# How to find the border points of a particular shape

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.

## Answers

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, ])

These functions (packages) seem adequate:

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