# Tag: space effect

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

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 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, star), (star, star))
star = star - 1
if star < 0:
star = SCREEN_W
star = 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) by the screen width (SCREEN_W) and the vertical position (star) 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 https://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, star), (star, star))
star = star - 1
if star < 0:
star = SCREEN_W
star = random.randint(0, SCREEN_H)
screen.blit(background, (0,0))
pygame.display.flip()

if __name__ == '__main__': main()
```