Game development HOWTO

This HOWTO is current as of July 2007. Rapid changes are occurring; expect things to change.

Note: The Pygame wrapper requires at least build 432 to work. See the reference manual at Pygame wrapper. See also Game development.

Crash Course on Pygame

The slides from Noah's lecture at the start of the game jam are online at http://dev.laptop.org/~coderanger/ (both PDF and PowerPoint form).

Development Environment

Start with Pygame. If you are running Mac OS X, check out the Mac setup instructions.

You don't need a laptop for simple Pygame development. When creating your Pygame game, use this new boilerplate:

import sys
import pygame
from pygame.locals import *

def main():
	window = pygame.display.set_mode((400, 225))
	pygame.event.set_blocked(MOUSEMOTION)
	pygame.init()

	while True:
		for event in [ pygame.event.wait() ] + pygame.event.get( ):
			print event
			if event.type == KEYUP:
				# Quit on 'q'
				if event.key == 113: sys.exit(0)

if __name__=="__main__":
    main()

The 'main' method will be called by the activity wrapper later on, so it must be called 'main'. Using pygame.event.wait() instead of pygame.event.get() reduces the cpu-load from 99% to 0.7 - 4%.

Wrapping and Testing

If you want to test using laptop software, but you don't have a real XO, you could set up an emulated environment.

To make your game run as an Activity, you will need the 'olpcgames' wrapper, otherwise known simply as "the wrapper". Eventually, the wrapper will be included as part of the standard laptop software distribution, but for now you must include it in your Activity.

git clone git://dev.laptop.org/projects/games-misc

This will download several games-related projects. There are a few game Activities checked in, as well as the wrapper.

First construct an activity development bundle as in Sugar Activity Tutorial.

The wrapper requires the following boilerplate as the main class of your activity as specified in activity.info. Generally this is called 'activity.py' and your Pygame app retains its original name:

import olpcgames

# Class name must match 'class' property in activity/activity.info:
class ExampleActivity(olpcgames.PyGameActivity):
    """An example of using a Pygame game as a Sugar activity."""
    
    game_name = 'examplemodule:main'        # game_name must match name of your Pygame module
    game_title = 'Example'
    game_size = (1200,825)

The game_name parameter has the format "module:method". The module will be imported and the method called to start your Pygame application. Method is optional and defaults to main. (FIXME: Use Gettext to localize the game_title parameter.)

The game_size is what sets the display mode in Pygame.

The last step: you need to copy or link the 'olpcgames' directory from the Git repository you checked out into the root of your Activity. (Eventually, 'olpcgames' will be part of the main laptop build, so you will no longer need to do this.)

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.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)

Troubleshooting

Ensure you are using build >=432. Also ensure you are using a checkout of the wrapper (recent as of Jul 12) from the Git repository.

Check your log files. Alt+0 on the XO will bring up the developer console. Look at the Log Viewer and select your activity's logfile, and look at the output/traceback.

lincolnquirk is the current maintainer of the wrapper, and resides in the #olpc-content IRC channel (Freenode, like other OLPC channels). Contact him if you get stuck, and bring your logfiles.