Revision 9fc33b4ac23fdcf8e2e93aacb1e9fc5842d57bf3 authored by Adrian Baddeley on 18 December 2007, 00:00:00 UTC, committed by Gabor Csardi on 18 December 2007, 00:00:00 UTC
1 parent 6db367d
rthin.Rd
\name{rthin}
\alias{rthin}
\title{Random Thinning}
\description{
Applies independent random thinning to a point pattern.
}
\usage{
rthin(X, P, \dots)
}
\arguments{
\item{X}{
A point pattern (object of class \code{"ppp"})
that will be thinned.
}
\item{P}{
Data giving the retention probabilities, i.e. the probability
that each point in \code{X} will be retained.
Either a single number, or a vector of numbers,
or a \code{function(x,y)}, or a pixel image (object of class
\code{"im"}).
}
\item{\dots}{
Additional arguments passed to \code{P}, if it is a function.
}
}
\details{
In a random thinning operation, each point of the pattern \code{X}
is randomly either deleted or retained (i.e. not deleted).
The result is a point pattern,
consisting of those points of \code{X} that were retained.
Independent random thinning means that the retention/deletion of each
point is independent of other points.
The argument \code{P} determines the probability of \bold{retaining}
each point. It may be
\describe{
\item{a single number,}{so that each point will be retained with
the same probability \code{P};
}
\item{a vector of numbers,}{so that the \code{i}th point of \code{X}
will be retained with probability \code{P[i]};
}
\item{a function \code{P(x,y)},}{so that a point at a location
\code{(x,y)} will be retained with probability \code{P(x,y)};
}
\item{a pixel image,}{containing values of the retention probability
for all locations in a region encompassing the point pattern.
}
}
If \code{P} is a function, it should be \sQuote{vectorised}, that is,
it should accept vector arguments \code{x,y} and should yield a
numeric vector of the same length. The function may have extra
arguments which are passed through the \code{\dots} argument.
}
\value{
A point pattern (object of class \code{"ppp"}).
}
\examples{
data(redwood)
plot(redwood, main="thinning")
# delete 20\% of points
Y <- rthin(redwood, 0.8)
points(Y, col="green", cex=1.4)
# function
f <- function(x,y) { ifelse(x < 0.4, 1, 0.5) }
Y <- rthin(redwood, f)
# pixel image
Z <- as.im(f, redwood$window)
Y <- rthin(redwood, Z)
}
\author{Adrian Baddeley
\email{adrian@maths.uwa.edu.au}
\url{http://www.maths.uwa.edu.au/~adrian/}
and Rolf Turner
\email{rolf@math.unb.ca}
\url{http://www.math.unb.ca/~rolf}
}
\keyword{spatial}
\keyword{datagen}
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