Filling alphahull with ggplot2 - r

Question

I used alphahull package to delineate dots on a map.
I plot the contours with a geom_segment.
My question is : how to fill the delineation given by the segment with a color ?
Here is a reproducible example :
set.seed(2)
dat <- data.frame(x = rnorm(20, 10, 5), y = rnorm(20, 20, 5), z = c(rep(1, 6), rep(2, 4)))
library(ggplot2)
library(alphahull)
alpha <- 100
alphashape1 <- ashape(dat[which(dat$z==1), c("x", "y")], alpha = alpha)
alphashape2 <- ashape(dat[which(dat$z==2), c("x", "y")], alpha = alpha)
map <- ggplot(dat, aes(x = x, y = y)) +
geom_point(data = dat, aes(x = x, y = y, colour = as.factor(dat$z))) +
geom_segment(data = data.frame(alphashape1$edges), aes(x = x1, y = y1, xend = x2, yend = y2, colour = levels(as.factor(dat$z))[1])) +
geom_segment(data = data.frame(alphashape2$edges), aes(x = x1, y = y1, xend = x2, yend = y2, colour = levels(as.factor(dat$z))[2]))
map

Answer 1

I believe this works w/o the need for graph ops:
fortify.ashape <- function(ashape_res) {
xdf <- data.frame(ashape_res$edges)
xdf <- do.call(
rbind,
lapply(1:nrow(xdf), function(i) {
rbind(
data.frame(x=xdf$x1[i], y=xdf$y1[i]),
data.frame(x=xdf$x2[i], y=xdf$y2[i])
)
})
)
xdf <- xdf[order(-1 * atan2(
xdf$y - mean(range(xdf$y)),
xdf$x - mean(range(xdf$x)))), c("x", "y")]
xdf <- rbind.data.frame(xdf[nrow(xdf),], xdf[1:(nrow(xdf)-1),])
xdf
}
alphashape1 <- ashape(dat[which(dat$z == 1), c("x", "y")], alpha = 15)
alphashape2 <- ashape(dat[which(dat$z == 2), c("x", "y")], alpha = 15)
ggplot() +
geom_point(data = dat, aes(x = x, y = y, colour = as.factor(dat$z))) +
geom_polygon(data=alphashape1, aes(x, y), fill="red", alpha=2/3) +
geom_polygon(data=alphashape2, aes(x, y), fill="blue", alpha=2/3)

Answer 2

This is because the ashape function only returns segments, not in any order.
The only way I found to reconstruct the order was by using the node information to form a graph and then find the shortest path along that graph.
A detailed example is here: https://rpubs.com/geospacedman/alphasimple - the code needs wrapping into a single function, which should be fairly easy to do. Once you have that order sorted, geom_polygon will draw it with filled shading in ggplot2.

Answer 3

Based on Spacedman's answer, I ordered separately the two sets of points and came up with this solution.
It could be optimized with a function that does it for each group automatically.
set.seed(2)
dat <- data.frame(x = rnorm(20, 10, 5), y = rnorm(20, 20, 5), z = c(rep(1, 6), rep(2, 4)))
library(ggplot2)
library(alphahull)
alpha <- 100
alphashape1 <- ashape(dat[which(dat$z==1), c("x", "y")], alpha = alpha)
alphashape2 <- ashape(dat[which(dat$z==2), c("x", "y")], alpha = alpha)
map <- ggplot(dat, aes(x = x, y = y)) +
geom_point(data = dat, aes(x = x, y = y, colour = as.factor(dat$z))) +
geom_segment(data = data.frame(alphashape1$edges), aes(x = x1, y = y1, xend = x2, yend = y2, colour = levels(as.factor(dat$z))[1])) +
geom_segment(data = data.frame(alphashape2$edges), aes(x = x1, y = y1, xend = x2, yend = y2, colour = levels(as.factor(dat$z))[2]))
map
alpha <- 15 # transparency argument
# First contour
alphashape1 <- ashape(dat[which(dat$z == 1), c("x", "y")], alpha = alpha)
alphashape1_ind <- alphashape1$edges[, c("ind1", "ind2")]
class(alphashape1_ind) = "character"
alphashape1_graph <- graph.edgelist(alphashape1_ind, directed = FALSE)
cut_graph1 <- alphashape1_graph - E(alphashape1_graph)[1] # Cut the first edge
ends1 <- names(which(degree(cut_graph1) == 1)) # Get two nodes with degree = 1
path1 <- get.shortest.paths(cut_graph1, ends1[1], ends1[2])$vpath[[1]]
path_nodes1 <- as.numeric(V(alphashape1_graph)[path1]$name)
# Second contour
alphashape2 <- ashape(dat[which(dat$z == 2), c("x", "y")], alpha = alpha)
alphashape2_ind <- alphashape2$edges[, c("ind1", "ind2")]
class(alphashape2_ind) = "character"
alphashape2_graph <- graph.edgelist(alphashape2_ind, directed = FALSE)
cut_graph2 <- alphashape2_graph - E(alphashape2_graph)[1] # Cut the first edge
ends2 <- names(which(degree(cut_graph2) == 1)) # Get two nodes with degree = 1
path2 <- get.shortest.paths(cut_graph2, ends2[1], ends2[2])$vpath[[1]]
path_nodes2 <- as.numeric(V(alphashape2_graph)[path2]$name)
# Updating of previous plot (see question)
map +
geom_polygon(data = dat[which(dat$z == 1), c("x", "y")][path_nodes1, ], aes(x = x, y = y),
fill = "red", colour = "red", size = 0.5, alpha = 0.3) +
geom_polygon(data = dat[which(dat$z == 2), c("x", "y")][path_nodes2, ],
aes(x = x, y = y), colour = "blue", fill = "blue", size = 0.5, alpha = 0.3)