Classify cycles as outer rings or holes based on winding

Usage

classify_cycles(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"

sf: negative area = outer, positive = hole
ogc: positive area = outer, negative = hole

Value

A data frame with cycle index, signed area, and type (outer/hole); when path provenance is present, also .path and .feature

Details

When the pool carries path provenance, ring roles are structural: the first ring of each part is the exterior and later rings are its holes, regardless of winding, and convention is not consulted. The signed area still reports the observed winding. Pools without provenance are classified by winding under the requested convention.

Examples

x <- 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)))"
))

pool <- establish_topology(x)
merged <- merge_coincident(pool)
classify_cycles(merged)
#>   cycle     area  type .path .feature
#> 1     1 -0.76200 outer     1        1
#> 2     2  0.09000  hole     2        1
#> 3     3 -0.19695 outer     3        2