[pdal] Filtering out coplanar points

[Etienne B. Racine]

Hi list,

I'm looking for a way of thinning a point cloud by scoring points according
to the information they bring according to an interpolated surface (such as
a DEM). PDAL can filter points randomly either on a grid or systematically,
but I'd like to keep points that indicate an inflection and remove points
that are within a plane tolerance (since they don't bring much information
if the points are linearly interpolated to a surface). I know PCL can
compute normals, could that be leveraged to provide a smarter way to thin a
point cloud or harnessing the progressive morphological filter? I haven't
found much relevant literature on that, but there must be. I probably don't
have the right terminology. If you have any insights, I'd appreciate.

One way to think about it would be to

   - Generate a DEM with the whole set of points;
   - For each point in dataset:
   - remove a point and generate another DEM;
      - compare the change in elevation (or volume or whatever metric you
      want) using e.g. RMSE;
      - associate RMSE value to the removed point;
   - Remove point with lowest RMSE, repeat until you reach a user-defined
   threshold.

This would be tedious, there's probably a better way. I thought it might
trigger some ideas or help people send me in the right direction.

Etienne
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