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.

Examples

Example: Concave hulls and alpha values

In this example, we will generate a series of concave hulls for a set of property addresses while varying alpha values.

The input features are in a UTM83-10 coordinate system, with ground units in meters, and so the alpha value represents a measurement in meters as well.

Alpha Value	Concave Hull
10 - Too small for useful results.
50 - Tightly fitting, some input points fall outside alpha distance.
Default (blank) - optimal value calculated to produce a single area.
500 - Loosely fitting but not convex.
10000 - Identical to the convex hull.

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.

Configuration

Input Ports

Output Ports

Parameters

Group Processing

Group By

If Group By attributes are specified, a hull output feature is generated for each set of input features that have the same values for all those attributes.

Otherwise, a single feature representing the hull of all input features is output.

Complete Groups

Select the point in processing at which groups are processed:

When All Features Received	This is the default behavior. Processing will only occur in this transformer once all input is present.
When Group Changes (Advanced)	This transformer will process input groups in order. Changes of the value of the Group By parameter on the input stream will trigger processing on the currently accumulating group. This may improve overall speed (particularly with multiple, equally-sized groups), but could cause undesired behavior if input groups are not truly ordered.

Hull

Hull Type

Select the hull type to be calculated:

Convex (): The minimum enclosing convex polygon, where no interior angle is greater than 180 degrees.
Concave)(: A hull based on Alpha Value, which has no angle restrictions and may consist of multiple polygons and donuts. It generally fits tighter to the shape of the dataset than the convex hull.

Alpha Value

When Hull Type is Concave)(, either:

Provide a specific value in ground units for the hull calculation, or
Leave the parameter blank (or enter zero) to have the optimal alpha calculated to produce a single hull area.

See Examples above for alpha value usage.

Output Dimension

Output

Select the dimensionality of the output hull:

If any input features are 3D, their z coordinates will be ignored.

The hull will consider 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.

Default Z Value

When Output is 3D, provide a Z value to assign to converted 2D features.

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.

How to Set Parameter Values

Defining Values

There are several ways to define a value for use in a Transformer. The simplest is to simply type in a value or string, which can include functions of various types such as attribute references, math and string functions, and workspace parameters.

Using the Text Editor

The Text Editor provides a convenient way to construct text strings (including regular expressions) from various data sources, such as attributes, parameters, and constants, where the result is used directly inside a parameter.

Text Editor

Using the Arithmetic Editor

The Arithmetic Editor provides a convenient way to construct math expressions from various data sources, such as attributes, parameters, and feature functions, where the result is used directly inside a parameter.

Arithmetic Editor

Conditional Values

Set values depending on one or more test conditions that either pass or fail.

Parameter Condition Definition Dialog

Content

Expressions and strings can include a number of functions, characters, parameters, and more.

When setting values - whether entered directly in a parameter or constructed using one of the editors - strings and expressions containing String, Math, Date/Time or FME Feature Functions will have those functions evaluated. Therefore, the names of these functions (in the form @<function_name>) should not be used as literal string values.

Content Types

String Functions	These functions manipulate and format strings.
Special Characters	A set of control characters is available in the Text Editor.
Math Functions	Math functions are available in both editors.
Date/Time Functions	Date and time functions are available in the Text Editor.
Math Operators	These operators are available in the Arithmetic Editor.
FME Feature Functions	These return primarily feature-specific values.
FME Parameters	FME and workspace-specific parameters may be used.
Creating and Modifying User Parameters	Create your own editable parameters.

Dialog Options - Tables

Table Tools

Transformers with table-style parameters have additional tools for populating and manipulating values.

Row Reordering

Enabled once you have clicked on a row item. Choices include:

Add a row
Remove a row
Move current row up one
Move current row down one
Move current row to top
Move current row to bottom

Cut, Copy, and Paste

Enabled once you have clicked on a row item. Choices include:

Cut a row - delete and copy to clipboard
Copy a row to the clipboard
Paste a row from the clipboard

Cut, copy, and paste may be used within a transformer, or between transformers.

Filter

Start typing a string, and the matrix will only display rows matching those characters. Searches all columns. This only affects the display of attributes within the transformer - it does not alter which attributes are output.

Import

Import populates the table with a set of new attributes read from a dataset. Specific application varies between transformers.

Reset/Refresh

Generally resets the table to its initial state, and may provide additional options to remove invalid entries. Behavior varies between transformers.

Note: Not all tools are available in all transformers.

For more information, see Transformer Parameter Menu Options.

Reference

Processing Behavior	Group-Based
Feature Holding	Yes
Dependencies	None
Aliases	ConvexHullAccumulator
History

FME Community

The FME Community has a wealth of FME knowledge with over 20,000 active members worldwide. Get help with FME, share knowledge, and connect with users globally.

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.