The source code example below is not much of a game, but it demonstrates the uses of layers for sprites and Parallax_scrolling (the mountains). Also, you can hide penguins behind blocks and mountains. Additionally, you can aim with the mouse pointer and press <key>p</key> to print out information about the layers (at the text console).

Trivia:

According to wikipedia, Parallax scrolling was first introduced to computer games by the arcade game Moon_Patrol 1982. The mountains (and in higher levels, futuristic buildings) moved at different speed to create the illusion of a side-scrolling computer game.

proposals to tinker:

• Change the layer of the three types of class.moutains. IF you switch -3 with -1, the view effect will be like looking at a rotating disc.
• Not enough action ? Apply the thingls learned at the pevious step, let the Birds change _layer by random and explode them by mouseclick !

Do you remember the sprite groups from the last step ? Sprite groups are basically containers where the sprites “live” in. As soon as a sprite is killed (by it's kill() functoin) the sprite is not also removed from the screen but also from all containers holding it. In the last two steps, i had two uses for sprite groups:

1. The allgroup (every sprite was member of this group), to clear, draw, and update all sprites from within the main loop
2. other groups like birdgroup, used to check each sprite in this group for collision detection (crashgroup)

You will note that there exist different variants of sprite groups:

• the old spritegroup
• the newer LayeredUpdate group

When using pygame.sprite.Layeredupdate() instead of pygame.sprite.Group() you can give each sprite a variable _layer as well as a variable groups to influence the drawing order of the sprite. In the previous step those variables were defined outside the sprite class in the mainloop. It makes more sense to define those variables inside the sprite class itself. The ideal place to do so is the __init__(self) method of each sprite class. Note two things:

1. The sprite groups must exist (be defined in the mainloop) before you can assign sprites to the groups. That means, inside the mainloop, before you create a Bird sprite or assign images to the Bird class, you must define the spritegroups.
2. Inside the class, you must assign groups and _layer before you call pygame.sprite.Sprite.__init__(self, *groups):

<note>#… means that i let away some not so important code lines</note>

class Bird(pygame.sprite.Sprite):
        #...
        def __init__(self, startpos=screen.get_rect().center):
            self.groups = birdgroup, allgroup # assign groups BEFORE calling pygame.sprite.Sprite.__init__
            self._layer = 7                   # assign _layer BEFORE calling pygame.sprite.Sprite.__init__ 
            pygame.sprite.Sprite.__init__(self,  self.groups ) #call parent class. NEVER FORGET !
            self.pos = starpos
            # ...
# define sprite groups 
birdgroup = bargroup = pygame.sprite.Group() # simple group for collision detection
allgroup  = pygame.sprite.LayeredUpdates() # more sophisticated and modern group

# assign images to the Bird class or create a Bird sprite
Bird()

---

please see the official pygame doumentation at: http://www.pygame.org/docs/ref/sprite.html#pygame.sprite.LayeredUpdates

---

The cool things about layers is: you can change them even at runtime, to place sprites more in the foreground or more in the background (see code example below). The change_layer method does exaclty that. Important: you need only to change the layer of the LayeredUpdates-group that actually draws the sprites on the screen. In the code example below, this is the allgroup. (Special thanks to Gummbum for helping me out here). Because i want only to change the layer of the Bird sprites and their Lifebar sprites i loop over all sprites in the groups birdgroup and bargroup. Each Bird sprite is a member of the allgroup as well as of the birdgroup. Each Lifebar sprite is a member of the allgroup as well as of the bargroup.

if pygame.mouse.get_pressed()[0]:
  if birdlayer < 10:
     birdlayer += 1
     cooldowntime = .5 # seconds
     cry.play()
     for bird in birdgroup:
         allgroup.change_layer(bird, birdlayer) # allgroup draws the sprite 
     for bar in bargroup:
         allgroup.change_layer(bar, birdlayer) # allgroup draws the sprite 

Up to now, i blitted all text to the screen or to the background with the write function. In this example, you will see a new class called Text, also printing a msg to the screen. The Text sprite has no advantages over blitting directly to the background yet, but you can use it later to change the _layer of the Text sprite or if you want the Text sprite to move around. Note that the class Text has it's own method newmsg to update the displaying text string.

def newmsg(self, birdlayer):
    self.image =  write("current Bird _layer = %i" % birdlayer)
    self.rect = self.image.get_rect()
    self.rect.center = (screen.get_width()/2,10)

Take a look at the Bird class. Something is new ! There is a new class variable Bird.waittime indicating how long a Bird should stay “invisible”. Instead of messing around with drawing and not drawing, during his “invisible” waittime, each bird is simply teleported to the position (-100,-100), that is outside your screen. If the waittime is over, the Bird sprite is teleported to his Bird.pos position and act like a normal Bird sprite - speeding around, crashing into walls and other birds, exploding.

Class Bird(pygame.sprite.Sprite):
    waittime = 1.0 #seconds
    #...
    def update(self, seconds):
        #---make Bird only visible after waiting time
        self.lifetime += seconds
        if self.lifetime > (self.waittime) and self.waiting:
           self.newspeed()
           self.waiting = False
           self.rect.centerx = round(self.pos[0],0)
           self.rect.centery = round(self.pos[1],0)
        if self.waiting:
           self.rect.center = (-100,-100)
        else:
           # speedcheck
           #.. all the other things

But why the waittime ? The answer is a modification to the Fragment class - A bird gets now Fragment when killed (red) and also when appearing (blue). Before you look at the Fragment class, check out the __init__ method of the Bird class:

def __init__(self, layer=4):
   #...
   Lifebar(self.number) #create a Lifebar for this Bird. 
   # starting implosion of blue fragments
   for _ in range(8):
         Fragment(self.pos, True)

See the for loop ? I simply wanted 8 (blue) Fragments, so i do not use an variable like a or b or x inside the for loop but simply an underscore. The True parameter in the Fragment class (see below) indicate a blue Fragment instead of the “normal” red Fragment.

The fragment class get now a paramter (bluefrag) into it's __init__ method indicating if this is a “born” (blue) or a “kill” (red) Fragment. Then the __init__ method splits using a if-else construct. The new part is only valid for “bluefrag” Fragments. After choosing per random from where (screen edge) the Fragment start, self.dx and self.dy are calculated… The Fragment should fly from the screen edge toward the position of the Bird sprite (self.target). But how fast does the fragment fly ? Because in the update method of the fragment class dx and dy is multiplied by the amount of seconds passed (at a Framerate of 30 FPS, this would be 1/30 of a second), the speed unit of a sprite is measured in pixel per second. Meaning each blue sprite will need exactly one second to travel from it's origin to the Bird sprite position.

But what if the waittime for each Birdsprite is 2 seconds ? or just a half second ? For this reason, dx and dy are multiplied by the factor ( 1.0 / Bird.waittime). So if the Bird waits 2 seconds before appearing, the blue Fragments have more time and should travel slower: 1.0 / 2.0 = 0.5, the speed is reduced. On the other hand, if the waittime is shorter (say 0.5 seconds) the Fragments should fly faster: ( 1.0 / 0.5 = 2); dx and dy is doubled.

For aesthetic reasons, i allow the blue Fragments to live up to a half second after reaching their target by adding to self.lifetime a value between 0 and 0.5: random.random() * 0.5. Random.random() creates a float value between 0 and 1.

class Fragment(pygame.sprite.Sprite):
    #...     
    def __init__(self, pos, bluefrag = False):
       #...
       self.bluefrag = bluefrag
       self.pos=[0.0,0.0]
       self.target = pos
       if self.bluefrag:      # blue frament implodes from screen edge toward Bird
          self.color = (0,0,random.randint(25,255)) # blue
          self.side = random.randint(1,4)
          if self.side == 1:  # left side
               self.pos[0] = 0   
               self.pos[1] = random.randint(0,screen.get_height())
          elif self.side == 2: # top
               self.pos[0] = random.randint(0,screen.get_width()) 
               #...
          # calculating flytime for one second.. Bird.waittime should be 1.0
          self.dx = (self.target[0] - self.pos[0]) * 1.0 / Bird.waittime
          self.dy = (self.target[1] - self.pos[1]) * 1.0 / Bird.waittime
          self.lifetime = Bird.waittime + random.random() * .5 # a bit more livetime after the Bird appears
       else: # red fragment explodes from the bird toward screen edge
          #...  all the stuff for red Fragments
          
    def update(self, seconds):
       # ...
       self.pos[0] += self.dx * seconds
       self.pos[1] += self.dy * seconds
       # ...
       self.rect.centerx = round(self.pos[0],0)
       self.rect.centery = round(self.pos[1],0)

You will notice that at the start of the game, the moutains “walk” in from right to left. All moutnains are instances of the same mountain class. The different types of moutains each have their own type parameter (making blue, red or pink mountains).

Most of the work in this class is done in it's __ini__-method:

class Mountain(pygame.sprite.Sprite):
        #...
        def __init__(self, type):
            self.type = type
            if self.type == 1:
                self._layer = -1
                self.dx = -100
                self.color = (0,0,255) # blue mountains, close
            elif self.type == 2:
                self._layer = -2
                self.color = (200,0,255) # pink mountains, middle
                self.dx = -75
            else:
                self._layer = -3
                self.dx = -35
                self.color = (255,0,0) # red mountains, far away
            self.groups = allgroup, mountaingroup
            pygame.sprite.Sprite.__init__(self, self.groups) # THE Line
            #...

Maybe the most interesting part here is the creation of the actual mountain. This is done with the help of the random.random() function (creates an decimal number between 0.0 and 1.0) and the pygame.draw.polygon-method. On each mountain surface, a filled triangle (polygon) is created from the lower left corner to a corner in the middle (x/2) and at a random height (the moutain peak) and back to the lower right corner. The syntax for pygame.draw.poligon is: pygame.draw.polygon(surface, color, pointlist, width=0)

class Mountain(pygame.sprite.Sprite):
   #...
   def __init__(self, type):
            #...
            self.dy = 0
            x = 100 * self.type * 1.5
            y = screen.get_height() / 2 + 50 * (self.type -1)
            self.image = pygame.Surface((x,y))
            self.image.set_colorkey((0,0,0)) # black is transparent
            pygame.draw.polygon(self.image, self.color,
               ((0,y),
                (0,y-10*self.type), 
                (x/2, int(random.random()*y/2)),
                (x,y-10*self.type),
                (x,y),
                (9,y)),0) # width=0 fills the polygon
            self.image.convert_alpha()
            self.rect = self.image.get_rect()

Each mountain has a .parent attribute, set to False in the __ini__-method. In the update-method of the moutain class, it is checked if the mountain' center is visible. If yes, a new mountain of the same type is created and placed directly to it's right side (yet invisible because outside the screen border). If a mountain travels too far to the left side, it is killed.

class Mountain(pygame.sprite.Sprite):
  #...
  def update(self, time):
     if self.rect.centerx + self.rect.width/2+10 < 0:
        self.kill()
        # create new mountains if necessary
     if not self.parent:
        if self.rect.centerx  < screen.get_width():
           self.parent = True
           Mountain(self.type) # new Mountain coming from the right side
            

To run this example you need:

View/Edit/Download the file directly in Github: https://github.com/horstjens/ThePythonGameBook/blob/master/pygame/016_layers.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
016_layers.py
pygame sprites with different layers and parallax scrolling
url: http://thepythongamebook.com/en:part2:pygame:step016
author: horst.jens@spielend-programmieren.at
licence: gpl, see http://www.gnu.org/licenses/gpl.html


change the sprite layer by clicking with left or right mouse button
the birdsprites will appear before or behind the blocks

point on a sprite and pres "p" to print out more information about that sprite
part of www.pythongamebook.com by Horst JENS


works with python3.4 and python2.7
"""
#the next line is only needed for python2.x and not necessary for python3.x
from __future__ import print_function, division

def game():
        
    import pygame
    import os
    import random


    pygame.mixer.pre_init(44100, -16, 2, 2048) # setup mixer to avoid sound lag
    pygame.init()
    screen=pygame.display.set_mode((640,480)) # try out larger values and see what happens !
    #winstyle = 0  # |FULLSCREEN # Set the display mode
    print("pygame version", pygame.ver)

    BIRDSPEEDMAX = 200
    BIRDSPEEDMIN = 10
    FRICTION =.999
    FORCE_OF_GRAVITY = 9.81
    
    
    def write(msg="pygame is cool"):
        """write text into pygame surfaces"""
        myfont = pygame.font.SysFont("None", 32)
        mytext = myfont.render(msg, True, (0,0,0))
        mytext = mytext.convert_alpha()
        return mytext
    
    class Text(pygame.sprite.Sprite):
        """ display a text"""
        def __init__(self, msg ):
            self.groups = allgroup, textgroup
            self._layer = 99
            pygame.sprite.Sprite.__init__(self, self.groups)
            self.newmsg(msg)

            
        def update(self, time):
            pass
        
        def newmsg(self, msg="i have nothing to say"):
            self.image =  write(msg)
            self.rect = self.image.get_rect()
            self.rect.center = (screen.get_width()/2,10)

    class Mountain(pygame.sprite.Sprite):
        """generate a mountain sprite for the background, to 
           demonstrate parallax scrolling. Like in the classic
           'moonbuggy' game. Mountains slide from right to left"""
        def __init__(self, type):
            self.type = type
            if self.type == 1:
                self._layer = -1
                self.dx = -100
                self.color = (0,0,255) # blue mountains, close
            elif self.type == 2:
                self._layer = -2
                self.color = (200,0,255) # pink mountains, middle
                self.dx = -75
            else:
                self._layer = -3
                self.dx = -35
                self.color = (255,0,0) # red mountains, far away
            self.groups = allgroup, mountaingroup
            pygame.sprite.Sprite.__init__(self, self.groups) # THE Line
            self.dy = 0
            x = 100 * self.type * 1.5
            y = screen.get_height() / 2 + 50 * (self.type -1)
            self.image = pygame.Surface((x,y))
            #self.image.fill((0,0,0)) # fill with black
            self.image.set_colorkey((0,0,0)) # black is transparent
            pygame.draw.polygon(self.image, self.color,
               ((0,y),
                (0,y-10*self.type), 
                (x/2, int(random.random()*y/2)),
                (x,y-10*self.type),
                (x,y),
                (9,y)),0) # width=0 fills the polygon
            self.image.convert_alpha()
            self.rect = self.image.get_rect()
            self.pos = [0.0,0.0]
            # start right side from visible screen
            self.pos[0] = screen.get_width()+self.rect.width/2
            self.pos[1] = screen.get_height()-self.rect.height/2
            self.rect.centerx = round(self.pos[0],0)
            self.rect.centery = round(self.pos[1],0)
            self.parent = False
            
        def update(self, time):
            self.pos[0] += self.dx * time
            self.pos[1] += self.dy * time
            self.rect.centerx = round(self.pos[0],0)
            self.rect.centery = round(self.pos[1],0)
            # kill mountains too far to the left
            if self.rect.centerx + self.rect.width/2+10 < 0:
                self.kill()
            # create new mountains if necessary
            if not self.parent:
                if self.rect.centerx  < screen.get_width():
                    self.parent = True
                    Mountain(self.type) # new Mountain coming from the right side
            
           
    
    class Block(pygame.sprite.Sprite):
        """a block with a number indicating it's layer.
           Blocks move horizontal and bounce on screen edges"""
        def __init__(self, blocknumber=1):
            self.blocknumber = blocknumber
            self.color = (random.randint(10,255),
                          random.randint(10,255),
                          random.randint(10,255))
            self._layer = self.blocknumber
            self.groups = allgroup, blockgroup
            pygame.sprite.Sprite.__init__(self, self.groups) # THE line
            self.area = screen.get_rect()
            self.image = pygame.Surface((100,100))
            self.image.fill(self.color)
            self.image.blit(write(str(self.blocknumber)),(40,40))
            self.image = self.image.convert()
            self.rect = self.image.get_rect()
            self.rect.centery = screen.get_height() / 2
            self.rect.centerx = 100 * self.blocknumber + 50
            #self.rect.centery = (screen.get_height() / 10.0) * self.blocknumber + self.image.get_height() / 2
            self.pos = [0.0,0.0]
            self.pos[0] = self.rect.centerx
            self.pos[1] = self.rect.centery
            self.dy = random.randint(50,100) * random.choice((-1,1))
            self.dx = 0
            
        def newspeed(self):
            self.dy *= -1
            
        def update(self, time):
            if not self.area.contains(self.rect):
                # --- compare self.rect and area.rect
                if self.pos[1]  < self.area.top:
                    self.pos[1] = self.area.top
                    self.newspeed() # calculate a new direction
                elif self.pos[1] > self.area.bottom:
                    self.pos[1] = self.area.bottom
                    self.newspeed() # calculate a new direction
            self.pos[0] += self.dx * time
            self.pos[1] += self.dy * time
            self.rect.centerx = round(self.pos[0],0)
            self.rect.centery = round(self.pos[1],0)
            
    class BirdCatcher(pygame.sprite.Sprite):
        """circle around the mouse pointer. Left button create new sprite, right button kill sprite"""
        def __init__(self):
            self._layer = 9
            self.groups = allgroup, stuffgroup
            pygame.sprite.Sprite.__init__(self, self.groups)
            self.image = pygame.Surface((100,100)) # created on the fly
            self.image.set_colorkey((0,0,0)) # black transparent
            pygame.draw.circle(self.image, (255,0,0), (50,50), 50, 2) # red circle
            self.image = self.image.convert_alpha()
            self.rect = self.image.get_rect()
            self.radius = 50 # for collide check

        def update(self, seconds):
            # no need for seconds but the other sprites need it
            self.rect.center = pygame.mouse.get_pos()

    class Lifebar(pygame.sprite.Sprite):
        """shows a bar with the hitpoints of a Bird sprite
           with a given bossnumber, the Lifebar class can 
           identify the boos (Bird sprite) with this codeline:
           Bird.birds[bossnumber] """
        def __init__(self, bossnumber):
            self.groups = allgroup, bargroup
            self.bossnumber = bossnumber
            self._layer = Bird.birds[self.bossnumber]._layer
            pygame.sprite.Sprite.__init__(self, self.groups)
            self.image = pygame.Surface((Bird.birds[self.bossnumber].rect.width,7))
            self.image.set_colorkey((0,0,0)) # black transparent
            pygame.draw.rect(self.image, (0,255,0), (0,0,Bird.birds[self.bossnumber].rect.width,7),1)
            self.rect = self.image.get_rect()
            self.oldpercent = 0
            
        def update(self, time):
            self.percent = Bird.birds[self.bossnumber].hitpoints / Bird.birds[self.bossnumber].hitpointsfull * 1.0
            if self.percent != self.oldpercent:
                pygame.draw.rect(self.image, (0,0,0), (1,1,Bird.birds[self.bossnumber].rect.width-2,5)) # fill black
                pygame.draw.rect(self.image, (0,255,0), (1,1,
                                 int(Bird.birds[self.bossnumber].rect.width * self.percent),5),0) # fill green
            self.oldpercent = self.percent
            self.rect.centerx = Bird.birds[self.bossnumber].rect.centerx
            self.rect.centery = Bird.birds[self.bossnumber].rect.centery - Bird.birds[self.bossnumber].rect.height /2 - 10
            #check if boss is still alive
            if Bird.birds[self.bossnumber].hitpoints < 1:
                self.kill() # kill the hitbar
      
    class Bird(pygame.sprite.Sprite):
        """a nice little sprite that bounce off walls and other sprites"""
        image=[]  # list of all images
        birds = {} # a dictionary of all Birds, each Bird has its own number
        number = 0  
        waittime = 1.0 # seconds
        def __init__(self, layer = 4 ):
            self.groups = birdgroup, allgroup # assign groups 
            self._layer = layer                   # assign level
            #self.layer = layer
            pygame.sprite.Sprite.__init__(self,  self.groups  ) #call parent class. NEVER FORGET !
            #pygame.sprite.Sprite.__init__(self,  *args ) #call parent class. NEVER FORGET !
            self.pos = [random.randint(50,screen.get_width()-50),
                        random.randint(25,screen.get_height()-25)] 
            self.area = screen.get_rect()
            self.image = Bird.image[0]
            self.hitpointsfull = float(100) # maximal hitpoints
            self.hitpoints = float(100) # actual hitpoints
            self.rect = self.image.get_rect()
            self.radius = max(self.rect.width, self.rect.height) / 2.0
            self.dx = 0   # wait at the beginning
            self.dy = 0            
            self.waittime = Bird.waittime # 1.0 # one second
            #self.newspeed()
            self.lifetime = 0.0
            self.waiting = True
            self.rect.center = (-100,-100) # out of visible screen
            self.cleanstatus()
            self.catched = False
            self.crashing = False
            #--- not necessary:
            self.number = Bird.number # get my personal Birdnumber
            Bird.number+= 1           # increase the number for next Bird
            Bird.birds[self.number] = self # store myself into the Bird dictionary
            #print "my number %i Bird number %i " % (self.number, Bird.number)
            Lifebar(self.number) #create a Lifebar for this Bird. 
            # starting implosion of blue fragments
            for _ in range(8):
                Fragment(self.pos, True)
            
        def newspeed(self):
            # new birdspeed, but not 0
            speedrandom = random.choice([-1,1]) # flip a coin
            self.dx = random.randint(BIRDSPEEDMIN,BIRDSPEEDMAX) * speedrandom 
            self.dy = random.randint(BIRDSPEEDMIN,BIRDSPEEDMAX) * speedrandom 

        def cleanstatus(self):
            self.catched = False   # set all Bird sprites to not catched
            self.crashing = False
        
        def kill(self):
            # a shower of red fragments, exploding outward
            for _ in range(15):
                Fragment(self.pos)
            pygame.sprite.Sprite.kill(self) # kill the actual Bird 
            
        
        def update(self, seconds):
            #---make Bird only visible after waiting time
            self.lifetime += seconds
            if self.lifetime > (self.waittime) and self.waiting:
                self.newspeed()
                self.waiting = False
                self.rect.centerx = round(self.pos[0],0)
                self.rect.centery = round(self.pos[1],0)
            if self.waiting:
                self.rect.center = (-100,-100)
            else:
                # speedcheck
                # friction make birds slower
                if abs(self.dx) > BIRDSPEEDMIN and abs(self.dy) > BIRDSPEEDMIN:
                    self.dx *= FRICTION
                    self.dy *= FRICTION
                # spped limit
                if abs(self.dx) > BIRDSPEEDMAX:
                    self.dx = BIRDSPEEDMAX * self.dx / self.dx
                if abs(self.dy) > BIRDSPEEDMAX:
                    self.dy = BIRDSPEEDMAX * self.dy / self.dy
                # movement
                self.pos[0] += self.dx * seconds
                self.pos[1] += self.dy * seconds
                # -- check if Bird out of screen
                if not self.area.contains(self.rect):
                    self.crashing = True # change colour later
                    # --- compare self.rect and area.rect
                    if self.pos[0] + self.rect.width/2 > self.area.right:
                        self.pos[0] = self.area.right - self.rect.width/2
                    if self.pos[0] - self.rect.width/2 < self.area.left:
                        self.pos[0] = self.area.left + self.rect.width/2
                    if self.pos[1] + self.rect.height/2 > self.area.bottom:
                        self.pos[1] = self.area.bottom - self.rect.height/2
                    if self.pos[1] - self.rect.height/2 < self.area.top:
                        self.pos[1] = self.area.top + self.rect.height/2
                    self.newspeed() # calculate a new direction
                #--- calculate actual image: crasing, catched, both, nothing ?
                self.image = Bird.image[self.crashing + self.catched*2]
                #--- calculate new position on screen -----
                self.rect.centerx = round(self.pos[0],0)
                self.rect.centery = round(self.pos[1],0)
                #--- loose hitpoins
                if self.crashing:
                    self.hitpoints -=1
                #--- check if still alive
                if self.hitpoints <= 0:
                    self.kill()
            
    class Fragment(pygame.sprite.Sprite):
        """a fragment of an exploding Bird"""
        gravity = False # fragments fall down ?
        def __init__(self, pos, bluefrag = False):
            self._layer = 9
            self.groups = allgroup, stuffgroup
            pygame.sprite.Sprite.__init__(self, self.groups)
            self.bluefrag = bluefrag
            self.pos = [0.0,0.0]
            self.target = pos
            self.fragmentmaxspeed = BIRDSPEEDMAX * 2 # try out other factors !
            if self.bluefrag:
                # blue frament implodes from screen edge toward Bird
                self.color = (0,0,random.randint(25,255)) # blue
                self.side = random.randint(1,4)
                if self.side == 1:  # left side
                    self.pos[0] = 0   
                    self.pos[1] = random.randint(0,screen.get_height())
                elif self.side == 2: # top
                    self.pos[0] = random.randint(0,screen.get_width()) 
                    self.pos[1] = 0
                elif self.side == 3: #right
                    self.pos[0] = screen.get_width() 
                    self.pos[1] = random.randint(0,screen.get_height())
                else: #bottom
                    self.pos[0] = random.randint(0,screen.get_width()) 
                    self.pos[1] = screen.get_height()
                # calculating flytime for one second.. Bird.waittime should be 1.0
                self.dx = (self.target[0] - self.pos[0]) * 1.0 / Bird.waittime
                self.dy = (self.target[1] - self.pos[1]) * 1.0 / Bird.waittime
                self.lifetime = Bird.waittime + random.random() * .5 # a bit more livetime after the Bird appears
            else: # red fragment explodes from the bird toward screen edge
                self.color = (random.randint(25,255),0,0) # red            
                self.pos[0] = pos[0]
                self.pos[1] = pos[1]
                self.dx = random.randint(-self.fragmentmaxspeed,self.fragmentmaxspeed)
                self.dy = random.randint(-self.fragmentmaxspeed,self.fragmentmaxspeed)
                self.lifetime = 1 + random.random()*3 # max 3 seconds
            self.image = pygame.Surface((10,10))
            self.image.set_colorkey((0,0,0)) # black transparent
            pygame.draw.circle(self.image, self.color, (5,5), random.randint(2,5))
            self.image = self.image.convert_alpha()
            self.rect = self.image.get_rect()
            self.rect.center = self.pos #if you forget this line the sprite sit in the topleft corner
            self.time = 0.0
            
        def update(self, seconds):
            self.time += seconds
            if self.time > self.lifetime:
                self.kill() 
            self.pos[0] += self.dx * seconds
            self.pos[1] += self.dy * seconds
            if Fragment.gravity and not self.bluefrag:
                self.dy += FORCE_OF_GRAVITY # gravity suck fragments down
            self.rect.centerx = round(self.pos[0],0)
            self.rect.centery = round(self.pos[1],0)
    
        
    background = pygame.Surface((screen.get_width(), screen.get_height()))
    background.fill((255,255,255))     # fill white
    background.blit(write("press left mouse button to increase Bird's layer"),(50,40))
    background.blit(write("press right mouse button to decrease Bird's layer."),(50,65))
    background.blit(write("layer of mountains are: -1 (blue), -2 (pink), -3 (red)"),(50,90))
    background.blit(write("Press ESC to quit, p to print info at mousepos"), (50,115))
    # secret keys: g (gravity), p (print layers)
    
    background = background.convert()  # jpg can not have transparency
    screen.blit(background, (0,0))     # blit background on screen (overwriting all)

    #define sprite groups. Do this before creating sprites 
    blockgroup = pygame.sprite.LayeredUpdates()
    birdgroup = pygame.sprite.Group() 
    textgroup = pygame.sprite.Group()
    bargroup = pygame.sprite.Group()
    stuffgroup = pygame.sprite.Group()
    mountaingroup = pygame.sprite.Group()
    # only the allgroup draws the sprite, so i use LayeredUpdates() instead Group()
    allgroup = pygame.sprite.LayeredUpdates() # more sophisticated, can draw sprites in layers 



    
    try: # load images into classes (class variable !). if not possible, draw ugly images
        Bird.image.append(pygame.image.load(os.path.join("data","babytux.png")))
        Bird.image.append(pygame.image.load(os.path.join("data","babytux_neg.png")))
    except:
        print("no image files 'babytux.png' and 'babytux_neg.png' in subfolder 'data'")
        print("therfore drawing ugly sprites instead")
        image = pygame.Surface((32,36))
        image.fill((255,255,255))
        pygame.draw.circle(image, (0,0,0), (16,18), 15,2)
        image.set_colorkey((255,255,255))
        Bird.image.append(image) # alternative ugly image
        image2 = image.copy()
        pygame.draw.circle(image2, (0,0,255), (16,18), 13,0)
        Bird.image.append(image2)
    Bird.image.append(Bird.image[0].copy()) # copy of first image
    pygame.draw.rect(Bird.image[2], (0,0,255), (0,0,32,36), 1) # blue border
    Bird.image.append(Bird.image[1].copy()) # copy second image
    pygame.draw.rect(Bird.image[3], (0,0,255), (0,0,32,36), 1) # blue border
    Bird.image[0] = Bird.image[0].convert_alpha()
    Bird.image[1] = Bird.image[1].convert_alpha()
    Bird.image[2] = Bird.image[2].convert_alpha()
    Bird.image[3] = Bird.image[3].convert_alpha()

    
    try: # ------- load sound -------
        cry = pygame.mixer.Sound(os.path.join('data','claws.ogg'))  #load sound
    except:
        raise(SystemExit, "could not load sound claws.ogg from 'data'")
        #print"could not load sound file claws.ogg from folder data. no sound, sorry"


    #create Sprites
    hunter = BirdCatcher() # display the BirdCatcher and name it "hunter"

    for x in range(screen.get_width()//100):
        Block(x) # add more Blocks if you y screen resolution is bigger
    
    othergroup =  [] # important for good collision detection
    badcoding = False
    clock = pygame.time.Clock()        # create pygame clock object 
    mainloop = True
    FPS = 60                           # desired max. framerate in frames per second. 
   
    birdlayer = 4
    birdtext = Text("current Bird _layer = %i" % birdlayer) # create Text sprite
    cooldowntime = 0 #sec
    
    # start with some Birds
    for _ in range(15):
        Bird(birdlayer)  # one single Bird
    
    # create the first parallax scrolling mountains
    Mountain(1) # blue
    Mountain(2) # pink
    Mountain(3) # red

    while mainloop: # ----------------- mainloop ----------------------
        milliseconds = clock.tick(FPS)  # milliseconds passed since last frame
        seconds = milliseconds / 1000.0 # seconds passed since last frame
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                mainloop = False # pygame window closed by user
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_ESCAPE:
                    mainloop = False # user pressed ESC
                #elif event.key == pygame.K_g:
                    #Fragment.gravity = not Fragment.gravity # toggle gravity class variable
                elif event.key == pygame.K_p: # get sprites at mouse position, print info
                    print("=========================")
                    print("-----Spritelist---------")
                    spritelist = allgroup.get_sprites_at(pygame.mouse.get_pos())
                    for sprite in spritelist:
                        print(sprite, "Layer:",allgroup.get_layer_of_sprite(sprite))
                    print("------------------------")
                    print("toplayer:", allgroup.get_top_layer())
                    print("bottomlayer:", allgroup.get_bottom_layer())
                    print("layers;", allgroup.layers())
                    print("=========================")
                    
                elif event.key == pygame.K_g:
                    Fragment.gravity = not Fragment.gravity # toggle gravity class variable
   

        # change birdlayer on mouseclick
        if cooldowntime <= 0: # to 
            if pygame.mouse.get_pressed()[0]:
                if birdlayer < 10:
                    birdlayer += 1
                    cooldowntime = .5 # seconds
                    cry.play()
                    for bird in birdgroup:
                        allgroup.change_layer(bird, birdlayer) # allgroup draws the sprite 
                    for bar in bargroup:
                        allgroup.change_layer(bar, birdlayer) # allgroup draws the sprite 
            if pygame.mouse.get_pressed()[2]:
                if birdlayer > -4:
                    birdlayer -= 1
                    cooldowntime = .5
                    cry.play()
                    for bird in birdgroup:
                        allgroup.change_layer(bird, birdlayer) # allgroup draws the sprite !
                    for bar in bargroup:
                        allgroup.change_layer(bar, birdlayer) # allgroup draws the sprite 
        else:
            cooldowntime -= seconds # to avoid speedclicking

        pygame.display.set_caption("fps: %.2f birds: %i grav: %s" % (clock.get_fps(), len(birdgroup), Fragment.gravity))
        

        birdtext.newmsg("current Bird _layer = %i" % birdlayer) # update text for birdlayer
    
        # ------ collision detection
        for bird in birdgroup:
            bird.cleanstatus() 
            
        #pygame.sprite.spritecollide(sprite, group, dokill, collided = None): return Sprite_list
        crashgroup = pygame.sprite.spritecollide(hunter, birdgroup, False, pygame.sprite.collide_circle)
        # pygame.sprite.collide_circle works only if one sprite has self.radius
        # you can do without that argument collided and only the self.rects will be checked
        for crashbird in crashgroup:
            crashbird.catched = True # will get a blue border from Bird.update()
        
        for bird in birdgroup:  # test if a bird collides with another bird
            # check the Bird.number to make sure the bird is not crashing with himself
            crashgroup = pygame.sprite.spritecollide(bird, birdgroup, False )
            for crashbird in crashgroup:
                if crashbird.number != bird.number: #different number means different birds
                    bird.crashing = True
                    if not bird.waiting:
                        bird.dx -= crashbird.pos[0] - bird.pos[0]
                        bird.dy -= crashbird.pos[1] - bird.pos[1]
        
        # create 10 new Birds if fewer than 11 birds alive
        if len(birdgroup) < 10:
            for _ in range(random.randint(1,5)):
                Bird(birdlayer)
                    
        # ----------- clear, draw , update, flip -----------------  
        allgroup.clear(screen, background)
        allgroup.update(seconds)
        allgroup.draw(screen)           
        pygame.display.flip()         

if __name__ == "__main__":
    game()
else:
    print("i was imported by", __name__)