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carla/Docs/ref_sensors.md

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Sensors' documentation

##Collision detector

This sensor registers an event each time its parent actor collisions against something in the world. Several collisions may be detected during a single simulation step.
To ensure that collisions with any kind of object are detected, the server creates "fake" actors for elements such as buildings or bushes so the semantic tag can be retrieved to identify it.

Collision detectors do not have any configurable attribute.

Output attributes:

Sensor data attribute Type Description
frame int Frame number when the measurement took place.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Location and rotation in world coordinates of the sensor at the time of the measurement.
actor carla.Actor Actor that measured the collision (sensor's parent).
other_actor carla.Actor Actor against whom the parent collided.
normal_impulse carla.Vector3D Normal impulse result of the collision.

##Depth camera

  • Blueprint: sensor.camera.depth
  • Output: carla.Image per step (unless sensor_tick says otherwise).

The camera provides a raw data of the scene codifying the distance of each pixel to the camera (also known as depth buffer or z-buffer) to create a depth map of the elements.

The image codifies depth value per pixel using 3 channels of the RGB color space, from less to more significant bytes: R -> G -> B. The actual distance in meters can be decoded with:

normalized = (R + G * 256 + B * 256 * 256) / (256 * 256 * 256 - 1)
in_meters = 1000 * normalized

The output carla.Image should then be saved to disk using a carla.colorConverter that will turn the distance stored in RGB channels into a [0,1] float containing the distance and then translate this to grayscale.
There are two options in carla.colorConverter to get a depth view: Depth and Logaritmic depth. The precision is milimetric in both, but the logarithmic approach provides better results for closer objects.

ImageDepth

Basic camera attributes

Blueprint attribute Type Default Description
image_size_x int 800 Image width in pixels.
image_size_y int 600 Image height in pixels.
fov float 90.0 Horizontal field of view in degrees.
sensor_tick float 0.0 Simulation seconds between sensor captures (ticks).

Camera lens distortion attributes

Blueprint attribute Type Default Description
lens_circle_falloff float 5.0 Range: [0.0, 10.0]
lens_circle_multiplier float 0.0 Range: [0.0, 10.0]
lens_k float -1.0 Range: [-inf, inf]
lens_kcube float 0.0 Range: [-inf, inf]
lens_x_size float 0.08 Range: [0.0, 1.0]
lens_y_size float 0.08 Range: [0.0, 1.0]

Output attributes

Sensor data attribute Type Description
frame int Frame number when the measurement took place.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Location and rotation in world coordinates of the sensor at the time of the measurement.
width int Image width in pixels.
height int Image height in pixels.
fov float Horizontal field of view in degrees.
raw_data bytes Array of BGRA 32-bit pixels.

##GNSS sensor

  • Blueprint: sensor.other.gnss
  • Output: carla.GNSSMeasurement per step (unless sensor_tick says otherwise).

Reports current gnss position of its parent object. This is calculated by adding the metric position to an initial geo reference location defined within the OpenDRIVE map definition.

GNSS attributes

Blueprint attribute Type Default Description
noise_alt_bias float 0.0 Mean parameter in the noise model for altitude.
noise_alt_stddev float 0.0 Standard deviation parameter in the noise model for altitude.
noise_lat_bias float 0.0 Mean parameter in the noise model for latitude.
noise_lat_stddev float 0.0 Standard deviation parameter in the noise model for latitude.
noise_lon_bias float 0.0 Mean parameter in the noise model for longitude.
noise_lon_stddev float 0.0 Standard deviation parameter in the noise model for longitude.
noise_seed int 0 Initializer for a pseudorandom number generator.
sensor_tick float 0.0 Simulation seconds between sensor captures (ticks).

Output attributes

Sensor data attribute Type Description
frame int Frame number when the measurement took place.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Location and rotation in world coordinates of the sensor at the time of the measurement.
latitude double Latitude of the actor.
longitude double Longitude of the actor.
altitude double Altitude of the actor.

##IMU sensor

  • Blueprint: sensor.other.imu
  • Output: carla.IMUMeasurement per step (unless sensor_tick says otherwise).

Provides measures that accelerometer, gyroscope and compass would retrieve for the parent object. The data is collected from the object's current state.

IMU attributes

Blueprint attribute Type Default Description
noise_accel_stddev_x float 0.0 Standard deviation parameter in the noise model for acceleration (X axis).
noise_accel_stddev_y float 0.0 Standard deviation parameter in the noise model for acceleration (Y axis).
noise_accel_stddev_z float 0.0 Standard deviation parameter in the noise model for acceleration (Z axis).
noise_gyro_bias_x float 0.0 Mean parameter in the noise model for the gyroscope (X axis).
noise_gyro_bias_y float 0.0 Mean parameter in the noise model for the gyroscope (Y axis).
noise_gyro_bias_z float 0.0 Mean parameter in the noise model for the gyroscope (Z axis).
noise_gyro_stddev_x float 0.0 Standard deviation parameter in the noise model for the gyroscope (X axis).
noise_gyro_stddev_y float 0.0 Standard deviation parameter in the noise model for the gyroscope (Y axis).
noise_gyro_stddev_z float 0.0 Standard deviation parameter in the noise model for the gyroscope (Z axis).
noise_seed int 0 Initializer for a pseudorandom number generator.
sensor_tick float 0.0 Simulation seconds between sensor captures (ticks).

Output attributes

Sensor data attribute Type Description
frame int Frame number when the measurement took place.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Transform in world.
accelerometer carla.Vector3D Measures linear acceleration in m/s^2.
gyroscope carla.Vector3D Measures angular velocity in rad/sec.
compass float Orientation in radians. North is (0.0, -1.0, 0.0) in UE.

##Lane invasion detector

Registers an event each time its parent crosses a lane marking.
The sensor uses road data provided by the OpenDRIVE description of the map to determine whether the parent vehicle is invading another lane by considering the space between wheels.
However there are some things to be taken into consideration:

  • Discrepancies between the OpenDRIVE file and the map will create irregularities such as crossing lanes that are not visible in the map.
  • The output retrieves a list of crossed lane markings: the computation is done in OpenDRIVE and considering the whole space between the four wheels as a whole. Thus, there may be more than one lane being crossed at the same time.

This sensor does not have any configurable attribute.

!!! Important This sensor works fully on the client-side.

Output attributes

Sensor data attribute Type Description
frame int Frame number when the measurement took place.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Location and rotation in world coordinates of the sensor at the time of the measurement.
actor carla.Actor Vehicle that invaded another lane (parent actor).
crossed_lane_markings list(carla.LaneMarking) List of lane markings that have been crossed.

##Lidar raycast sensor

  • Blueprint: sensor.lidar.ray_cast
  • Output: carla.LidarMeasurement per step (unless sensor_tick says otherwise).

This sensor simulates a rotating Lidar implemented using ray-casting.
The points are computed by adding a laser for each channel distributed in the vertical FOV. The rotation is simulated computing the horizontal angle that the Lidar rotated in a frame. The point cloud is calculated by doing a ray-cast for each laser in every step:
points_per_channel_each_step = points_per_second / (FPS * channels)

A Lidar measurement contains a packet with all the points generated during a 1/FPS interval. During this interval the physics are not updated so all the points in a measurement reflect the same "static picture" of the scene.

This output contains a cloud of simulation points and thus, can be iterated to retrieve a list of their carla.Location:

for location in lidar_measurement:
    print(location)

!!! Tip Running the simulator at fixed time-step it is possible to tune the rotation for each measurement. Adjust the step and the rotation frequency to get, for instance, a 360 view each measurement.

LidarPointCloud

Lidar attributes

Blueprint attribute Type Default Description
channels int 32 Number of lasers.
range float 10.0 Maximum distance to measure/raycast in meters (centimeters for CARLA 0.9.6 or previous).
points_per_second int 56000 Points generated by all lasers per second.
rotation_frequency float 10.0 Lidar rotation frequency.
upper_fov float 10.0 Angle in degrees of the highest laser.
lower_fov float -30.0 Angle in degrees of the lowest laser.
sensor_tick float 0.0 Simulation seconds between sensor captures (ticks).

Output attributes

Sensor data attribute Type Description
frame int Frame number when the measurement took place.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Location and rotation in world coordinates of the sensor at the time of the measurement.
horizontal_angle float Angle (radians) in the XY plane of the lidar this frame.
channels int Number of channels (lasers) of the lidar.
get_point_count(channel) int Number of points per channel captured this frame.
raw_data bytes Array of 32-bits floats (XYZ of each point).

##Obstacle detector

Registers an event every time the parent actor has an obstacle ahead.
In order to anticipate obstacles, the sensor creates a capsular shape ahead of the parent vehicle and uses it to check for collisions.
To ensure that collisions with any kind of object are detected, the server creates "fake" actors for elements such as buildings or bushes so the semantic tag can be retrieved to identify it.

Blueprint attribute Type Default Description
distance float 5 Distance to trace.
hit_radius float 0.5 Radius of the trace.
only_dynamics bool false If true, the trace will only consider dynamic objects.
debug_linetrace bool false If true, the trace will be visible.
sensor_tick float 0.0 Simulation seconds between sensor captures (ticks).

Output attributes

Sensor data attribute Type Description
frame int Frame number when the measurement took place.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Location and rotation in world coordinates of the sensor at the time of the measurement.
actor carla.Actor Actor that detected the obstacle (parent actor).
other_actor carla.Actor Actor detected as an obstacle.
distance float Distance from actor to other_actor.

##Radar sensor

The sensor creates a conic view that is translated to a 2D point map of the elements in sight and their speed regarding the sensor. This can be used to shape elements and evaluate their movement and direction. Due to the use of polar coordinates, the points will concentrate around the center of the view.

Points measured are contained in carla.RadarMeasurement as an array of carla.RadarDetection, which specifies their polar coordinates, distance and velocity.
This raw data provided by the radar sensor can be easily converted to a format manageable by numpy:

# To get a numpy [[vel, altitude, azimuth, depth],...[,,,]]:
points = np.frombuffer(radar_data.raw_data, dtype=np.dtype('f4'))
points = np.reshape(points, (len(radar_data), 4))

The provided script manual_control.py uses this sensor to show the points being detected and paint them white when static, red when moving towards the object and blue when moving away:

ImageRadar

Blueprint attribute Type Default Description
horizontal_fov float 30 Horizontal field of view in degrees.
points_per_second int 1500 Points generated by all lasers per second.
range float 100 Maximum distance to measure/raycast in meters.
sensor_tick float 0.0 Simulation seconds between sensor captures (ticks).
vertical_fov float 30 Vertical field of view in degrees.

Output attributes

Sensor data attribute Type Description
raw_data list(carla.RadarDetection) The list of points detected
RadarDetection attributes Type Description
altitude float Altitude angle in radians.
azimuth float Azimuth angle in radians.
depth float Distance in meters.
velocity float Velocity towards the sensor.

##RGB camera

  • Blueprint: sensor.camera.rgb
  • Output: carla.Image per step (unless sensor_tick says otherwise)..

The "RGB" camera acts as a regular camera capturing images from the scene.
carla.colorConverter

If enable_postprocess_effects is enabled, a set of post-process effects is applied to the image for the sake of realism:

  • Vignette: darkens the border of the screen.
  • Grain jitter: adds some noise to the render.
  • Bloom: intense lights burn the area around them.
  • Auto exposure: modifies the image gamma to simulate the eye adaptation to darker or brighter areas.
  • Lens flares: simulates the reflection of bright objects on the lens.
  • Depth of field: blurs objects near or very far away of the camera.

The sensor_tick tells how fast we want the sensor to capture the data. A value of 1.5 means that we want the sensor to capture data each second and a half. By default a value of 0.0 means as fast as possible.

ImageRGB

Basic camera attributes

Blueprint attribute Type Default Description
fov float 90.0 Horizontal field of view in degrees.
fstop float 1.4 Opening of the camera lens. Aperture is 1 / fstop with typical lens going down to f / 1.2 (larger opening). Larger numbers will reduce the Depth of Field effect.
image_size_x int 800 Image width in pixels.
image_size_y int 600 Image height in pixels.
iso float 1200.0 The camera sensor sensitivity.
gamma float 2.2 Target gamma value of the camera.
sensor_tick float 0.0 Simulation seconds between sensor captures (ticks).
shutter_speed float 60.0 The camera shutter speed in seconds (1.0 / s).

Camera lens distortion attributes

Blueprint attribute Type Default Description
lens_circle_falloff float 5.0 Range: [0.0, 10.0]
lens_circle_multiplier float 0.0 Range: [0.0, 10.0]
lens_k float -1.0 Range: [-inf, inf]
lens_kcube float 0.0 Range: [-inf, inf]
lens_x_size float 0.08 Range: [0.0, 1.0]
lens_y_size float 0.08 Range: [0.0, 1.0]

Advanced camera attributes

Since these effects are provided by UE, please make sure to check their documentation:

Blueprint attribute Type Default Description
min_fstop float 1.2 Maximum aperture.
blade_count int 5 Number of blades that make up the diaphragm mechanism.
exposure_mode str "manual" Can be "manual" or "histogram". More in UE4 docs.
exposure_compensation float 3.0 Logarithmic adjustment for the exposure. 0: no adjustment, -1:2x darker, -2:4 darker, 1:2x brighter, 2:4x brighter.
exposure_min_bright float 0.1 In exposure_mode: "histogram". Minimum brightness for auto exposure. The lowest the eye can adapt within. Must be greater than 0 and less than or equal to exposure_max_bright.
exposure_max_bright float 2.0 In exposure_mode: "histogram". Maximum brightness for auto exposure. The highestthe eye can adapt within. Must be greater than 0 and greater than or equal to exposure_min_bright.
exposure_speed_up float 3.0 In exposure_mode: "histogram". Speed at which the adaptation occurs from dark to bright environment.
exposure_speed_down float 1.0 In exposure_mode: "histogram". Speed at which the adaptation occurs from bright to dark environment.
calibration_constant float 16.0 Calibration constant for 18% albedo.
focal_distance float 1000.0 Distance at which the depth of field effect should be sharp. Measured in cm (UE units).
blur_amount float 1.0 Strength/intensity of motion blur.
blur_radius float 0.0 Radius in pixels at 1080p resolution to emulate atmospheric scattering according to distance from camera.
motion_blur_intensity float 0.45 Strength of motion blur [0,1].
motion_blur_max_distortion float 0.35 Max distortion caused by motion blur. Percentage of screen width.
motion_blur_min_object_screen_size float 0.1 Percentage of screen width objects must have for motion blur, lower value means less draw calls.
slope float 0.88 Steepness of the S-curve for the tonemapper. Larger values make the slope steeper (darker) [0.0, 1.0].
toe float 0.55 Adjusts dark color in the tonemapper [0.0, 1.0]
shoulder float 0.26 Adjusts bright color in the tonemapper [0.0, 1.0]
black_clip float 0.0 This should NOT be adjusted. Sets where the crossover happens and black tones start to cut off their value [0.0, 1.0].
white_clip float 0.04 Set where the crossover happens and white tones start to cut off their value. Subtle change in most cases [0.0, 1.0].
temp float 6500.0 White balance in relation to the temperature of the light in the scene. White light: when this matches light temperature. Warm light: When higher than the light in the scene, it is a yellowish color. Cool light: When lower than the light. Blueish color.
tint float 0.0 White balance temperature tint. Adjusts cyan and magenta color ranges. This should be used along with the white balance Temp property to get accurate colors. Under some light temperatures, the colors may appear to be more yellow or blue. This can be used to balance the resulting color to look more natural.
chromatic_aberration_intensity float 0.0 Scaling factor to control color shifting, more noticeable on the screen borders.
chromatic_aberration_offset float 0.0 Normalized distance to the center of the image where the effect takes place.
enable_postprocess_effects bool True Post-process effects activation.

Output attributes

Sensor data attribute Type Description
frame int Frame number when the measurement took place.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Location and rotation in world coordinates of the sensor at the time of the measurement.
width int Image width in pixels.
height int Image height in pixels.
fov float Horizontal field of view in degrees.
raw_data bytes Array of BGRA 32-bit pixels.

##Semantic segmentation camera

  • Blueprint: sensor.camera.semantic_segmentation
  • Output: carla.Image per step (unless sensor_tick says otherwise).

This camera classifies every object in sight by displaying it in a different color according to its tags (e.g., pedestrians in a different color than vehicles).
When the simulation starts, every element in scene is created with a tag. So it happens when an actor is spawned. The objects are classified by their relative file path in the project. For example, meshes stored in Unreal/CarlaUE4/Content/Static/Pedestrians are tagged as Pedestrian.

The server provides an image with the tag information encoded in the red channel: A pixel with a red value of x belongs to an object with tag x.
This raw carla.Image can be stored and converted it with the help of CityScapesPalette in carla.ColorConverter to apply the tags information and show picture with the semantic segmentation.
The following tags are currently available:

Value Tag Converted color
0 Unlabeled ( 0, 0, 0)
1 Building ( 70, 70, 70)
2 Fence (190, 153, 153)
3 Other (250, 170, 160)
4 Pedestrian (220, 20, 60)
5 Pole (153, 153, 153)
6 Road line (157, 234, 50)
7 Road (128, 64, 128)
8 Sidewalk (244, 35, 232)
9 Vegetation (107, 142, 35)
10 Car ( 0, 0, 142)
11 Wall (102, 102, 156)
12 Traffic sign (220, 220, 0)

!!! Note Adding new tags: It requires some C++ coding. Add a new label to the ECityObjectLabel enum in "Tagger.h", and its corresponding filepath check inside GetLabelByFolderName() function in "Tagger.cpp".

ImageSemanticSegmentation

Basic camera attributes

Blueprint attribute Type Default Description
fov float 90.0 Horizontal field of view in degrees.
image_size_x int 800 Image width in pixels.
image_size_y int 600 Image height in pixels.
sensor_tick float 0.0 Simulation seconds between sensor captures (ticks).

Camera lens distortion attributes

Blueprint attribute Type Default Description
lens_circle_falloff float 5.0 Range: [0.0, 10.0]
lens_circle_multiplier float 0.0 Range: [0.0, 10.0]
lens_k float -1.0 Range: [-inf, inf]
lens_kcube float 0.0 Range: [-inf, inf]
lens_x_size float 0.08 Range: [0.0, 1.0]
lens_y_size float 0.08 Range: [0.0, 1.0]

Output attributes

Sensor data attribute Type Description
fov float Horizontal field of view in degrees.
frame int Frame number when the measurement took place.
height int Image height in pixels.
raw_data bytes Array of BGRA 32-bit pixels.
timestamp double Simulation time of the measurement in seconds since the beginning of the episode.
transform carla.Transform Location and rotation in world coordinates of the sensor at the time of the measurement.
width int Image width in pixels.