HullAccumulator

Categories

Spatial Analysis

Related Transformers

2DBoxReplacer

BoundingBoxAccumulator

BoundingBoxReplacer

BoundsExtractor

HullReplacer

RasterExtentsCoercer

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

Jump to Configuration

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

In this example, we have a set of address points and want to generate a convex hull around them.

The features are routed into a HullAccumulator.

In the parameters dialog, Hull Type is Convex ().

The output hull contains all input features.

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

Features with 2D and/or 3D geometry.

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

Output Ports

Hull geometry as specified in parameters. Typically a polygon, but possibly a line or a point.

Features without valid geometry are output via this port, as are features with non-numeric values for Alpha or 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.

Rejected Feature Handling: can be set to either terminate the translation or continue running when it encounters a rejected feature. This setting is available both as a default FME option and as a workspace parameter.

Parameters

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.

There are two typical reasons for using When Group Changes (Advanced) . The first is incoming data that is intended to be processed in groups (and is already so ordered). In this case, the structure dictates Group By usage - not performance considerations.

The second possible reason is potential performance gains.

Performance gains are most likely when the data is already sorted (or read using a SQL ORDER BY statement) since less work is required of FME. If the data needs ordering, it can be sorted in the workspace (though the added processing overhead may negate any gains).

Sorting becomes more difficult according to the number of data streams. Multiple streams of data could be almost impossible to sort into the correct order, since all features matching a Group By value need to arrive before any features (of any feature type or dataset) belonging to the next group. In this case, using Group By with When All Features Received may be the equivalent and simpler approach.

Note  Multiple feature types and features from multiple datasets will not generally naturally occur in the correct order.

As with many scenarios, testing different approaches in your workspace with your data is the only definitive way to identify performance gains.

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.

Accumulation Mode

Drop Incoming Attributes: The hull receives no attributes.

Merge Incoming Attributes: The hull receives all attributes from input features.

Use Attributes From One Feature: The hull receives all attributes from one representative feature.

Output

Select the dimensionality of the output hull:

2D

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

3D

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

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.

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. There are a number of tools and shortcuts that can assist in constructing values, generally available from the drop-down context menu adjacent to the value field.

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.

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

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.

Reference

Processing Behavior

Group-Based

Feature Holding

Yes
Dependencies None
Aliases ConvexHullAccumulator
History  

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