fmesher (version 0.3.0)

fm_nonconvex_hull: Compute an extension of a spatial object

Description

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 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 matrix
fm_nonconvex_hull(x, ...)
# S3 method for sf
fm_nonconvex_hull(x, ...)
# S3 method for Spatial
fm_nonconvex_hull(x, ...)
# S3 method for sfg
fm_nonconvex_hull(x, ...)

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.

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

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.

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.

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)
}