# difference_closed_contours_xld (Operator)

## Name

`difference_closed_contours_xld` — Compute the difference of closed contours.

## Signature

`difference_closed_contours_xld(Contours, Sub : ContoursDifference : : )`

## Description

The operator `difference_closed_contours_xld` calculates the difference of the regions enclosed by the contours `Contours` and `Sub`. The boundaries of this difference are returned in `ContoursDifference`.

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

Internally, first, the regions enclosed by each set of contours (`Contours` and `Sub`, respectively) are determined as described below. Then, the difference of these two regions is calculated.

In the following, the two sets of input contours are referred to as boundary sets. A boundary set consists of an arbitrary number of boundaries (the individual contours of `Contours` and `Sub`, 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.

The resulting boundaries `ContoursDifference` are automatically classified into boundaries that enclose regions and boundaries that enclose holes. This information is stored in the global contour attribute 'is_hole'. For boundaries that enclose holes, the global contour attribute 'is_hole' is set to 1. Otherwise, it is set to 0. For further information about global contour attributes see `get_contour_global_attrib_xld`.

Note that subsequent points whose row and column coordinates differ by less than 1e-06 pixels are considered as one point.

## Execution Information

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

## Parameters

`Contours` (input_object)  xld_cont(-array) `→` object

Contours enclosing the region from which the second region is subtracted.

`Sub` (input_object)  xld_cont(-array) `→` object

Contours enclosing the region that is subtracted from the first region.

`ContoursDifference` (output_object)  xld_cont(-array) `→` object

Contours enclosing the difference.

## Result

`difference_closed_contours_xld` returns TRUE if all parameters are correct. If necessary, an exception is raised.

## Possible Predecessors

`gen_contour_region_xld`, `zero_crossing_sub_pix`, `threshold_sub_pix`

## Alternatives

`difference`

`intersection_closed_contours_xld`, `symm_difference_closed_contours_xld`, `union2_closed_contours_xld`, `difference_closed_polygons_xld`