chull {base}R Documentation

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.

Details

xy.coords is used to interpret the specification of the points. The algorithm is that given by Eddy (1977).

‘Peeling’ as used in the S function chull can be implemented by calling chull recursively.

Value

An integer vector giving the indices of the points lying on the convex hull, in clockwise order.

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

require(graphics)
X <- matrix(rnorm(2000), ncol=2)
plot(X, cex=0.5)
hpts <- chull(X)
hpts <- c(hpts, hpts[1])
lines(X[hpts, ])
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