How to get the Difference blending mode?

Question

I'm trying to make a night effect for my game in Pygame. So I'm gonna blit a black image on the screen then, change its blending mode to Difference just like in Photoshop in order to make the screen look darker. However, I still don't know how to do that as I haven't used the blending modes in Pygame yet. Any help ?

Answer 1

The blending mode can be changed by setting the optional special_flags argument of pygame.Surface.blit:

blit(source, dest, area=None, special_flags=0) -> Rect
[...]
New in pygame 1.8: Optional special_flags: BLEND_ADD, BLEND_SUB, BLEND_MULT, BLEND_MIN, BLEND_MAX.
New in pygame 1.8.1: Optional special_flags: BLEND_RGBA_ADD, BLEND_RGBA_SUB, BLEND_RGBA_MULT, BLEND_RGBA_MIN, BLEND_RGBA_MAX, BLEND_RGB_ADD, BLEND_RGB_SUB, BLEND_RGB_MULT, BLEND_RGB_MIN, BLEND_RGB_MAX.
New in pygame 1.9.2: Optional special_flags: BLEND_PREMULTIPLIED
New in pygame 2.0.0: Optional special_flags: BLEND_ALPHA_SDL2 [...]

e.g.:

screen.blit(image, (x, y), special_flags = pygame.BLEND_RGBA_SUB)

---

Unfortunately, Pygame doesn't have a blending mode that gives the absolute difference of 2 images. However it can be achieved with

MAX(SUB(image1, imgage2), SUB(image2, image1))

e.g.:

image1 = pygame.image.load('image2.png')
image2 = pygame.image.load('image1.png')
temp_image = image1.copy() 
temp_image.blit(image2, (0, 0), special_flags = pygame.BLEND_RGBA_SUB)
final_image = image2.copy() 
final_image.blit(image1, (0, 0), special_flags = pygame.BLEND_RGBA_SUB)
final_image.blit(temp_image, (0, 0), special_flags = pygame.BLEND_RGBA_MAX)