grDevices (version 3.4.1)

chull: Compute Convex Hull of a Set of Points

Description

Computes the subset of points which lie on the convex hull of the set of points specified.

Usage

chull(x, y = NULL)

Arguments

x, y

coordinate vectors of points. This can be specified as two vectors x and y, a 2-column matrix x, a list x with two components, etc, see xy.coords.

Value

An integer vector giving the indices of the unique points lying on the convex hull, in clockwise order. (The first will be returned for duplicate points.)

Details

xy.coords is used to interpret the specification of the points. Infinite, missing and NaN values are not allowed.

The algorithm is that given by Eddy (1977).

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

Eddy, W. F. (1977) A new convex hull algorithm for planar sets. ACM Transactions on Mathematical Software, 3, 398--403.

Eddy, W. F. (1977) Algorithm 523. CONVEX, A new convex hull algorithm for planar sets[Z]. ACM Transactions on Mathematical Software, 3, 411--412.

See Also

xy.coords, polygon

Examples

X <- matrix(stats::rnorm(2000), ncol = 2)
chull(X)

  # Example usage from graphics package
  plot(X, cex = 0.5)
  hpts <- chull(X)
  hpts <- c(hpts, hpts[1])
  lines(X[hpts, ])