BLEND_OVERLAY blend mode

[itzpr3d4t0r]

Closes #2373, featuring:
BLEND_OVERLAY and BLEND_RGB_OVERLAY flags and algorithm with:

• 1 pixel at a time implementation that's hardware agnostic
• 8 pixels at a time with AVX2
• 4 pixels at a time with SSE2

For a visual test i'm using the following image and program:

import pygame

pygame.init()
TEXT_COLOR = "black"
screen = pygame.display.set_mode((1000, 500))

img = pygame.image.load("base.png").convert()
img = pygame.transform.scale(img, (250, 250))

background = pygame.Surface(img.get_size())
background.fill((255, 100, 50))

background2 = pygame.Surface(img.get_size())
background2.fill((255, 0, 100))

font = pygame.font.SysFont("Arial", 28, True)
BLEND_OVERLAY = font.render("OVERLAY", True, TEXT_COLOR)
BLEND_ADD = font.render("ADD", True, TEXT_COLOR)
BLEND_MULT = font.render("MULT", True, TEXT_COLOR)
BASE = font.render("BASE", True, TEXT_COLOR)
MIN = font.render("MIN", True, TEXT_COLOR)
MAX = font.render("MAX", True, TEXT_COLOR)
SUB = font.render("SUB", True, TEXT_COLOR)

keep = True
clock = pygame.Clock()

while keep:
    clock.tick(60)

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            keep = False

    screen.blits(
        [
            (img, (0, 250)),
            (img, (250, 250)),
            (img, (500, 250)),
            (img, (750, 250)),
            (img, (0, 0)),
            (img, (250, 0)),
            (img, (500, 0)),
            (img, (750, 0)),
            (background, (0, 0), None, pygame.BLEND_OVERLAY),
            (background, (250, 0), None, pygame.BLEND_ADD),
            (background, (500, 0), None, pygame.BLEND_MULT),
            (background, (250, 250), None, pygame.BLEND_MIN),
            (background, (500, 250), None, pygame.BLEND_MAX),
            (background, (750, 250), None, pygame.BLEND_SUB),
            (BLEND_OVERLAY, BLEND_OVERLAY.get_rect(center=(125, 20))),
            (BLEND_ADD, BLEND_ADD.get_rect(center=(375, 20))),
            (BLEND_MULT, BLEND_MULT.get_rect(center=(625, 20))),
            (BASE, BASE.get_rect(center=(125, 270))),
            (MIN, MIN.get_rect(center=(375, 270))),
            (MAX, MAX.get_rect(center=(625, 270))),
            (SUB, SUB.get_rect(center=(875, 270))),
        ]
    )

    pygame.display.flip()

[MyreMylar]

OK, this all seems to work OK, the tests make sense and I'm convinced that it has value as a blend mode.

Only two things lacking really:

• An SSE2/NEON version for decent performance on macs/arm.
• Documentation on the special flags page - ideally we should include the formula in this one as it is more complicated than the existing set of MUL, ADD & SUB blend modes. Something like the description on the wikipedia page:
  

Would help I think.

I think we could have the SSE2 version in another PR - because there will at least be a version for all platforms after this one, but we definitely need the docs or nobody will know it exists.

[itzpr3d4t0r]

Since this has been up for a long time I've also added the SSE2 RGB blitter.

[MyreMylar]

Great!

The new docs look lovely:

And the blend mode still works locally, as well as passing the unit tests on all platforms now.

Approved!