Overview of Lua scripting

Language and sandboxing

OpenMW supports scripts written in Lua 5.1 with some extensions (see below) from Lua 5.2. There are no plans to switch to any newer version of the language, because newer versions are not supported by LuaJIT.

Here are starting points for learning Lua:

• Programing in Lua (first edition, aimed at Lua 5.0)
• Lua 5.1 Reference Manual

Each script works in a separate sandbox and doesn’t have any access to the underlying operating system. Only a limited list of allowed standard libraries can be used: coroutine, math (except math.randomseed – it is called by the engine on startup and not available from scripts), string, table, os (only os.date, os.difftime, os.time). These libraries are loaded automatically and are always available.

Allowed basic functions: assert, error, ipairs, next, pairs, pcall, print, select, tonumber, tostring, type, unpack, xpcall, rawequal, rawget, rawset, getmetatable, setmetatable.

Supported Lua 5.2 features:

• goto and ::labels::;
• hex escapes \x3F and \* escape in strings;
• math.log(x [,base]);
• string.rep(s, n [,sep]);
• in string.format(): %q is reversible, %s uses __tostring, %a and %A are added;
• String matching pattern %g;
• __pairs and __ipairs metamethods;
• Function table.unpack (alias to Lua 5.1 unpack).

Loading libraries with require('library_name') is allowed, but limited. It works this way:

1. If library_name is one of the standard libraries, then return the library.
2. If library_name is one of the built-in API packages, then return the package.
3. Otherwise search for a Lua source file with such name in data folders. For example require('my_lua_library.something') will try to open one of the files my_lua_library/something.lua or my_lua_library/something/init.lua.

Loading DLLs and precompiled Lua files is intentionally prohibited for compatibility and security reasons.

Basic concepts

Game object

Any object that exists in the game world and has a specific location. Player, actors, items, and statics are game objects.

Record

Persistent information about an object. Includes starting stats and links to assets, but doesn’t have a location. Game objects are instances of records. Some records (e.g. a unique NPC) have a single instance, some (e.g. a specific potion) may correspond to multiple objects.

Note

Don’t be confused with MWSE terminology. In MWSE game objects are “references” and records are “objects”.

Cell

An area of the game world. A position in the world is a link to a cell and X, Y, Z coordinates in the cell. At a specific moment in time each cell can be active or inactive. Inactive cells don’t perform physics updates.

Global scripts

Lua scripts that are not attached to any game object and are always active. Global scripts can not be started or stopped during a game session. Lists of global scripts are defined by omwscripts files, which should be registered in openmw.cfg.

Local scripts

Lua scripts that are attached to some game object. A local script is active only if the object it is attached to is in an active cell. There are no limitations to the number of local scripts on one object. Local scripts can be attached to (or detached from) any object at any moment by a global script. In some cases inactive local scripts still can run code (for example during saving and loading), but while inactive they can not see nearby objects.

Note

Currently scripts on objects in a container or an inventory are considered inactive. Probably later this behaviour will be changed.

Player scripts

A specific kind of local scripts; player script is a local script that is attached to a player. It can do everything that a normal local script can do, plus some player-specific functionality (e.g. control UI and camera).

This scripting API was developed to be conceptually compatible with multiplayer. In multiplayer the server is lightweight and delegates most of the work to clients. Each client processes some part of the game world. Global scripts are server-side and local scripts are client-side. Because of this, there are several rules for the Lua scripting API:

1. A local script can see only some area of the game world (cells that are active on a specific client). Any data from inactive cells can’t be used, as they are not synchronized and could be already changed on another client.
2. A local script can only modify the object it is attached to. Other objects can theoretically be processed by another client. To prevent synchronization problems the access to them is read-only.
3. Global scripts can access and modify the whole game world including unloaded areas, but the global scripts API is different from the local scripts API and in some aspects limited, because it is not always possible to have all game assets in memory at the same time.
4. Though the scripting system doesn’t actually work with multiplayer yet, the API assumes that there can be several players. That’s why any command related to UI, camera, and everything else that is player-specific can be used only by player scripts.

How to run a script

Let’s write a simple example of a Player script:

-- Save to my_lua_mod/example/player.lua

local ui = require('openmw.ui')

return {
    engineHandlers = {
        onKeyPress = function(key)
            if key.symbol == 'x' then
                ui.showMessage('You have pressed "X"')
            end
        end
    }
}

The script will be used only if it is specified in one of content files. OpenMW Lua is an inclusive OpenMW feature, so it can not be controlled by ESP/ESM. The options are:

1. Create text file “my_lua_mod.omwscripts” with the following line:

PLAYER: example/player.lua

2. (not implemented yet) Add the script in OpenMW CS on “Lua scripts” view and save as “my_lua_mod.omwaddon”.

Enable it in openmw.cfg the same way as any other mod:

data=path/to/my_lua_mod
content=my_lua_mod.omwscripts  # or content=my_lua_mod.omwaddon

Now every time the player presses “X” on a keyboard, a message is shown.

Format of .omwscripts

# Lines starting with '#' are comments

GLOBAL: my_mod/some_global_script.lua

# Script that will be automatically attached to the player
PLAYER: my_mod/player.lua

# Local script that will be automatically attached to every NPC and every creature in the game
NPC, CREATURE: my_mod/some_other_script.lua

# Local script that can be attached to any object by a global script
CUSTOM: my_mod/something.lua

# Local script that will be automatically attached to any Container AND can be
# attached to any other object by a global script.
CONTAINER, CUSTOM: my_mod/container.lua

Each script is described by one line: <flags>: <path to .lua file in virtual file system>. The order of lines determines the script load order (i.e. script priorities).

Possible flags are:

• GLOBAL - a global script; always active, can not be stopped;
• CUSTOM - dynamic local script that can be started or stopped by a global script;
• PLAYER - an auto started player script;
• ACTIVATOR - a local script that will be automatically attached to any activator;
• ARMOR - a local script that will be automatically attached to any armor;
• BOOK - a local script that will be automatically attached to any book;
• CLOTHING - a local script that will be automatically attached to any clothing;
• CONTAINER - a local script that will be automatically attached to any container;
• CREATURE - a local script that will be automatically attached to any creature;
• DOOR - a local script that will be automatically attached to any door;
• INGREDIENT - a local script that will be automatically attached to any ingredient;
• LIGHT - a local script that will be automatically attached to any light;
• MISC_ITEM - a local script that will be automatically attached to any miscellaneous item;
• NPC - a local script that will be automatically attached to any NPC;
• POTION - a local script that will be automatically attached to any potion;
• WEAPON - a local script that will be automatically attached to any weapon;
• APPARATUS - a local script that will be automatically attached to any apparatus;
• LOCKPICK - a local script that will be automatically attached to any lockpick;
• PROBE - a local script that will be automatically attached to any probe tool;
• REPAIR - a local script that will be automatically attached to any repair tool.

Several flags (except GLOBAL) can be used with a single script. Use space or comma as a separator.

Hot reloading

It is possible to modify a script without restarting OpenMW. To apply changes, open the in-game console and run the command: reloadlua. This will restart all Lua scripts using the onSave and onLoad handlers the same way as if the game was saved or loaded. It reloads all .omwscripts files and .lua files that are not packed to any archives. .omwaddon files and scripts packed to BSA can not be changed without restarting the game.

Lua console

It is also possible to run Lua commands directly from the in-game console.

To enter the Lua mode run one of the commands:

• lua player or luap - enter player context
• lua global or luag - enter global context
• lua selected or luas - enter local context on the selected object

Script structure

Each script is a separate file in the game assets. Starting a script means that the engine runs the file, parses the table it returns, and registers its interface, event handlers, and engine handlers. The handlers are permanent and exist until the script is stopped (if it is a local script, because global scripts can not be stopped).

Here is an example of a basic script structure:

local util = require('openmw.util')

local function onUpdate(dt)
    ...
end

local function onSave()
    ...
    return data
end

local function onLoad(data)
    ...
end

local function myEventHandler(eventData)
    ...
end

local function somePublicFunction(params, ...)
    ...
end

return {
    interfaceName = 'MyScriptInterface',
    interface = {
        somePublicFunction = somePublicFunction,
    },

    eventHandlers = { MyEvent = myEventHandler },

    engineHandlers = {
        onUpdate = onUpdate,
        onSave = onSave,
        onLoad = onLoad,
    }
}

Note

Every instance of every script works in a separate enviroment, so it is not necessary to make everything local. It’s local just because it makes the code a bit faster.

All sections in the returned table are optional. If you just want to do something every frame, it is enough to write the following:

return {
    engineHandlers = {
        onUpdate = function()
            print('Hello, World!')
        end
    }
}

Engine handlers

An engine handler is a function defined by a script, that can be called by the engine. I.e. it is an engine-to-script interaction. Not visible to other scripts. If several scripts register an engine handler with the same name, the engine calls all of them according to the load order (i.e. the order of content= entries in openmw.cfg) and the order of scripts in omwaddon/omwscripts.

Some engine handlers are allowed only for global, or only for local/player scripts. Some are universal. See Engine handlers reference.

onSave and onLoad

When a game is saved or loaded, the engine calls the engine handlers onSave or onLoad to save or load each script. The value that onSave returns will be passed to onLoad when the game is loaded. It is the only way to save the internal state of a script. All other script variables will be lost after closing the game. The saved state must be serializable.

Note that onLoad means loading a script rather than loading a game. If a script did not exist when a game was saved then onLoad will not be called, but onInit will.

onSave and onLoad can be called even for objects in inactive state, so it shouldn’t use openmw.nearby.

An example:

...

local scriptVersion = 3  -- increase it every time when `onSave` is changed

local function onSave()
    return {
        version = scriptVersion
        some = someVariable,
        someOther = someOtherVariable
    }
end

local function onLoad(data)
    if not data or not data.version or data.version < 2 then
        print('Was saved with an old version of the script, initializing to default')
        someVariable = 'some value'
        someOtherVariable = 42
        return
    end
    if data.version > scriptVersion then
        error('Required update to a new version of the script')
    end
    someVariable = data.some
    if data.version == scriptVersion then
        someOtherVariable = data.someOther
    else
        print(string.format('Updating from version %d to %d', data.version, scriptVersion))
        someOtherVariable = 42
    end
end

return {
    engineHandlers = {
        onUpdate = update,
        onSave = onSave,
        onLoad = onLoad,
    }
}

Serializable data

Serializable value means that OpenMW is able to convert it to a sequence of bytes and then (probably on a different computer and with different OpenMW version) restore it back to the same form.

Serializable value is one of:

• nil value
• a number
• a string
• a game object
• a value of a type, defined by openmw.util
• a table whith serializable keys and values

Serializable data can not contain:

• Functions
• Tables with custom metatables
• Several references to the same table. For example { x = some_table, y = some_table } is not allowed.
• Circular references (i.e. when some table contains itself).

API packages

API packages provide functions that can be called by scripts. I.e. it is a script-to-engine interaction. A package can be loaded with require('<package name>'). It can not be overloaded even if there is a lua file with the same name. The list of available packages is different for global and for local scripts. Player scripts are local scripts that are attached to a player.

Package	Can be used	Description
openmw.interfaces	everywhere	Public interfaces of other scripts.
openmw.util	everywhere	Defines utility functions and classes like 3D vectors, that don’t depend on the game world.
openmw.storage	everywhere	Storage API. In particular can be used to store data between game sessions.
openmw.core	everywhere	Functions that are common for both global and local scripts
openmw.types	everywhere	Functions for specific types of game objects.
openmw.async	everywhere	Timers (implemented) and coroutine utils (not implemented)
openmw.world	by global scripts	Read-write access to the game world.
openmw.self	by local scripts	Full access to the object the script is attached to.
openmw.nearby	by local scripts	Read-only access to the nearest area of the game world.
openmw.input	by player scripts	User input.
openmw.ui	by player scripts	Controls user interface.
openmw.camera	by player scripts	Controls camera.
openmw.postprocessing	by player scripts	Controls post-process shaders.
openmw.debug	by player scripts	Collection of debug utils.

openmw_aux

openmw_aux.* are built-in libraries that are themselves implemented in Lua. They can not do anything that is not possible with the basic API, they only make it more convenient. Sources can be found in resources/vfs/openmw_aux. In theory mods can override them, but it is not recommended.

Built-in library	Can be used	Description
openmw_aux.calendar	everywhere	Game time calendar
openmw_aux.util	everywhere	Miscellaneous utils
openmw_aux.time	everywhere	Timers and game time utils
openmw_aux.ui	by player scripts	User interface utils

They can be loaded with require the same as API packages. For example:

local time = require('openmw_aux.time')
time.runRepeatedly(doSomething, 15 * time.second)  -- run `doSomething()` every 15 seconds

Script interfaces

Each script can provide a named interface for other scripts. It is a script-to-script interaction. This mechanism is not used by the engine itself.

A script can use an interface of another script either if both are global scripts, or both are local scripts on the same object. In other cases events should be used.

Defining an interface:

return {
    interfaceName = "SomeUtils"
    interface = {
        version = 1,
        doSomething = function(x, y) ... end,
    }
}

Overriding the interface and adding a debug output:

local baseInterface = nil  -- will be assigned by `onInterfaceOverride`
interface = {
    version = 1,
    doSomething = function(x, y)
        print(string.format('SomeUtils.doSomething(%d, %d)', x, y))
        baseInterface.doSomething(x, y)  -- calls the original `doSomething`

        -- WRONG! Would lead to an infinite recursion.
        -- local interfaces = require('openmw.interfaces')
        -- interfaces.SomeUtils.doSomething(x, y)
    end,
}

return {
    interfaceName = "SomeUtils",
    interface = interface,
    engineHandlers = {
        onInterfaceOverride = function(base) baseInterface = base end,
    },
}

A general recommendation about overriding is that the new interface should be fully compatible with the old one. So it is fine to change the behaviour of SomeUtils.doSomething, but if you want to add a completely new function, it would be better to create a new interface for it. For example SomeUtilsExtended with an additional function doSomethingElse.

Using the interface:

local interfaces = require('openmw.interfaces')

local function onUpdate()
    interfaces.SomeUtils.doSomething(2, 3)
end

return { engineHandlers = {onUpdate = onUpdate} }

The order in which the scripts are started is important. So if one mod should override an interface provided by another mod, make sure that load order (i.e. the sequence of lua-scripts=… in openmw.cfg) is correct.

Interfaces of built-in scripts

Interface	Can be used	Description
AI	by local scripts	Control basic AI of NPCs and creatures.
Camera	by player scripts	Allows to alter behavior of the built-in camera script without overriding the script completely.
Settings	by player and global scripts	Save, display and track changes of setting values.
MWUI	by player scripts	Morrowind-style UI templates.

Event system

This is another kind of script-to-script interactions. The differences:

• Any script can send an event to any object or a global event to global scripts.
• Events are delivered with a small delay (in single player the delay is always one frame).
• Event handlers can not return any data to the sender.
• Event handlers have a single argument eventData (must be serializable)

Events are the main way of interacting between local and global scripts. They are not recommended for interactions between two global scripts, because in this case interfaces are more convenient.

If several scripts register handlers for the same event, the handlers will be called in reverse order (opposite to engine handlers). I.e. the handler from the last script in the load order will be called first. Return value ‘false’ means “skip all other handlers for this event”. Any other return value (including nil) means nothing.

An example. Imagine we are working on a mod that adds some “dark power” with special effects. We attach a local script to an item that can explode. At some moment it will send the ‘DamagedByDarkPower’ event to all nearby actors:

local self = require('openmw.self')
local nearby = require('openmw.nearby')

local function onActivated()
    for i, actor in ipairs(nearby.actors) do
        local dist = (self.position - actor.position):length()
        if dist < 500 then
            local damage = (1 - dist / 500) * 200
            actor:sendEvent('DamagedByDarkPower', {source=self.object, damage=damage})
        end
    end
end

return { engineHandlers = { onActivated = onActivated } }

And every actor should have a local script that processes this event:

local function damagedByDarkPower(data)
    ...  -- apply `data.damage` to stats / run custom animation / etc
end

return {
    eventHandlers = { DamagedByDarkPower = damagedByDarkPower },
}

Someone may create an additional mod that adds a protection from the dark power. The protection mod attaches an additional local script to every actor. The script intercepts and modifies the event:

local protectionLevel = ...

local function reduceDarkDamage(data)
    data.damage = data.damage - protectionLevel  -- reduce the damage
    return data.damage > 0  -- it skips the original handler if the damage becomes <= 0
end

return {
    eventHandlers = { DamagedByDarkPower = reduceDarkDamage },
}

In order to be able to intercept the event, the protection script should be placed in the load order below the original script.

See the list of events that are used by built-in scripts.

Timers

Timers are in the openmw.async package. They can be set either in simulation time or in game time.

• Simulation time: the number of seconds in the game world (i.e. seconds when the game is not paused), passed from starting a new game.
• Game time: current time of the game world in seconds. Note that game time generally goes faster than the simulation time.

When the game is paused, all timers are paused as well.

When an object becomes inactive, timers on this object are not paused, but callbacks are called only when the object becomes active again. For example if there were 3 timers with delays 30, 50, 90 seconds, and from the 15-th to the 65-th second the object was inactive, then the first two callbacks are both evaluated on the 65-th second and the third one – on the 90-th second.

There are two types: reliable and unsavable timers.

Reliable timer

Reliable timers are automatically saved and restored when the game is saved or loaded. When the game is saved each timer record contains only name of a callback, the time when the callback should be called, and an argument that should be passed to the callback. The callback itself is not stored. That’s why callbacks must be registered when the script is initialized with a function async:registerTimerCallback(name, func). Name is an arbitrary string.

An example:

local async = require('openmw.async')

local teleportWithDelayCallback = async:registerTimerCallback('teleport',
function(data)
    data.actor:teleport(data.destCellName, data.destPos)
end)

local function teleportWithDelay(delay, actor, cellName, pos)
    async:newSimulationTimer(delay, teleportWithDelayCallback, {
        actor = actor,
        destCellName = cellName,
        destPos = pos,
    })
end

Unsavable timer

Unsavable timers can be created from any function without registering a callback in advance, but they can not be saved. If the player saves the game when an unsavable timer is running, then the timer will be lost after reloading. So be careful with unsavable timers and don’t use them if there is a risk of leaving the game world in an inconsistent state.

An example:

local async = require('openmw.async')
local ui = require('openmw.ui')

return {
    engineHandlers = {
        onKeyPress = function(key)
            if key.symbol == 'x' then
                async:newUnsavableSimulationTimer(
                    10,
                    function()
                        ui.showMessage('You have pressed "X" 10 seconds ago')
                    end)
            end
        end,
    }
}

Also in openmw_aux is the helper function runRepeatedly, it is implemented on top of unsavable timers:

local core = require('openmw.core')
local time = require('openmw_aux.time')

-- call `doSomething()` at the end of every game day.
-- the second argument (`time.day`) is the interval.
-- the periodical evaluation can be stopped at any moment by calling `stopFn()`
local timeBeforeMidnight = time.day - core.getGameTime() % time.day
local stopFn = time.runRepeatedly(doSomething, time.day, {
    initialDelay = timeBeforeMidnight,
    type = time.GameTime,
})

Using IDE for Lua scripting

Find the directory resources/lua_api in your installation of OpenMW. It describes OpenMW LuaAPI in LDT Documentation Language. It is the source from which the API reference is generated.

If you write scripts using Lua Development Tools (eclipse-based IDE), you can import these files to get code autocompletion and integrated OpenMW API reference. Here are the steps:

• Install and run LDT.
• Press File / New / Lua Project in menu.

• Specify project name (for example the title of your omwaddon)
• Set Targeted Execution Environment to No Execution Environment, and Target Grammar to lua-5.1.

• Press Next, choose the Libraries tab, and click Add External Source Folder.
• Specify there paths to resources/lua_api and resources/vfs in your OpenMW installation.

• Press Finish. Create a new Lua file.
• Now you have code completion! Press Ctrl+Space in any place to see the variants.

In some cases LDT can deduce types automatically, but it is not always possible. You can add special hints to give LDT more information:

• Before function definition: --- @param TYPE argName
• Before variable definition: --- @field TYPE variableName

--- @param openmw.core#GameObject obj
local function doSomething(obj)
    -- autocompletion now works with `obj`
end

--- @field openmw.util#Vector3 c
local c

-- autocompletion now works with `c`

In order to have autocompletion for script interfaces the information where to find these interfaces should be provided. For example for the camera interface (defined in resources/vfs/scripts/omw/camera.lua):

--- @type Interfaces
-- @field scripts.omw.camera#Interface Camera
-- ... other interfaces here
--- @field #Interfaces I
local I = require('openmw.interfaces')

I.Camera.disableZoom()

See LDT Documentation Language for more details.