QGIS/resources/function_help/json/boundary

{
  "name": "boundary",
  "type": "function",
  "groups": ["GeometryGroup"],
  "description":"Returns the closure of the combinatorial boundary of the geometry (ie the topological boundary of the geometry). For instance, a polygon geometry will have a boundary consisting of the linestrings for each ring in the polygon. Some geometry types do not have a defined boundary, e.g., points or geometry collections, and will return NULL.",
  "arguments": [ {"arg":"geometry","description":"a geometry"} ],
  "examples": [ { "expression":"geom_to_wkt(boundary(geom_from_wkt('Polygon((1 1, 0 0, -1 1, 1 1))')))", "returns":"'LineString(1 1,0 0,-1 1,1 1)'"},
                { "expression":"geom_to_wkt(boundary(geom_from_wkt('LineString(1 1,0 0,-1 1)')))", "returns":"'MultiPoint ((1 1),(-1 1))'"}
              ]
}