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Hull of a point set

Source: R/place.R
vl_hull.Rd

vl_hull() returns the convex hull, or a concave one that follows the point set more closely — useful for outlining a cluster, or for building an exclusion zone that annotations should avoid.

Usage

vl_hull(x, y, concavity = Inf)

Arguments

x, y

Point coordinates (plain numerics, in whatever space you are working in — the hull is scale-free).

concavity

Threshold as above; larger is more convex, and Inf (the default) is the convex hull.

Value

A data frame of the hull ring's x/y, in order, ready for polygon_grob(). The ring is not closed (the last point does not repeat the first).

Details

The concave hull digs into the convex hull, replacing an edge with two edges through the nearest unused point whenever the edge is long relative to the detour. concavity is that threshold, and larger means more convex:

  • Inf (the default) is exactly the convex hull.

  • 8 follows the points loosely.

  • 4 is a good tight outline for a scattered cluster.

  • Below about 3 the boundary starts to self-intersect.

Self-intersection is inherent to the method, not a defect to be fixed here — a boundary that threads between interior points has to cross itself eventually. Inspect the result if you push it hard.

Examples

set.seed(1)
pts <- data.frame(x = runif(40), y = runif(40))
h <- vl_hull(pts$x, pts$y)
vl_scene(3, 3, dpi = 96, bg = "white") |>
  draw(polygon_grob(h$x, h$y, gp = vl_gpar(fill = "#DCE7F5", col = "steelblue"))) |>
  draw(points_grob(pts$x, pts$y, gp = vl_gpar(fill = "grey25", col = NA)))