(Discussion) Initial refactor resampleCloudSpatially for multi-threading

[nigels-com]

Note: This branch is untested. It does compile.

I have a heavily refactored version of resampleCloudSpatially that supports multi-threading.
In terms of licensing, ideally I could contribute that upstream rather than LGPL the surrounding code.
So the purpose of this PR is to open a discussion of what might be agreeable all around.

The reasoning for this change:

• Overload with a marker/filtered count back-end version of resampleCloudSpatially that does not allocate the markers or the output point cloud.
• In our use case we prefer the markers directly rather than converting from the ReferenceCloud
• In our use case the ReferenceCloud isn't needed, so don't build it
• Incrementally building the output cloud is problematic concurrently, better as a "gather" step once markers are finalised.
• Also, it resolves the problem of reserving the appropriate storage and then shrinking it again at the end.
• The pre-existing resampleCloudSpatially continues behaving in a compatible manner.

And to be clear this PR does not multi-thread but is more aligned with my more severe refactor.

Further thoughts:

• Perhaps move octree build up and out to the pre-existing overload, to share it read-only across worker threads.

To broadly describe the multi-threading approach, have concurrent neighbour searching, but lock access to the markers with a mutex and check if the current point is still filtered-in before proceeding to filter-out the neighbourhood. We do see some occasional non-determinism due to the non-deterministic order of points being processed, but the speedup is worthwhile. With 12 threads we don't seem quite bottle-necked on updating markers, but that likely depends on the minimum distance and the typical neighbour count.

[nigels-com]

On a related note, I'm also interested in the potential for multi-threaded octree building.
With 12 threads the resampling step is taking a similar amount of time to the octree indexing.

[dgirardeau]

Hi,

From what I see in this PR (at this point) the changes to the existing code base seem reasonable.

And if you wish to contribute your parallel version upstream, no problem ;)

[dgirardeau]

Hi @nigels-com, are you going to push this effort forward in the future?

[nigels-com]

It has been a fair while, indeed.
I don't well recall the details of this, but will take a fresh look.