difference_closed_contours_xld — Compute the difference of closed contours.
difference_closed_contours_xld calculates the difference
of the regions enclosed by the contours
The boundaries of this difference are returned in
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
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
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
Note that subsequent points whose row and column coordinates differ by less than 1e-06 pixels are considered as one point.
Contours enclosing the region from which the second region is subtracted.
Contours enclosing the region that is subtracted from the first region.
Contours enclosing the difference.
difference_closed_contours_xld returns TRUE if all parameters are
correct. If necessary, an exception is raised.