lab 7a: sprites I

Lab 7a: Sprites and pygame (part I)

In this lab we'll focus on a special pygame class to handle moving and interactive objects in our games: Sprites.

First, as always, move into your games directory, create a directory for this week's lab, and move into that. E.g.


   cd games
   mkdir lab7
   cd lab7

Next, copy across some files for this week's lab: backdrop.gif, ship.gif, star.gif.

Today we'll be creating our python files from scratch, e.g.
gedit lab7a.py &

Once the file has been created, you'll need to make it executable, e.g.
chmod u+x *.py

Pre-amble: clean program structure

#! /usr/bin/python
import sys, pygame, math

# =====================================================================
# GLOBAL VARIABLES
#   - list all variables that need to be globally accessible

# define the size of the pygame display window
scrSize = scrWidth, scrHeight = 640,480

gameScreen = None   # the main display screen
backImage = None    # the loaded background image
scrRefreshRate = 40 # the pause (in milliseconds) between updates


# =====================================================================
# SETUP ROUTINE   
#   - initializes the pygame display screen and background image

def gameSetup():
   global gameScreen, scrSize, backImage
   pygame.init()
   gameScreen = pygame.display.set_mode(scrSize)
   backImage = pygame.image.load('backdrop.gif')
   gameScreen.blit(backImage, (0,0))


# =====================================================================
# GAME OBJECTS
#   - define the different types of in game objects (e.g. ships,
#     characters, etc)


# =====================================================================
# EVENT HANDLING ROUTINE
#    - processes any pending in game events,
#      returns True if the game should keep playing,
#      returns False if the game should quit

def processEvents():

   # process each pending event
   for event in pygame.event.get():

      # if the user closed the window send False back
      #    to tell the game to quit playing
      if event.type == pygame.QUIT:
         return False

   # if this point is reached then no quit event was
   #    encountered, so return True to keep playing
   return True


# =====================================================================
# MAIN GAME CONTROL ROUTINE
#   - sets up the game and runs the main game update loop
#     until instructed to quit

def main():

   # identify any global variables the main routine needs to access
   global gameScreen, backImage, scrRefreshRate

   # initialize pygame and the game's display screen
   gameSetup()

   # run the main game loop
   keepPlaying = True
   while keepPlaying:

      # handle any pending events
      keepPlaying = processEvents()

      # switch to the new display image
      pygame.display.flip()

      # pause before initiating the next loop cycle
      pygame.time.delay(scrRefreshRate)

   # the main game loop has completed, so exit the game
   sys.exit()


# =====================================================================
# INITIATE THE GAME
#    - calls the main() routine
if __name__ == "__main__":
   main()

Sprites for in-game objects

Part of our previous lab looked at reorganizing our game code to use Python classes to hold information about the various types of objects in our game.

There is also a special PyGame class called Sprites that have a variety of builtin properties and routines that we will look at today.

We will create a GameObject class, based on the PyGame Sprites class, that allows us to do some cleanly automated updating of objects.

Our game object class will have two routines (or methods) associated with it:

a constructor, or initialization routine, that sets up the initial position, speed, image, etc for the object, and
an update routine that regularly adjusts the object's current position and image

# =====================================================================
# GAME OBJECT
#   - controls the basic movable in-game objects (ships in this case)

class GameObject(pygame.sprite.Sprite):

   # the constructor (initialization routine) for the
   #     movable game objects
   def __init__(self, image, x, y, direction, speed):

      # initialize a pygame sprite for the object
      pygame.sprite.Sprite.__init__(self)

      # load the image for the object
      self.origImage = pygame.image.load(image)
      self.image = self.origImage

      # set up the initial position for the object
      self.position = self.x, self.y = x, y

      # set up the initial direction for the object
      self.facingDir = direction

      # set up the initial velocity of the object
      self.speed = speed


   # the update routine adjusts the object's current position and
   #     image based on its speed and direction
   def update(self):

      # update the object's position
      self.x = self.x + self.speed[0]
      self.y = self.y + self.speed[1]
      self.position = self.x, self.y

      # update the object's image
      self.image = pygame.transform.rotate(self.origImage, self.facingDir)

      # center the object's image in its new position
      self.rect = self.image.get_rect()
      self.rect.center = self.position

The class definition above defines how a "GameObject" works, but we still need to add code to our main routine to create the actual game objects we want to see, and to provide specific initial values for their speed, facing, position, etc.

We'll revise our main routine to:

create a list of game objects (three of them in this case), providing initial values for the speed, position, facing, and image for each object in the list,
create a special group for these objects, and
issue commands to the group (rather than having to go through and update them individually).

# =====================================================================
# MAIN GAME CONTROL ROUTINE
#   - sets up the game and runs the main game update loop
#     until instructed to quit

def main():

   # identify any global variables the main routine needs to access
   global gameScreen, backImage, scrRefreshRate

   # initialize pygame and the game's display screen
   gameSetup()

   # create an array of objects to add to the display,
   # giving each of them 
   #      an image, xcoord, ycoord, facing, and speed
   gameObjList = [ 
               GameObject('ship.gif', 80, 60, 270, [1,1]),
               GameObject('ship.gif', 560, 60, 180, [-1,1]),
               GameObject('ship.gif', 560, 420, 90, [-1,-1]) 
	       ]

   # create a group for the game objects
   objGroup = pygame.sprite.RenderUpdates(*gameObjList)

   # run the main game loop
   keepPlaying = True
   while keepPlaying:

      # handle any pending events
      keepPlaying = processEvents()

      # update the display of the object groups
      objGroup.clear(gameScreen, backImage)

      # run the updates on each object in the group
      objGroup.update()

      # switch to the new display image
      pygame.display.flip()

      # pause before initiating the next loop cycle
      pygame.time.delay(scrRefreshRate)

   # the main game loop has completed, so exit the game
   sys.exit()

Updating the display

We're getting closer, but the game display hasn't been told how/when to redraw the portions of the screen that need to change based on the movement of our game objects.

This involves two steps:

setting up a list of rectangles that keep track of the pieces of the screen that need to be redrawn because some of our game objects have moved off of or onto them (this is much more efficient than redrawing the whole display every time), and
adding code to the while keepPlaying loop in our main routine to handle the screen revisions.

# =====================================================================
# MAIN GAME CONTROL ROUTINE
#   - sets up the game and runs the main game update loop
#     until instructed to quit

def main():

   # identify any global variables the main routine needs to access
   global gameScreen, backImage, scrRefreshRate

   # initialize pygame and the game's display screen
   gameSetup()

   # create the list of screen update sections (rectangles)
   updateSections = None

   # create an array of objects to add to the display,
   # giving each of them 
   #      an image, xcoord, ycoord, facing, and speed
   gameObjList = [ 
               GameObject('ship.gif', 80, 60, 270, [1,1]),
               GameObject('ship.gif', 560, 60, 180, [-1,1]),
               GameObject('ship.gif', 560, 420, 90, [-1,-1]) 
	       ]

   # create a group for the game objects
   objGroup = pygame.sprite.RenderUpdates(*gameObjList)

   # run the main game loop
   keepPlaying = True
   while keepPlaying:

      # handle any pending events
      keepPlaying = processEvents()

      # update the display of the object groups
      objGroup.clear(gameScreen, backImage)

      # run the updates on each object in the group
      objGroup.update()

      # update the display rectangles
      updateSections = objGroup.draw(gameScreen)

      # update the buffered display
      pygame.display.update(updateSections)

      # switch to the new display image
      pygame.display.flip()

      # pause before initiating the next loop cycle
      pygame.time.delay(scrRefreshRate)

   # the main game loop has completed, so exit the game
   sys.exit()

Complete version

Hopefully we can now see three ships moving towards the center of the screen. (A copy of the complete code is included below.)

#! /usr/bin/python
import sys, pygame, math

# =====================================================================
# GLOBAL VARIABLES
#   - list all variables that need to be globally accessible

# define the size of the pygame display window
scrSize = scrWidth, scrHeight = 640,480

gameScreen = None   # the main display screen
backImage = None    # the loaded background image
scrRefreshRate = 40 # the pause (in milliseconds) between updates


# =====================================================================
# SETUP ROUTINE   
#   - initializes the pygame display screen and background image

def gameSetup():
   global gameScreen, scrSize, backImage
   pygame.init()
   gameScreen = pygame.display.set_mode(scrSize)
   backImage = pygame.image.load('backdrop.gif')
   gameScreen.blit(backImage, (0,0))


# =====================================================================
# GAME OBJECT
#   - controls the basic movable in-game objects (ships in this case)

class GameObject(pygame.sprite.Sprite):

   # the constructor (initialization routine) for the
   #     movable game objects
   def __init__(self, image, x, y, direction, speed):

      # initialize a pygame sprite for the object
      pygame.sprite.Sprite.__init__(self)

      # load the image for the object
      self.origImage = pygame.image.load(image)
      self.image = self.origImage

      # set up the initial position for the object
      self.position = self.x, self.y = x, y

      # set up the direction for the object
      self.facingDir = direction

      # set up the initial velocity of the object
      self.speed = speed


   # the update routine adjusts the object's current position and
   #     image based on its speed and direction
   def update(self):

      # update the object's position
      self.x = self.x + self.speed[0]
      self.y = self.y + self.speed[1]
      self.position = self.x, self.y

      # update the object's image
      self.image = pygame.transform.rotate(self.origImage, self.facingDir)

      # center the object's image in its new position
      self.rect = self.image.get_rect()
      self.rect.center = self.position


# =====================================================================
# EVENT HANDLING ROUTINE
#    - processes any pending in game events,
#      returns True if the game should keep playing,
#      returns False if the game should quit

def processEvents():

   # process each pending event
   for event in pygame.event.get():

      # if the user closed the window send False back
      #    to tell the game to quit playing
      if event.type == pygame.QUIT:
         return False

   # if this point is reached then no quit event was
   #    encountered, so return True to keep playing
   return True


# =====================================================================
# MAIN GAME CONTROL ROUTINE
#   - sets up the game and runs the main game update loop
#     until instructed to quit

def main():

   # identify any global variables the main routine needs to access
   global gameScreen, backImage, scrRefreshRate

   # initialize pygame and the game's display screen
   gameSetup()

   # create the list of screen update sections (rectangles)
   updateSections = None

   # create an array of objects to add to the display,
   # giving each of them 
   #      an image, xcoord, ycoord, facing, and speed
   gameObjList = [ 
               GameObject('ship.gif', 80, 60, 270, [1,1]),
               GameObject('ship.gif', 560, 60, 180, [-1,1]),
               GameObject('ship.gif', 560, 420, 90, [-1,-1]) 
	       ]

   # create a group for the game objects
   objGroup = pygame.sprite.RenderUpdates(*gameObjList)

   # run the main game loop
   keepPlaying = True
   while keepPlaying:

      # handle any pending events
      keepPlaying = processEvents()

      # update the display of the object groups
      objGroup.clear(gameScreen, backImage)

      # run the updates on each object in the group
      objGroup.update()

      # update the display rectangles
      updateSections = objGroup.draw(gameScreen)

      # update the buffered display
      pygame.display.update(updateSections)

      # switch to the new display image
      pygame.display.flip()

      # pause before initiating the next loop cycle
      pygame.time.delay(scrRefreshRate)

   # the main game loop has completed, so exit the game
   sys.exit()


# =====================================================================
# INITIATE THE GAME
#    - calls the main() routine
if __name__ == "__main__":
   main()

Lab 7a: Sprites and pygame (part I)

Pre-amble: clean program structure

Sprites for in-game objects

Updating our in-game objects

Updating the display

Complete version