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pygame / docs / tut / SpriteIntro.html

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TUTORIAL:Sprite Module Introduction
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  <title>Pygame Tutorials - Sprite Module Intoduction</title>
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<h1 align="center"><font size="-1">Pygame Tutorials</font><br>
 Sprite Module Introduction</h1>
   
<h2 align="center">by Pete Shinners<br>
 <font size="-1">pete@shinners.org</font></h2>
   
<h3 align="center">Revision 1.1, April 12th, 2002</h3>
  <br>
 <br>
    Pygame version 1.3 comes with a new module, pygame.sprite. This module 
is written in python and includes some higher level classes to manage your 
game objects. By using this module to its full potential, you can easily 
manage and draw your game objects. The sprite classes are very optimized, 
so it's likely your game will run faster with the sprite module than without.<br>
 <br>
 The sprite module is also meant to be very generic. It turns out you can 
use it with nearly any type of gameplay. All this flexibility comes with a
slight penalty, it needs a little understanding to properly use it. The <a href="../ref/pygame_sprite.html">
reference documentation</a> for the sprite module can keep you running, but
you'll probably need a bit more explanation of how to use pygame.sprite in
your own game.<br>
 <br>
 Several of the pygame examples (like "chimp" and "aliens") have been updated 
to use the sprite module. You may want to look into those first to see what 
this sprite module is all about. The chimp module even has it's own line-by-line 
tutorial, which may help get more an understanding of programming with python 
and pygame.<br>
 <br>
 Note that this introduction will assume you have a bit of experience programming 
with python, and are somewhat framiliar with the different parts of creating 
a simple game. In this tutorial the word "reference" is occasianally used. 
This represents a python variable. Variables in python are references, so 
you can have several variables all pointing to the same object.<br>
 <br>
 
<h2>History Lesson</h2>
  The term "sprite" is a holdover from older computer and game machines.
These older boxes were unable to draw and erase normal graphics fast enough
for them to work as games. These machines had special hardware to handle
game like objects that needed to animate very quickly. These objects were
called 'sprites' and had special limitations, but could be drawn and updated
very fast. They usually existed in special overlay buffers in the video.
These days computers have become generally fast enough to handle sprite like
objects without dedicated hardware. The term sprite is still used to represent
just about anything in a 2D game that is animated.<br>
   <br>
   <br>
     
<h2>The Classes</h2>
 The sprite module comes with two main classes. The first is Sprite, which 
should be used as a base class for all your game objects. This class doesn't 
really do anything on its own, it just includes several functions to help 
manage the game object. The other type of class is Group. The Group class 
is a container for different Sprite objects. There are actually several different 
types of group classes. Some of the Groups can draw all the elements they 
contain, for example.<br>
 <br>
 This is all there really is to it. We'll start with a description of what 
each type of class does, and then discuss the proper ways to use these two 
classes.<br>
 <br>
     
<h2>The Sprite Class</h2>
 As mentioned before, the Sprite class is designed to be a base class for 
all your game objects. You cannot really use it on its own, as it only has 
several methods to help it work with the different Group classes. The sprite 
keeps track of which groups it belongs to. The class constructor (__init__ 
method) takes an argument of a Group (or list of Groups) the Sprite instance 
should belong to. You can also change the Group membership for the Sprite 
with the add() and remove() methods. There is also a groups() method, which 
returns a list of the current groups containing the sprite.<br>
 <br>
 When using the your Sprite classes it's best to think of them as "valid" 
or "alive" when they are belonging to one or more Groups. When you remove 
the instance from all groups pygame will clean up the object. (Unless you 
have your own references to the instance somewhere else.) The kill() method 
removes the sprite from all groups it belongs to. This will cleanly delete 
the sprite object. If you've put some little games together, you'll know sometimes
cleanly deleting a game object can be tricky. The sprite also comes with
an alive() method, which returns true if it is still a member of any groups.<br>
 <br>
 <br>
 
<h2>The Group Class</h2>
 The Group class is just a simple container. Similar to the sprite, it has 
an add() and remove() method which can change which sprites belong to the 
group. You also can pass a sprite or list of sprites to the contructor (__init__ 
method) to create a Group instance that contains some initial sprites.<br>
 <br>
 The Group has a few other methods like empty() to remove all sprites from 
the group and copy() which will return a copy of the group with all the same 
members. Also the has() method will quickly check if the Group contains a 
sprite or list of sprites.<br>
 <br>
 The other function you will use frequently is the sprites() method. This 
returns an object that can be looped on to access every sprite the group contains.
Currently this is just a list of the sprites, but in later version of python
this will likely use iterators for better performance.<br>
 <br>
 As a shortcut, the Group also has an update() method, which will call an 
update() method on every sprite in the group. Passing the same arguments to
each one. Usually in a game you need some function that updates the state 
of a game object. It's very easy to call your own methods using the Group.sprites() 
method, but this is a shortcut that's used enough to be included. Also note 
that the base Sprite class has a "dummy" update() method that takes any sort 
of arguments and does nothing.<br>
 <br>
 Lastly, the Group has a couple other methods that allow you to use it with 
the builtlin len() method, getting the number of sprites it contains. and 
the "truth" operator, which allows you to do "if mygroup:" to check if the 
group has any sprites.<br>
 <br>
 
<h2>Mixing Them Together</h2>
 At this point the two classes seem pretty basic. Not doing a lot more than 
you can do with a simple list and your own class of game objects. But there 
are some big advantages to using the Sprite and Group together. A sprite can
belong to as many groups as you want. Remember as soon as it belongs to no
groups, it will usually be cleared up (unless you have other "non-group" references
to that object)<br>
 <br>
 The first big thing is a fast simple way to categorize sprites. For example, 
say we had a pacman-like game. We could make separate groups for the different 
types of objects in the game. Ghosts, Pac, and Pellets. When pac eats a power 
pellet, we can change the state for all ghost objects by effecting everything 
in the Ghost group. This is quicker and simpler than looping through a list 
of all the game objects and checking which ones are ghosts.<br>
 <br>
 Adding and removing groups and sprites from each other is a very fast operation, 
quicker than using lists to store everything. Therefore you can very 
efficiently change group memberships. Groups can be used to work 
like simple attributes for each game object. Instead of tracking some attribute 
like "close_to_player" for a bunch of enemy objects, you could add them to 
a separate group. Then when you need to access all the enemies that are near 
the player, you already have a list of them, instead of going through a list 
of all the enemies, checking for the "close_to_player" flag. Later on your 
game could add multiple players, and instead of adding more "close_to_player2", 
"close_to_player3" attributes, you can easily add them to different groups 
for each player.<br>
 <br>
 Another important benefit of using the Sprites and Groups, the groups cleanly 
handle the deleting (or killing) of game objects. In a game where many objects 
are referencing other objects, sometimes deleting an object can be the hardest 
part, since it can't go away until it is not referenced by anyone. Say we 
have an object that is "chasing" another object. The chaser can keep a simple 
Group that references the object (or objects) it is chasing. If the object 
being chased happens to be destroyed, we don't need to worry about notifying 
the chaser to stop chasing. The chaser can see for itself that its group is
now empty, and perhaps find a new target.<br>
 <br>
 Again, the thing to remember is that adding and removing sprites from groups 
is a very cheap/fast operation. You may be best off by adding many groups 
to contain and organize your game objects. Some could even be empty for large 
portions of the game, there isn't any penalties for managing your game like 
this.<br>
 <br>
 
<h2>The Many Group Types</h2>
 The above examples and reasons to use Sprites and Groups are only a tip
of the iceberg. Another advantage is that the sprite module comes with 
several different types of Groups. These groups all work just like a regular 
old Group, but they also have added functionality (or slightly different functionality).
Here's a list of the Group classes included with the sprite module.<br>
 
<blockquote><b>Group</b><br>
 This is the standard "no frills" group mainly explained above. Most of the 
other Groups are derived from this one, but not all.<br>
   <br>
   <b>GroupSingle<br>
   </b>This works exactly like the regular Group class, but it only contains 
the most recently added sprite. Therefore when you add a sprite to this group, 
it "forgets" about any previous sprites it had. Therefore it always contains 
only one or zero sprites.<br>
   <br>
   <b>RenderPlain<br>
   </b>This is a standard group derived from Group. It has a draw() method 
that draws all the sprites it contains to the screen (or any Surface). For 
this to work, it requires all sprites it contains to have a "image" and "rect" 
attributes. It uses these to know what to blit, and where to blit it. <br>
   <br>
   <b>RenderClear<br>
   </b>This is derived from the RenderPlain group, and adds a method named 
clear(). This will erase the previous position of all drawn sprites. It uses 
a background image to fill in the areas where the sprite were. It is smart 
enough to handle deleted sprites and properly clear them from the screen when
the clear() method is called.<br>
   <br>
   <b>RenderUpdates</b><br>
 This is the cadillac of rendering Groups. It is inherited from RenderClear, 
but changes the draw() method to also return a list of pygame Rects, which represent 
all the areas onscreen that have been changed.<br>
   <br>
   </blockquote>
 That is the list of different groups available We'll discuss more about
these rendering groups in the next section. There's nothing stopping you
from creating your own Group classes as well. They are just python code,
so you can inherit from one of these and add/change whatever you want. In
the future I hope we can add a couple more Groups to this list. A GroupMulti
which is like the GroupSingle, but can hold up to a given number of sprites
(in some sort of circular buffer?). Also a super-render group that can clear
the position of the old sprites without needing a background image to do
it (by grabbing a copy of the screen before blitting). Who knows really,
but in the future we can add more useful classes to this list.<br>
   <br>
   
  <h2>The Rendering Groups</h2>
 From above we can see there are three different rendering groups. We could 
probably just get away with the RenderUpdates one, but it adds overhead not 
really needed for something like a scrolling game. So we have a couple tools 
here, pick the right one for the right job.<br>
   <br>
 For a scrolling type game, where the background completely changes every 
frame. We obviously don't need to worry about python's update rectangles in
the call to display.update(). You should definitely go with the RenderPlain 
group here to manage your rendering.<br>
   <br>
 For games where the background is more stationary, you definitely don't
want pygame updating the entire screen (since it doesn't need to). This type
of game usually involves erasing the old position of each object, then drawing 
it in a new place for each frame. This way we are only changing what is necessary. 
Most of the time you will just want to use the RenderUpdates class here. Since
you will also want to pass this list of changes to the display.update() function.<br>
 <br>
 The RenderUpdates class also does a good job an minimizing 
overlapping areas in the list of updated rectangles. If the previous position 
and current position of an object overlap, it will merge them into a single 
rectangle. Combine this with the fact that is properly handles deleted objects 
and this is one powerful Group class. If you've written a game that manages 
the changed rectangles for the objects in a game, you know this the cause 
for a lot of messy code in your game. Especially once you start to throw in
objects that can be deleted at anytime. All this work is reduced down to
a clear() and draw() method with this monster class. Plus with the overlap 
checking, it is likely faster than if you did it yourself.<br>
   <br>
 Also note that there's nothing stopping you from mixing and matching these
render groups in your game. You should definitely use multiple rendering groups
when you want to do layering with your sprites. Also if the screen is split into
multiple sections, perhaps each section of the screen should use an appropriate
render group?<br>
 <br>


  <h2>Collision Detection</h2>
 The sprite module also comes with two very generic collision detection functions. 
For more complex games, these really won't work for you, but you can easily 
grab the sourcecode for them, and modify them as needed.<br>
 Here's a summary of what they are, and what they do.<br>
   
  <blockquote><b>spritecollide(sprite, group, dokill) -&gt; list<br>
     </b>This checks for collisions between a single sprite and the sprites 
in a group. It requires a "rect" attribute for all the sprites used. It returns 
a list of all the sprites that overlap with the first sprite. The "dokill" 
argument is a boolean argument. If it is true, the function will call the 
kill() method on all the sprites. This means the last reference to each sprite 
is probably in the returned list. Once the list goes away so do the sprites. 
A quick example of using this in a loop,<br>
     
    <pre>	&gt;&gt;&gt; for bomb in sprite.spritecollide(player, bombs, 1):<br>	...     boom_sound.play()<br>	...     Explosion(bomb, 0)<br></pre>
 This finds all the sprites in the "bomb" group that collide with the player. 
Because of the "dokill" argument it deletes all the crashed bombs. For each 
bomb that did collide, it plays a "boom" sound effect, and creates a new Explosion
where the bomb was. (Note, the Explosion class here knows to add each instance
to the appropriate class, so we don't need to store it in a variable, that
last line might feel a little "funny" to you python programmers.<br>
     <br>
     <b>groupcollide(group1, group2, dokill1, dokill2) -&gt; dictionary</b><br>
 This is similar to the spritecollide function, but a little more complex. 
It checks for collisions for all the sprites in one group, to the sprites 
in another. There is a dokill argument for the sprites in each list. When 
dokill1 is true, the colliding sprites in group1 will be kill()ed. When dokill2 
is true, we get the same results for group2. The dictionary it returns works 
like this; each key in the dictionary is a sprite from group1 that had a collision.
The value for that key is a list of the sprites that it collided with. Perhaps
another quick code sample explains it best<br>
     
    <pre>	&gt;&gt;&gt; for alien in sprite.groupcollide(aliens, shots, 1, 1).keys()
    ...     boom_sound.play()
    ...     Explosion(alien, 0)
    ...     kills += 1
</pre>
 This code checks for the collisions between player bullets and all the aliens 
they might intersect. In this case we only loop over the dictionary keys, 
but we could loop over the values() or items() if we wanted to do something 
to the specific shots that collided with aliens. If we did loop over the values()
we would be looping through lists that contain sprites. The same sprite may
even appear more than once in these different loops, since the same "shot"
could have collided against multiple "aliens".<br>
     <br>
     <br>
     </blockquote>
 Those are the basic collision functions that come with pygame. It should 
be easy to roll your own that perhaps use something differen than the "rect" 
attribute. Or maybe try to fine-tweak your code a little more by directly 
effecting the collision object, instead of building a list of the collision? 
The code in the sprite collision functions is very optimized, but you could 
speed it up slightly by taking out some functionality you don't need.<br>
     <br>
     
    <h2>Common Problems</h2>
   <p>Currently there is one main problem that catches new users. When you
derive your new sprite class with the Sprite base, you <b>must</b> call the
Sprite.__init__() method from your own class __init__() method. If you forget
to call the Sprite.__init__() method, you get a cryptic error, like this;
<tt>AttributeError: 'mysprite' instance has no attribute '_Sprite__g'</tt>.
</p>
    <br>

    <h2>Extending Your Own Classes <i>(Advanced)</i></h2>

   <p>Because of speed concerns, the current Group classes try to
only do exactly what they need, and not handle a lot of general situations. If you
decide you need extra features, you may want to create your own Group class.</p>

    <p>The Sprite and Group classes were designed to be extended, so feel free
to create your own Group classes to do specialized things. The best place to start
is probably the actual python source code for the sprite module. Looking at the
current Sprite groups should be enough example on how to create your own.</p>

   <p>For example, here is the source code for a rendering Group that calls a
render() method for each sprite, instead of just blitting an "image" variable from it.
Since we want it to also handle updated areas, we will start with a copy of the original
RenderUpdates group, here is the code:
<pre>
class RenderUpdatesDraw(RenderClear):
    """call sprite.draw(screen) to render sprites"""
    def draw(self, surface):
        dirty = self.lostsprites
        self.lostsprites = []
        for s, r in self.spritedict.items():
            newrect = s.draw(screen) #Here's the big change
            if r is 0:
                dirty.append(newrect)
            else:
                dirty.append(newrect.union(r))
            self.spritedict[s] = newrect
        return dirty
</pre></p>

    <p>Following is more information on how you could create your own Sprite
and Group objects from scratch. </p>
     
    <p>The Sprite objects only "require" two methods. "add_internal()" and "remove_internal()".
These are called by the Group classes when they are removing a sprite from
themselves. The add_internal() and remove_internal() have a single argument
which is a group. Your Sprite will need some way to also keep
track of the Groups it belongs to. You will likely want to try to match the
other methods and arguments to the real Sprite class, but if you're not going
to use those methods, you sure don't need them.</p>
     
    <p>It is almost the same requirements for creating your own Group. In 
fact, if you look at the source you'll see the GroupSingle isn't derived from
the Group class, it just implements the same methods so you can't really tell
the difference. Again you need an "add_internal()" and "remove_internal()" method
that the sprites call when they want to belong or remove themselves from
the group. The add_internal() and remove_internal() have a single argument 
which is a sprite. The only other requirement for the Group classes is they 
have a dummy attribute named "_spritegroup". It doesn't matter what the value 
is, as long as the attribute is present. The Sprite classes can look for this
attribute to determine the difference between a "group" and any ordinary python
container. (This is important, because several sprite methods can take an
argument of a single group, or a sequence of groups. Since they both look
similar, this is the most flexible way to "see" the difference.)</p>
     
    <p>You should through the code for the sprite module. While
the code is a bit "tuned", it's got enough comments to help you follow along.
There's even a todo section in the source if you feel like contributing.</p>
                              
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