Actors not only include vehicles and walkers, but also sensors, traffic signs, traffic lights, and the spectator. It is crucial to have fully 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://forum.carla.org/).
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.
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.
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).
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_.
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__.
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).
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.
__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 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.
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 or eased. It is defined by the helper class [carla.AttachmentType](python_api.md#carla.AttachmentType).
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.
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.
* 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>(6.13 in the link provided)</small>.
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.
So far, CARLA only is aware of some signs: stop, yield and speed limit. Traffic lights are considered an inherited class from the more general traffic sign. __None of these can be found in the blueprint library__ and thus, cannot be spawned. They set traffic conditions, so they are mindfully 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.
Vehicles will only notice a traffic light if the light is red.
[__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.
[__carla.Vehicle__](python_api.md#carla.Vehicle) are a special type of actor. They are remarkable for having better physics. This is achieved applying four types of different controls.
* __[carla.VehiclePhysicsControl](python_api.md#carla.VehiclePhysicsControl)__ defines physical attributes of the vehicle. Besides many different attribute, this controller contains two more controllers. [carla.GearPhysicsControl](python_api.md#carla.GearPhysicsControl) for the gears. The other is a list of [carla.WheelPhysicsControl](python_api.md#carla.WheelPhysicsControl), that provide specific control over the different wheels.
Vehicles include other functionalities unique to them.
* The __autopilot mode__ will subscribe them to the [Traffic manager](adv_traffic_manager.md), and simulate real urban conditions. This module is hard-coded, not based on machine learning.
* __Vehicle lights__ created specifically for each vehicle model. They can be accessed from the API. In order to turn them on/off, the vehicle uses flag values. These are defined in [carla.VehicleLightState](python_api.md#carla.VehicleLightState) and binary operations are used to combine them.
So far, vehicle lights have been implemented only for a specific set of vehicles, listed in the [release post](http://carla.org/2020/03/09/release-0.9.8/#vehicle-lights).
* [__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.
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.
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.
