LiDAR accuracy incorrect

[thomasTUK]

Bug report

• AirSim Version/#commit: N/A latest probably
• UE/Unity version: UE 4.26
• autopilot version: simple_flight
• OS Version: Windows 10

What's the issue you encountered?

I have run a point cloud simulation, because I want to calculate the point spaces beforehand the simulation.

During the experiment, I have notived that the point cloud during a single shot produces twice as many points as it should be. As you can see in the screenshots.

Point Cloud is here: points_14_25_48_0_0_-2.txt

Settings

        "LidarSensor1": {
            "SensorType": 6,
            "Enabled" : true,
            "NumberOfChannels": 16,
            "Range": 100,
            "PointsPerSecond": 288000,
            "RotationsPerSecond": 20,
            "HorizontalFOVStart": 0,
            "HorizontalFOVEnd": 0,
            "VerticalFOVUpper": 15,
            "VerticalFOVLower": -15,
            "X": 0,
            "Y": 0,
            "Z": -1,
            "Roll": 0,
            "Pitch": 0,
            "Yaw" : 0,
            "DataFrame": "SensorLocalFrame",
            "DrawDebugPoints": true

How can the issue be reproduced?

1. Run a test with drone_lidar.py and write to disk

[zimmy87]

Can you elaborate on how you're selecting points in your third screenshot? It looks like you're using CloudCompare, but it's unclear to me how you're selecting a subset of points that corresponds to those produced by a "single shot". I did some testing locally with the settings you provided and with the default update frequency of 10, one single iteration of the lidar obtaining a point cloud should do 29377 line traces (see here for where this is calculated), so you should see a lot more than just 4 points for a single shot. Also, the timestamp at which a point is detected isn't stored in the point cloud, so I don't know of a way for selecting the subset for a single shot other than manually counting a certain number.

[thomasTUK]

HI @zimmy87 , Thanks for the message! The third picture is just a zoom in the second picture. Can you provide me some code of how to integrate the time stamps?

[zimmy87]

The timestamp is included in the lidar output here. You can read this value by accessing the LidarData.time_stamp field returned by a call to getLidarData in the python client. For example, in a python script, you can print the timestamp by executing the following (not tested locally):

import airsim

client = airsim.MultirotorClient()
client.confirmConnection()
client.enableApiControl(True)

lidarData = client.getLidarData()
print(lidarData.time_stamp)

[thomasTUK]

I'll have a look @zimmy87! Are you able to answer my question, posted here? It is regarding the timestamp but also the Lidar parameters. https://github.com/microsoft/AirSim/discussions/4406

[thomasTUK]

I have change in the source file (UnrealEngine\EnvironmentName\Plugins\AirSim\Source\AirLib\include\sensors\lidar\LidarSimpleParams.hpp) the value of real T update_frequency to 5 or 20 Hz, but I got with the same settings.json the exact number of point (29377).

So, in my case, delta_time is not derived from the update frequency - why is that? Looking forward to your anwser :)