Polygon Sponsor

Polygon Set Concept

The polygon_set concept tag is polygon_set_concept

The semantic of a polygon_set is zero or more geometry regions.  A Polygon Set Concept may be defined with floating point coordinates, but a snap rounding distance of one integer unit will still be applied, furthermore, geometry outside the domain where one integer unit is sufficient to provide robustness may lead to undefined behavior in algorithms.  It is recommended to use integer coordinates for robust operations.  In the case that data represented contains only Manhattan geometry a runtime check will default to the Manhattan algorithm.  The results of which are identical to what the general algorithm would do, but obtained more efficiently.  In the case that the data represented contains only Manhattan and 45-degree geometry a runtime check will default to the faster 45-degree algorithm.  The results of which may differ slight from what the general algorithm would do because non-integer intersections will be handled differently.

Users are recommended to use std::vector and std::list of user defined polygons or library provided polygon_set_data<coordinate_type> objects.  Lists and vectors of models of polygon_concept or polygon_with_holes_concept are automatically models of polygon_set_concept.

Example code custom_polygon_set.cpp demonstrates mapping a user defined class to the library polygon_set_concept

An object that is a model of polygon_set_concept can be viewed as a model of polygon_90_set_concept or polygon_45_set_concept if it is determined at runtime to conform to the restrictions of those concepts.  This concept casting is accomplished through the view_as<>() function.

view_as<polygon_90_set_concept>(polygon_set_object)
view_as<polygon_45_set_concept>(polygon_set_object)

The return value of view_as<>() can be passed into any interface that expects an object of the conceptual type specified in its template parameter.  Polygon sets cannot be viewed as single polygons or rectangles since it generally cannot be known whether a polygon set contains only a single polygon without converting to polygons.

Operators

The return type of some operators is the polygon_set_view operator template type.  This type is itself a model of the polygon 90 set concept, but furthermore can be used as an argument to the polygon_set_data constructor and assignment operator.  The operator template exists to eliminate temp copies of intermediate results when Boolean operators are chained together.

Operators are declared inside the namespace boost::polygon::operators.

Functions

Polygon Set Data Object

The polygon set data type encapsulates the internal data format that serves as the input to the sweep-line algorithm that implements polygon-clipping Boolean operations.  It also internally keeps track of whether that data has been sorted or scanned and maintains the invariant that when its flags indicate that the data is sorted or scanned the data has not been changed to violate that assumption.  Using the Polygon Set Data type directly can be more efficient than using lists and vectors of polygons in the functions above because of the invariants it can enforce which provide the opportunity to maintain the data is sorted form rather than going all the way out to polygons then resorting those vertices for a subsequent operation.

The declaration of Polygon Set Data is the following:

template <typename T>
class polygon_set_data;

The class is parameterized on the coordinate data type.  Algorithms that benefit from knowledge of the invariants enforced by the class are implemented as member functions to provide them access to information about those invariants. 

Example code polygon_set_usage.cpp demonstrates using the library provided polygon set data types and functions

Member Functions