org.opengis.geometry.package.html Maven / Gradle / Ivy
Show all versions of gt-opengis Show documentation
package org.opengis.geometry
Root package for {@linkplain org.opengis.geometry.Geometry geometries}. The following is adapted from
Feature Geometry (Topic 1) specification.
The geometry packages contain the various classes for coordinate geometry.
All of these classes through the root class {@link org.opengis.geometry.Geometry} inherit an optional
association to a coordinate reference system. All direct positions exposed through the interfaces
defined in this specification shall be in the coordinate reference system of the geometric object
accessed. All elements of a geometric complex, composite, or aggregate shall be associated to the
same coordinate reference system. When instances of {@link org.opengis.geometry.Geometry} are aggregated
in another {@link org.opengis.geometry.Geometry} (such as a {@link org.opengis.geometry.aggregate.Aggregate},
or {@link org.opengis.geometry.complex.Complex}) which already has a coordinate reference system specified,
then these elements are assumed to be in that same coordinate reference system unless otherwise
specified.
The geometry package has several internal packages that separate primitive
geometric objects, aggregates and complexes, which have a more elaborate internal structure
than simple aggregates.
Any object that inherits the semantics of the {@link org.opengis.geometry.Geometry}
acts as a set of direct positions. Its behavior will be determined by which direct positions it
contains. Objects under {@link org.opengis.geometry.primitive.Primitive} will be open, that is, they
will not contain their boundary points; curves will not contain their end points, surfaces will
not contain their boundary curves, and solids will not contain their bounding surfaces. Objects
under {@link org.opengis.geometry.complex.Complex} will be closed, that is, they will contain their
boundary points. This leads to some apparent ambiguity. A representation of a line as a primitive
must reference its end points, but will not contain these points as a set of direct positions. A
representation of a line as a complex will also reference its end points, and will contain these
points as a set of direct positions. This means that identical digital representations will have
slightly different semantics depending on whether they are accessed as primitives or complexes.
This difference of semantics is most striking in the
{@link org.opengis.geometry.complex.CompositeCurve}. Composite curves are used to represent features whose
geometry could also be represented as curve primitives. From a cartographic point of view, these
two representations are not different. From a topological point of view, they are different. This
distinction appears as an inheritance relationship between {@link org.opengis.geometry.complex.CompositeCurve}
and {@link org.opengis.geometry.primitive.OrientableCurve}. The primary semantics of a
{@link org.opengis.geometry.complex.CompositeCurve} is as a closed {@link org.opengis.geometry.Geometry}, but it
may also act as an open {@link org.opengis.geometry.Geometry} under {@link org.opengis.geometry.primitive.Primitive}
operations. Interface protocols depending upon the topological details of this object will have to
be distinguished as to whether they have been inherited from {@link org.opengis.geometry.primitive.Primitive}
or {@link org.opengis.geometry.complex.Complex}, where the distinction first occurs. Even though these protocols
have been inherited from the same operations defined at {@link org.opengis.geometry.Geometry}, they will act
differently depending upon the branch of the inheritance tree from which they have inherited semantics.
Creators of implementation profiles may take this into account and use a proxy mechanism for realization
relationships that cause semantic dissonance.
{@link org.opengis.geometry.Geometry} and {@link org.opengis.geometry.primitive.Primitive}
are purely abstract in the sense that no object or data structure from an application can
instantiate them directly. Instances of these classes must be instances of one of their non-abstract
subtypes, such as {@link org.opengis.geometry.primitive.Point}, {@link org.opengis.geometry.primitive.Curve}, or
{@link org.opengis.geometry.primitive.Surface}. This is not the case for {@link org.opengis.geometry.complex.Complex},
which can be directly instantiated by an application, and need not be an instance of one of the
non-abstract subclasses of {@link org.opengis.geometry.complex.Composite}.