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 tosf::st_simplify()
. The default ispmin(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 byconvex + concave
followed by erosion byconcave
. This is equivalent to dilation byconvex
followed by closing (dilation + erosion) byconcave
.
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.