Find the largest empty rectangle in a scene — vl_empty

Answers "where is there room?" — for a legend, an annotation, a watermark, or anything else that has to go somewhere the marks are not.

Usage

vl_empty_region(
  scene,
  avoid = NULL,
  within = NULL,
  grid = 200,
  unit = c("npc", "mm")
)

Arguments

scene: A vl_scene() (or anything with an as_vellum_scene() method).
avoid: Names of the grobs to treat as obstacles. NULL (default) takes every node that is not a label.
within: Region to search, as c(x0, y0, x1, y1) in npc of the page. Defaults to the whole page.
grid: Grid resolution (cells across the longer side).
unit: Unit of the returned rectangle: "npc" (default) or "mm".

Value

A named numeric c(x0, y0, x1, y1), in unit, using the usual y-grows-up convention. All zeros if there is no free space.

Details

Occupancy is rasterised onto a grid and the answer is exact on that grid. The approximation is deliberate: the exact maximal empty rectangle over n boxes is superquadratic, and nothing is placed to sub-pixel tolerance. Raise grid to trade time for precision. Boxes are rounded outward, so the result is conservative — it will never claim space that is in fact occupied.

Examples

set.seed(1)
s <- vl_scene(4, 3, dpi = 96, bg = "white") |>
  draw(points_grob(runif(40) * 0.5, runif(40),
                   gp = vl_gpar(fill = "steelblue", col = NA)))
vl_empty_region(s)
#>   x0   y0   x1   y1 
#> 0.52 0.00 1.00 1.00