does lidar sensor output data have spatial locality?

Question

i have a question about the output of lidar sendor (like velodyne vlp-16, etc)

Is there any spatial or temporal locality for output data which is from lidar sensor? Does the produced output data from lidar sensor are stored in memory position sequentially or randomly? (which means that if a data from 0x000 and 0x001 point are spatially or temporally related.)

I see that lidar output data is obtained by spherical coordinate system and converted to cartesian coordiate system. (FIG 9-1, https://greenvalleyintl.com/wp-content/uploads/2019/02/Velodyne-LiDAR-VLP-16-User-Manual.pdf)

And there is a driver code to make un-ordered point cloud data to ordered data (https://github.com/ros-drivers/velodyne/blob/master/velodyne_pointcloud/include/velodyne_pointcloud/organized_cloudXYZIRT.h)

So, i guess lidar sensor is rotating 360' and sequentially produce the output and save them to certain memory location. So, is that possible to presume that memory address could give us hint for point cloud data information even before the ordering process?

Thank you. If i have some lidar sensor, then this question could be solved easily. but i don't have one.

Answer 1

For each given sample there is a spacial assumption that you can make. And for each sequential sample there are temporal and spacial assumptions that you can make. These assumptions might change slightly based on the operating parameters of the system, such as the scanning speed. Also keep in mind that the measurements may be interlaced in position across the scan. This was true of the HDL64 lasers I'm not familiar with the newer ones.

And if you're using processed or aggregated data you will want to make sure that you fully understand each of the previous steps in the pipeline to make sure that you understand the data structure.

Answer 2

You can’t assume the order in memory corresponds to a particular point in space. The Velodyne you are referring rotates to collect data to make a point cloud, but not necessarily in the order that is intuitive without understanding the hardware.

However, the driver of the sensor may have some particular ordering in memory corresponding to the spherical coordinates of the points, but you would need to check the documentation.