chullPolygon {tigers}

Convex Hull of Polygon

Description

Finds the convex hull of a polygon.

Note that the function chull (see link below) finds the convex hull of a set of points and is about twice slower than the present one when applied to a polygon.

Usage

chullPolygon(x, y = NULL)

Arguments

x, y

the coordinates of the points given in the usual way in R.

Details

This internal implementation requires the polygon to be open and in clockwise order (a crash will happen otherwise). Clockwise order is checked and possibly handled before calling the C code.

Value

a vector of integers which give the indices of the vertices of the input polygon defining the convex hull.

Author(s)

Emmanuel Paradis

References

Graham, R. L. and Yao, F. F. (1983) Finding the convex hull of a simple polygon. Journal of Algorithms, 4, 324–331. <doi:10.1016/0196-6774(83)90013-5>

See Also

chull

Examples

XY <- rbind(c(0, 0),
            c(1, 0),
            c(.25, .25),
            c(.5, .5),
            c(1.2, .8),
            c(1, .78),
            c(0, 1))
(i <- chullPolygon(XY))
plot(XY, type = "n", asp = 1)
polygon(XY, lwd = 5, border = "lightgrey")
text(XY, labels = 1:nrow(XY), cex = 2/1.5)
polygon(XY[i, ], border = "blue", lty = 2, lwd = 3)