How can I make the background of a surface transparent in pygame

In pygame I draw a figure on a Surface. I want the Surface itself to be transparent but the image visible. I've tried to use set_alpha on the Surface, but the figure itself stays transparent.

    self.original_image = Surface((self.rect.w, self.rect.h))
    self.original_image.set_alpha(0)

    pg.draw.polygon(self.original_image,
                    self.color,
                    [Vector2(gui.PATCH_SIZE, gui.PATCH_SIZE),
                     Vector2(gui.HALF_PATCH_SIZE( ), 0),
                     Vector2(0, gui.PATCH_SIZE),
                     Vector2(gui.HALF_PATCH_SIZE( ), gui.PATCH_SIZE*3/4),
                     ])

    self.image = self.original_image

When I draw it I see nothing. When I leave out self.original_image.set_alpha(0) I get the figure I want, but on a black background rectangle, which obscures whatever it is over. I get the same result if I try self.original_image.fill((0, 0, 0, 0)) instead of self.original_image.set_alpha(0). Any suggestions would be appreciated. Thanks.

P.S. I'm trying to draws a NetLogo standard "Turtle."

Asked by: Grace734 | Posted: 30-11-2021

Answer 1

Example fill background with green color and then it draw surface with red rectangle/poligon.

First uses convert_alpha(), next fill with fill([0,0,0,0]), and you can draw.

In this version you can use colors [R,G,B,Alpha] to set different transparency for every pixel. But you can't uses set_alpha(128) to change transparency for all pixels at one.

import pygame

# --- constants --- (UPPER_CASE_NAMES)

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600

FPS = 25

BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
GREEN = (0, 255, 0)
RED   = (255, 0, 0)

# --- main ---

pygame.init()

screen = pygame.display.set_mode( (SCREEN_WIDTH, SCREEN_HEIGHT) )

surface = pygame.surface.Surface(screen.get_size()).convert_alpha()
surface.fill([0,0,0,0])
pygame.draw.polygon(surface, RED, [(100,100), (200,200), (300,100)])

clock = pygame.time.Clock()

running = True
while running:

    # --- events ---
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

        elif event.type == pygame.KEYUP:
            if event.key == pygame.K_ESCAPE:
                running = False

    # --- draws ---

    screen.fill(GREEN)

    screen.blit(surface, (0,0))

    pygame.display.flip()

    # --- FPS ---

    ms = clock.tick(FPS)
    #pygame.display.set_caption('{}ms'.format(ms)) # 40ms for 25FPS, 16ms for 60FPS
    fps = clock.get_fps()
    pygame.display.set_caption('FPS: {}'.format(fps))

# --- end ---

pygame.quit()

Use set_colorkey([0,0,0]) and blit() will not copy pixels with color [0,0,0].

In this version you can use set_alpha(128) to set the same transparency for all pixels but you can't uses colors [R,G,B,Alpha] to set different transparency for every pixel separatelly.

import pygame

# --- constants --- (UPPER_CASE_NAMES)

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600

FPS = 25 # for more than 220 it has no time to update screen

BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
GREEN = (0, 255, 0)
RED   = (255, 0, 0)

# --- main ---

pygame.init()

screen = pygame.display.set_mode( (SCREEN_WIDTH, SCREEN_HEIGHT) )

surface = pygame.surface.Surface(screen.get_size())
surface.set_colorkey([0,0,0]) # don't copy color [0,0,0] on screen
pygame.draw.polygon(surface, RED, [(100,100), (200,200), (300,100)])

clock = pygame.time.Clock()

running = True
while running:

    # --- events ---
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

        elif event.type == pygame.KEYUP:
            if event.key == pygame.K_ESCAPE:
                running = False

    # --- draws ---

    screen.fill(GREEN)

    #surface.set_alpha(128)
    screen.blit(surface, (0,0))

    pygame.display.flip()

    # --- FPS ---

    ms = clock.tick(FPS)
    #pygame.display.set_caption('{}ms'.format(ms)) # 40ms for 25FPS, 16ms for 60FPS
    fps = clock.get_fps()
    pygame.display.set_caption('FPS: {}'.format(fps))

# --- end ---

pygame.quit()

EDIT:

import pygame

# --- constants --- (UPPER_CASE_NAMES)

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600

FPS = 25 # for more than 220 it has no time to update screen

BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
GREEN = (0, 255, 0)
RED   = (255, 0, 0)
BLUE  = (0, 0, 255)
YELLOW = (255,255,0)

# --- main ---

pygame.init()

screen = pygame.display.set_mode( (SCREEN_WIDTH, SCREEN_HEIGHT) )

# ---

background = pygame.surface.Surface(screen.get_size())

color = (128,128,128)
for x in range(0, SCREEN_WIDTH, 40):
    for y in range(0, SCREEN_HEIGHT, 40):
        pygame.draw.rect(background, color, [x,y,40,40])
        if color == (128,128,128):
            color = (192,192,192)
        else:
            color = (128,128,128)

# ---

surface = pygame.surface.Surface(screen.get_size())
surface.set_colorkey([0,0,0]) # don't copy color [0,0,0] on screen
pygame.draw.polygon(surface, RED, [(100,100), (200,200), (300,100)])
pygame.draw.polygon(surface, GREEN, [(100,300), (200,200), (300,300)])
pygame.draw.polygon(surface, BLUE, [(100,100), (200,200), (100,300)])
pygame.draw.polygon(surface, YELLOW, [(300,100), (200,200), (300,300)])
surface.set_alpha(128) # semi transparent all pixels

# --- mainloop ---

clock = pygame.time.Clock()

running = True
while running:

    # --- events ---

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

        elif event.type == pygame.KEYUP:
            if event.key == pygame.K_ESCAPE:
                running = False

    # --- draws ---

    screen.blit(background, (0,0))

    screen.blit(surface, (0,0))

    pygame.display.flip()

    # --- FPS ---

    ms = clock.tick(FPS)
    #pygame.display.set_caption('{}ms'.format(ms)) # 40ms for 25FPS, 16ms for 60FPS
    fps = clock.get_fps()
    pygame.display.set_caption('FPS: {}'.format(fps))

# --- end ---

pygame.quit()

Answered by: Sarah242 | Posted: 01-01-2022

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How can I make the background of a surface transparent in pygame

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