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Postmortem: Thief: The Dark Project

July 9, 1999 Article Start Previous Page 2 of 4 Next

What Went Right

1. Designing data-driven tools. Our experience on previous titles taught us that one of the impediments to timely game development is the mutual dependence of artists, designers, and programmers at every development stage. One of the development goals for the Dark Engine, on which Thief is built, was to create a set of tools that enabled programmers, artists, and designers to work more effectively and independently. The focus of this effort was to make the game highly data-driven and give non-programmers a high degree of control over the integration of their work. Media and game systems were to be easily and intuitively plugged in and edited by the team members responsible for their creation, without requiring the direct involvement of programmers.

The Dark Object System stood at the heart of our strategy. Primarily designed by programmer Marc "Mahk" LeBlanc, the Object System was a general database for managing the individual objects in a simulation. It provided a generic notion of properties that an object might possess, and relations that might exist between two objects. It allowed game-specific systems to create new kinds of properties and relations easily, and provided automatic support for saving, loading, versioning, and editing properties and relations. It also supported a game-specific hierarchy of object types, which could be loaded, saved, and edited by designers. Programmers specified the available properties and relations, and the interface used for editing, using a set of straightforward classes and structures. Using GUI tools, the designers specified the hierarchy and composition of game objects independent of the programming staff. In Thief there was no code-based game object hierarchy of any kind.

Hand-to-hand combat is sometimes necessary.

Although the implementation of the system was much more work than we expected, and management of the object hierarchy placed significant demands on lead designer Tim Stellmach, it turned out to be one of the best things in the project. Once the set of available properties and relations exposed by programmers was mature, the Object System allowed the designers to specify most of the behaviors of the game without scripting or programmer intervention. Additionally, the relative ease with which variables could be made available to designers in order to tweak the game encouraged programmers to empower the designers thoroughly.

The second major component of our strategy was our resource management system. The resource management system gave the game high-level management control of source data, such as texture maps, models, and digital sounds. It helped manage the game's use of system memory, and provided the data flow functions necessary for configuration management.

Looking Glass's previous resource management system provided similar functionality, but identified resources by an integer ID and required a special resource compilation step. This technique often required recompilation of the game executable in order to integrate new art, and required that the team exit the game when resources were published to the network. The new system referred to a resource by its file name without its extension, used a file system directory structure for namespace management, didn't leave files open while working, and required no extra compilation step. Developers simply dropped art into their local data tree and started using it. To expose art to the rest of the team, lead artist Mark Lizotte just copied art into the shared project directories. Compound resources were treated as extensions to the file system and were built using the standard .zip format. This allowed us to use off-the-shelf tools for constructing, compressing, and viewing resource files. The system facilitated content development by allowing programmers, artists and designers to add new data to an existing game quickly.

The data-driven approach worked so well that through much of our development, Thief and System Shock 2 (two very different games) used the same executable and simply chose a different object hierarchy and data set at run time.

2. Sound as a game design focus. Sound plays a more central role in Thief than in any other game I can name. Project director Greg LoPiccolo had a vision of Thief that included a rich aural environment where sound both enriched the environment and was an integral part of gameplay. The team believed in and achieved this vision, and special credit goes to audio designer Eric Brosius.

As an element of the design, sound played two roles in Thief. First, it was the primary medium through which the AIs communicated both their location and their internal state to the player. In Thief we tried to design AIs with a broader range of awareness than the typical two states that AIs exhibit: "oblivious" and "omniscient." Such a range of internal states would be meaningless if the player could not perceive it, so we used a broad array of speech broadcast by the AIs to clue in the player. While very successful for humanoid AIs, we feel the more limited expressibility of non-human creatures is the heart of why many customers didn't like our "monster levels."

Second, the design used sounds generated by objects in the game, especially the player, to inform AIs about their surroundings. In Thief, the AIs rarely "cheat" when it comes to knowledge of their environment. Considerable work went into constructing sensory components sufficient to permit the AIs to make decisions purely based on the world as they perceive it. This allowed us to use player sounds as an integral part of gameplay, both as a way that players can reveal themselves inadvertently to the AIs and as a tool for players to distract or divert an AI. Moreover, AIs communicated with each other almost exclusively through sound. AI speech and sounds in the world, such as the sound of swords clashing, were assigned semantic values. In a confrontation, the player could expect nearby AIs to become alarmed by the sound of combat or cries for help, and was thus encouraged to ambush opponents as quietly as possible.

In order for sound to work in the game as designed, we needed to implement a sound system significantly more sophisticated than many other games. When constructing a Thief mission, designers built a secondary "room database" that reflected the connectivity of spaces at a higher level than raw geometry. Although this was also used for script triggers and AI optimizations, the primary role of the room database was to provide a representation of the world simple enough to allow realistic real-time propagation of sounds through the spaces. Without this, it is unlikely the sound design could have succeeded, as it allowed the player and the AIs to perceive sounds more as they are in real life and better grasp the location of their opponents in the mission spaces.

3. Focus, focus, focus. Early on, the Thief plan was chock full of features and metagame elements: lots of player tools and a modal inventory user interface to manage them; multiplayer cooperative, death-match and "Theft-match" modes; a form of player extra-sensory perception; player capacity to combine world objects to create new tools; and branching mission structures. These and other "cool ideas" were correctly discarded.

Instead, we focused in on creating a single-player, linear, mission-based game centered exclusively around stealth, with a player toolset that fit within the constraints of an extension of the Quake user interface. The notion came into full force with two decisions we made about seven months before we shipped. First, the project was renamed Thief from the working title "The Dark Project," a seemingly minor decision that in truth gave the team a concrete ideological focus. Second, we decided preemptively to drop multiplayer support, not simply due to schedule concerns, but also to allow us as much time as possible to hone the single-player experience. In the end, some missions didn't achieve the stealth focus we wanted, particularly those originally designed for "Dark Camelot," but the overall agenda was the right one.

Concept sketch of a burrick [zoom]

4. Objectives and difficulty. One of the Thief team's favorite games during development was Goldeneye on the N64. We were particularly struck by the manner in which levels of difficulty were handled. Each level of difficulty had a different overlapping set of objectives for success, and missions were subtly changed at each level in terms of object placement and density. Relatively late in the development of Thief, we decided such a system would work well in our game. Extending the concept, we added a notion that as difficulty increased, the level of toleration of murder of human beings decreased. We also allowed players to change their difficulty level at the beginning of each mission. The system was a success in two ways. First, it made clear to the player exactly what "difficulty" meant. Second, it allowed the designers to create a very different experience at each level of difficulty, without changing the overall geometry and structure of a mission. This gave the game a high degree of replayability at a minimum development cost.

5. Multiple narrow-purpose scripting solutions. Although the Object System provided a lot of flexibility, we also needed a scripting language to fully specify object behaviors. Rather than create a single all-encompassing scripting system, we chose to develop several more focused tools for scripting. This tiered scripting solution worked well.

In creating our core "high-end" object scripting technology, we wanted to allow designers with moderate programming skill to create complex object behaviors easily. Scripts were event-driven objects attached at run time to game objects, and contained data, methods, and message handlers. The game provided a selection of services to allow the script to query the world state and the game object state, and also to perform complex tasks. Our goal was to create a scripting language that offered source-level debugging, was fast, and was dynamic. The solution was essentially C++ in .DLLs, compiled by the C++ compiler, using a combination of classes and preprocessor macros to ease interface publishing, handle dynamic linking, and provide designers a clear programming model. Though used by both programming-savvy designers and programmers, the fact that it was a real programming language prevented widespread use by all of the designers.

Most designers were interested in customizing AI behaviors. For the AI we created a simpler scripting system, "Pseudo-scripts," that were implemented as properties within the Object System. Pseudo-scripts took the burden of coding scripts off of the designers. The AI provided a stock set of triggers, such as "I see the player near an object" or "I see a dead body"; the designer provided the consequence of the trigger. Each Pseudo-script was edited in a dialog box presenting parameters to tweak the "if" clause of the trigger, and space for a list of simple, unconditional actions to perform when the trigger fired. In this way, the custom behavioral possibilities of the AI at any moment were described by the aggregate of Pseudo-scripts that were attached to that AI. This approach had three benefits. First, it was simple enough so that designers with no programming experience were comfortable using it. Second, it narrowed the range of triggers a designer could use to a good pre-selected set, rather than giving them an open-ended system that might not have worked as well. Finally, when and how to evaluate AI triggers, a potential run-time expense if not carefully constructed, could be custom built by a programmer.

Concept sketch of Hammer

The final scripting system built into Thief was the Tagged Schema system. When the game required motions and sounds, it requested them as concepts modified by optional qualifiers, rather than directly. For example, an AI who had just heard the player would request the concept "moderate alert," qualified with an optional tag like "+sense:sound." A potential set of resources was then chosen using a pattern matcher; in this example, it would choose all samples in that AI's voice expressing a generic "something's not right," all samples expressing "I heard something fishy," but no samples expressing "I saw something fishy." From this set, the specific resource was then chosen using a weighted random selection. The tables used were specified by the designers using a simple language. Specifying motion and sound selection this way, designers created an interesting variety of randomized environments and behaviors without changing the code of the game.

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