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There are a lot of articles about games. Most of these are about particular aspects of a game like rendering or physics. All engines, however, have a binding structure that ties all aspects of the game together. Usually there is a base class (Object, Actor or Entity are common names) that all objects in the game derive from, but very little is written on the subject. Only very recently a couple of talks on game|tech have briefly touched on the subject [Bloom], [Butcher], [Stelly]. Still, choosing a structure to build your game on is very important. The end user might not “see” the difference between a good and a bad structure, but this choice will affect many aspects of the development process. A good structure will reduce risk and increase the efficiency of the team.
When we started Shellshock: Nam ‘67 (SSN67) at Guerrilla Games we were looking for a structure that would be flexible enough to handle all of our ideas, yet strict enough to force us into a structured way of working. The development of our first game Killzone was already well underway so we had a good opportunity to look at their structure and improve on it. After a couple of weeks the base structure for our game had been designed and over the course of the next 2 years a lot changed but our basic structure remained more or less the same.
The core design decisions we made at the start of SSN67 are:
In the remainder of this article these points will be worked out to show how they were implemented in SSN67.
|Shellshock Nam ‘67|
In SSN67 the base class for all game objects is called Entity. We make a clear distinction between objects that have game state and those that don't. For example, the static world geometry and its collision model are not Entities, neither is a moving cloud in the sky. The player can't change the state of these objects so they are not an Entity. An oil barrel, for example, is an Entity because when it explodes it can harm the player and therefore influences the game state.
Using a definition like this makes it very easy to separate objects that are important to the game from objects that are not. Our streaming system, for example, can stream textures and other rendering data in and out without affecting the game state. Another system that benefits is the save game system. The state of an Entity is saved and on reload all current Entities are destroyed and replaced by Entities from the save game. Because all static geometry is unaffected saving and loading is very quick and the resulting save game is small. To go back to our example of a moving cloud: In SSN67 clouds are not Entities and therefore not saved. If you look carefully you can see this.
Most of the Entities in SSN67 need a position and orientation in the world so our Entity by default contains a 4x4 matrix. For those Entities that do not need a position we just accept the overhead.Data Driven
Data driven systems use data instead of code to determine the properties of a system where possible. In SSN67 we used the EntityResource class as base class for Entity properties. EntityResources are defined in a text file and edited by hand. The following example shows what the configuration for an NPC could look like:
Name = Ramirez
Model = models/soldiers/ramirez
Speed = 5
When these text files are loaded by our serialization system the corresponding objects are automatically created and each variable is mapped onto a member variable. After an EntityResource is loaded we validate the range of each variable and check if all variables together form a correct configuration. If this is not the case the game will display an error and refuse to run until the problem is corrected.
In SSN67 we do not allow Entities to be created without an EntityResource. Using an EntityResource makes sure that a specific object acts the same in all levels. This generally reduces the amount of bug fixing when the code for an Entity changes, because there is no need to test every level but only the EntityResources that are affected. We use a simple folder structure to store our EntityResources with one object per file.
The Factory pattern [DP] is used to create an Entity from its EntityResource. This means that if you have an EntityResource you can create a corresponding Entity ready to be used in game. We use the factory mechanism, for example, in our Maya plug-in. Level designers can create an Entity in a level by selecting the corresponding EntityResource file. Maya uses the factory to create an Entity for it and allows the level designer to position it. Entities that need specific (dynamic) behavior are controlled from our scripting language LUA [LUA]. The properties that can be set from LUA usually boil down to giving high level commands to the AI so that most of the configuration of an Entity is still contained in the EntityResource. Supporting a new Entity class in Maya is as simple as recompiling the Maya plug-in.Model-View-Controller
The Model-View-Controller pattern [DP] is used in many areas. In a word processor, for example, the Controller handles mouse and keyboard input and translates these to simple actions. The Model manages the text and executes the actions dictated by the Controller. The View is responsible for drawing (a portion of) the text on screen and communicates with the display drivers.
In a game we can use the same pattern. In our case the Model is called Entity, the View is called EntityRepresentation and the Controller is called Controller. We will now discuss the roles of each of these classes in game.
Entities try to keep a minimal state of the object. Any state that is only needed for visualization is not part of the Entity. Most Entities in SSN67 are simple state machines.
Ideally the Entity should not know about its animations but unfortunately most animations affect the collision volume of the Entity and therefore the game state. Still, we try to keep Entities and their animations as loosely coupled as possible. A walking humanoid, for example, looks at its EntityResource for its maximum walk speed and not at the walk animation. The EntityResource precalculates the speed of the walk animation and speeds up / slows down the animation based on the current speed. This makes it possible to use a simple model for a walking humanoid (linear movement) while the animation always matches the current speed perfectly.
The EntityRepresentation is responsible for drawing the Entity and all of its effects.
Examples of things that an EntityRepresentation manages are:
An EntityRepresentation can look at but not change the state of the Entity. The current state of an Entity is usually enough to calculate the state of the EntityRepresentation. In some cases this is not practical however. For example, when a human gets hit we want a blood spray from the wound. Simply looking at the previous health and the current health does not tell us where the enemy was hit. For these cases we use a messaging system. All messages derive from a class called EntityEvent, which contains an ID that indicates the type of the message. The getting hit message also contains the type and amount of damage and where the hit occurred. The Entity fills in the structure and sends it to a message handler in the EntityRepresentation. It won't receive a response to this message so the communication is one way only.
To achieve a game object that looks realistic the timing of effects is usually critical and needs a lot of code to manage. By separating this code from the essential game logic we reduce the risk of bugs. Another advantage is that if all objects follow the rules, the game state is not dependent on any of the EntityRepresentations and the game can run without them. This principle can be very useful for creating a dedicated multiplayer server where graphical output is not needed.
The Controller provides abstract input for an Entity. For every Entity that supports Controllers we create a base class that defines the controllable parts of the Entity. From this version we can derive an AI and a player version of the Controller. For example, a Humanoid has a HumanoidController and we derive a HumanoidAIController and a HumanoidPlayerController from this controller. The player version of the controller can be further split up to provide mouse and keyboard support (for PC) or joystick support (for XBOX and PS2).
The role of the Controller is to specify what the Entity should do. The Controller can't directly change the state of the Entity. Every update the Controller is polled by the Entity and the Entity tries to follow the Controllers instructions.
For example, the Controller would never call a MoveTo() function on a Humanoid but instead the Humanoid would poll the Controller's GetDesiredSpeed() function and then try to reach this speed while performing collision detection to make sure not to clip through walls.
The Controller's functions are more abstract than IsButtonPressed() or GetJoystickAxisX(), this to make it easier for the AI to use the Controller.
To make a Humanoid interact with a mounted gun, for example, the HumanoidController implements a function called GetUseObject(). This function is polled every update by the Humanoid. The AI uses reasoning to determine if it is beneficial to use a specific mounted gun. When it chooses to use a mounted gun it walks to the correct location and returns the mounted gun through GetUseObject(). When the player stands next to a mounted gun the HumanoidPlayerController detects that the mounted gun is in range and handles the logic of the use menu. The HUD takes care of drawing the menu. When the selection is made it is returned through GetUseObject(). Next time the Humanoid polls the HumanoidController it will start up the mount process. The HumanoidController is disabled and the mounted gun receives a MountedGunAIController or MountedGunPlayerController so that it can be used.
We will illustrate the Model-View-Controller mechanism with an example of a tank.
The tank is split up in an Entity (Tank), EntityRepresentation (TankRepresentation) and a Controller (TankController).
The TankController has functions like GetAcceleration(), GetSteerDirection(), GetAimTarget() and Fire(). The AI steers the tank using a TankAIController, the player uses a TankPlayerController.
The Tank keeps track of the physics state of the vehicle. It looks at the controller and applies forces to the physical model and responds to collisions. It keeps track of damage state, turret direction and implements firing logic.
The TankRepresentation draws the tank. It creates sparks particles and corresponding sound when the tank collides. It plays the engine sound and changes its pitch when the speed of the tank changes. It creates smoke particles when the tank fires and produces track marks when the tank is driving. It can also fine tune the position / rotation of the tracks so that they follow the terrain exactly (without influencing the collision volume of the tank).
When a human fires a gun, the bullets exit from the muzzle of the gun. In SSN67 this sometimes led to problems. Every weapon had its own aiming animations that were slightly different. These differences led to inconsistencies with our precalculated cover positions for the AI. The AI would sometimes think that they could fire from a specific location but when they got to that position and tried they would shoot into a rock.
To improve the situation we separated the logic of firing from the visual effect (the tracers). The bullets now exit from a fixed offset (roughly where the shoulder is) based only on the current position and stance of the humanoid. The tracers are handled by the EntityRepresentation and come from the muzzle of the gun (which is taken from the animation) and move in the same direction as the bullet. They slowly converge towards the real path of the bullet so that the impact point and time for the bullet and tracer is the same.
For the player it turned out that with our 3rd person camera it sometimes looked like you could fire over a piece of cover where in reality you couldn't. We tweaked the firing offsets for the player so that he was shooting from eye height instead of shoulder height and fixed the problem.
This simple model in the Entity is easy to use for the AI and guarantees consistency throughout many bits of code that are related to aiming and firing. It trades complicated code throughout the system for complicated code localized in the EntityRepresentation where it can only influence the EntityRepresentation itself.