rgeos::gBuffer shrink without loss of points

I need to be able to shrink a polygon of lat/lon data without loss of points; more the point, I need the points to be effectively "smooshed" in the correct direction. Typically, gBuffer works fine, but there is no assurance on the number of points nor the relative spacing of them. Ultimately, there are properties with each point that I need to preserve, and splines, smoothing, and other "nice efficiencies" with gBuffer and polygon growing/shrinking do not allow me to preserve those properties with sufficient confidence of a 1-to-1 mapping.

Example:

library(rgeos)   # gBuffer

dat <- structure(list(x = c(6, 5.98, 5.94, 5.86, 5.75, 5.62, 5.47, 5.31, 5.13, -4.87, -5.04, -5.22, -5.39, -5.55, -5.69, -5.81, -5.9, -5.96, -6, -6, -6, -5.96, -5.9, -5.81, -5.69, -5.55, -5.39, -5.22, -5.04, -3.04, -2.87, -2.69, -2.53, -2.38, -2.25, -2.14, -2.06, -2.02, -2, -2, -1.96, -1.9, -1.81, -1.69, -1.55, -1.39, -1.22, -1.04, -0.87, 1.13, 1.31, 1.47, 1.62, 1.75, 1.86, 1.94, 1.98, 2, 2, 2, 2.04, 2.1, 2.19, 2.31, 2.45, 2.61, 2.78, 2.96, 4.96, 5.13, 5.31, 5.47, 5.62, 5.75, 5.86, 5.94, 5.98, 6), y = c(5, 5.18, 5.35, 5.51, 5.66, 5.78, 5.88, 5.95, 5.99, 5.99, 6, 5.97, 5.92, 5.83, 5.72, 5.59, 5.43, 5.27, 5.09, -4.91, -5.09, -5.27, -5.43, -5.59, -5.72, -5.83, -5.92, -5.97, -6, -6, -5.99, -5.95, -5.88, -5.78, -5.66, -5.51, -5.35, -5.18, -5, -1.91, -1.73, -1.57, -1.41, -1.28, -1.17, -1.08, -1.03, -1, -1.01, -1.01, -1.05, -1.12, -1.22, -1.34, -1.49, -1.65, -1.82, -2, -4.91, -5.09, -5.27, -5.43, -5.59, -5.72, -5.83, -5.92, -5.97, -6, -6, -5.99, -5.95, -5.88, -5.78, -5.66, -5.51, -5.35, -5.18, 5)), row.names = c(NA, -78L), class = "data.frame")

# "shrink-wrap"
sp <- sp::SpatialPolygons(list(sp::Polygons(list(sp::Polygon( as.matrix(dat) )), "dat")))
sp2 <- gBuffer(sp, width = -0.5)
dat2 <- as.data.frame(sp2@polygons[[1]]@Polygons[[1]]@coords)

c(nrow(dat), nrow(dat2))
# [1] 78 97

Immediately we see the change in the number of points. I recognize that most of the time, this is a desired trait of gBuffer, so perhaps rgeos is not the best tool for this transformation.

library(ggplot2) # just for vis here
ggplot(dat, aes(x, y)) +
  geom_path() + geom_point() +
  geom_path(data = dat2, color = "red") + geom_point(data = dat2, color = "red")  

This image has the effect on the overall shape I want, but it has increased the number of points, which means that I can no longer rely on a 1-to-1 relationship to the original points.

In general, the polygons are not symmetric, and many have in-croppings like this where many methods of "pulling" points in a particular direction will be biased or in the wrong direction.

I can find no option in gBuffer nor other functions in rgeos to be able to preserve the number and basic spatial-relationship of the points. I don't need "perfect" shrinking, if that changes things, but it should not deviate significantly.

This might work if you're satisfied with how you're currently shrinking the polygons. It builds on that to get a 1:1 point mapping from the old (large) points to the new (smaller) polygon.

library(rgeos)   # gBuffer
library(sf)
library(tidyverse)
dat <- structure(list(x = c(6, 5.98, 5.94, 5.86, 5.75, 5.62, 5.47, 5.31, 5.13, -4.87, -5.04, -5.22, -5.39, -5.55, -5.69, -5.81, -5.9, -5.96, -6, -6, -6, -5.96, -5.9, -5.81, -5.69, -5.55, -5.39, -5.22, -5.04, -3.04, -2.87, -2.69, -2.53, -2.38, -2.25, -2.14, -2.06, -2.02, -2, -2, -1.96, -1.9, -1.81, -1.69, -1.55, -1.39, -1.22, -1.04, -0.87, 1.13, 1.31, 1.47, 1.62, 1.75, 1.86, 1.94, 1.98, 2, 2, 2, 2.04, 2.1, 2.19, 2.31, 2.45, 2.61, 2.78, 2.96, 4.96, 5.13, 5.31, 5.47, 5.62, 5.75, 5.86, 5.94, 5.98, 6), y = c(5, 5.18, 5.35, 5.51, 5.66, 5.78, 5.88, 5.95, 5.99, 5.99, 6, 5.97, 5.92, 5.83, 5.72, 5.59, 5.43, 5.27, 5.09, -4.91, -5.09, -5.27, -5.43, -5.59, -5.72, -5.83, -5.92, -5.97, -6, -6, -5.99, -5.95, -5.88, -5.78, -5.66, -5.51, -5.35, -5.18, -5, -1.91, -1.73, -1.57, -1.41, -1.28, -1.17, -1.08, -1.03, -1, -1.01, -1.01, -1.05, -1.12, -1.22, -1.34, -1.49, -1.65, -1.82, -2, -4.91, -5.09, -5.27, -5.43, -5.59, -5.72, -5.83, -5.92, -5.97, -6, -6, -5.99, -5.95, -5.88, -5.78, -5.66, -5.51, -5.35, -5.18, 5)), row.names = c(NA, -78L), class = "data.frame")

# "shrink-wrap"
sp <- sp::SpatialPolygons(list(sp::Polygons(list(sp::Polygon( as.matrix(dat) )), "dat")))
sp2 <- gBuffer(sp, width = -0.5)
dat2 <- as.data.frame(sp2@polygons[[1]]@Polygons[[1]]@coords)

## New methods begin here
# change objects to type `sf`
sp_sf <- st_as_sf(sp)
sp2_sf <- st_as_sf(sp2)

dat_sf <- dat %>% st_as_sf(coords = c('x', 'y'))
dat2_sf <- dat2 %>% st_as_sf(coords = c('x', 'y'))

# The plot so far, saved for building on further down
p <- ggplot() + 
  geom_sf(data = sp_sf, color = 'blue', fill = NA) + 
  geom_sf(data = dat_sf, color = 'blue') +
  geom_sf(data = sp2_sf, color = 'red', fill = NA) + 
  geom_sf(data = dat2_sf, color = 'red')




# Using st_nearest_points original points to new small polygon
#  results in (perpendicular?) lines from old points to new small polygon
near_lines <- st_nearest_points(dat_sf, sp2_sf)

#plotted together:
p + geom_sf(data = near_lines, color = 'black')

## Zooming in on a problem area
p + geom_sf(data = near_lines, color = 'black') + 
  coord_sf(xlim = c(-3, 0), ylim = c(-2,0))

# Get only 1:1 points for shrunken polygon
# a small buffer had to be added, as some points were not showing up
# you may need to adjust the buffer, depending on your data & projection
new_points <- st_intersection(st_buffer(near_lines, .001), sp2_sf)

# All together now:
p + geom_sf(data = near_lines, color = 'black') + 
  geom_sf(data = new_points, color = 'green', size = 4) +
  coord_sf(xlim = c(-3, 0), ylim = c(-2,0))

^{Created on 2020-12-20 by the reprex package (v0.3.0)}