Get hole points for constrained triangulation

pool_holes() and hole_points() are the same function: pool_holes() is the name that matches the other pool_*() accessors, and hole_points() is kept for existing code.

Usage

pool_holes(x, convention = c("sf", "ogc"))

hole_points(x, convention = c("sf", "ogc"))

Arguments

x: A wkpool (ideally after merge_coincident)
convention: Which winding convention to use when the pool has no path provenance: "sf" (default) or "ogc"

Value

A matrix of hole points, one point inside each hole cycle, or NULL if no holes

Details

For use with RTriangle::triangulate(). Each row is a point inside a hole, which tells the triangulator to exclude that region.

The point is found on a horizontal line across the hole: the middle of the widest stretch of that line that starts at the hole's boundary and crosses no segment of the pool. It is inside the hole even when the hole is not convex, where the centroid of the ring's vertices can fall outside it (and a triangulator would then erase the polygon around the hole instead), and it is clear of any island in the hole.

A hole that is exactly filled by another polygon in the pool (an enclave in a coverage, such as a lake island that is its own feature) is not a hole of the pool and is not returned.

When the pool carries path provenance, holes are identified structurally (non-first rings of their part) and convention is not consulted.

Examples

polygons_with_holes <- wk::as_wkb(c(
  paste0(
    "MULTIPOLYGON (((0 0, 0 1, 0.75 1, 1 0.8, 0.5 0.7, ",
    "0.8 0.6, 0.69 0, 0 0), (0.2 0.2, 0.5 0.2, ",
    "0.5 0.4, 0.3 0.6, 0.2 0.4, 0.2 0.2)))"),
  "MULTIPOLYGON (((0.69 0, 0.8 0.6, 1.1 0.63, 1.23 0.3, 0.69 0)))"
))

x <- establish_topology(polygons_with_holes)
merged <- merge_coincident(x)
pool_holes(merged)
#>          x   y
#> [1,] 0.325 0.5
if (requireNamespace("RTriangle", quietly = TRUE)) {
  pslg <- as_pslg(merged)
  holes <- pool_holes(merged)
  tri <- RTriangle::triangulate(
    RTriangle::pslg(P = pslg$P, S = pslg$S, H = holes)
  )
}