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Compute an extension of a spatial object

Source: R/nonconvex_hull.R
fm_nonconvex_hull.Rd

Constructs a potentially nonconvex extension of a spatial object by performing dilation by convex + concave followed by erosion by concave. This is equivalent to dilation by convex followed by closing (dilation + erosion) by concave.

Usage

fm_nonconvex_hull(x, ...)

# S3 method for class 'sfc'
fm_nonconvex_hull(
  x,
  convex = -0.15,
  concave = convex,
  preserveTopology = TRUE,
  dTolerance = NULL,
  crs = fm_crs(x),
  ...
)

fm_extensions(x, convex = -0.15, concave = convex, dTolerance = NULL, ...)

# S3 method for class 'matrix'
fm_nonconvex_hull(x, ...)

# S3 method for class 'sf'
fm_nonconvex_hull(x, ...)

# S3 method for class 'Spatial'
fm_nonconvex_hull(x, ...)

# S3 method for class 'sfg'
fm_nonconvex_hull(x, ...)

Arguments

x

A spatial object

...

Arguments passed on to the fm_nonconvex_hull() sub-methods

convex

numeric vector; How much to extend

concave

numeric vector; The minimum allowed reentrant curvature. Default equal to convex

preserveTopology

logical; argument to sf::st_simplify()

dTolerance

If not zero, controls the dTolerance argument to sf::st_simplify(). The default is pmin(convex, concave) / 40, chosen to give approximately 4 or more subsegments per circular quadrant.

crs

Options crs object for the resulting polygon

Value

fm_nonconvex_hull() returns an extended object as an sfc polygon object (regardless of the x class).

fm_extensions() returns a list of sfc objects.

Details

Morphological dilation by convex, followed by closing by concave, with minimum concave curvature radius concave. If the dilated set has no gaps of width between $$2 \textrm{convex} (\sqrt{1+2\textrm{concave}/\textrm{convex}} - 1) $$ and \(2\textrm{concave}\), then the minimum convex curvature radius is convex.

The implementation is based on the identity $$\textrm{dilation}(a) \& \textrm{closing}(b) = \textrm{dilation}(a+b) \& \textrm{erosion}(b)$$ where all operations are with respect to disks with the specified radii.

When convex, concave, or dTolerance are negative, fm_diameter * abs(...) is used instead.

Differs from sf::st_buffer(x, convex) followed by sf::st_concave_hull() (available from GEOS 3.11) in how the amount of allowed concavity is controlled.

Functions

  • fm_nonconvex_hull(): Basic nonconvex hull method.

  • fm_extensions(): Constructs a potentially nonconvex extension of a spatial object by performing dilation by convex + concave followed by erosion by concave. This is equivalent to dilation by convex followed by closing (dilation + erosion) by concave.

INLA compatibility

For mesh and curve creation, the fm_rcdt_2d_inla(), fm_mesh_2d_inla(), and fm_nonconvex_hull_inla() methods will keep the interface syntax used by INLA::inla.mesh.create(), INLA::inla.mesh.2d(), and INLA::inla.nonconvex.hull() functions, respectively, whereas the fm_rcdt_2d(), fm_mesh_2d(), and fm_nonconvex_hull() interfaces may be different, and potentially change in the future.

References

Gonzalez and Woods (1992), Digital Image Processing

See also

fm_nonconvex_hull_inla()

Examples

inp <- matrix(rnorm(20), 10, 2)
out <- fm_nonconvex_hull(inp, convex = 1)
plot(out)
points(inp, pch = 20)

if (TRUE) {
  inp <- sf::st_as_sf(as.data.frame(matrix(1:6, 3, 2)), coords = 1:2)
  bnd <- fm_extensions(inp, convex = c(0.75, 2))
  plot(fm_mesh_2d(boundary = bnd, max.edge = c(0.25, 1)), asp = 1)
}