# 4th. Sensors and data

Sensors are actors that retrieve data from their surroundings. They are crucial to create learning environment for driving agents.  

This page summarizes everything necessary to start handling sensors. It introduces the types available and a step-by-step guide of their life cycle. The specifics for every sensor can be found in the [sensors reference](ref_sensors.md).

* [__Sensors step-by-step__](#sensors-step-by-step)  
	*   [Setting](#setting)  
	*   [Spawning](#spawning)  
	*   [Listening](#listening)  
	*   [Data](#data)  
* [__Types of sensors__](#types-of-sensors)  
	*   [Cameras](#cameras)  
	*   [Detectors](#detectors)  
	*   [Other](#other)  

---
## Sensors step-by-step  

The class [carla.Sensor](python_api.md#carla.Sensor) defines a special type of actor able to measure and stream data.  

* __What is this data?__ It varies a lot depending on the type of sensor. All the types of data are inherited from the general [carla.SensorData](python_api.md#carla.SensorData). 
* __When do they retrieve the data?__ Either on every simulation step or when a certain event is registered. Depends on the type of sensor. 
* __How do they retrieve the data?__ Every sensor has a `listen()` method to receive and manage the data.  

Despite their differences, all the sensors are used in a similar way. 

### Setting

As with every other actor, find the blueprint and set specific attributes. This is essential when handling sensors. Their attributes will determine the results obtained. These are detailed in the [sensors reference](ref_sensors.md). 

The following example sets a dashboard HD camera.

```py
# Find the blueprint of the sensor.
blueprint = world.get_blueprint_library().find('sensor.camera.rgb')
# Modify the attributes of the blueprint to set image resolution and field of view.
blueprint.set_attribute('image_size_x', '1920')
blueprint.set_attribute('image_size_y', '1080')
blueprint.set_attribute('fov', '110')
# Set the time in seconds between sensor captures
blueprint.set_attribute('sensor_tick', '1.0')
``` 

### Spawning

`attachment_to` and `attachment_type`, are crucial. Sensors should be attached to a parent actor, usually a vehicle, to follow it around and gather the information. The attachment type will determine how its position is updated regarding said vehicle. 

* __Rigid attachment.__ Movement is strict regarding its parent location. This is the proper attachment to retrieve data from the simulation.  
* __SpringArm attachment.__ Movement is eased with little accelerations and decelerations. This attachment is only recommended to record videos from the simulation. The movement is smooth and "hops" are avoided when updating the cameras' positions.  

```py
transform = carla.Transform(carla.Location(x=0.8, z=1.7))
sensor = world.spawn_actor(blueprint, transform, attach_to=my_vehicle)
```
!!! Important
    When spawning with attachment, location must be relative to the parent actor.  

### Listening

Every sensor has a [`listen()`](python_api.md#carla.Sensor.listen) method. This is called every time the sensor retrieves data.  

The argument `callback` is a [lambda function](https://www.w3schools.com/python/python_lambda.asp). It describes what should the sensor do when data is retrieved. This must have the data retrieved as an argument.  

```py
# do_something() will be called each time a new image is generated by the camera.
sensor.listen(lambda data: do_something(data))

...

# This collision sensor would print everytime a collision is detected. 
def callback(event):
    for actor_id in event:
        vehicle = world_ref().get_actor(actor_id)
        print('Vehicle too close: %s' % vehicle.type_id)

sensor02.listen(callback)
```

### Data

Most sensor data objects have a function to save the information to disk. This will allow it to be used in other environments.  

Sensor data differs a lot between sensor types. Take a look at the [sensors reference](ref_sensors.md) to get a detailed explanation. However, all of them are always tagged with some basic information.  

<table class ="defTable">
<thead>
<th>Sensor data attribute</th>
<th>Type</th>
<th>Description</th>
</thead>
<tbody>
<td><code>frame</code> </td>
<td>int</td>
<td>Frame number when the measurement took place.</td>
<tr>
<td><code>timestamp</code> </td>
<td>double</td>
<td>Timestamp of the measurement in simulation seconds since the beginning of the episode.</td>
<tr>
<td><code>transform</code> </td>
<td><a href="../python_api#carlatransform">carla.Transform</a></td>
<td>World reference of the sensor at the time of the measurement.</td>
</tbody>
</table>
<br>

!!! Important
    `is_listening` is a __sensor attribute__ that enables/disables data listening at will.  
    `sensor_tick` is a __blueprint attribute__ that sets the simulation time between data received.  

---
## Types of sensors
 
### Cameras

Take a shot of the world from their point of view. The helper class [carla.ColorConverter](python_api.md#carla.ColorConverter) will modify said image to represent different information.

* __Retrieve data__ every simulation step.  

<table class ="defTable">
<thead>
<th>Sensor</th>
<th>Output</th>
<th>Overview</th>
</thead>
<tbody>
<td>Depth</td>
<td><a href="../python_api#carlaimage">carla.Image</a></td>
<td>Renders the depth of the elements in the field of view in a gray-scale map.</td>
<tr>
<td>RGB</td>
<td><a href="../python_api#carlaimage">carla.Image</a></td>
<td>Provides clear vision of the surroundings. Looks like a normal photo of the scene.</td>
<tr>
<td>Semantic segmentation</td>
<td><a href="../python_api#carlaimage">carla.Image</a></td>
<td>Renders elements in the field of view with a specific color according to their tags.</td>
</tbody>
</table>
<br>

### Detectors

Retrieve data when the object they are attached to registers a specific event.  

* __Retrieve data__ when triggered.  

<table class ="defTable">
<thead>
<th>Sensor</th>
<th>Output</th>
<th>Overview</th>
</thead>
<tbody>
<td>Collision</td>
<td><a href="../python_api#carlacollisionevent">carla.CollisionEvent</a></td>
<td>Retrieves collisions between its parent and other actors.</td>
<tr>
<td>Lane invasion</td>
<td><a href="../python_api#carlalaneinvasionevent">carla.LaneInvasionEvent</a></td>
<td>Registers when its parent crosses a lane marking.</td>
<tr>
<td>Obstacle</td>
<td><a href="../python_api#carlaobstacledetectionevent">carla.ObstacleDetectionEvent</a></td>
<td>Detects possible obstacles ahead of its parent.</td>
</tbody>
</table>
<br>

### Other

Different functionalities such as navigation, measurement of physical properties and 2D/3D point maps of the scene.  

* __Retrieve data__ every simulation step.  

<table class ="defTable">
<thead>
<th>Sensor</th>
<th>Output</th>
<th>Overview</th>
</thead>
<tbody>
<td>GNSS</td>
<td><a href="../python_api#carlagnssmeasurement">carla.GNSSMeasurement</a></td>
<td>Retrieves the geolocation of the sensor.</td>
<tr>
<td>IMU</td>
<td><a href="../python_api#carlaimumeasurement">carla.IMUMeasurement</a></td>
<td>Comprises an accelerometer, a gyroscope, and a compass.</td>
<tr>
<td>LIDAR raycast</td>
<td><a href="../python_api#carlalidarmeasurement">carla.LidarMeasurement</a></td>
<td>A rotating LIDAR. Generates a 3D point cloud modelling the surroundings.</td>
<tr>
<td>Radar</td>
<td><a href="../python_api#carlaradarmeasurement">carla.RadarMeasurement</a></td>
<td>2D point map modelling elements in sight and their movement regarding the sensor. </td>
<tr>
<td>RSS</td>
<td><a href="../python_api#carlarssresponse">carla.RssResponse</a></td>
<td>Modifies the controller applied to a vehicle according to safety checks. This sensor works in a different manner than the rest, and there is specific <a href="../adv_rss">RSS documentation</a> for it. </td>
</tbody>
</table>
<br>

---
That is a wrap on sensors and how do these retrieve simulation data.  

Thus concludes the introduction to CARLA. However there is yet a lot to learn.

* __Gain some practise.__ It may be a good idea to try some of the code recipes provided in this documentation. Combine them with the example scripts, test new ideas. 

<div class="build-buttons">
<p>
<a href="../ref_code_recipes" target="_blank" class="btn btn-neutral" title="Code recipes">
Code recipes</a>
</p>
</div>

* __Continue learning.__ There are some advanced features in CARLA: rendering options, traffic manager, the recorder, and some more. This is a great moment to learn on them. 
 
<div class="build-buttons">
<p>
<a href="../adv_synchrony_timestep" target="_blank" class="btn btn-neutral" title="Synchrony and time-step">
Synchrony and time-step</a>
</p>
</div>

* __Experiment freely.__ Take a look at the __References__ section of this documentation. It contains detailed information on the classes in the Python API, sensors, and much more. 

<div class="build-buttons">
<p>
<a href="../python_api" target="_blank" class="btn btn-neutral" title="Python API reference">
Python API reference</a>
</p>
</div>


* __Give your two cents.__ Any doubts, suggestions and ideas are welcome in the forum.

<div class="build-buttons">
<p>
<a href="https://forum.carla.org/" target="_blank" class="btn btn-neutral" title="Go to the CARLA forum">
CARLA forum</a>
</p>
</div>