[filters/VoxelGrid] Provide a way to override CentroidPoint or add my own Accumulators

[jackjansen]

Context

I have defined my own points, with x, y, z and color, and an extra field that is essentially a bit mask.
I have been using this for years, and it works fine everywhere, with the exception of VoxelGrid.

VoxelGrid uses CentroidPoint to combine points, and it uses a set of predefined Accumulators to combine the fields within points according to the correct semantics for those fields.

This works fine for coordinates, normals, colours and a few other attribute types that pcl support, but there does not appear to be a way to extend the field types supported by supplying my own Accumulator type for my bit mask fields (which would is essence do a bitwise or on the fields in the incoming points).

With the current code structure it is impossible to override any of the implementations without copying everything up to VoxelGrid into my own code, which seems a bit gross.

Expected behavior

I would like to have full control over the way attributes of my points are treated, especially attributes that pcl doesn't know about as distributed because they are specific to my usage, but I can also see use cases for enabling use of different combination strategies for "standard" attributed like position or color.

Current Behavior

Currently I keep the leaf layout and then make an extra pass over the source points, call getCentroidIndex() and then manually modify the field.

Describe the solution you'd like

If I could override CentroidPoint when instantiating VoxelGrid I would only need to subclass CentroidPoint to do what I want. And I think it would allow all sorts of potentially useful other uses as well (for example create a CentroidPoint that uses a completely different way of combining points. I can think of random selection, for example, or modifying some attributes based on the values of some other attributes.

Describe alternatives you've considered

An alternative that would work for my use case is if I could add my own Accumulator implementation to the Accumulator.types list, but I'm not sure that there is a construct in C++ that would allow that.

Additional context

Add any other context or screenshots about the feature request here.

[mvieth]

Hi, your desire to customize the centroid computation is totally understandable. Looking at the way that CentroidPoint is currently used in VoxelGrid, I guess it would be necessary to add a CentroidPoint member in VoxelGrid that the user can replace via a setter-method before calling filter. But CentroidPoint, as well as all existing accumulators, would then also require a new method that resets the internal state. Additionally, to perhaps be able to parallelize VoxelGrid in the future, it would be nice to have some method to clone a CentroidPoint (so that each thread has its own CentroidPoint).

An alternative idea, that changes less of the existing code, could be to give the user the option to pass a function to VoxelGrid, which perhaps receives a point cloud and a vector of indices (the points to use for the centroid computation), and returns a single point (the centroid). This approach seems less invasive to me, while still giving total freedom to the user. Let me know what you think.