\name{triangulate}
\alias{triangulate}
\title{
Triangulate a two-dimensional polygon.
}
\description{
This algorithm decomposes a general polygon into simple
polygons and uses the \dQuote{ear-clipping} algorithm to triangulate it.
Polygons with holes are supported.
}
\usage{
triangulate(x, y = NULL, z = NULL, random = TRUE, plot = FALSE, partial = NA)
}
\arguments{
  \item{x, y, z}{
Coordinates of a two-dimensional polygon in a format supported by \code{\link{xyz.coords}}.  
See Details for how \code{z} is handled.
}
  \item{random}{
Whether to use a random or deterministic triangulation.
}
  \item{plot}{
Whether to plot the triangulation; mainly for debugging purposes.
}
  \item{partial}{
If the triangulation fails, should partial results be returned?
}
}
\details{
Normally \code{triangulate} looks only at the \code{x} and \code{y}
coordinates.  However, if one of those is constant, it is replaced
with the \code{z} coordinate if present.

The algorithm works as follows.  First, it breaks the polygon into 
pieces separated by \code{NA} values in \code{x} or \code{y}.
Each of these pieces should be a simple, non-self-intersecting
polygon, separate from the other pieces. 
(Though some minor exceptions to this rule may work, none
are guaranteed).  The nesting of these pieces is determined.

The \dQuote{outer} polygon(s) are then merged with the
polygons that they immediately contain, and each of these
pieces is triangulated using the ear-clipping algorithm.  

Finally, all the triangulated pieces are put together into one
result.
}
\value{
A three-by-n array giving the indices of the vertices of 
each triangle.  (No vertices are added; only the original
vertices are used in the triangulation.)

The array has an integer vector attribute \code{"nextvert"}
with one entry per vertex, giving the index of the next 
vertex to proceed counter-clockwise around outer
polygon boundaries, clockwise around inner boundaries.
}
\references{
See the Wikipedia article \dQuote{polygon triangulation}
for a description of the ear-clipping algorithm. 
}
\author{
Duncan Murdoch
}
\note{
Not all inputs will succeed, even when a triangulation is
possible.  Generally using \code{random = TRUE} will find
a successful triangulation if one exists, but it may
occasionally take more than one try.
}
\seealso{
\code{\link{extrude3d}} for a solid extrusion of a polygon, \code{\link{polygon3d}} for
a flat display; both use \code{triangulate}.
}
\examples{
theta <- seq(0, 2*pi, len = 25)[-25]
theta <- c(theta, NA, theta, NA, theta, NA, theta, NA, theta)
r <- c(rep(1.5, 24), NA, rep(0.5, 24), NA, rep(0.5, 24), NA, rep(0.3, 24), NA, rep(0.1, 24))
dx <- c(rep(0, 24), NA, rep(0.6, 24), NA, rep(-0.6, 24), NA, rep(-0.6, 24), NA, rep(-0.6, 24))
x <- r*cos(theta) + dx
y <- r*sin(theta)
plot(x, y, type = "n")
polygon(x, y)
triangulate(x, y, plot = TRUE)
open3d()
polygon3d(x, y, x - y, col = "red")
}
\keyword{ graphics }
