Snippet to draw an alpha shape using ggplot2

AlphaShape_ggplot2.R

# Script to plot an alpha shape on ggplot2 from a given sets of points
# Inspired by: https://rpubs.com/geospacedman/alphasimple

# Packages --------------------------------------------------------------------
library(ggplot2)
library(dplyr)
library(alphahull)
library(igraph)


# Data Generation -------------------------------------------------------------
set.seed(1001)
ex = data.frame(x = rnorm(100), y = rnorm(100))


# Computation And Order Alpha Shape -------------------------------------------
ex_ashape = ashape(ex, alpha = 1.5)

# Take the coordinates of points on the edges of the alpha shape
ex_mat = ex_ashape$edges[, c("ind1", "ind2")]

# Convert 'numeric' matrix to 'character' matrix, to avoid wrong node orders 
# (Otherwise 'graph.edgelist()' interprets number as order of nodes)
class(ex_mat) = "character"

# Make the graph of points
ex_graph = graph.edgelist(ex_mat, directed = F)

# Verify its a cyclic graph
plot(ex_graph)

# Cut open the graph to walk through it in order
cut_graph = ex_graph - E(ex_graph)[1]  # Cut the first edge

ends = names(which(degree(cut_graph) == 1))   # Get two nodes with degree = 1

# Compute a path
path = get.shortest.paths(cut_graph, ends[1], ends[2])$vpath[[1]]

# Get node names (= row numbers in original data.frame)
path_nodes = as.numeric(V(ex_graph)[path]$name)


# General plot ----------------------------------------------------------------
p_no_ashape = ggplot(ex, aes(x = x, y = y)) +
  geom_point()

p_no_ashape

p_ashape = p_no_ashape +
  geom_polygon(data = ex[path_nodes, ], fill = "purple", alpha = 0.5)

p_ashape