Return the convex hull of a triangulation object

Given a triangulation tri.obj of \(n\) points in the plane, this subroutine returns two vectors containing the coordinates of the nodes on the boundary of the convex hull.

ConvexHull is an experimental C++ implementation of Grahams Scan without previous triangulation, should be much faster.

convex.hull(tri.obj, plot.it=FALSE, add=FALSE,...)
       ConvexHull(x,y)

Arguments

tri.obj

object of class triSht

plot.it

logical, if TRUE the convex hull of tri.obj will be plotted.

add

logical. if TRUE (and plot.it=TRUE), add to a current plot.

...

additional plot arguments

x

only for ConvexHull(): x coordinates for C++ call to ConvexHull

y

only for ConvexHull(): see x

Note

In case that there are several collinear nodes on the convex hull convex.hull will return them all while ConvexHull will only give edge points.

Value

x

x coordinates of boundary nodes.

y

y coordinates of boundary nodes.

Author

Albrecht Gebhardt <albrecht.gebhardt@aau.at>, Roger Bivand <roger.bivand@nhh.no>

See also

triSht, print.triSht, plot.triSht, summary.triSht, triangles.

Examples

## random points:
rand.tr<-tri.mesh(runif(10),runif(10))
plot(rand.tr)
rand.ch<-convex.hull(rand.tr, plot.it=TRUE, add=TRUE, col="red")

## use a part of the quakes data set:
data(quakes)
quakes.part<-quakes[(quakes[,1]<=-17 & quakes[,1]>=-19.0 &
                     quakes[,2]<=182.0 & quakes[,2]>=180.0),]
quakes.tri<-tri.mesh(quakes.part$lon, quakes.part$lat, duplicate="remove")
plot(quakes.tri)
convex.hull(quakes.tri, plot.it=TRUE, add=TRUE, col="red")