Engine overview

Overview of the GemRB Engine

GemRB is a game engine that strives to be compatible with Bioware’s Infinity Engine (IE), the engines that drives Baldur’s Gate, Icewind Dale and Planescape: Torment games.

GemRB stands for Game Engine Made with preRendered Backgrounds. This indeed describes the fundamental characteristics of Infinity Engine-based games: The world, or more correctly the areas where you move, be it cold empty plains of Icewind Dale, busy streets of Waterdeep or damp catacombs under Sigil, was painted and/or rendered in advance, during game development, into a series of static pictures.

The engine then transfers portions of those pictures on the screen, puts animated player and non-player avatars on top of it, and uses clever tricks to make the world appear lively and three-dimensional. But remember that deep inside it’s just animated sprites over a static picture.

Note that actual engine of each of the original games differs from that of other games. That of Baldur’s Gate 1 is the most primitive, PS:T is the most atypical, and Baldur’s Gate 2 the one most useful for modding.

Simplified Infinity Engine Ancestry

GemRB has to be compatible with all of them, either by adopting superset of the features, or choosing between mutually exclusive features by means of GameType, which in turn selects appropriate Game Flags, GemRB’s own override files, plugins and scripts. GameType is set in the main config file (usually autodetected).

GemRB Sources

GemRB sources consist of:

  • Main program
  • Core library
    • Interface class
    • UI layer
    • Plugin manager class
    • Gamescript
    • Scriptables
    • and much more
  • Plugins
    • I/O drivers
    • Resource loaders
    • Scripting language(s)
    • Effects
  • GUIScripts
  • Override and unhardcoded
  • Includes
  • Docs

Main program: gemrb/GemRB.cpp

Trivial main() function that just calls the engine’s Init() and Main() functions.

Core library: gemrb/core/

Core library is the main part of GemRB. It contains parts of the engine common to all the games. The goal is to make it as universal as possible, but some AD\&D rules-specific stuff is hard to get rid of.

Interface Class: gemrb/core/Interface.cpp

The library’s central hub is Interface class, which binds the various parts of the engine together (including the plugins), contains global initialization, main loop, utility functions etc. It needs to lose some weight!

PluginManager Class: gemrb/core/PluginManager.cpp

This is the class responsible for loading GemRB plugins.

User Interface: gemrb/core/GUI

These are classes defining window management, GUI controls (eg. buttons) and our text handling subsystem.

Plugins: gemrb/plugins

Plugins in GemRB are used for:

  • I/O drivers
  • Resource loaders
  • Scripting language
  • Effects
I/O Drivers

These plugins are used for graphic and audio output (SDLVideo, SDLAudio, OpenALAudio, NullSound) and input (SDLVideo again).

Resource Loaders

These plugins implement readers and writers for data files used by the engine, many of which are in custom formats (INIImporter, AREImporter, MVEPlayer, …) and in case of audio/video files also provide streaming (ACMReader, …).

Scripting Language

GemRB’s support for Python scripts is contained in the GUIScript plugin. It is used in place of Lua, which the originals used to power the cheat console.

Note that GameScript, scripting language used in Infinity Engine scripts (sometimes called IEScritp), is implemented in the Core Library instead.


These plugins implement effects used in the Infinity Engine spells and scripts like RetreatFrom, SetPoisonedState, Damage … Common effects are in the FXOpcodes plugin, game-while specific ones are in IWDOpcodes and PSTOpcodes.

GUIScripts: gemrb/GUIScripts/

These are Python scripts implementing GemRB’s user interface and many game rules, which were hardwired into the executable in the original games. The scripts are organized by game type, while the top dir contains shared files. Hacking the scripts provides a gentle path into GemRB development and you can find specific documentation here.

Overrides and unhardcoded data: gemrb/override/

gemrb/override/ and gemrb/unhardcoded/ are two directories containing GemRB’s own data files for all (shared) or a given game type. This includes tables which were hardwired in the original engine. Also the all important gemrb.ini internal engine settings file is found here.

The difference between the two is that unhardcoded contains only new files, while override is meant as what its name suggests. We want the latter to contain as few files as possible to reduce potential conflicts with mods.

See this tabular overview to better understand how they relate to the data shipped with the games.

Includes: gemrb/includes

Common include files used by Core Library and the plugins.

Docs: gemrb/docs

Initial GemRB documentation. Tables/ contains descriptions of GemRB’s override tables (not for long) and the rest are mostly very very specific or already found on this website, like the GUIScript docs.