Title:	Extract Centerline from Closed Polygons
Version:	0.2.4
Maintainer:	Anatoly Tsyplenkov <atsyplenkov@fastmail.com>
Description:	Generates skeletons of closed 2D polygons using Voronoi diagrams. It provides methods for 'sf', 'terra', and 'geos' objects to compute polygon centerlines based on the generated skeletons. Voronoi, G. (1908) <doi:10.1515/crll.1908.134.198>.
License:	MIT + file LICENSE
URL:	https://centerline.anatolii.nz, https://github.com/atsyplenkov/centerline
BugReports:	https://github.com/atsyplenkov/centerline/issues
Depends:	R (≥ 4.1.0)
Imports:	wk (≥ 0.9), sf (≥ 1.0), geos (≥ 0.2.4), sfnetworks (≥ 0.6), checkmate
Suggests:	smoothr (≥ 1.0.0), testthat (≥ 3.0.0), geomtextpath (≥ 0.1.5), terra (≥ 1.7), igraph (≥ 2.0.0), ggplot2 (≥ 3.1.0), raybevel (≥ 0.2.0)
Config/testthat/edition:	3
Encoding:	UTF-8
RoxygenNote:	7.3.2
Language:	en-US
Config/Needs/website:	mapgl, rmapshaper, bench, htmlwidgets
NeedsCompilation:	no
Packaged:	2025-09-09 02:54:33 UTC; TsyplenkovA
Author:	Anatoly Tsyplenkov [aut, cre, cph]
Repository:	CRAN
Date/Publication:	2025-09-09 07:20:15 UTC

Find the shortest path between start and end points within a polygon

Description

Find the shortest path between start and end points within a polygon

Usage

cnt_path(skeleton, start_point, end_point)

Arguments

Details

The following function uses the sfnetworks::st_network_paths() approach to connect start_point with end_point by using the skeleton of a closed polygon as potential routes.

It is important to note that multiple starting points are permissible, but there can only be one ending point. Should there be two or more ending points, the algorithm will return an error.

Neither starting nor ending points are required to be located on the edges of a polygon (i.e., snapped to the boundary); they can be positioned wherever possible inside the polygon.

The algorithm identifies the closest nodes of the polygon's skeleton to the starting and ending points and then connects them using the shortest path possible along the skeleton. Therefore, if more precise placement of start and end points is necessary, consider executing the cnt_skeleton() function with the keep = 1 option. In doing so, the resulting skeleton may be more detailed, increasing the likelihood that the starting and ending points are already situated on the skeleton paths.

Value

a list of sf, sfc, SpatVector or geos_geometry class objects of a LINESTRING geometry

Examples

library(sf)
library(geos)
# Load Polygon and points data
polygon <-
  sf::st_read(
    system.file("extdata/example.gpkg", package = "centerline"),
    layer = "polygon",
    quiet = TRUE
  ) |>
  geos::as_geos_geometry()

points <-
  sf::st_read(
    system.file("extdata/example.gpkg", package = "centerline"),
    layer = "polygon_points",
    quiet = TRUE
  ) |>
  geos::as_geos_geometry()

# Find polygon's skeleton
pol_skeleton <- cnt_skeleton(polygon)

# Connect points
pol_path <-
  cnt_path(
    skeleton = pol_skeleton,
    start_point = points[2],
    end_point = points[1]
  )

# Plot
plot(polygon)
plot(pol_skeleton, col = "blue", add = TRUE)
plot(points[1:2], col = "red", add = TRUE)
plot(pol_path, lwd = 3, add = TRUE)

Guess polygon's centerline

Description

This function, as follows from the title, tries to guess the polygon centerline by connecting the most distant points from each other. First, it finds the point most distant from the polygon's centroid, then it searches for a second point, which is most distant from the first. The line connecting these two points will be the desired centerline.

Usage

cnt_path_guess(input, skeleton = NULL, return_geos = FALSE, ...)

Arguments

Value

An sf, sfc or SpatVector class object of a LINESTRING geometry

Examples

library(sf)
library(geos)
lake <-
  sf::st_read(
    system.file("extdata/example.gpkg", package = "centerline"),
    layer = "lake",
    quiet = TRUE
  ) |>
  geos::as_geos_geometry()
# Find lake's centerline
lake_centerline <- cnt_path_guess(input = lake, keep = 1)
# Plot
plot(lake)
plot(lake_centerline, col = "firebrick", lwd = 2, add = TRUE)

Create a skeleton of a closed polygon object

Description

This function generates skeletons of closed polygon objects.

Usage

cnt_skeleton(input, keep = 0.5, method = "voronoi")

Arguments

Details

Polygon simplification/densification

• If keep = 1, no transformation will occur. The function will use the original geometry to find the skeleton.

• If the keep parameter is below 1, then the geos::geos_simplify() function will be used. So the original input geometry would be simplified, and the resulting skeleton will be cleaner but maybe more edgy. The current realisation of simplification is similar (but not identical) to rmapshaper::ms_simplify() one with Douglas-Peuker algorithm. However, due to geos superpower, it performs several times faster. If you find that the built-in simplification algorithm performs poorly, try rmapshaper::ms_simplify() first and then find the polygon skeleton with keep = 1, i.e. cnt_skeleton(rmapshaper::ms_simplify(polygon_sf), keep = 1)

• If the keep is above 1, then the densification algorithm is applied using the geos::geos_densify() function. This may produce a very large object if keep is set more than 2. However, the resulting skeleton would potentially be more accurate.

Skeleton method

• If method = "voronoi" (default), the skeleton will be generated using the geos::geos_voronoi_edges() function. This is application of the Voronoi diagram algorithm (Voronoi, 1908). A Voronoi diagram partitions space into regions based on the distance to the polygon's vertices. The edges of these cells form a network of lines (skeletons) that represent the structure of the polygon while preserving its overall shape.

• If method = "straight", the skeleton will be generated using the raybevel::skeletonize() function. See https://www.tylermw.com/posts/rayverse/raybevel-introduction.html

Value

a sf, sfc, SpatVector or geos_geometry class object of a MULTILINESTRING geometry

References

Voronoi, G. (1908). Nouvelles applications des paramètres continus à la théorie des formes quadratiques. Journal für die reine und angewandte Mathematik, 134, 198-287. doi:10.1515/crll.1908.134.198

Examples

library(sf)

polygon <-
  sf::st_read(system.file("extdata/example.gpkg", package = "centerline"),
    layer = "polygon",
    quiet = TRUE
  )

plot(polygon)

pol_skeleton <- cnt_skeleton(polygon)

plot(pol_skeleton)

Plot centerline with ggplot2

Description

Binding for ggplot2::geom_sf(), therefore it supports only sf objects.

Usage

geom_cnt(
  mapping = ggplot2::aes(),
  data = NULL,
  stat = "sf",
  position = "identity",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE,
  keep = 0.5,
  method = c("voronoi", "straight"),
  simplify = TRUE,
  ...
)

Arguments

Value

A Layer ggproto object that can be added to a plot.

CRS

coord_sf() ensures that all layers use a common CRS. You can either specify it using the crs param, or coord_sf() will take it from the first layer that defines a CRS.

Combining sf layers and regular geoms

Most regular geoms, such as geom_point(), geom_path(), geom_text(), geom_polygon() etc. will work fine with coord_sf(). However when using these geoms, two problems arise. First, what CRS should be used for the x and y coordinates used by these non-sf geoms? The CRS applied to non-sf geoms is set by the default_crs parameter, and it defaults to NULL, which means positions for non-sf geoms are interpreted as projected coordinates in the coordinate system set by the crs parameter. This setting allows you complete control over where exactly items are placed on the plot canvas, but it may require some understanding of how projections work and how to generate data in projected coordinates. As an alternative, you can set default_crs = sf::st_crs(4326), the World Geodetic System 1984 (WGS84). This means that x and y positions are interpreted as longitude and latitude, respectively. You can also specify any other valid CRS as the default CRS for non-sf geoms.

The second problem that arises for non-sf geoms is how straight lines should be interpreted in projected space when default_crs is not set to NULL. The approach coord_sf() takes is to break straight lines into small pieces (i.e., segmentize them) and then transform the pieces into projected coordinates. For the default setting where x and y are interpreted as longitude and latitude, this approach means that horizontal lines follow the parallels and vertical lines follow the meridians. If you need a different approach to handling straight lines, then you should manually segmentize and project coordinates and generate the plot in projected coordinates.

See Also

geom_cnt_text(), geom_cnt_label(), ggplot2::geom_sf()

Examples

library(sf)
library(ggplot2)

lake <-
  sf::st_read(
    system.file("extdata/example.gpkg", package = "centerline"),
    layer = "lake",
    quiet = TRUE
  )

ggplot() +
  geom_sf(data = lake) +
  geom_cnt(
    data = lake,
    keep = 1,
    simplify = TRUE
  ) +
  theme_void()

Plot label or text on centerline with ggplot2

Description

Binding for geomtextpath::geom_textsf() and geomtextpath::geom_labelsf()

Usage

geom_cnt_text(
  mapping = ggplot2::aes(),
  data = NULL,
  stat = "sf",
  position = "identity",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE,
  keep = 0.5,
  method = c("voronoi", "straight"),
  simplify = TRUE,
  ...
)

geom_cnt_label(
  mapping = ggplot2::aes(),
  data = NULL,
  stat = "sf",
  position = "identity",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE,
  keep = 0.5,
  method = c("voronoi", "straight"),
  simplify = TRUE,
  ...
)

Arguments

Details

Aesthetics

geom_cnt_text() understands the following aesthetics:

• x

• y

• label

• alpha

• angle

• colour

• family

• fontface

• group

• hjust

• linecolour

• lineheight

• linetype

• linewidth

• size

• spacing

• textcolour

• vjust

In addition to aforementioned aesthetics, geom_cnt_label() also understands:

• boxcolour

• boxlinetype

• boxlinewidth

• fill

See Also

geom_cnt(), geomtextpath::geom_textsf(), geomtextpath::geom_labelsf(), ggplot2::geom_sf()

Examples

library(sf)
library(ggplot2)

lake <-
  sf::st_read(
    system.file("extdata/example.gpkg", package = "centerline"),
    layer = "lake",
    quiet = TRUE
  )

# Plot centerline and lake name as text
ggplot() +
  geom_sf(data = lake) +
  geom_cnt_text(
    data = lake,
    aes(label = "Lake Ohau"),
    size = 8,
    simplify = TRUE
  ) +
theme_void()

# Plot lake name as label
ggplot() +
  geom_sf(data = lake) +
  geom_cnt_label(
    data = lake,
    aes(label = "Lake Ohau"),
    linecolor = NA, # disable line drawing
    size = 10,
    method = "s",
    simplify = TRUE
  ) +
theme_void()