Pyrogenesis Engine Overview

Introduction

Pyrogenesis is the game enginefor 0 A.D. Its design goals include cross-platform compatibility (Windows, Linux and Mac OS X), efficiency (including hardware-accelerated 3D graphics), and extensibility through game mods and scripts. The core engine is written in C++. However, much of the high level game logic and user interface is implemented by scripts, which are written in JavaScript. This document focuses on the C++ engine (present in the source folder in SVN). We first describe the "life cycle" of a game and how the different engine modules interact. We then provide an overview of each module in the engine.

Life of a Game Session

When you launch 0 A.D, the game first loads from a combination of plain files in binaries/data as well as a zip archive containing commonly used files. The Virtual File System (VFS) module (implemented in lib/file) makes both files in the archive and regular files look like the same kind of File object to the game. Once the game is loaded, it displays a main menu GUI. The GUI engine is implemented by the gui module. It supports defining a GUI page layout in XML and then responding to events using JavaScript. These files are present in binaries/data/mods/official/gui/test. They interact with the game using the script functions exposed by the various C++ modules, which are generally in scripting/ScriptGlue.cpp. From the main menu, players may either start a single player game, host a multiplayer game, or join a multiplayer game.

The game logic code works the same way whether the game is multiplayer or single player - all that differs is how commands are treated. We'll first describe how the game logic code is organized before explaining the details of setting up multiplayer. The game logic is implemented in the simulation module, most notably in CSimulation, which handles all updates, and CEntity, which represents and updates an in-game object (unit, building, etc). In both multiplayer and single player, the game state is updated in turns, which are about 150 milliseconds each. Each turn, objects move to some new position, units may gather resources or do damage (depending on their gather rates), etc. A turn is handled in the CSimulation::Update method. In addition, between turns, the graphical objects in the game are animated continuously as fast as the frame rate permits by interpolating between their previous turn's position and their next turn's position. This is done by CSimulation::Interpolate. There is a CTurnManager object which says when it is time for a new turn (the simulation queries this object each frame). While the game is running, commands from the player (obtained through the GUI) are queued up using CTurnManager::QueueLocalCommand. Each turn, the turn manager provides a batch of commands to the simulation based on what was queued up. The only difference between single player and multiplayer is which turn manager is used: The multiplayer turn manager sends commands to the host, which buffers a batch of orders and then broadcasts them to all players, thus making sure that all clients get the same batches of commands, keeping their simulations in lockstep. The single player turn manager sends commands directly to the simulation. All commands are validated before being played, to prevent cheating. When AI is implemented, it will be done by simply sending orders from the AI player to the order queue as if a human player had issued them.

Most of the game logic happens in CEntity and its related scripts (binaries/data/mods/official/entity_functions.js) and helper classes (like CEntityManager). All objects that a player may interact with in the game are entities - including units, buildings, trees, any obstacle that has an effect on pathfinding, etc. Each entity is represented in the game world by an actor - a graphical mesh with potentially some other meshes attached to it at prop points (things like shields, weapons, or the rider on a horse). There are also purely decorative objects that are not entities, such as grass blades or shrubs. These are implemented by having an actor with no entity. Actor drawing and animation is handled in the graphics package, including loading meshes and animations from 0 A.D.'s custom compressed formats, PMD (Pyrogenesis MoDel) and PSA (PyrogenesiS Animation).

Each entity has a hierarchical tree of properties as well as a number of event handlers implemented in JavaScript. Entity types (such as different soldier types) are defined as XML "template" files in binaries/data/mods/official/entities. These XML templates can inherit from each other, so that, for example, all spearmen can inherit some properties from template_infantry_spearman.xml, but Celtic and Persian spearmen can be differentiated by modifying celt_infantry_spearman.xml and pers_infantry_spearman.xml. A property such as maximum health might be implemented in an XML file as <traits><health><max>100</max></health></traits> and will then be accessible through scripts and C++ code as traits.health.max. The XML files also define script functions that can handle various events for an entity. For example, an entity may choose to react when it's damaged by attacking the entity that damaged it. Most orders which require the entity to do some work are implemented as events. For example, if a unit has an attack with attack rate of 1 hit per 2 seconds, then as long as it is trying to attack a target, it will get a "handle generic order" event with action "attack" ever 2 seconds, and then a JavaScript function will decide how much damage to inflict on the target and do so. The C++ code only does the "heavy-duty" work like pathfinding and collision detection, allowing most of the game logic to be handled in JavaScript for a quick development cycle. One last thing to note about entities is that although they have many properties, only the properties that differ from those in the template, such as "current health", are maintained for each CEntity object. The rest are held by the template (CEntityTemplate). This means that it's possible to, for example, upgrade the attack of all current and future entities of some type by applying a change to their template. Entity properties and event handlers are described in detail at XML.Entity. Scripting helper functions are implemented in the scripting module - Most notably, the CJSObject class, which makes it possible to expose C++ instance variables and methods in an object to JavaScript fields and functions.

The graphics in 0 A.D. are handled by two packages - graphics, which manages graphical objects like meshes, animations and textures, and renderer, which figures out what items to draw and also manages global effects such as shadows and water rendering. Sound is implemented in the sound module and provides basic support for playing music and playing short sounds on various events. The game view (including camera) is maintained in CGameView (graphics/GameView.cpp). Player input is handled in engine/Interact.cpp.

One final thing to note about game life is how the map editor, Atlas, works. Atlas is launched by running pyrogenesis with the -editor option. It creates a window using the wxWidgets library, and then runs a special copy of the game within this window. This makes it possible to render objects in Atlas using the same code that renders them in the game, and to start testing the game at any time by pressing the Play Simulation button in Atlas. The Atlas GUI is also scriptable. It is implemented in binaries/data/tools/atlas/scripts. Atlas also supports hotloading GUI scripts, so it is possible to edit a script and immediately see its effect in a running Atlas window. The goal is to eventually support hotloading for the game GUI and possibly even game logic scripts in 0 A.D.

Code Organization

Module Descriptions

Low-Level

Simulation

Rendering

Scripting

Network

Input

Sound

Atlas (Map Editor)

Standalone Tools

Developing Pyrogenesis

Build Process

Automated Tests

Useful Features for Testing

Outline for this document (from Gobby):

engine overview
  life of a game session from start to finish (pyrogenesis startup, VFS, menu GUI,
    creation of game object, loading, gameplay, scripts, etc), mentioning the
    components involved in each part
  explanation of multiplayer sessions
  explanation of how Atlas fits in

how to build the distribution
  supported compilers
  
  build system
    * only one copy of settings for Mac, Win, Unix (helps keep workspaces in sync)
    * what it means to update-workspaces
    * organization of the tree as static libraries
    * external libraries and how they are linked in
    * required libraries for Linux and OS X ("manual" installation of the game)

finding your way around: brief description of all repository directories
  merge the below descriptions of svn/source and those already found in "finding your way" 


overview of svn/source directories:
  collada: routines for loading COLLADA 3d models
  dcdt: triangulation library used by the pathfinder
  graphics: manages objects drawn on the screen, like textures, animated models,
    terrain patches, etc
  gui: a homemade OpenGL in-game GUI based on XML files
  i18n: routines for internationalization (translating in-game text)
  lib: Jan's collection of mostly low-level routines.
    allocators: memory (sub)allocators
    posix: POSIX emulation when on Windows
      file: efficient file loading code.
        http://www.wildfiregames.com/users/code/wiki/index.php?title=Virtual_File_System
        http://www.stud.uni-karlsruhe.de/~urkt/articles/study_thesis.pdf 
    res: resource handling (textures and sounds)
    sysdep: bridges differences between systems and allows 'portable' code
    debug (asserts), error handling, timing bit bashing, etc. - all the dirty details.
  maths: math code (linear algebra, geometry)
  network: the network engine (based on Enet; serializes game messages)
  pch: pre-compiled headers (this directory is required by the build system)
  ps: pyrogenesis engine - basically, 'everything else'
    utility classes, console, profiler, XML loader
  renderer: rendering algorithms, including sorting, quality settings, shadows and water
  script: scripting engine (!JavaScript)
  simulation: most of the actual RTS game logic
    simulation turns, entities, techs, unit AI, pathfinding, ..
  sound: (WIP) high-level sound and music engine
    builds on an abstraction of OpenAL
  tools
    particle engine, archive builder, map editor, random map generator, PMD exporter 
  overview of scripts in data/mods/official/scripts (entity functions & game startup)
  
misc topics:
  self-test
    * purpose
    * mechanism in VC IDE
    * where they lie 
  
  overview of art formats
  
  useful features for testing/development
    copy from the finding your way around - doc