如何使用pygame在精灵后面留下痕迹？

Question

众所周知，如果你移动一个精灵而不在它留下轨迹之前填充屏幕，但是，我想在移动其他东西时留下一个很酷的轨迹（这意味着我不能简单地停止填充屏幕。 在此先感谢您的帮助

Answer 1

一种解决方案是创建另一个透明表面（此处称为alpha_surf），其大小与您在其上用轨迹对对象进行blit 的屏幕大小相同。它必须是每像素的 alpha 表面，您可以通过传递 pygame.SRCALPHA 特殊标志来创建它。

通过用透明白色填充对象并减少每帧alpha_surf 上所有像素的 alpha，并传递pygame.BLEND_RGBA_MULT 标志，以便仅影响 alpha 通道。

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()

或者，您可以创建具有不同 alpha 值的精灵图像的多个版本，并存储精灵的先前位置。然后在前面的位置用较低的 alpha 对图像进行 blit。

如果您想创建不同类型的轨迹，例如烟雾，您也可以使用其他图像或粒子来代替 self.image。

这是另一个变体，带有单独的不同图像，用于在精灵的 self.images 被 blitted 之前被 blitted，因此它会出现在它们下方：

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()