The Automation 4 Lua scripting engine is based on the version 5.1 series of the Lua scripting language.

This manual will not deal with the Lua language itself nor the standard libraries shipping with Lua, but only the additional functions and data structures provided by the Aegisub Automation 4 Lua interface. Please see the Lua 5.1 manual for details on the language itself and its standard libraries.

General overview of the organisation of Automation 4 Lua

The smallest legal Automation 4 Lua script is an empty file, but that won't be able to do anything interesting.

There's a number of global variables a script can set to provide information about itself. This information will be displayed in the Automation/Manager window: script_name, script_description, script_author and script_version.

Automation 4 Lua implements both of the currently defined "features" of Automation 4: Macro and Export Filter. One script (one file) can define zero, one or many of each of those features. For example, the Karaoke Templater script defines one macro and one export filter.

When an Automation 4 Lua script is loaded, its top-level code is executed once. You can put variable initialisations and such at the top level, but what you usually will do is define some of the script information globals, import some modules, write some functions that do the script's work and then call the feature registration functions. This is described on the Registration page. The only fields of the aegisub object that should be touched during script loading are lua_automation_version and the registration functions. Most others will simply do nothing and return nil.

When the user activates a feature from the Aegisub interface (such as by selecting a macro from the Automation menu) the registered script function is run. One of the parameters passed to the function is a subtitles object, the primary interface to the subtitle data the script will manipulate. To some extent, the subtitles object works as an integer-indexed array, but it exposes some special interfaces to add, remove and modify subtitle lines. The subtitles object allows you to access every line in the subtitle file, including headers, style definitions, dialogue lines and comment lines. This is described on the Subtitle file interface page.

Automation 4 Lua also provides a number of helper functions in the core API for getting information on e.g. the video frame timestamps and how large a piece of text will be when rendered with a given style.

Most things that can be implemented in clean Lua code, i.e. that don't depend directly on Aegisub internal data structures, have been implemented outside the core API as Lua modules. While it is possible to write Automation 4 Lua scripts without using the provided standard include files you will find that for anything but the simplest scripts you will need some of the functions provided by the includes.

The Automation 4 Lua core API

Automation 4 Lua provides various APIs that can be grouped in these general categories.

Script and feature registration
Deals with advertising what features a script provides and a few other script meta data.
Subtitle file interface
Deals with use of the subtitles object, the principal way of accessing and modifying the subtitle data.
Progress reporting and debug output
providing feedback to the user while a script is running, outputting hints and warnings to the user and printing debug information.
Displaying dialogue boxes and getting user input
requesting user input during macro execution by dialogue boxes and providing a configuration interface for export filters.
Miscellaneous APIs
for e.g. getting the rendered size of text and getting video frame rate information.