FME Transformers: 2024.1

Categories
Spatial Analysis
Related Transformers
2DBoxReplacer
BoundingBoxAccumulator
BoundingBoxReplacer
BoundsExtractor
HullReplacer
MinimumSpanningCircleReplacer
RasterExtentsCoercer

# HullAccumulator

Creates a convex or concave hull based on a group of input features.

### Typical Uses

• Creating hull geometry over multiple features.

## How does it work?

The HullAccumulator receives features with geometry and calculates either a Convex or Concave hull around them.

A convex hull is a polygon covering the extents of all features, where no interior angle is greater than 180 degrees. It resembles an elastic band surrounding the features.

A concave hull (or alpha hull) has no restrictions on angle and may follow the general shape of the data more tightly. It is calculated using the Alpha Value parameter and can have multiple parts and donuts.

If an output hull has zero area (the hull of a single point or vertical line, for example), it will become a point or line as appropriate.

3D hulls may be generated when input features are either 3D or a mix of 3D and 2D. If mixed, 2D features are assigned a default Z value.

### Concave hulls and alpha values

Concave hulls vary in shape. They are calculated by first triangulating the dataset, then measuring the circumradius of the resulting triangles (the radius of a circle that touches all three points of the triangle). Triangles that are larger than the specified Alpha Value are discarded, and the rest are dissolved to form the hull. Measurements are in ground units.

If the Alpha Value is explicitly specified, it is used for the calculation. If it is zero (0) or not specified, the alpha is approximated as the smallest value that produces a single area. If the alpha is too small it may return a null geometry. Larger numbers generally produce larger areas, and a large enough alpha value produces the same result as a convex hull.

See Examples below to see the effect of varying the Alpha Value.

Geometries that are farther away than Alpha Value from the rest of the geometries may be discarded if they do not constitute a valid area on their own.

Concave hull generation works best with points, but line and area geometries will be accepted and converted to points if input.

## Usage Notes

• Point clouds are considered single features. To create a hull from a point cloud’s individual points, use a PointCloudToPointCoercer beforehand, setting the Output Geometry parameter to either Single Multipoint or Individual Points.
• For triangulated surfaces, see the SurfaceModeller, Triangles output.

## Editing Transformer Parameters

Transformer parameters can be set by directly entering values, using expressions, or referencing other elements in the workspace such as attribute values or user parameters. Various editors and context menus are available to assist. To see what is available, click beside the applicable parameter.

## Reference

 Processing Behavior Feature Holding Yes Dependencies None Aliases ConvexHullAccumulator History

## FME Community

The FME Community is the place for demos, how-tos, articles, FAQs, and more. Get answers to your questions, learn from other users, and suggest, vote, and comment on new features.

Search for all results about the HullAccumulator on the FME Community.

Examples may contain information licensed under the Open Government Licence – Vancouver, Open Government Licence - British Columbia, and/or Open Government Licence – Canada.