How to Implement an AI Actor

AI Actor is the way Limon enables custom behaviour for NPCs or enemies. Limon engine has a class named ActorInterface under src/AI, that it used to implement custom AI behaviour.

Limon editor allows selecting Actor per model. After Actor selected, a new instance of the selected Actor is created, and called each frame with current information about the world. It is also possible to expose some settings to be filled by level designer using the same interface of editor.

Actors follow the unified parameter contract. Like Triggers and Player Extensions, ActorInterface provides a protected parameters member with base getParameters() / setParameters() implementations, so a simple actor can seed its defaults into that member in the constructor and rely on the base methods - no override required.

Most actors, however, read their configuration every frame in play() and prefer real typed members (float aggroRange, int health, …) over scanning an untyped vector. Because both methods are virtual, an actor is free to keep its configuration in whatever form it likes: override getParameters() to convert its typed members into a GenericParameter vector - each entry filled with its current value - and override setParameters() to apply edited or loaded values back onto those members. The same vector drives editor widgets, map serialization, and load either way. The bundled actors use this “typed-member” style, and it is the recommended approach for non-trivial AI.

ActorInterface Class

ActorInterface class has two helper structs used to pass information between engine and AI. Those are ActorInformation struct and InformationRequest struct. details are below.

Warning

Information requests are prepared by separate threads, and no guarantees made for when they will return. Check routeReady flag on ActorInformation before using the route.

	ActorInterface(uint32_t id, LimonAPI *limonAPI)
std::vector<LimonTypes::GenericParameter>	getParameters()
void	setParameters(std::vector<LimonTypes::GenericParameter>parameters)
void	play(long time, ActorInformation& information)
bool	interaction(std::vector<LimonTypes::GenericParameter>parameters)
std::string	getName() const

ActorInterface(uint32_t id, LimonAPI *limonAPI)

The constructor of the interface.

Note

All Actors must have the same signature, no other parameters should be required.

std::vector<LimonTypes::GenericParameter> getParameters()

Returns a vector of LimonTypes::GenericParameter. These parameters are going to be set by the map designer using the editor, and must be filled with their current values, because the Load/Save logic uses them to persist AI information. The base implementation returns the protected parameters member - seed your defaults into it in the constructor and they are returned here, exactly as for Triggers and Player Extensions. Override this method when the actor owns its configuration as its own typed members and assembles the vector from them; this is what the bundled actors do.

void setParameters(std::vector<LimonTypes::GenericParameter> parameters)

The parameters set by map designer will be passed to this method. It might be just set, or they might be loading as part of map load. The base implementation stores them in the protected parameters member; override it to apply the values onto the actor’s own typed members instead.

void play(long time, ActorInformation &information)

Called on each frame, with current information about player and world, in form of ActorInformation struct

bool interaction(std::vector<LimonTypes::GenericParameter> &interactionInformation)

called by other entities, like Actors or Player. Used to pass information like hits, or alarming etc.

std::string getName() const

Returns the name of the Actor.

Warning

The name must be unique, or the results will be undefined.

ActorInformation struct

This struct is feeded for each frame, and meant to contain information to trigger AI behaviour. It contains the following information

Type	Name	Description
bool	canSeePlayerDirectly	Is there any object between Actor and Player.
bool	isPlayerLeft	Is Player at left of Actor.
bool	isPlayerRight	Is Player at right of Actor.
bool	isPlayerUp	Is Player higher up than Actor.
bool	isPlayerDown	Is Player lower than Actor.
bool	isPlayerFront	Is Player in front of the Actor.
bool	isPlayerBack	Is Player at back of the Actor.
float	cosineBetweenPlayer	What is the cosine of player and actor front vector.
glm::vec3	playerDirection	What is the direction vector from actor to player.
float	playerDistance	What is the distance between actor and player (unit is close to meters).
float	cosineBetweenPlayerForSide	cosine of the angle between right vector of actor and player.
bool	playerDead	Is player dead?
uint32_t	maximumRouteDistance(128)	how deep the route search should go. (maximum ~128 meters default)
std::vector<glm::vec3>	routeToRequest	Points to follow to reach the player.
bool	routeFound	Was route course successful?
bool	routeReady	Was route course done?

The first part of the information will be filled for each frame. The Route will not be filled until requested, and routeFound/routeReady will be false. To request route to player, check InformationRequest struct below.

InformationRequest struct

This struct is part of ActorInterface, and each frame Limon Engine checks all Actors for request changes. When a request is checked, its information will be reset to prevent multiple requests.

Type	Name	Description
bool	routeToPlayer	Request a route to Player
bool	routeToCustomPosition	Request a route to custom position
glm::vec3	customPosition	Position to course path

Python Actors

Actors can also be written in Python by subclassing ActorInterface from actor_interface.py (see Python Limon API Usage for how scripts are discovered and the conventions they must follow). The actor type is registered under the class name, so a Python class PythonCowboyEnemy(ActorInterface) appears in the editor as the actor PythonCowboyEnemy. Method and field names use Python casing: play, interaction, get_parameters, set_parameters, get_name, and the ActorInformation passed to play exposes the same fields in snake_case (can_see_player_directly, player_distance, cosine_between_player, route_to_request, route_ready …).

The Python base provides the actor-state helpers that C++ actors get as members/methods. They are valid from construction onward — the model is attached before the actor is created:

• self.model_id / self.get_model_id() - world-object ID of the model this actor drives. It is distinct from self.actor_id / self.get_world_id() (the actor’s own ID). Pass model_id to every model-targeted API call (animations, translate, orientation, transform queries).

• self.get_position() - the model’s world position as a Vec3 (mirror of C++ getPosition()).

• self.get_front_vector() - the model’s forward direction as a Vec3 (mirror of C++ getFrontVector()).

To request a navigation route - the Python equivalent of writing the InformationRequest struct - set the flags on self.information_request inside play():

def play(self, time, information):
    if not self.route_to_request:
        self.information_request.route_to_player = True   # ask the engine for a route
    # ... follow information.route_to_request once information.route_ready is True

The engine reads the request once per tick and clears it (the same one-shot semantics as the C++ getRequests() path), so re-raise it whenever you still need a route. Setting route_to_custom_position together with custom_position (a Vec3) requests a route to an arbitrary point instead of the player.

Note

Route nodes in information.route_to_request arrive as {x, y, z} dicts, and VEC4 parameter values as 4-tuples. Convert them to Vec3 before doing vector math - python_cowboy_enemy.py includes small helpers for this.

How to enable Dynamic Library discovery

Limon engine will try to load custom actors on engine startup, from libcustomTriggers file (extension based on platform). If the file is found, engine will check for a method with following signature:

void registerActors(std::map<std::string, ActorInterface*(*)(uint32_t, LimonAPI*)>* actorMap)

This method should fill the actorMap passed, with all the custom actors, like this:

(*actorMap)["$ACTOR_NAME1$"] = &createActorT<$ActorClass1$>;
(*actorMap)["$ACTOR_NAME2$"] = &createActorT<$ActorClass2$>;

Sample Actors

The engine ships with sample actors under samples/ that implement ActorInterface:

• HumanEnemy - a navigation-mesh driven enemy that chases and attacks the player.

• CowboyEnemyAI - the western-demo gunslinger AI built on the same interface.

Two Python actor samples ship under Engine/Scripts/, both subclassing the Python base in actor_interface.py (see Python Actors):

• python_cowboy_enemy.py (the PythonCowboyEnemy class) - a faithful Python port of the C++ CowboyEnemyAI above, and the fullest example: it drives its own position, requests routes, plays animations, and interacts with the player. Registers as the actor type PythonCowboyEnemy, distinct from the C++ Cowboy Enemy so both coexist in the editor.

• simple_guard_actor.py (the SimpleGuardActor class) - a minimal starting point that reacts to player presence without moving or animating.