Tanks Game Server Code

For both the tanks game as well as the jigsaw puzzle game server code, we used Adobe Flash* to render the images and animations on the shared screen. This section describes how we dealt with interfacing the game framework to Flash, followed by an example of accepting a player's request to move a tank forward.

It should be noted that while many games built on this framework may use Adobe Flash, the game framework code does not mandate the use of Flash.

Interfacing the Game Framework to Flash

While Flash afforded us many comforts in animating the game, it posed one primary challenge: the game framework provides a JavaScript/PHP interface, not a Flash interface. Therefore, our main challenge was how to get player creation and action commands from JavaScript/PHP into Flash. This trick here is to export a few Flash ActionScript functions and make those available to the game server's JavaScript. The game server's JavaScript interacts with the PHP code through HTTP POSTs/GETs. To make this concrete, in the game's action script code we exported calls to various functions as shown in Figure 7.

// set up all external calls
ExternalInterface.addCallback( "addTank", this, addTank );
ExternalInterface.addCallback( "removeTank", this, removeTank );
ExternalInterface.addCallback( "commandTank", this, commandTank );
ExternalInterface.addCallback( "newGame", this, newGame );
ExternalInterface.addCallback( "restartGame", this, restartGame );

Figure 7 - Exporting ActionScript Functions.

Once the ActionScript interfaces are exported, we can call them by packaging up an XML string representing the ActionScript function to call along with its arguments and calling the Flash object's “CallFunction” method, as show in Figure 8.

var _invoke = function( cmdName, args ) {
var _cmd_str = [
' '" returntype="javascript">',
'' + _toXML(args) +
'',
''].join('');
return _flashRef.CallFunction( _cmd_str );
}

Figure 8 - Calling ActionScript from JavaScript.

Once we can call ActionScript from JavaScript, we are left with the simple task of mapping player commands into ActionScript calls, as is shown in Figure 9 for moving a tank forward.

react: function( opts ) {
var cmd = '';
var params = '';
var _message = '';
switch( parseInt(opts.type) ) {
…
case 2: // move cmd = 'commandTank';
params = [ opts.source_id, 0,
opts.message ].join('_');
break;
…
}

if( cmd != '' ) {
var _ret = _invoke( cmd, params );
}
},

Figure 9 - JavaScript player action command parsing

Conclusions and Future Work

We've presented a new usage model for multiplayer games that takes advantage of players being in a single location looking at a shared view of the game and controlling the game with mobile devices. We explained important aspects of the game framework we developed to implement this usage model borrowing ideas and technologies such as AJAX from the web development world. We presented a detailed walk through of a game built on this framework.

The last thing we'll do is to make our code available to the public and let you have a go at this usage model that we've applied to games but can be extended to the other areas. The next steps are yours - take the code and make your own games or applications. Again, the code is available for download via Intel's website (email address required) or via Gamasutra. There is also a video available here with more information.

Remember that games are just one example of the usage model. A number of future directions are possible for this work:

• Porting existing games to use this framework and usage model
• Developing new games -- or even new game genres -- based on this usage model
• Focus on improving performance and response time
• Focusing more on graphics capabilities of the server that are not possible on the client
• Trying this out in more and more locations, with more and more players at a time
• Trying this for non-game usages: audience polls, group creativity, and whatever else you can think of

We are excited to see and hear where this technology goes. We would love to hear from you about your uses of this code. Please contact us!