HullAccumulator

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.

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.

If Group By attributes are specified, then 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.

Process At End (Blocking): This is the default behavior. Processing will only occur in this transformer once all input is present.

Process 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.

Considerations for Using Group By

There are two typical reasons for using Process 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 Process At End (Blocking) 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.

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.

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.

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