The architecture and implementation of Half-Life for Multimodal Multimedia Interaction (HL4MI) is discussed. HL4MI is a project to explore ubiquitous computing and gaming. The commer-cial game Half-Life serves as a simulator for military tactical decision scenarios. Commodity PDAs, commercial wireless lan technology, and Internet based protocols are used to extend the game interface onto auxiliary devices.
The HL4MI architecture relies heavily on Python. Python is embedded into a modified Half-Life game server. The integrated interpreter announces interesting game events to distributed clients. A remote procedure call server also runs in the interpreter. This allows for clients to remotely control the game. Finally, clients on handheld devices are implemented using the C and Java versions of Python.
Half-Life for Multimodal Multimedia Interfaces (HL4MI) is an innovative application of Python to gaming and ubiquitous computing. The project’s goal is to improve decision mak-ing in military command and control. The thesis of the project is that ubiquitous computing concepts can help achieve this end. Towards that end Python is used to implement a gam-ing/ simulation infrastructure that supports the use of wirelessly connected personal digital assistants (PDAs) during game play.
HL4MI touches on almost every strength of the Python language. Python is embedded in the first person shooter game Half-Life  to provide a sensor network on the game’s virtual world. Python provides the glue around a content based messaging service. This messaging service distributes the sensor data. An RPC server is also implemented within Half-Life. This allows for remote control of the game server. Clients that monitor the game state and control the server are written in Python. Python’s portability and integration with GUI services are a key advantage here.
Figure 1 illustrates the HL4MI architecture at a high level. HL4MI can serve as a design example of a Python based ubiquitous computing environment. The design presents a simple, elegant system for remote observation and manipulation of a complex system.
This work is being conducted as part of the DARPA Command Post of the Future (CPOF) Project. The CPOF program is charged with investigating how computing can improve com-mand and control decisions in combat. These situations involve distributed group communi-cation, fast decision making, high rates of information generation, and personnel mobility.
Our thesis is that ad hoc collections of computers can lead to improvements in tacti-cal decision making. A system that coordinates small constellations of interactive devices (desktop, handheld, large displays) can provide the right interactive affordances at the right time.
Gaining access to a militarily realistic experimental environment is extremely difficult. First person shooter games, such as Half-Life, exhibit many of the decision making aspects of interest. Thus, such games can serve as first order approximations to the desired environment.
The approach of using a game in this fashion has a number of advantages. The simulation platform is readily accessible to a civilian population. Experimental subjects are easy to find and train. Maintenance and deployment issues are reduced. Since the simulation engine is software, the virtual world can be perfectly instrumented. The only major drawback is the level of fidelity to true military situations.
A key question is how to construct the ubiquitous computing infrastructure. The result must enable three major capabilities. First, observation of interesting events in the tactical environment must be possible. Second, there must be a mechanism for distributed clients to receive these events. This reception should not require tight coordination with the game server. Third, remote clients must have a means to affect the game world.
In the abstract, a distributed computing system is the end result. Distributed systems have a long history and are a continued area of research. There are any number of existing technologies to build such systems. Certainly widely popular object oriented solutions such as CORBA, DCOM, and Jini could have been brought to bear.
However, a scripting language based approach was chosen. One key reason is the ability to embed such languages within systems code. This makes integration with pre–existing systems easier. Successful scripting languages simplify network programming and support most popular network protocols. Architectural flexibility in HL4MI is an overarching concern. Scripting language support for gluing existing libraries and components together allows us to rapidly prototype connections to tools in the field. The accessibility of scripting langauges decreases the cost of bringing new programmers onto the project. Finally, deployment is simpler relative to most distributed object systems.
Given the scripting language approach, there are any number of implementations to choose from. Perl and Python, as two of the most mature and featureful languages, were considered. Python was settled upon for the following reasons. Python is almost trivially embedded within any application extensible through C or C++ code. Python can easily participate in most component systems and supports programming of almost all popular and emerging Internet protocols. Python implementations exist across a wide range of platforms, including the handheld devices of interest. Python code performance was satisfactory for most of our purposes and bottlenecks can be easily engineered around. All of our needs could be satisfied by Python, but the language interoperates well with other languages. Python is more consistent than Perl making the language more accessible to undergraduates, a major source of programmers for this project. Also, Python’s interaction with threads seems to be better implemented than Perl’s. Finally, the Jython implementation of Python turned out to be particularly advantageous. The differences in Windows CE C/C++, relative to the desktop, force severe limitations in ports of the most popular scripting languages. Insignia’s Jeode  Java virtual machine for Windows CE supports Jython on stock commercial handheld devices such as the Compaq iPaq.
Besides Python, the HL4MI architecture has three other major elements. The first is the game Half-Life, used as a virtual tactical decision environment. The second part is Elvin , a content based messaging system. Elvin is used to distribute game observations to interested parties. Lastly, clients on various platforms receive and process these observations. Clients present the information to end users or perform automated processing on their behalf.
There are a number of commercial games that could have been chosen for this project. Half-Life was selected due to the gameplay design and the extension capabilities of the game engine.
Half-Life is commendably extensible for a commercial game. Game play in Half-Life’s game server is embodied by a set of C++ classes. The source for this code is freely available. These classes are built into a dynamic link library (DLL) that is loaded at runtime into the main game engine. The engine periodically calls into the DLL to effect game logic. Numerous new games have been constructed with players, non-players, weapons, and game rules differing highly from stock Half-Life. As an example, one such commercial extension, Counter-Strike, simulates a hostage rescue situation.
This leads into how Half-Life’s gameplay style is advantageous. Straight Half-Life is a single player, single machine mission. However, Half-Life also supports network based team play. While still somewhat of a chaotic frenzy, elements of group communication and coordination come into play. In the real world of the military, such issues are at the core of tactical effectiveness. Network Half-Life is then a basis for more tactically sophisticated game play.
Elvin is a daemon based system for content based, asynchronous messaging. Messages are essentially bags of named properties. The names are simple identifiers, while the actual prop-erty values can be strings, integers, longs, and opaque sequences of bytes. Long lived Elvin servers listen for client event announcements. Clients also inform Elvin servers what events they are interested in by providing subscriptions, patterns that match events of interest. Elvin daemons are then responsible for routing events to the appropriate registered clients. Figure 2 illustrates the Elvin architecture. Elvin supports ad hoc discovery of servers and
Using Elvin to distribute game observations provides HL4MI with two key capabilities. Distributed asynchronous messaging supports networked clients that do not centrally coordi-nate with the game server. The game server simply generates events of interest as frequently as possible with as much information as possible. Elvin’s content based routing then supports filtering of these events on a client’s behalf. This makes clients much simpler to program rel-ative to other messaging infrastructures such as pure IP Multicast, heavyweight message queues such as the Java Messaging Service, or advanced group communication systems such as Spread .
HL4MI is an extension module (mod for short) for Half-Life. The HL4MI mod retains the networked, team game play from the publisher’s software developers kit. Embedded within the mod is a Python interpreter.
Various game events of significance are noted and forwarded to the interpreter for pro-cessing. These events are then transmitted to clients using Elvin. In addition, the interpreter implements a SOAP  based remote procedure call (RPC) server. Figure 3 provides an illus-tration of the architecture. The figures on the right represent diverse collections of interactive devices, the ultimate target of this work.
The C implementation of Python has been designed to be easily embedded within C and C++ code. This made it straightforward to embed C-Python in a Half-Life mod. For each game instance, the game engine instantiates an instance of a gamerules class. Within this class we have an opportunity to initialize the Python interpreter. The mod implements a C++ Python module to support calls into the game world as part of the RPC server. Also as part of the initialization process, a pure Python module, pyhl, is imported into the interpreter. This module provides the dispatchMessage procedure.
dispatchMessage is intended to handle event callbacks. Game events are sent in as Python dictionaries. Dictionaries encourage callbacks to include as much information as possible. dispatchMessage can easily ignore information it does not know about. These events can be processed anyway the pyhl module sees fit. A key flexibility of this scheme is that event processing can be replaced without rebuilding HL4MI.
Generating Elvin events from the Python interpreter embedded into Half-Life is quite easy. Here are some examples of the types of events that are currently distributed to clients. Roughly once per second, every player’s position is announced. When players acquire or lose items, such as weapons, an announcement is made. Changes to player health as well as player deaths are announced. Player connections and disconnections, level intermissions, and game completions also generate notifications.
Figure 4 illustrates a portion of a client designed to just listen for position announcements from one particular player. import elvin # The callback for position announcements def sub_cb(sub, nfn, insec, rock): print "bmd’s new location: %d, %d" % (nfn[’x’], nfn[’y’]) # Establish a connection to the nearest Elvin # daemon responsible for generating HL4MI events connection = client.connection() connection.set_discovery(1) connection.set_scope("hl4mi") connection.open() # Listen only for position announcements about me sub_string = ’require(HL4MI) && type == "position" && player == "bmd"’ sub = connection.subscribe(sub_string) sub.add_listener(sub_cb, connection) sub.register() #-- wait for delivery connection.run()
Clients can connect to the HL4MI infrastructure after a game has started. Forcing clients to recover strictly by listening for events would be inefficient and non-deterministic. In ad-dition to asynchronous events, the embedded Python interpreter implements a synchronous RPC server. Clients can use this mechanism to query the game state.
The SOAP protocol is used to implement the RPC server. SOAP is an emerging Web based standard, primarily for doing RPC using XML over HTTP. The benefits of using SOAP for this project were numerous. SOAP in Python is supported by a diverse number of packages. Deploying SOAP clients is easy, requiring only a TCP stack and XML parsing. This is especially advantageous for underpowered, mobile, handheld devices. SOAP supports interoperation with a wide range of languages such as Perl and Java. Our SOAP server library is currently SOAP.py version 0.9.7 . Figure 5 illustrates Python code to remotely adjust a
import SOAP, random HL4MIEndPointURL = ’http://hl4mi.cs.nwu.edu/’ NamespaceURI = ’urn:hl4mi’ server = SOAP.SOAPProxy(HL4MIEndPointURL, namespace=NamespaceURI) roster = server.getRoster() unlucky = random.choice(roster) server.changeHealth(unlucky[’id’], -10)
A simple intial set of RPC calls is currently provided in HL4MI. Lists of the current teams and rosters for each team can be requested. The equipment a particular player has can be retrieved. Similarly, clients can query the current health and current team of a player. Text messages that appear on a player’s console can be sent from any SOAP enabled client. The same can be done for the game server’s log messages. Finally, clients can affect the game world by remotely changing the health of a player or causing the player to lose an item.
The current set of clients are fairly primitive, but Python is still a central element of the process. Figure 6 presents some client screen captures from handheld devices. The leftmost capture is a two dimensional overhead view of the relative positions of the players within the game world. This is currently implemented in C++. The capture on the right is a client which displays the current health of a team of players. This second client is written using Jython running on Insignia’s Jeode Java virtual machine. The clients execute on Compaq iPaqs running Microsoft PocketPC 3.0. These handhelds are equipped with 802.11b wireless LAN cards. Clients are also being prototyped using C-Python on iPaqs running Handheld Linux.
Clients deployed on advanced tablet machines running Windows 2000 are being proto-typed using ActiveState Python. These tablets have a touch screen for pen based interaction. Python’s strong integration with a variety of graphical user interface toolkits (Tk, wxWin-dows, gtk, Qt) is advantageous. The tablet machines are Fujitsu Stylistic 3600s, equipped with a Pentium III, 256 Mb of memory, a 10 GB hard drive, and an 802.11b wireless lan card.
HL4MI currently provides a basic experimental infrastructure. Much work remains. Our ini-tial client set is starting to reveal programming patterns within the HL4MI framework. This knowledge can guide the construction of Python based class libraries, object frameworks, and/or language extensions to support programming in this domain. Further contributions will be in exploring how the availability of ubiquitous computing capabilities changes game play. These efforts fall into two areas: exploding the HUD out into multiple ubiquitous com-puting devices and pulling the context in to the game play.
HL4MI provides mechanism to build ubiquitous clients for command and control. There is little experience building such clients. Asynchronous events lead to non-obvious patterns of control flow. Abstraction and composition of events and their descriptions is not a part of HL4MI. Can Pythonic programming styles ease the task of writing clients for this infras-tructure? The Elvin subscription language is limited, being a means for writing tri-valued logic expressions . A lack of abstraction and composition constructs in the language constrains the complexity of subscriptions that can be expressed. Beyond a few conjunctions, subscrip-tions become unreadable. Wrapping a high level Pythonic toolkit around the subscription language should remove both of these problems.
Asynchronously generated events are a core element of HL4MI. However, event driven programming is not familiar and accessible to many programmers. Experience from GUI toolkits , iterators, state machines, coroutines, and continuations can provide guidance. Recent versions of the Python language (2.2) and alternative implementations, such as Stack-less Python , provide rich support for such programming constructs. Similar to the sub-scription language, a framework for HL4MI event processing could be built on top of these facilities.
Finally, large potential exists to incorporate automated proactive assistance in HL4MI. This step moves in the direction of artificial intelligence. Python based agent and machine learning frameworks, such as Narval , should be easy to integrate.
The Python based sensor network embedded in Half-Life can be used in a more sophisticated fashion. Significant tactical events can be detected internally. For example, from current game events, a simple qualitative analysis of how “spread out” a team is can be calculated. This analysis could be constantly displayed ambiently on an unobtrusive device not connected to a player’s HUD. If a team becomes too spread out, then a team leader can be automatically informed.
Within HL4MI the varying affordances of different devices can be leveraged. The result of game analyses described above can be delivered to different devices. For example, using auditory channels might be more appropriate for alerting a commander to the fact that his team is too spread out. A wirelessly connected PDA with earbud headphones and streaming audio support could fill this role.
A colleague here at Northwestern University, Professor Ian Horswill, studies autonomous mobile robot systems. Some of Horswill’s students have applied his techniques to non-player characters in Half-Life, termed Flexbots . Merging HL4MI and Flexbots is a prime ob-jective. The monitoring and control of Flexbots is a good opportunity to apply ubiquitous computing techniques. The point of autonomy is to allow an otherwise engaged player not to interact with his robotic teammates. However, the player does want to monitor these team-mates while consuming little cognitive bandwidth. As well, directing these robotic teammates is occasionally called for. In an ubiquitous enviromment, what are appropriate mechanisms to do so?
“Pulling context in”, refers to incorporating real world context into the game play. People can play games in different physical environments. Technologies such as Active Badges  and iButtons  support sensing of people’s location in the physical world. Can we take this information and change how Half-Life is played? In the near future, a player may be in a physical environment that has wall sized displays, sophisticated audio input and output, and network accessible data storage and processing. Some memebers of the team may be physically located with the player, while others are remote.
With an appropriate ubiquitous computing infrastructure, the game engine can incorpo-rate these varied contexts and adjust game play accordingly. For example, when an expe-rienced commander is colocated with members of his team, the commander may become a force multiplier, improving the combat effectiveness of the team’s virtual players. This is in addition to the fact that the team probably coordinates better thanks to physical proximity. Even more striking would be a game that increases the complication of play when a rich information environment is detected.
The most relevant related work is Pederson’s QWPython project . QWPython wraps a Python interpreter around the C++ core of QuakeWorld, a freely available server for mul-tiplayer Quake, a game similar to Half-Life. In QWPython, Python replaces the QuakeC language for implementing game logic. In contrast to HL4MI, QWPython is oriented toward implementing new gameplay styles. HL4MI’s first goal is integrating a game into an ubiqui-tous computing environment. Future efforts to incorporate ubquitous computing capabilities into the game play will be informed by QWPython.
As well, Asbahr  describes an embedding of Python in a next generation version of Ultima Online. Ultima Online is a multiplayer role-playing game; a much different style from Half-Life. Eve Online  is a similar massively multiplayer online game that relies on Stackless Python to implement game logic.
The MoPY  project has somewhat similar goals to HL4MI in terms of integrating Python and ubiquitous computing. MoPY takes a CORBA based approach to building the distributed infrastructure. The application space is oriented towards highly mobile clients supported over a wider area. Thus, network bandwidth and reliability constraints are much tighter. Also, the communication rate between the handhelds and applications is significantly reduced in comparison to HL4MI.
Rakotonirainy and Chilvers  describe a mobile computing infrastructure based upon the Hector  distributed object system. The project focused on transparent adaptive be-havior under mobility along with object migration in a distributed system. Interestingly the work seems to have been based on an earlier version of Elvin, validating the choice of Elvin for communication in HL4MI.
The Half-Life for Multimodal Multimedia Interaction architecture, (HL4MI), was presented. In HL4MI, Python was heavily used to integrate a first person shooter game into an ubiqui-tous computing infrastucture. HL4MI embeds Python into a network server for the commer-cial game Half-Life. The interpreter implements a remotely accessible sensor network on the virtual world. The Elvin messaging system allows clients to remotely observe the game state. A SOAP based RPC server for remotely controlling the game server was also incorporated.
Distributed clients that leverage this infrastructure run on a variety of networked devices. Besides stock desktop tools, tablet and PDA based prototype clients have been implemented. C-Python on Linux based handhelds and Jython on WinCE machines were used.
HL4MI is an ongoing concern. Future work will include more development of distributed multimodal interfaces. Game play design in an HL4MI environment will also be examined. Constructing Python based frameworks and toolkits for programming HL4MI tools will be a major effort.
HL4MI’s key lesson is that a combination of Python’s capabilities, a robust content based messaging system (Elvin), and an open RPC protocol (SOAP), lead to a rich and flexible distributed application infrastructure. This infrastructure can be used to elegantly integrate a commercial game with clients for ubiquitous computing. The HL4MI design serves as an example for any system which supports distributed observation of a complex, dynamic system.