3.5.9. CITYDB_ENVELOPE
The package CITYDB_ENVELOPE
provides functions that allow a user to
calculate the maximum 3D bounding volume of a CityObject identified by
its ID. For each feature type, a corresponding function is provided
starting with env_
prefix. In PostgreSQL, they are part of an instance
schema like ‘citydb’ and not ‘citydb_pkg’ due to unforeseen schema
changes by adding CityGML ADEs.
The bounding volume is calculated by evaluating all geometries of the
city object in all LoDs including implicit geometries. In PostGIS, they
are first collected and then fed to the ST_3DExtent
aggregate function
which returns a BOX3D
object. In Oracle the aggregate function
SDO_AGGR_MBR
is used which produces a 3D optimized rectangle with only
two points. The box2envelope
function turns this output into a diagonal
cutting plane through the calculated bounding volume. This surface
representation follows the definition of the ENVELOPE column of the
CITYOBJECT table as discussed in Section 3.2.2
(see also Fig. 3.27).
All functions in this package return such a geometry.
The CITYDB_ENVELOPE
API also allows for updating the ENVELOPE column of
the city objects with the calculated value (by simply setting the
set_envelope
argument that is available for all functions to 1).
This is useful, for instance, whenever one of the geometry
representations of the city object has been changed or if the ENVELOPE
column could not be (correctly) filled during import and, for example,
is NULL
.
To calculate and update the ENVELOPE of all city objects of a given
feature type, use the get_envelope_cityobjects
function and provide the
OBJECTCLASS_ID
as parameter. If 0 is passed as OBJECTCLASS_ID
, then
the ENVELOPE columns of all city objects are updated. To update only
those ENVELOPE columns having NULL
as value, set the only_if_null
parameter to 1.
Function

Return Type

Explanation

box2envelope (BOX3D)

GEOMETRY

Takes a BOX3D and returns a 3D polygon that
represents a diagonal cutting plane through this
box. Under Oracle the input is an optimized 3D
rectangle (SDO_INTERPRETATION = 3)

env_cityobject (cityobject_id,
set_envelope)

GEOMETRY

Returns the current envelope representation of
the given CityObject and optionally updates the
ENVELOPE column

get_envelope_cityobjects
(objectclass_id, set_envelope,
only_if_null)

GEOMETRY

Returns the current envelope representation of
all CityObjects of given object class and
optionally updates the ENVELOPE column with
the individual bounding boxes

get_envelope_implicit_geometry
(implicit_rep_id, reference_point,
transformation_matrix)

GEOMETRY

Returns the envelope of an implicit geometry
which has been transformed based on the
passed reference point and transformation
matrix

update_bounds (old_box,
new_box)

GEOMETRY

Takes two GEOMETRY objects to call
box2envelope and returns the result. If one
side is NULL, the nonempty input is
returned.
