dms_overlay_polygon

Geometric functions dms_overlay_polygon

syntax

dms_overlay_polygon(first_polygon_data_item, second_polygon_data_item)
dms_overlay_polygon(polygon_data_item)

description

dms_overlay_polygon finds the pairs of polygons that overlap and computes the intersection geometry of each pair, resulting in a new uint32 Domain unit with one entry per pair. In QGIS this operation is known as intersect.

With two arguments the pairs run between two polygon sets: every polygon of the first with every polygon of the second that it overlaps. With one argument the pairs are those within a single set; each pair occurs once, with first_rel < second_rel, and a polygon is never paired with itself.

Unlike the four library variants, neither argument has to be a valid polygon: each pair is read under the even-odd rule of the dms polygon operators family, which is what makes an overlay possible on a source that self-intersects or has flipped rings without repairing it first.

Two polygons form a pair when their intersection encloses an area. Polygons that only touch, along a shared edge or in one point, are not a pair, as with bp_overlay_polygon and unlike geos_overlay_polygon.

result

The result is a new domain unit (uint32) with:

geometry: the intersection polygon of that pair
first_rel: a relation towards the domain of the first (or only) argument
second_rel: a relation towards the domain of the second argument, or of the only argument in the one-argument form

A relation is absent when its argument is a Parameter (a Void domain).

applies to

Attribute arguments with a polygon Value type: spoint, ipoint, wpoint, upoint, fpoint or dpoint

conditions

The composition type of the arguments needs to be polygon.
The Values unit of the two arguments must match.

since version

20.20.0

example

unit<uint32> intersect := dms_overlay_polygon(building/geometry, district/geometry)
{
    attribute<ipoint>   geometry;
    attribute<building> first_rel;
    attribute<district> second_rel;
}