Using Surface.copy() sometimes loses transparency

Question

For some (but not all!) images, copying a surface using surface.copy() loses the transparency. So I've got two questions?

1. Why does copy lose the transparency? The docs sound like everything about the new surface should be the same, but that's obviously not happening.

2. Why does this happen with some images and not others?

Here is an example "bad" image -- when copied, the transparency is lost

Here is an example "good" image -- when copied, the transparency is not lost.

And here is the code that you can run to see the difference:

import pygame

def test():
    screen = pygame.display.set_mode((320, 240))
    bad_original = pygame.image.load('bad-image.gif') 
    bad_copied = bad_original.copy()

    good_original = pygame.image.load('good-image.gif') 
    good_copied = good_original.copy()

    while True:
        for event in pygame.event.get():
            if (event.type == pygame.KEYDOWN and event.key == pygame.K_ESCAPE or
                event.type == pygame.QUIT):
                pygame.quit()

        screen.fill((150, 150, 150))

        screen.blit(bad_original, (0,0))
        screen.blit(bad_copied, (100, 0))
        screen.blit(good_original, (0,100))
        screen.blit(good_copied, (100, 100))
        pygame.display.flip()

if __name__ == '__main__':
    test()

And heck, for completion, here's what a screenshot of running the above code looks like.

Please note that I'm not looking for workarounds; I just want to know what I am not understanding about surface.copy, or anything you think I may not understand about working with Pygame surfaces.

I'm using Python 3.3 and Pygame 1.9.2pre on a Windows 7 machine.

Answer 1

You need to use .convert_alpha()

Try:

pygame.image.load('my_image.gif').convert_alpha()

See: http://www.pygame.org/docs/ref/surface.html#pygame.Surface.convert_alpha

"Creates a new copy of the surface with the desired pixel format. The new surface will be in a format suited for quick blitting to the given format with per pixel alpha. If no surface is given, the new surface will be optimized for blitting to the current display.

Unlike the Surface.convert() method, the pixel format for the new image will not be exactly the same as the requested source, but it will be optimized for fast alpha blitting to the destination."

In pygame anytime you load and image, or create a surface, with the intent of displaying it you should .convert() it if it has no transparency, or .convert_alpha() it if it has transparency. This yields both a big speedup AND solves the mystery of, 'Why is my transparency doing that?'.