Carla: Lidar to World Transformation

[luigui2906]

Hi,

First of all, I would like to thank all the CARLA team for the great work done with the simulator. I find it really useful.

I have been working with Lidar data and I am trying to take all the frames received from the sensor at different times to a same coordinate system. I followed the example given in the file point_cloud_example.py, where this is done for a pointcloud obtained from the depth map extracted from camera. However, it seems that there are problems with the transformation using the Lidar sensor.

This is how I am defining my sensor, I included only the position parameters:

lidar.set_position(x=(0.81-0.05), y=0, z=1.73)
lidar.set_rotation(pitch=0, yaw=0, roll=0)

Outside the game loop I am taking what should be the transformation matrix from the Lidar to the vehicle by doing:
lidar_to_vehicle_transform = lidar.get_unreal_transform()

And inside my game loop, for each frame I do:

world_transform = Transform(measurements.player_measurements.transform) lidar_to_world_transform = world_transform * lidar_to_vehicle_transform measurement.point_cloud.apply_transform(lidar_to_world_transform)

And save the measurement into disk. However if I integrate all frames together, is possible to see that the resulting pointcloud have transformation errors from a frame to the other. I can't see how the coordinate axis of the Lidar and the vehicle are defined in order to calculate and apply the transformation by myself.

Thanks in advance.

[minhaoxu1995]

Hi Luis,

392 this might help.

[luigui2906]

Thank you @minhaoxu1995,

That solves the problem. There's still something weird however. When reconstructing the environment with the data from the Lidar everything seems to be inverted, like a mirror. A right turn made by the vehicle seems like a left turn in the reconstructed data. It's not really important, but I would like just to know why this happens.

[marcgpuig]

@minhaoxu1995 thank you for your answer!

@luigui2906 take a look at Unreal's coordinate system:

Depending on how you are making the transformation and how you are visualizing the data is normal that you can see a _flip_ on some axis. Could be this the case?

[luigui2906]

Hi @marcgpuig,

Yep, I think is because the Lidar coordinate system is defined following left-hand rule (YxX gives Z) just as you show in the image. While the vehicle's is defined right-handed (XxY gives Z). That's why we need the scale(z=-1) on the unreal transform. And this is where the flip comes from I think.

[xinshuoweng]

can anyone help how to do scale(z=-1) in the lastest Carla version? it seems like that the feature has disappeared since a version.
@marcgpuig