# filters.hexbin

A common questions for users of point clouds is what the spatial extent of a point cloud collection is. Files generally provide only rectangular bounds, but often the points inside the files only fill up a small percentage of the area within the bounds.

Hexbin output shows boundary of actual points in point buffer, not just rectangular extents.

The hexbin filter reads a point stream and writes out a metadata record that contains a boundary, expressed as a well-known text polygon. The filter counts the points in each hexagonal area to determine if that area should be included as part of the boundary. In order to write out the metadata record, the pdal pipeline command must be invoked using the “–pipeline-serialization” option:

Streamable Stage

This stage supports streaming operations

## Example 1

The following pipeline file and command produces an JSON output file containing the pipeline’s metadata, which includes the result of running the hexbin filter:

    [
        "/Users/me/pdal/test/data/las/autzen_trim.las",
        {
            "type" : "filters.hexbin"
        }
    ]

    $ pdal pipeline hexbin-pipeline.json --metadata hexbin-out.json

    {
      "stages":
      {
        "filters.hexbin":
        {
          "area": 746772.7543,
          "avg_pt_per_sq_unit": 22.43269935,
          "avg_pt_spacing": 2.605540869,
          "boundary": "MULTIPOLYGON (((636274.38924399 848834.99817891, 637242.52219686 848834.99817891, 637274.79329529 849226.26445367, 637145.70890157 849338.05481789, 637242.52219686 849505.74036422, 636016.22045656 849505.74036422, 635983.94935813 849114.47408945, 636113.03375184 848890.89336102, 636274.38924399 848834.99817891)))",
          "boundary_json": { "type": "MultiPolygon", "coordinates": [ [ [ [ 636274.38924399, 848834.99817891 ], [ 637242.52219686, 848834.99817891 ], [ 637274.79329529, 849226.26445367 ], [ 637145.70890157, 849338.05481789 ], [ 637242.52219686, 849505.74036422 ], [ 636016.22045656, 849505.74036422 ], [ 635983.94935813, 849114.47408945 ], [ 636113.03375184, 848890.89336102 ], [ 636274.38924399, 848834.99817891 ] ] ] ] },
          "density": 0.1473004999,
          "edge_length": 0,
          "estimated_edge": 111.7903642,
          "hex_offsets": "MULTIPOINT (0 0, -32.2711 55.8952, 0 111.79, 64.5422 111.79, 96.8133 55.8952, 64.5422 0)",
          "sample_size": 5000,
          "threshold": 15
        }
    },
    ...

## Example 2

As a convenience, the `pdal info` command will produce similar output:

    $ pdal info --boundary /Users/me/test/data/las/autzen_trim.las

    {
      "boundary":
      {
        "area": 746772.7543,
        "avg_pt_per_sq_unit": 22.43269935,
        "avg_pt_spacing": 2.605540869,
        "boundary": "MULTIPOLYGON (((636274.38924399 848834.99817891, 637242.52219686 848834.99817891, 637274.79329529 849226.26445367, 637145.70890157 849338.05481789, 637242.52219686 849505.74036422, 636016.22045656 849505.74036422, 635983.94935813 849114.47408945, 636113.03375184 848890.89336102, 636274.38924399 848834.99817891)))",
        "boundary_json": { "type": "MultiPolygon", "coordinates": [ [ [ [ 636274.38924399, 848834.99817891 ], [ 637242.52219686, 848834.99817891 ], [ 637274.79329529, 849226.26445367 ], [ 637145.70890157, 849338.05481789 ], [ 637242.52219686, 849505.74036422 ], [ 636016.22045656, 849505.74036422 ], [ 635983.94935813, 849114.47408945 ], [ 636113.03375184, 848890.89336102 ], [ 636274.38924399, 848834.99817891 ] ] ] ] },
        "density": 0.1473004999,
        "edge_length": 0,
        "estimated_edge": 111.7903642,
        "hex_offsets": "MULTIPOINT (0 0, -32.2711 55.8952, 0 111.79, 64.5422 111.79, 96.8133 55.8952, 64.5422 0)",
        "sample_size": 5000,
        "threshold": 15
      },
      "filename": "\/Users\/acbell\/pdal\/test\/data\/las\/autzen_trim.las",
      "pdal_version": "1.6.0 (git-version: 675afe)"
    }

## Options

edge_size

  If not set, the hexbin filter will estimate a hex size based on a sample of the data. If set, hexbin will use the provided size in constructing the hexbins to test.

sample_size

  How many points to sample when automatically calculating the edge size? Only applies if edge_size is not explicitly set. [Default: 5000]

threshold

  Number of points that have to fall within a hexagon boundary before it is considered “in” the data set. [Default: 15]

precision

  Minimum number of significant digits to use in writing out the well-known text of the boundary polygon. [Default: 8]

preserve_topology

  Use GEOS SimplifyPreserveTopology instead of Simplify for polygon simplification with smooth option. [Default: true]

smooth

  Use GEOS simplify operations to smooth boundary to a tolerance [Default: true]

where

  An expression that limits points passed to a filter. Points that don’t pass the expression skip the stage but are available to subsequent stages in a pipeline. [Default: no filtering]

where_merge

  A strategy for merging points skipped by a ‘where’ option when running in standard mode. If `true`, the skipped points are added to the first point view returned by the skipped filter. If `false`, skipped points are placed in their own point view. If `auto`, skipped points are merged into the returned point view provided that only one point view is returned and it has the same point count as it did when the filter was run. [Default: `auto`]