# Actors and blueprints

Actors in CARLA are the elements that perform actions within the simulation, and they can affect other actors. Actors in CARLA includes vehicles and walkers and also sensors, traffic signs, traffic lights and the spectator. It is crucial to have full understanding on how to operate on them.  

This section will cover spawning, destruction, types, and how to manage them. However, the possibilities are almost endless. Experiment, take a look at the __tutorials__ in this documentation and share doubts and ideas in the [CARLA forum](https://github.com/carla-simulator/carla/discussions/).  

- [__Blueprints__](#blueprints)  
	- [Managing the blueprint library](#managing-the-blueprint-library)  
- [__Actor life cycle__](#actor-life-cycle)  
	- [Spawning](#spawning)  
	- [Handling](#handling)  
	- [Destruction](#destruction)  
- [__Types of actors__](#types-of-actors)  
	- [Sensors](#sensors)  
	- [Spectator](#spectator)  
	- [Traffic signs and traffic lights](#traffic-signs-and-traffic-lights)  
	- [Vehicles](#vehicles)  
	- [Walkers](#walkers)  

---
## Blueprints

These layouts allow the user to smoothly incorporate new actors into the simulation. They are already-made models with animations and a series of attributes. Some of these are modifiable and others are not. These attributes include, among others, vehicle color, amount of channels in a lidar sensor, a walker's speed, and much more.

Available blueprints are listed in the [blueprint library](bp_library.md), along with their attributes. Vehicle and walker blueprints have a generation attribute that indicates if they are a new (gen 2) or old (gen 1) asset.

### Managing the blueprint library

The [carla.BlueprintLibrary](python_api.md#carla.BlueprintLibrary) class contains a list of [carla.ActorBlueprint](python_api.md#carla.ActorBlueprint) elements. It is the world object who can provide access to it.
```py
blueprint_library = world.get_blueprint_library()
```
Blueprints have an ID to identify them and the actors spawned with it. The library can be read to find a certain ID, choose a blueprint at random, or filter results using a [wildcard pattern](https://tldp.org/LDP/GNU-Linux-Tools-Summary/html/x11655.htm).

```py
# Find a specific blueprint.
collision_sensor_bp = blueprint_library.find('sensor.other.collision')
# Choose a vehicle blueprint at random.
vehicle_bp = random.choice(blueprint_library.filter('vehicle.*.*'))
```

Besides that, each [carla.ActorBlueprint](python_api.md#carla.ActorBlueprint) has a series of [carla.ActorAttribute](python_api.md#carla.ActorAttribute) that can be _get_ and _set_.
```py
is_bike = [vehicle.get_attribute('number_of_wheels') == 2]
if(is_bike)
    vehicle.set_attribute('color', '255,0,0')
```
!!! Note
    Some of the attributes cannot be modified. Check it out in the [blueprint library](bp_library.md).

Attributes have an [carla.ActorAttributeType](python_api.md#carla.ActorAttributeType) variable. It states its type from a list of enums. Also, modifiable attributes come with a __list of recommended values__. 

```py
for attr in blueprint:
    if attr.is_modifiable:
        blueprint.set_attribute(attr.id, random.choice(attr.recommended_values))
```
!!! Note
    Users can create their own vehicles. Check the __Tutorials (assets)__ to learn on that. Contributors can [add their new content to CARLA](tuto_D_contribute_assets.md). 

---
## Actor life cycle  

!!! Important
    This section mentions different methods regarding actors. The Python API provides for __[commands](python_api.md#command.SpawnActor)__ to apply batches of the most common ones, in just one frame. 

### Spawning

__The world object is responsible of spawning actors and keeping track of these.__ Spawning only requires a blueprint, and a [carla.Transform](python_api.md#carla.Transform) stating a location and rotation for the actor.  

The world has two different methods to spawn actors.  

* [`spawn_actor()`](python_api.md#carla.World.spawn_actor) raises an exception if the spawning fails.
* [`try_spawn_actor()`](python_api.md#carla.World.try_spawn_actor) returns `None` if the spawning fails.

```py
transform = Transform(Location(x=230, y=195, z=40), Rotation(yaw=180))
actor = world.spawn_actor(blueprint, transform)
```

!!! Important
    CARLA uses the [Unreal Engine coordinates system](https://carla.readthedocs.io/en/latest/python_api/#carlarotation). Remember that [`carla.Rotation`](https://carla.readthedocs.io/en/latest/python_api/#carlarotation) constructor is defined as `(pitch, yaw, roll)`, that differs from Unreal Engine Editor `(roll, pitch, yaw)`. 

The actor will not be spawned in case of collision at the specified location. No matter if this happens with a static object or another actor. It is possible to try avoiding these undesired spawning collisions.  

* `map.get_spawn_points()` __for vehicles__. Returns a list of recommended spawning points. 

```py
spawn_points = world.get_map().get_spawn_points()
```

* `world.get_random_location()` __for walkers__. Returns a random point on a sidewalk. This same method is used to set a goal location for walkers.  

```py
spawn_point = carla.Transform()
spawn_point.location = world.get_random_location_from_navigation()
```

An actor can be attached to another one when spawned. Actors follow the parent they are attached to. This is specially useful for sensors. The attachment can be rigid (proper to retrieve precise data) or with an eased movement according to its parent. It is defined by the helper class [carla.AttachmentType](python_api.md#carla.AttachmentType).  

The next example attaches a camera rigidly to a vehicle, so their relative position remains fixed. 

```py
camera = world.spawn_actor(camera_bp, relative_transform, attach_to=my_vehicle, carla.AttachmentType.Rigid)
```
!!! Important
    When spawning attached actors, the transform provided must be relative to the parent actor. 

Once spawned, the world object adds the actors to a list. This can be easily searched or iterated on. 
```py
actor_list = world.get_actors()
# Find an actor by id.
actor = actor_list.find(id)
# Print the location of all the speed limit signs in the world.
for speed_sign in actor_list.filter('traffic.speed_limit.*'):
    print(speed_sign.get_location())
```

### Handling

[carla.Actor](python_api.md#carla.Actor) mostly consists of _get()_ and _set()_ methods to manage the actors around the map. 

```py
print(actor.get_acceleration())
print(actor.get_velocity())

location = actor.get_location()
location.z += 10.0
actor.set_location(location)
```

The actor's physics can be disabled to freeze it in place. 

```py
actor.set_simulate_physics(False)
```
Besides that, actors also have tags provided by their blueprints. These are mostly useful for semantic segmentation sensors. 

!!! Warning
    Most of the methods send requests to the simulator asynchronously. The simulator has a limited amount of time each update to parse them. Flooding the simulator with _set()_ methods will accumulate a significant lag.


### Destruction

Actors are not destroyed when a Python script finishes. They have to explicitly destroy themselves.  

```py
destroyed_sucessfully = actor.destroy() # Returns True if successful
```

!!! Important
    Destroying an actor blocks the simulator until the process finishes. 

---
## Types of actors  
### Sensors

Sensors are actors that produce a stream of data. They have their own section, [4th. Sensors and data](core_sensors.md). For now, let's just take a look at a common sensor spawning cycle.  

This example spawns a camera sensor, attaches it to a vehicle, and tells the camera to save the images generated to disk.  

```py
camera_bp = blueprint_library.find('sensor.camera.rgb')
camera = world.spawn_actor(camera_bp, relative_transform, attach_to=my_vehicle)
camera.listen(lambda image: image.save_to_disk('output/%06d.png' % image.frame))
```
* Sensors have blueprints too. Setting attributes is crucial.  
* Most of the sensors will be attached to a vehicle to gather information on its surroundings. 
* Sensors __listen__ to data. When data is received, they call a function described with a __[Lambda expression](https://docs.python.org/3/reference/expressions.html)__ <small>(6.14 in the link provided)</small>. 

### Spectator

Placed by Unreal Engine to provide an in-game point of view. It can be used to move the view of the simulator window. The following example would move the spectator actor, to point the view towards a desired vehicle. 

```py
spectator = world.get_spectator()
transform = vehicle.get_transform()
spectator.set_transform(carla.Transform(transform.location + carla.Location(z=50),
carla.Rotation(pitch=-90)))

```

### Traffic signs and traffic lights

Only stops, yields and traffic lights are considered actors in CARLA so far. The rest of the OpenDRIVE signs are accessible from the API as [__carla.Landmark__](python_api.md#carla.Landmark). Their information is accessible using these instances, but they do no exist in the simulation as actors. Landmarks are explained more in detail in the following step, __3rd. Maps and navigation__.  

When the simulation starts, stop, yields and traffic light are automatically generated using the information in the OpenDRIVE file. __None of these can be found in the blueprint library__ and thus, cannot be spawned. 

!!! Note
    CARLA maps do not have traffic signs nor lights in the OpenDRIVE file. These are manually placed by developers.  

[__Traffic signs__](python_api.md#carla.TrafficSign) are not defined in the road map itself, as explained in the following page. Instead, they have a [carla.BoundingBox](python_api.md#carla.BoundingBox) to affect vehicles inside of it.  
```py
#Get the traffic light affecting a vehicle
if vehicle_actor.is_at_traffic_light():
    traffic_light = vehicle_actor.get_traffic_light()
``` 
[__Traffic lights__](python_api.md#carla.TrafficLight) are found in junctions. They have their unique ID, as any actor, but also a `group` ID for the junction. To identify the traffic lights in the same group, a `pole` ID is used.  

The traffic lights in the same group follow a cycle. The first one is set to green while the rest remain frozen in red. The active one spends a few seconds in green, yellow and red, so there is a period of time where all the lights are red. Then, the next traffic light starts its cycle, and the previous one is frozen with the rest.  

The state of a traffic light can be set using the API. So does the seconds spent on each state. Possible states are described with [carla.TrafficLightState](python_api.md#carla.TrafficLightState) as a series of enum values. 
```py
#Change a red traffic light to green
if traffic_light.get_state() == carla.TrafficLightState.Red:
    traffic_light.set_state(carla.TrafficLightState.Green)
    traffic_light.set_set_green_time(4.0)
``` 

!!! Note
    Vehicles will only be aware of a traffic light if the light is red.  

### Vehicles

[__carla.Vehicle__](python_api.md#carla.Vehicle) is a special type of actor. It incorporates special internal components that simulate the physics of wheeled vehicles. This is achieved by applying four types of different controls:  

* __[carla.VehicleControl](python_api.md#carla.VehicleControl)__ provides input for driving commands such as throttle, steering, brake, etc. 
```py
    vehicle.apply_control(carla.VehicleControl(throttle=1.0, steer=-1.0))
```
* __[carla.VehiclePhysicsControl](python_api.md#carla.VehiclePhysicsControl)__ defines physical attributes of the vehicle and contains two more controllers:

    * [carla.GearPhysicsControl](python_api.md#carla.GearPhysicsControl) which controls the gears. 
    * [carla.WheelPhysicsControl](python_api.md#carla.WheelPhysicsControl) which provides specific control over each wheel.  

```py
    vehicle.apply_physics_control(carla.VehiclePhysicsControl(max_rpm = 5000.0, center_of_mass = carla.Vector3D(0.0, 0.0, 0.0), torque_curve=[[0,400],[5000,400]]))
```

Vehicles have a [carla.BoundingBox](python_api.md#carla.BoundingBox) encapsulating them. This bounding box allows physics to be applied to the vehicle and enables collisions to be detected.  

```py
    box = vehicle.bounding_box
    print(box.location)         # Location relative to the vehicle.
    print(box.extent)           # XYZ half-box extents in meters.
```

The physics of vehicle wheels can be improved by enabling the [sweep wheel collision parameter][enable_sweep]. The default wheel physics uses single ray casting from the axis to the floor for each wheel but when sweep wheel collision is enabled, the full volume of the wheel is checked against collisions. It can be enabled as such:

```py
    physics_control = vehicle.get_physics_control()
    physics_control.use_sweep_wheel_collision = True
    vehicle.apply_physics_control(physics_control)
```

[enable_sweep]: https://carla.readthedocs.io/en/latest/python_api/#carla.VehiclePhysicsControl.use_sweep_wheel_collision


Vehicles include other functionalities unique to them:

* __Autopilot mode__ will subscribe a vehicle to the [Traffic Manager](adv_traffic_manager.md) to simulate real urban conditions. This module is hard-coded, not based on machine learning.  

```py
    vehicle.set_autopilot(True)
```
* __Vehicle lights__ have to be turned on and off by the user. Each vehicle has a set of lights listed in [__carla.VehicleLightState__](python_api.md#carla.VehicleLightState). Not all vehicles have lights integrated. At the time of writing, vehicles with integrated lights are as follows:  
	*   __Bikes:__ All bikes have a front and back position light.  
	*   __Motorcycles:__ Yamaha and Harley Davidson models.  
	*   __Cars:__ Audi TT, Chevrolet Impala, both Dodge police cars, Dodge Charger, Audi e-tron, Lincoln 2017 and 2020, Mustang, Tesla Model 3, Tesla Cybertruck, Volkswagen T2 and the Mercedes C-Class.  

The lights of a vehicle can be retrieved and updated anytime using the methods [carla.Vehicle.get_light_state](python_api.md#carla.Vehicle.get_light_state) and [carla.Vehicle.set_light_state](#python_api.md#carla.Vehicle.set_light_state). These use binary operations to customize the light setting.  

```py
# Turn on position lights
current_lights = carla.VehicleLightState.NONE
current_lights |= carla.VehicleLightState.Position
vehicle.set_light_state(current_lights)
```

### Walkers

[__carla.Walker__](python_api.md#carla.Walker) work in a similar way as vehicles do. Control over them is provided by controllers.  

* [__carla.WalkerControl__](python_api.md#carla.WalkerControl) moves the pedestrian around with a certain direction and speed. It also allows them to jump. 
* [__carla.WalkerBoneControl__](python_api.md#carla.WalkerBoneControl) provides control over the 3D skeleton. [This tutorial](tuto_G_control_walker_skeletons.md) explains how to control it. 

Walkers can be AI controlled. They do not have an autopilot mode. The [__carla.WalkerAIController__](python_api.md#carla.WalkerAIController) actor moves around the actor it is attached to.  

```py
walker_controller_bp = world.get_blueprint_library().find('controller.ai.walker')
world.SpawnActor(walker_controller_bp, carla.Transform(), parent_walker)
```
!!! Note
    The AI controller is bodiless and has no physics. It will not appear on scene. Also, location `(0,0,0)` relative to its parent will not cause a collision.  


__Each AI controller needs initialization, a goal and, optionally, a speed__. Stopping the controller works in the same manner. 

```py
ai_controller.start()
ai_controller.go_to_location(world.get_random_location_from_navigation())
ai_controller.set_max_speed(1 + random.random())  # Between 1 and 2 m/s (default is 1.4 m/s).
...
ai_controller.stop()
```
When a walker reaches the target location, they will automatically walk to another random point. If the target point is not reachable, walkers will go to the closest point from their current location.

A snipet in [carla.Client](python_api.md#carla.Client.apply_batch_sync) uses batches to spawn a lot of walkers and make them wander around.

!!! Important
    __To destroy AI pedestrians__, stop the AI controller and destroy both, the actor, and the controller. 

---
That is a wrap as regarding actors in CARLA. The next step takes a closer look into the map, roads and traffic in CARLA.  

Keep reading to learn more or visit the forum to post any doubts or suggestions that have come to mind during this reading. 
<div text-align: center>
<div class="build-buttons">
<p>
<a href="https://github.com/carla-simulator/carla/discussions/" target="_blank" class="btn btn-neutral" title="CARLA forum">
CARLA forum</a>
</p>
</div>
<div class="build-buttons">
<p>
<a href="../core_map" target="_blank" class="btn btn-neutral" title="3rd. Maps and navigation">
3rd. Maps and navigation</a>
</p>
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</div>