Zero-copy point cloud subscriber in ROS2

[ravijo]

The components in ROS2 provide Zero-copy communication. Luckily, the realsense package comes with this option.

Quoting the following from Efficient intra-process communication section:

Our ROS2 Wrapper node supports zero-copy communications if loaded in the same process as a subscriber node. This can reduce copy times on image topics (not point-cloud or others), especially with big frame resolutions and high FPS.

Does it mean that point clouds are not supported for zero-copy if loaded in the same process as a subscriber node?

[MartyG-RealSense]

Hi @ravijo Another way to interpret that statement would be that copy times are not reduced for pointclouds.

[ravijo]

Hi @MartyG-RealSense

Sorry for the confusion. Does that mean intra-process communication isn't helpful in the case of point cloud??? How to achieve zero-copy communication for point clouds then?

[MartyG-RealSense]

That would be my interpretation that zero-copy would not be useful for pointclouds.

Someone in the link below wrote a ROS2 module that was tested with ROS2 Galactic that supports pointclouds with zero-copy, though I cannot guarantee how well it will work with the RealSense ROS2 wrapper.

https://github.com/ZhenshengLee/ros2_shm_msgs

[ravijo]

I see.

It is using shared memory architecture underneath.

Thank you for the link.

[MartyG-RealSense]

Hi @ravijo Do you require further assistance with this case, please? Thanks!

[ravijo]

@MartyG-RealSense: I am not dealing with it at prersent. So it is better to close this issue. Thanks for the kind support.

[MartyG-RealSense]

You are very welcome. Thanks very much for the update!

[ZhenshengLee]

Someone in the link below wrote a ROS2 module that was tested with ROS2 Galactic that supports pointclouds with zero-copy, though I cannot guarantee how well it will work with the RealSense ROS2 wrapper.

@MartyG-RealSense @ravijo

Hi, this lib https://github.com/ZhenshengLee/ros2_shm_msgs has been used in the custom version of ros2_v4l2_camera
and rslidar_sdk, and the performance improvement is awesome!

see https://github.com/ZhenshengLee/ros2_v4l2_camera
and https://github.com/ros2driver/rslidar_sdk/tree/outdoor/dev_opt_shm

if you need any help to merge this module into the RealSense ROS2 , feel free to ping me.

[MartyG-RealSense]

Thanks so much @ZhenshengLee for sharing this with the RealSense ROS community!

[toan70411hd91]

Hello @MartyG-RealSense, I am using ROS2 Realsense Wrapper (Humble, https://github.com/IntelRealSense/realsense-ros/tree/ros2-development). As you mentioned the link https://github.com/ZhenshengLee/ros2_shm_msgs above (introduced by @ZhenshengLee ). I have made a comparison between ROS2 Realsense Wrapper (Humble) and ros2_shm_msgs (Humble) regarding to the topic frequency and CPU consumption. I have found that the performance of ROS2 Realsense Wrapper (Humble) and ros2_shm_msgs are almost same when publishing the pointcloud. I just wonder whether some modules in ros2_shm_msgs are merged into Realsense ROS2 or not? Or Type adaption is considered in ROS2 Realsense Wrapper (Humble) or not? I guess IT'S NOT since I could not found the related contents in the Wrapper source code. Can you please confirm this information?

[ZhenshengLee]

@toan70411hd91

I just wonder whether some modules in ros2_shm_msgs are merged into Realsense ROS2 or not?

No, nothing has been merged.

Or Type adaption is considered in ROS2 Realsense Wrapper (Humble) or not?

It's unknown for me, because I didn't track this feature.

[ZhenshengLee]

@toan70411hd91
You can use zero-copy through intra-process comm which is provided officially, check it in https://github.com/IntelRealSense/realsense-ros#efficient-intra-process-communication

[toan70411hd91]

@ZhenshengLee thank you so much for your information and quick helps. May I ask you some questions related to ros2_shm_msgs?

1. In folder "intra", i found that you added intra type adaption within cpu. Currently, ros2_shm_msgs only support type adaption (within CPU) for image only, but not for pointcloud?
2. Is it correct if "zero-copy" in ros2_shm_msgs called "minimal copy" when we publish topics?
3. In your opinion, whether the performance (regarding to speed and CPU consumption) much better if we use type adaption?

Even I did not use https://github.com/IntelRealSense/realsense-ros#efficient-intra-process-communication, the performance of ROS2 Realsense Wrapper (regarding to speed and CPU consumption) is almost same with ros2_shm-msgs. I am not sure the hints inside since i am quite new in this topic. Maybe, my tested point-cloud data is small (camera resolution 640x480, with decimation filter).