.Clock to wait 1/30th of a second
py.quit()

However, since this is a screen tearing problem the solution may be to stop using Pygame altogether. How to avoid tearing with pygame on Linux/X11 This question seems to cover what you may be looking for.

Good luck.