Operators

# symm_difference_closed_polygons_xld (Operator)

## Name

symm_difference_closed_polygons_xld — Compute the symmetric difference of closed polygons.

## Signature

symm_difference_closed_polygons_xld(Polygons1, Polygons2 : PolygonsDifference : : )

## Description

The operator symm_difference_closed_polygons_xld calculates the symmetric difference of the regions enclosed by the polygons Polygons1 and Polygons2. The boundaries of this symmetric difference are returned in PolygonsDifference.

If the individual input polygons are not closed, they are closed automatically by connecting their start and end points.

Internally, first, the regions enclosed by each set of polygons (Polygons1 and Polygons2, respectively) are determined as described below. Then, the symmetric difference of these two regions is calculated.

In the following, the two sets of input polygons are referred to as boundary sets. A boundary set consists of an arbitrary number of boundaries (the individual polygons of Polygons1 and Polygons2, respectively). Each boundary may be convex or concave and self-intersecting. Internal holes may be formed by the nesting of boundaries.

The region enclosed by such a boundary set is defined by the so-called Even-Odd-Rule. Thus, it consists of all points with the following property: The line constructed by connecting the point with another reference point that lies outside of the region has an odd number of intersections with boundaries.

More descriptive, this means that a boundary defines a hole in the region enclosed by another boundary if it lies completely inside that region. Similarly, the overlapping area of two separate boundaries of one boundary set is taken as a "hole", i.e., it does not belong to the region enclosed by the two boundaries. Note that the region enclosed by a boundary does not depend on the orientation of the boundary.

A self-intersecting boundary may be split at the intersection point or it is reordered such that it touches itself at the intersection point. The resulting boundaries are treated as separate boundaries.

## Attention

The resulting polygons PolygonsDifference contain no references to the XLD contours that are possibly referenced by Polygons1 and Polygons2. Hence, operators that access the contours associated with a polygon, e.g., split_contours_xld will not work correctly.

## Parallelization

• Multithreading type: reentrant (runs in parallel with non-exclusive operators).
• Processed without parallelization.

## Parameters

Polygons1 (input_object)  xld_poly(-array) object

Polygons enclosing the first region.

Polygons2 (input_object)  xld_poly(-array) object

Polygons enclosing the second region.

PolygonsDifference (output_object)  xld_poly(-array) object

Polygons enclosing the symmetric difference.

## Result

symm_difference_closed_polygons_xld returns 2 (H_MSG_TRUE) if all parameters are correct. If necessary, an exception is raised.