Daisychain connect multiple polygons by single nearest points - r

Question

I'm looking to create a masking polygon for multiple polygons.
It's easy enough for a single polygon:
How to apply a polygon mask layer in ggplot
but much trickier for multiple:
https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/definitions/definitions.html
I feel like I'm fairly close but I need a method of drawing a line between a point for the closest polygon, i.e. I'd like to try and connect nearest islands with a line.
While also connecting those polygons to an outer polygon box but only with a single line:
library(tidyverse)
library(sf)
library(albersusa)
usa <- usa_sf()
HI <- st_coordinates(usa %>%
filter(name %in% c("Hawaii"))) %>%
as.data.frame() %>%
select(X, Y)
rec_box <-
data.frame(
X = c(-108,-108,-101,-101,-108),
Y = c(24, 28, 28, 24, 24)
)
mask <- rbind(HI, rec_box)
eg <- st_as_sf(data.frame(mask), coords = c("X", "Y"))
poly <- st_convex_hull(eg)
ggplot() +
geom_sf(data = poly) +
geom_density2d_filled(data = HI, aes(x = X, y = Y)) +
geom_polygon(data = mask,
aes(x = X, y = Y),
color = "black",
fill = "white")

Answer 1

Edit:
For only a mask around the islands:
library(tidyverse)
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1
library(albersusa)
#library(concaveman)
#library(nngeo)
usa <- usa_sf()
# keep only the geometry from Hawaii (not the data)
HI <- usa %>%
filter(name %in% c("Hawaii")) %>%
st_cast('POLYGON') %>%
st_geometry() %>%
st_as_sf()
# HI as dataframe as your example
HI2 <- st_coordinates(usa %>%
filter(name %in% c("Hawaii"))) %>%
as.data.frame() %>%
select(X, Y)
# you may need a larger box. This uses sf:st_bbox to get exact
# rectangle of the polygons. adjust accordingly
hi_box <- st_bbox(HI) %>% st_as_sfc() %>% st_as_sf()
# find the difference betwee HI & hi_box
hi_mask <- st_difference(hi_box, st_union(HI))
ggplot() +
geom_density_2d_filled(data = HI2, aes(x = X, y = Y)) +
geom_sf(data = hi_mask, fill = 'black')
Created on 2021-04-04 by the reprex package (v0.3.0)
If you're happy with the HI mask created above, you can connect it to your defined bounding box using a single line with nngeo::st_connect().
This should find the shortest connecting line between the two. If you prefer the connecting line to be at a specific point (southwest corner in your example above), supply it as an sf object to the st_connect() function instead of the entire box.
library(tidyverse)
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1
library(albersusa)
library(concaveman)
library(nngeo)
usa <- usa_sf()
# Hawaii, cast to POLYGON rather than the original MULTIPOLYGON
HI <- (usa %>%
filter(name %in% c("Hawaii"))) %>%
st_cast('POLYGON')
connected_HI <- concaveman(HI)
# rec box as an sf object with same crs as HI
rec_box <-
data.frame(X = c(-108, -108, -101, -101, -108),
Y = c(24, 28, 28, 24, 24)) %>%
st_as_sf(coords = c('X', 'Y'
)) %>%
st_set_crs(st_crs(HI))
# connecting HI mask you made to your rec_box
connected_to_rec_box <- nngeo::st_connect(st_zm(connected_HI), rec_box)
ggplot() +
geom_sf(data = connected_HI, fill = 'turquoise', alpha = .4) +
geom_sf(data = connected_to_rec_box, color = 'red')
Created on 2021-04-04 by the reprex package (v0.3.0)