pygame Drawing on the screen Transparency


There are kinds 3 of transparency supported in pygame: colorkeys, Surface alphas, and per-pixel alphas.


Makes a color fully transparent, or more accurately, making a color simply not be blit. If you have an image with a black rect inside you could set a colorkey to prevent the black color from being blit.

BLACK = (0, 0, 0)
my_image.set_colorkey(BLACK)  # Black colors will not be blit.

A Surface can only have one colorkey. Setting another colorkey will overwrite the previous. Colorkeys cannot have different alpha values, it can only make a color not visible.

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Surface alphas

Makes the whole Surface transparent by an alpha value. With this method you can have different alpha values but it will affect the whole Surface.

my_image.set_alpha(100)  # 0 is fully transparent and 255 fully opaque.

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Per-pixel alpha

Makes every pixel in the Surface transparent by a individual alpha value. This gives you the most freedom and flexibility but is also the slowest method. This method also requires the Surface to be created as a per-pixel alpha Surface, and the color arguments needs to contain a fourth alpha integer.

size = width, height = (32, 32)
my_image = pygame.Surface(size, pygame.SRCALPHA)  # Creates an empty per-pixel alpha Surface.

The Surface will now draw transparency if the color contains the fourth alpha value.

BLUE = (0, 0, 255, 255)
pygame.draw.rect(my_image, BLUE, my_image.get_rect(), 10)

Unlike the other Surfaces, this Surface default color won't be black but transparent. That's why the black rectangle in the middle disappear.

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Combine colorkey and Surface alpha

Colorkeys and Surface alphas can be combined, but per-pixel alpha cannot. This can be useful if you don't want the slower performance of a per-pixel Surface.


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Full code

Copy this in an empty file and run it. Press the keys 1, 2, 3 or 4 to make the images appear. Press 2, 3 or 4 multiple times to make them more opaque.

import pygame

WHITE = (255, 255, 255)
BLACK = (0, 0, 0)
RED = (255, 0, 0)
GREEN = (0, 255, 0)
BLUE = (0, 0, 255, 50)  # This color contains an extra integer. It's the alpha value.
PURPLE = (255, 0, 255)

screen = pygame.display.set_mode((200, 325))
screen.fill(WHITE)  # Make the background white. Remember that the screen is a Surface!
clock = pygame.time.Clock()

size = (50, 50)
red_image = pygame.Surface(size)
green_image = pygame.Surface(size)
blue_image = pygame.Surface(size, pygame.SRCALPHA)  # Contains a flag telling pygame that the Surface is per-pixel alpha
purple_image = pygame.Surface(size)

# For the 'blue_image' it's the alpha value of the color that's been drawn to each pixel that determines transparency.

pygame.draw.rect(red_image, RED, red_image.get_rect(), 10)
pygame.draw.rect(green_image, GREEN, green_image.get_rect(), 10)
pygame.draw.rect(blue_image, BLUE, blue_image.get_rect(), 10)
pygame.draw.rect(purple_image, PURPLE, purple_image.get_rect(), 10)

while True:

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_1:
                screen.blit(red_image, (75, 25))
            elif event.key == pygame.K_2:
                screen.blit(green_image, (75, 100))
            elif event.key == pygame.K_3:
                screen.blit(blue_image, (75, 175))
            elif event.key == pygame.K_4:
                screen.blit(purple_image, (75, 250))