Tag Archives: pygame

Python Fast XML Parsing

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Here is a useful tip on Python XML decoding.

I was extending xml.sax.ContentHandler class in a example to decode maps for a Pygame application when my connection went down and I noticed that the program stop working raising a exception regarded a call to urlib (a module for retrieve resources by url). I noticed that the module was getting the remote DTD schema to validate the XML.

<!DOCTYPE map SYSTEM "http://mapeditor.org/dtd/1.0/map.dtd">

This is not a requirement for my applications and it’s a huge performance overhead when works (almost 1 second for each map loaded) and when the applications is running in a environment without Internet it just waits for almost a minute and then fail with the remain decoding. A dirty workaround is open the XML file and get rid of the line containing the DTD reference.

But the correct way to programming XML decoding when we are not concerned on validate a XML schema is just the xml.parsers.expat. Instead of using a interface you just have to set some callback functions with the behaviors we want. This is a example from the documentation:

import xml.parsers.expat
 
# 3 handler functions
def start_element(name, attrs):
    print 'Start element:', name, attrs
def end_element(name):
    print 'End element:', name
def char_data(data):
    print 'Character data:', repr(data)
 
p = xml.parsers.expat.ParserCreate()
 
p.StartElementHandler = start_element
p.EndElementHandler = end_element
p.CharacterDataHandler = char_data
 
p.Parse("""<?xml version="1.0"?>
<parent id="top"><child1 name="paul">Text goes here</child1>
<child2 name="fred">More text</child2>
</parent>""", 1)

The output:

Start element: parent {'id': 'top'}
Start element: child1 {'name': 'paul'}
Character data: 'Text goes here'
End element: child1
Character data: '\n'
Start element: child2 {'name': 'fred'}
Character data: 'More text'
End element: child2
Character data: '\n'
End element: parent

Pygame: Running Orcs

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Here is a Pygame Sprite animation using the approach presented by Joe Wreschnig and Nicolas Crovatti. It’s not yet exactly what I need but is very suitable.

import pygame, random
from pygame.locals import *
 
class Char(pygame.sprite.Sprite):
	x,y = (100,0)
	def __init__(self, img, frames=1, modes=1, w=32, h=32, fps=3):
		pygame.sprite.Sprite.__init__(self)
		original_width, original_height = img.get_size()
		self._w = w
		self._h = h
		self._framelist = []
		for i in xrange(int(original_width/w)):
			self._framelist.append(img.subsurface((i*w,0,w,h)))
		self.image = self._framelist[0]		
		self._start = pygame.time.get_ticks()
		self._delay = 1000 / fps
		self._last_update = 0
		self._frame = 0
		self.update(pygame.time.get_ticks(), 100, 100)	
 
	def set_pos(self, x, y):
		self.x = x
		self.y = y
 
	def get_pos(self):
		return (self.x,self.y)
 
	def update(self, t, width, height):
		# postion
		self.y+=1
		if(self.y>width):
			self.x = random.randint(0,height-self._w)
			self.y = -self._h
 
		# animation
		if t - self._last_update > self._delay:
			self._frame += 1
			if self._frame >= len(self._framelist):
				self._frame = 0
			self.image = self._framelist[self._frame]
			self._last_update = t
 
SCREEN_W, SCREEN_H = (320, 320)
 
def main():
	pygame.init()
	screen = pygame.display.set_mode((SCREEN_W, SCREEN_H))
	background = pygame.image.load("field.png")
	img_orc = pygame.image.load("orc.png")
	orc = Char(img_orc, 4, 1, 32, 48)
	while pygame.event.poll().type != KEYDOWN:
		screen.blit(background, (0,0))
		screen.blit(orc.image,  orc.get_pos())
		orc.update(pygame.time.get_ticks(), SCREEN_W, SCREEN_H)
		pygame.display.update()
		pygame.time.delay(10)
 
if __name__ == '__main__': main()

Here is it working:

Uptade: I put this source and images at the OpenPixel project in Github

Pygame Simple Key Handling

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Here’s a simple key handle in Pygame wheres you move a circle using keyboard.

import pygame
from pygame.locals import *
 
def main():
	x,y = (100,100)
	pygame.init()
	screen = pygame.display.set_mode((400, 400))
	while 1:
		pygame.time.delay(1000/60)
      # exit handle
		for event in pygame.event.get():
			if event.type == QUIT:
				return
			elif event.type == KEYDOWN and event.key == K_ESCAPE:
				return
 
      # keys handle 
		key=pygame.key.get_pressed()
		if key[K_LEFT]:
			x-=1
		if key[K_RIGHT]:
			x+=1
		if key[K_UP]:
			y-=1
		if key[K_DOWN]:
			y+=1
 
		# fill background and draw a white circle
		screen.fill((255,255,255))
		pygame.draw.circle(screen, (0,0,0), [x,y], 30)
		pygame.display.flip()
 
if __name__ == '__main__': main()

Here’s a video of it working:

Function pygame.key.get_pressed Returns a sequence of boolean values representing the state of every key on the keyboard. It’s very useful because usually on others game platforms I have to create it by myself.

This approach allow me to handle more than one key at time. For example, left and up keys can be pressed and each one is handled separately creating a diagonal movement.

Pygame, Simple Space Effect

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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 <me@silveiraneto.net>
# 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()