cgal_split_union_polygon - ObjectVision/GeoDMS GitHub Wiki

Geometric functions > cgal_split_union_polygon

• cgal_split_union_polygon(polygon_data_item)
• cgal_split_union_polygon(polygon_data_item, partition_attribute)

cgal_split_union_polygon is a combination of cgal_union_polygon and cgal_split_polygon. It first dissolves polygons (optionally grouped by a partition_attribute) and then splits any resulting multi-polygons into individual single-polygon parts.

Without a partition_attribute, all polygons are unioned into one geometry first, then split into their disconnected parts.

With a partition_attribute, polygons are first grouped and dissolved per group value, and then each group's result is split into individual parts. The result includes a part_rel attribute relating each output polygon back to the partition group.

The cgal_ prefix of the function name indicates that the implementation of the operator uses CGAL, which provides exact predicates and exact constructions for maximum numerical precision.

• attribute polygon_data_item with a polygon value type
• attribute partition_attribute (optional) with a categorical value type, domain matching polygon_data_item

1. The composition type of the polygon_data_item needs to be polygon.
2. The order of points in the polygon_data_item needs to be clockwise for exterior bounds and counterclockwise for holes (right-hand-rule).
3. When a partition_attribute is provided, its domain must match the domain of polygon_data_item.

The result is a container with:

• geometry: polygon attribute with the individual single polygons
• polygon_rel or part_rel: relation back to the source domain or partition

14.0

// Union all, then split into disconnected parts
unit<uint32> parts := cgal_split_union_polygon(district/geometry)
{
    attribute<dpoint>   geometry;
    attribute<district> polygon_rel;
}

// Union by province, then split into parts per province
unit<uint32> province_parts := cgal_split_union_polygon(district/geometry, district/province_rel)
{
    attribute<dpoint>   geometry;
    attribute<province> part_rel;
}

• cgal_split_polygon
• cgal_union_polygon
• geos_split_union_polygon
• bp_split_union_polygon