Game development HOWTO

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This document describes how to use the Pygame library for Python to create a new Game activity for the OLPC (Sugar) platform. Its intention is to allow a Python programmer who wants to learn (or already knows) Pygame to integrate their Pygame application into a Sugar-hosted activity using the OLPCGames Pygame wrapper.

This HOWTO is current as of December 2007.

Requirements

This HOWTO assumes that you know the basics of computer programming, how to navigate a file-system, and how to edit files on your machine. It also assumes that you will largely learn Pygame programming through the large number of available Pygame references and tutorials. We focus here on how to integrate your Pygame games into the Sugar environment.

Components

  • Pygame -- this is a Python wrapper around the Simple Direct-media Layer (SDL) library. It is used for lots of games coded in Python and can run on most machines (including Windows, Mac and Linux). If you are running on an OLPC-XO, Pygame should already be available. If not, use your system's package manager to install the Pygame distribution.
  • OLPCGames -- the OLPC Sugar specific library which provides the glue code that lets your Pygame game run inside a Sugar activity. It also gives you access to the various "special" features in the Sugar environment, such as the mesh network and the camera. If you are on an OLPC-XO, you can download the current OLPCGames distribution and unpack it.
    • Note: The OLPCGames Pygame wrapper requires at least build 432 to work for version 1.0 and at least an Update.2 build (649) for version 1.1 and above. See the reference manual at Pygame wrapper. See also Game development.

Environment

You will need a working Sugar Developers environment. If you are working directly on an OLPC-XO, you will need to know how to use a standard text editor, such as vi or nano, which are available within the "Terminal" activity in your activity toolbar.

  • You'll need to set aside a few hours to learn vi before you start this HOWTO if you don't already know it and want to use it well)
  • Nano is often considered easier to learn immediately because the major commands are all spelled out at the bottom of the screen (where you need to remember the vim commands yourself)

If you are working in an emulated environment, or a sugar-jhbuild environment, you can use whatever text editor you prefer to create the files we will be working on. There are many text editors with some Python support, and full IDEs are also available.

Skeleton Setup

To start, you will likely want to download the OLPCGames 1.2 source package. This package includes a skeleton script that lets you generate a new OLPCGames-based Pygame Activity with a single command.

Getting the Skeleton Script

To install the package, you will need to download the .zip or .tar.gz to your machine and extract it with either of: 

unzip OLPCGames-1.2.zip

or 

tar -zxf OLPCGames-1.2.tar.gz

which will create a directory named OLPCGames-1.2. Change to the OLPCGames-1.2/skeleton directory and run the command: 

./buildskel.py activityname "My Activity Name"

to create a new activity instance.

Installing and Testing

To test that you have your environment properly configured, we'll restart sugar and attempt to run the newly created (empty) activity. Change to the new activity directory (activityname.activity) and run: 

python setup.py dev

when you restart Sugar you should have a new activity in your Activity bar named "My Activity Name". Clicking on this activity should result in dark blue screen with a toolbar at the top of the window.

Testing Outside Sugar

The run.py script in the skeleton project is where you skeleton activity currently points for its "mainloop", particularly the "main" function within it. When you are just starting you you'll likely want to work within run.py to create new code and experiment. run.py is actually set up to be used as a python script via: 

python run.py

which will run on a non-Sugar environment (i.e. a normal Linux, Windows or Mac desktop with Pygame installed). You may, however, have to configure your system to have the current working directory in the Python path (this is the default on Sugar systems, including emulators and sugar-jhbuild shells).

Customising the Skeleton

Your Sugar-specific activity values are stored in two main locations; the activity.py file and the activity directory. The pydoc for the PyGameActivity class describes the various attributes/settings available for your Activity object. These include changing the file-name and method-name for your mainloop function, and changing the title of your activity.

The activity directory is used by Sugar to find things such as your svg icon, translated names and the like. See Activity Bundles for details.

Getting Started with Pygame

At this point, your OLPC Sugar activity is running as a host for a simple Pygame event loop. The code in run.py does some trickery to make the event loop reasonably efficient, by limiting the number of frames rendered per second using a "pygame.time.Clock()" instance. It also uses a complex iteration mechanism: 

for event in [ pygame.event.wait() ] + pygame.event.get( ):

which allows your activity to go completely quiet if there are no pending events, but still processes all pending events before issuing a "display.flip()" command. None of that is OLPC or Sugar specific, incidentally, it's just good practice to reduce your processing load when running on an OLPC machine.

  • Note: the event iteration mechanism reduces the cpu-load from 99% to 0.7 - 4% in our tests versus a simple pygame.event.get() loop.

Reference Links

  • Pygame wrapper pydoc -- the OLPCGames wrapper's reference manual and pydoc
  • Pygame Documentation -- the official collection of Pygame documentation, you will need this to get any Pygame programming done

Special Hardware

The Pygame page has some information on hardware.

Camera

OLPCGames provides you with a simple wrapper class that will use GStreamer to snap an image with the built-in camera on the XO. The camera module works by creating a GStreamer pipeline that starts with the camera and terminates in a png or jpeg file. Alternately the module can allow you to take a "picture" of the test source, for those testing on non-XO hardware.

You will need to import the module: 

from olpcgames import camera

when you want to take a picture with the camera, call snap_async, like so: 

camera.snap_async( 'picture 32', source='test' )

the gstreamer pipeline will be created and will iterate until complete. When the picture is available it will be read as a pygame image and returned via a camera.CAMERA_LOAD or camera.CAMERA_LOAD_FAIL event in your Pygame mainloop. These events have the following members:

  • filename -- filename into which the file was saved
  • success -- boolean indicating whether we succeeded/failed
  • token -- the object passed in as the first argument to snap_async
  • image -- the loaded image as a Pygame surface (image), or None on failure
  • err -- if an error occurred, the Exception instance

There is a cameratest activity in GIT which demonstrates basic use of the snap_async function.

Note that the camera module exposes an older synchronous API for backwards compatibility. You likely should *not* use that api, as it can hang your entire activity waiting for the GStreamer stream to complete (which can take an arbitrary amount of time).

Sharing and the Mesh

Your activity can automatically be shared (through the user clicking "Share" on the Sugar toolbar). You should be using build >=530 (or so) for mesh related stuff. When the user does this, your activity will appear in the Neighborhood screen and others can join it.

In order to meaningfully communicate, you must 

import olpcgames.mesh as mesh

Now, you can make use of events you receive that are mesh related. See Pygame_wrapper#Mesh for details.

Here is a simple example -- in your event loop, just add listening for certain mesh events: 

    for evt in [ pygame.event.wait() ] + pygame.event.get( ):
        if evt.type == pygame.KEYDOWN:
            # ...

        elif evt.type == mesh.PARTICIPANT_ADD:
            # Somebody joined. Add them to our list of people, and send them a message:
            self.participants.append(evt.handle)
            mesh.send_to(evt.handle, str(self.game_state))

        elif evt.type == mesh.MESSAGE_UNI:
            # Received a message! Display it (and/or decode it):
            print "Got a message from %s: %s" % (evt.handle, evt.content)
            # Figure out the nick of whoever sent it:
            print "It was from buddy %s" % (mesh.get_buddy(evt.handle).props.nick)

See Also:

  • Discussion of how Productive uses the mesh module and raw Telepathy

Troubleshooting

Ensure you are using at least an Update.2 (first manufacturing release) Sugar environment. Also ensure you are using a checkout of the wrapper from the Git repository.

Check your log files. On modern Sugar, use the Log Viewer activity to view the log for your activity. Open this activity and find your activity in the list of activity instances on the left. The numeric suffixes increase as you run your activity multiple times.

User:Mcfletch is the current maintainer of the wrapper. Contact him if you get stuck.

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