#
HullReplacer

Replaces the geometry of the feature with a polygon representing its hull.

## Input Ports

This transformer accepts both 2D and 3D features, but the z coordinates of 3D features will be ignored during the calculation of the hull. Depending on the value of the Output parameter, the convex hull of 3D features may be a polygon that uses unmodified 3D vertices from the input features. This may result in a polygon with varying z coordinates.

Arcs and ellipses will be stroked prior to the calculation of the hull. Text features will be treated as points.

## Output Ports

A feature representing the requested hull. Typically a polygon, but possibly a line or a point.

Features with non-numeric attributes for Alpha and Default Z Value

Rejected features will have an fme_rejection_code attribute with one of the following values: INVALID_GEOMETRY_VERTICES, INVALID_PARAMETER_ALPHA_VALUE, INVALID_PARAMETER_DEFAULT_Z_VALUE.

## Parameters

### Hull Parameters

The Hull Type parameter can specify either a convex or a concave hull. A convex hull is a polygon where no interior angle is greater than 180 degrees. For a concave hull this restriction does not apply.

The convex hull is defined as the minimum enclosing convex polygon. In lay terms, the effect is similar to tightening a rubber band around the feature. Note that the convex hull may be a line or a point if the resulting polygon has an area of zero.

The concave hull, also known as the alpha hull, is calculated based on the Alpha Value parameter. It is possible for the concave hull to more closely represent the outline of a feature’s geometry than the convex hull can. The concave hull may consist of multiple donuts or polygons.

When the Alpha Value parameter is specified, it is used in the alpha hull calculation directly; a larger number will generally produce larger areas. If a circle with a radius of Alpha Value ground units is able to pass between two points A and B of the feature without touching any other points of the feature, then the resulting hull will not directly connect A and B. Geometries that are farther away than Alpha Value from the rest of the geometries may be culled (removed) if they do not constitute a valid area on their own. It is possible that a null geometry will be returned if the specified alpha was too small for the spacing of the geometry.

When the Alpha Value parameter is not specified or is set to 0, an approximation to the optimal alpha, which is the smallest alpha that generates a single area, will be used to generate the concave hull. Concave hull generation works best with points, but line and area geometries will be accepted and converted to points if input.

### Output Dimension

Choose between 2D and 3D representation of coordinates of points in the hull.

When 2D is chosen, all z coordinates will be removed from any vertices in the output.

When 3D is chosen, the convex hull will be a polygon that uses unmodified 3D vertices from the input features. If there is a mixture of 2D and 3D vertices in the input features, then the 2D vertices will be converted to 3D using the specified Default Z Value. However, if all input features are 2D, the output will not be converted to 3D.

When Automatic is chosen, the output will be similar to choosing 3D, except that for a mixture of 2D and 3D vertices, the 2D vertices will be converted to 3D using the largest z value from the input features.

2D vertices that are converted to 3D will use the Default Z Value as the z coordinate when the Output parameter is 3D.

## Example

## Editing Transformer Parameters

Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click beside the applicable parameter. For more information, see Transformer Parameter Menu Options.

## Transformer Categories

## Transformer History

This transformer was previously named ConvexHullReplacer.

## Technical History

Associated FME function or factory: ConvexHullFactory

## Search FME Community

Search for samples and information about this transformer on the FME Community.

**Tags **Keywords: ConvexHullCreator ConvexHullReplacer