Pygame moving stars

Question

Am trying to program a moving starry background, where a random amount of white stars(dots) are drawn onto a black background and slowly drop to the bottom and once they do, reappear on top.

So far, I'm getting my starry background although static and with weird patterns ..

Bonus points for having new stars appear at the top of the screen :)

Thanks!

import pygame
import random

pygame.init()

WIDTH = 480
HEIGHT = 600

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

FPS = 60
clock = pygame.time.Clock()

screen = pygame.display
screen.set_caption("Starry Night")
screen = screen.set_mode((WIDTH, HEIGHT))
screen.fill(BLACK)

yspeed = 5

x = 1
y = 1

class Star(object):
    def __init__(self, x, y, yspeed):
        self.colour = WHITE
        self.radius = 1
        self.x = x
        self.y = y
        self.yspeed = yspeed

    def draw(self):
        pygame.draw.circle(screen, self.colour, (self.x, self.y), self.radius)

    def fall(self):
        self.y += self.yspeed

    def check_if_i_should_reappear_on_top(self):
        if self.y >= HEIGHT:
            self.y = 0


stars = []

for i in range(100):
    x = random.randint(1, WIDTH - 1)
    y = random.randint(1, HEIGHT - 1)
    stars.append(Star(x, y, yspeed))

GameOn = True

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

    for star in stars:
        star.draw()
        star.fall()
        star.check_if_i_should_reappear_on_top()

    pygame.display.flip()
    clock.tick(FPS)

pygame.quit()

score 1 · Accepted Answer · answered Jan 15 '21 at 18:44

You have to redraw the entire scene in each frame. Therefore you must clear the display in each frame:

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

    screen.fill(BLACK)                            # <--- CLAER DISPLAY

    for star in stars:
        star.draw()
        star.fall()
        star.check_if_i_should_reappear_on_top()

    pygame.display.flip()
    clock.tick(FPS)

The typical PyGame application loop has to:

handle the events by either pygame.event.pump() or pygame.event.get().
update the game states and positions of objects dependent on the input events and time (respectively frames)
clear the entire display or draw the background
draw the entire scene (blit all the objects)
update the display by either pygame.display.update() or pygame.display.flip()

Pygame moving stars

1 Answers1