How to scroll the background surface in PyGame?

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pygame.Surface.scroll() doesn’t do what you expect. It doesn’t seamless roll the surface. See the documentation:

scroll() Shift the surface image in place

[…] Areas of the surface that are not overwritten retain their original pixel values. […]

You’ve to write your own scroll functions, which place the part of the surface, which was shifted out, at the other side of thee surface.

e.g. write 2 functions (scrollX and scrollY), which can roll along an axis:

def scrollX(screenSurf, offsetX):
    width, height = screenSurf.get_size()
    copySurf = screenSurf.copy()
    screenSurf.blit(copySurf, (offsetX, 0))
    if offsetX < 0:
        screenSurf.blit(copySurf, (width + offsetX, 0), (0, 0, -offsetX, height))
    else:
        screenSurf.blit(copySurf, (0, 0), (width - offsetX, 0, offsetX, height))

def scrollY(screenSurf, offsetY):
    width, height = screenSurf.get_size()
    copySurf = screenSurf.copy()
    screenSurf.blit(copySurf, (0, offsetY))
    if offsetY < 0:
        screenSurf.blit(copySurf, (0, height + offsetY), (0, 0, width, -offsetY))
    else:
        screenSurf.blit(copySurf, (0, 0), (0, height - offsetY, width, offsetY))

if pressed[pygame.K_UP]:
    scrollY(screen, 2)
elif pressed[pygame.K_DOWN]:
    scrollY(screen, -2)
elif pressed[pygame.K_LEFT]:
    scrollX(screen, 2)
elif pressed[pygame.K_RIGHT]:
    scrollX(screen, -2)

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