How do I leave a trail behind a sprite with pygame?

Question

It's known that if you move a sprite without filling the screen before it leaves a trail, however, I want to leave a cool trail behind my while moving other stuff (which means I cannot simply stop filling the screen.

Answer 1

One solution would be to create another transparent surface (called alpha_surf here) with the size of the screen onto which you blit the objects with trails. It needs to be a per-pixel alpha surface which you can create by passing the pygame.SRCALPHA special flag.

Blit the objects and reduce the alpha of all pixels on the alpha_surf each frame by filling it with a transparent white and also pass the pygame.BLEND_RGBA_MULT flag so that only the alpha channel is affected.

import pygame as pg
from pygame.math import Vector2


class Player(pg.sprite.Sprite):

    def __init__(self, pos, *groups):
        super().__init__(*groups)
        self.image = pg.Surface((50, 50), pg.SRCALPHA)
        pg.draw.circle(self.image, pg.Color('dodgerblue'), (25, 25), 25)
        self.rect = self.image.get_rect(center=pos)
        self.vel = Vector2(0, 0)
        self.pos = Vector2(pos)

    def update(self):
        self.pos += self.vel
        self.rect.center = self.pos


def main():
    pg.init()
    screen = pg.display.set_mode((640, 480))
    # Blit objects with trails onto this surface instead of the screen.
    alpha_surf = pg.Surface(screen.get_size(), pg.SRCALPHA)
    clock = pg.time.Clock()
    all_sprites = pg.sprite.Group()
    player = Player((150, 150), all_sprites)

    while True:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                return
            elif event.type == pg.KEYDOWN:
                if event.key == pg.K_d:
                    player.vel.x = 5
                elif event.key == pg.K_a:
                    player.vel.x = -5
                elif event.key == pg.K_w:
                    player.vel.y = -5
                elif event.key == pg.K_s:
                    player.vel.y = 5
            elif event.type == pg.KEYUP:
                if event.key == pg.K_d and player.vel.x > 0:
                    player.vel.x = 0
                elif event.key == pg.K_a and player.vel.x < 0:
                    player.vel.x = 0
                elif event.key == pg.K_w:
                    player.vel.y = 0
                elif event.key == pg.K_s:
                    player.vel.y = 0

        # Reduce the alpha of all pixels on this surface each frame.
        # Control the fade speed with the alpha value.
        alpha_surf.fill((255, 255, 255, 220), special_flags=pg.BLEND_RGBA_MULT)

        all_sprites.update()
        screen.fill((20, 50, 80))  # Clear the screen.
        all_sprites.draw(alpha_surf)  # Draw the objects onto the alpha_surf.
        screen.blit(alpha_surf, (0, 0))  # Blit the alpha_surf onto the screen.
        pg.display.flip()
        clock.tick(60)


if __name__ == '__main__':
    main()
    pg.quit()

Alternatively, you could create several versions of the sprite image with different alpha values and also store the previous positions of the sprite. Then just blit the images with lower alpha at the previous positions.

You can also blit other images or particles instead of the self.image if you want to create a different kind of trail, for example smoke.

Here's another variant with a separate, different image for the trail which gets blitted before the self.images of the sprites are blitted, so that it'll appear below them:

import pygame as pg
from pygame.math import Vector2


class Player(pg.sprite.Sprite):

    def __init__(self, pos, *groups):
        super().__init__(*groups)
        self.image = pg.Surface((50, 70), pg.SRCALPHA)
        self.image.fill(pg.Color('sienna1'))
        self.rect = self.image.get_rect(center=pos)
        # A separate image for the trail (just a single-color circle).
        self.trail_image = pg.Surface((40, 40), pg.SRCALPHA)
        pg.draw.circle(self.trail_image, pg.Color('dodgerblue'), (20, 20), 20)
        self.trail_rect = self.trail_image.get_rect()

        self.vel = Vector2(0, 0)
        self.pos = Vector2(pos)

    def update(self):
        self.pos += self.vel
        self.rect.center = self.pos
        # Update the rect of the trail as well, because we'll blit it there.
        self.trail_rect.center = self.rect.center


def main():
    pg.init()
    screen = pg.display.set_mode((640, 480))
    # Blit objects with trails onto this surface instead of the screen.
    alpha_surf = pg.Surface(screen.get_size(), pg.SRCALPHA)
    clock = pg.time.Clock()
    all_sprites = pg.sprite.Group()
    sprites_with_trails = pg.sprite.Group()
    player = Player((150, 150), all_sprites, sprites_with_trails)

    while True:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                return
            elif event.type == pg.KEYDOWN:
                if event.key == pg.K_d:
                    player.vel.x = 5
                elif event.key == pg.K_a:
                    player.vel.x = -5
                elif event.key == pg.K_w:
                    player.vel.y = -5
                elif event.key == pg.K_s:
                    player.vel.y = 5
            elif event.type == pg.KEYUP:
                if event.key == pg.K_d and player.vel.x > 0:
                    player.vel.x = 0
                elif event.key == pg.K_a and player.vel.x < 0:
                    player.vel.x = 0
                elif event.key == pg.K_w:
                    player.vel.y = 0
                elif event.key == pg.K_s:
                    player.vel.y = 0

        # Reduce the alpha of all pixels on this surface each frame.
        # Control the fade speed with the alpha value.
        alpha_surf.fill((255, 255, 255, 244), special_flags=pg.BLEND_RGBA_MULT)

        all_sprites.update()
        screen.fill((20, 50, 80))  # Clear the screen.
        # Blit the trails onto the alpha_surf.
        for sprite in sprites_with_trails:
            alpha_surf.blit(sprite.trail_image, sprite.trail_rect)
        screen.blit(alpha_surf, (0, 0))  # Blit the alpha_surf onto the screen.
        all_sprites.draw(screen)  # Draw the objects onto the alpha_surf.
        pg.display.flip()
        clock.tick(60)


if __name__ == '__main__':
    main()
    pg.quit()