Buffer a polygon outward

Grows a ring by width in every direction — the exclusion zone around a region, the halo around a cluster outline, the standoff a leader line should keep.

Usage

vl_buffer(x, y, width, arc = 6)

Arguments

x, y

The ring's coordinates, in order and not closed.

width

Distance to grow by, in the same units as x/y.

arc

Points per quarter-turn on rounded corners. More is smoother.

Value

A data frame of the buffered ring's x/y.

Details

Corners are rounded, which is what a standoff usually wants and what keeps the result well-defined at sharp angles (a mitred corner runs away to infinity as the angle closes).

On concave rings: the offset boundary of a concave polygon can cross itself where the buffer is wider than a local feature. This function offsets the ring and does not repair that, because repairing it is a boolean union — a Phase 12 operation. Buffering a convex ring, or buffering by less than the narrowest feature, is always well-behaved. Passing the result through vl_hull() is a crude but effective fix when it is not.

Examples

tri <- data.frame(x = c(0.3, 0.7, 0.5), y = c(0.3, 0.35, 0.7))
buf <- vl_buffer(tri$x, tri$y, 0.08)
vl_scene(3, 3, dpi = 96, bg = "white") |>
  draw(polygon_grob(buf$x, buf$y, gp = vl_gpar(fill = "#F3E0DA", col = NA))) |>
  draw(polygon_grob(tri$x, tri$y, gp = vl_gpar(fill = "tomato", col = NA)))