[pygame] [Pygame] How can I have an accurate timing when I am showing an image?

["Francesco Martino" <francesco.k@xxxxxxxxx>]

I am using Pygame for building a psychological experiment.
The code should do something very easy: it should display an image for
a limited amount of time, then show a blank screen and then another
image. Images are stored as bitmap files (470 Kb each).
The main requisite for this program is that timing needs to be very
accurate, namely if I say that an image should be displayed for 800
msec the amount of error should be as little as possibile.

The code is very simple: each image is shown using

        startTime = time.time()
        test = image.load(image1).convert() #image1 is the name of the file
        surf.blit(test, (0,0))
        display.update()
        pygame.time.delay(TIME_TEST_IMAGE)
        stopTime = time.time()
	img_length = stopTime - startTime

Using the Python time module, I see that error is acceptable for my
purposes (the range is TIME_TEST_image +/-1, plus a costant error of
45 msec, so image is displayed for about 845 msec) in most of the
cases. Unfortunately, sometimes the image is displayed for a shorter
time (with a costant error of 11 msec, so image is displayed for about
811 msec) Since it happens with an irregular rate, it is very
difficult to understand the reason. Has anyone some ideas? Is there a
better way to display images for a fixed period of time with pygame?

Thank you for any idea
Francesco