How to find the border points of a particular shape

Question

Suppose I have a irregular, random, shape generated from a dataset. How do I find points that are situated on the shape's border?

I am using R. Are there any packages in R for this purpose? For simplicity, just assume that I have a 2d dataset of points.

Answer 1

You are looking for the chull() function to compute the convex hull of a 2D object, in package grdevices.

Here is the example from the online help:

require(stats)
X <- matrix(rnorm(2000), ncol = 2)
chull(X)
## Not run: 
# Example usage from graphics package
plot(X, cex = 0.5)
hpts <- chull(X)
hpts <- c(hpts, hpts[1])
lines(X[hpts, ])

Answer 2

These functions (packages) seem adequate:

• ahull (alphahull)
• convex.hull (tripack)
• chull (grDevices)
• in.chull (sgeostat)
• convhulln (geometry)
• convexhull.xy (spatstat)
• calcConvexHull (PBSmapping)