Silveira Neto Posts

This is a simple space effect of sliding stars using Pygame.

Direct link to video: simple_space_effect_01.ogv

We set some constants like the screen size and the number N of star we want.

N = 200
SCREEN_W, SCREEN_H = (640, 480)

Using list comprehension we create a list of random points in the screen, that will be our stars. The size of this list is N.

stars = [
  [random.randint(0, SCREEN_W),random.randint(0, SCREEN_H)]
  for x in range(N)
]

Each star is represented by one tuple on the stars list. The first star is on stars[0] and is a touple with [x, y] positions.

At each step from the game loop we draw and update the position of each star. A star is draw as a white line of one pixel. See the pygame.draw.line doc.

for star in stars:
  pygame.draw.line(background,
    (255, 255, 255), (star[0], star[1]), (star[0], star[1]))
  star[0] = star[0] - 1
  if star[0] < 0:
      star[0] = SCREEN_W
      star[1] = random.randint(0, SCREEN_H)

In this example we update the position of a star by decreasing its horizontal position. When the horizontal position is less than zero, it's not displayed on the screen anymore so we replace its horizontal position (star[0]) by the screen width (SCREEN_W) and the vertical position (star[1]) by a new random position. This will be like create a new star and guarantee always a different pattern of sliding stars.

The complete code:

#!/usr/bin/env python

# A simple effect of sliding stars to create a deep space sensation.
# by Silveira Neto 
# Free under the terms of GPLv3 license
# See http://silveiraneto.net/2009/08/12/pygame-simple-space-effect/

import os,sys,random
import pygame
from pygame.locals import *

# Constants 
N = 200
SCREEN_W, SCREEN_H = (640, 480)

def main():
	# basic start
	pygame.init()
	screen = pygame.display.set_mode((SCREEN_W,SCREEN_H))
	pygame.display.set_caption('Simple Space Effect by Silveira Neto')

	# create background
	background = pygame.Surface(screen.get_size())
	background = background.convert()

	# generate N stars
	stars = [
		[random.randint(0, SCREEN_W),random.randint(0, SCREEN_H)]
		for x in range(N)
	]

	# main loop
	clock = pygame.time.Clock()
	while 1:
		clock.tick(22)
		for event in pygame.event.get():
			if event.type == QUIT:
				return
			elif event.type == KEYDOWN and event.key == K_ESCAPE:
				return
		background.fill((0,0,0))
		for star in stars:
			pygame.draw.line(background,
				(255, 255, 255), (star[0], star[1]), (star[0], star[1]))
			star[0] = star[0] - 1
			if star[0] < 0:
				star[0] = SCREEN_W
				star[1] = random.randint(0, SCREEN_H)
		screen.blit(background, (0,0))
		pygame.display.flip()

if __name__ == '__main__': main()