redundant projection calculations

[dllu]

I noticed the pointRange is computed in by float pointDistance which is an expensive operation as it requires computing a square root for each point (and there may be over 2 million points per second with Ouster OS1-128).

But the raw data from the lidar already has the range, so it is not necessary to recompute it again.

The PointOS1 defined at https://github.com/ouster-lidar/ouster_example/blob/master/ouster_ros/include/ouster_ros/point_os1.h contains a useful range field which contains the raw range reading.

I suggest making the PointType a template parameter for pointDistance, and do a template specialization for PointOS1 to use the raw range reading directly without recomputing it.

Likewise the horizon angle is redundantly computed by an expensive call to atan2, which is not necessary, because the raw data from the lidar contains this information already in the raw UDP packets, both for Velodyne and Ouster lidars. Unfortunately using pcl::PointXYZI causes this information to be lost, requiring it to be recomputed. For Ouster lidars specifically, the horizon angle does not change between frames, and the driver always outputs all the points (even points without valid range readings), so it is possible to compute the horizon angle from the index of the point. However, the angle is not stored in PointOS1 so it may be more difficult to get the data from raw lidar data. But then atan2 is much slower than sqrt so it may be worth it.

atan2 typically costs 100 cycles, so high resolution lidars producing 2.6 million points per second (e.g. Ouster OS1-128 in 2048 mode) will be starting to be a significant performance penalty.

[TixiaoShan]

You are right about all the things you mentioned there. This part of the code is simply copied from LeGO-LOAM, which uses a range image for segmentation. I agree that the projection function here can significantly be improved.

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