as.mask.Rd
\name{as.mask}
\alias{as.mask}
\title{Pixel Image Approximation of a Window}
\description{
Obtain a discrete (pixel image) approximation of a given window
}
\usage{
as.mask(w, eps=NULL, dimyx=NULL, xy=NULL)
}
\arguments{
\item{w}{A window (object of class \code{"owin"}).}
\item{eps}{(optional) width and height of pixels.}
\item{dimyx}{(optional) pixel array dimensions}
\item{xy}{(optional) pixel coordinates}
}
\value{
A window (object of class \code{"owin"})
of type \code{"mask"} representing a binary pixel image.
}
\details{
This function generates a rectangular grid of locations in the plane,
tests whether each of these locations lies inside the
window \code{w}, and stores the results as a binary pixel image
or `mask' (an object of class \code{"owin"}, see \code{\link{owin.object}}).
The most common use of this function is to approximate the shape
of another window \code{w} by a binary pixel image. In this case,
we will usually want to have a very fine grid of pixels.
This function can also be used to generate a coarsely-spaced grid of
locations inside a window, for purposes such as subsampling
and prediction.
The grid spacing and location are controlled by the
arguments \code{eps}, \code{dimyx} and \code{xy},
which are mutually incompatible.
If \code{eps} is given, then the grid spacing will be
approximately \code{eps} in both the \eqn{x} and \eqn{y} directions.
If \code{dimyx} is given, then the pixel grid will be an
\eqn{m \times n}{m x n} rectangular grid
where \eqn{m, n} are given by \code{dimyx[2]}, \code{dimyx[1]}
respectively. \bold{Warning:} \code{dimyx[1]} is the number of
pixels in the \eqn{y} direction, and \code{dimyx[2]} is the number
in the \eqn{x} direction.
If \code{xy} is given, then this should be a structure
containing two elements \code{x} and \code{y} which are the
vectors of \eqn{x} and \code{y} coordinates of the margins
of the grid. The pixel coordinates will be generated
from these two vectors. In this case \code{w} may be omitted.
If neither \code{eps} nor \code{dimyx} nor \code{xy} is given,
the pixel raster dimensions are obtained from
\code{\link{spatstat.options}("npixel")}.
There is no inverse of this function!
}
\seealso{
\code{\link{owin.object}},
\code{\link{as.rectangle}},
\code{\link{spatstat.options}}
}
\examples{
w <- owin(c(0,10),c(0,10), poly=list(x=c(1,2,3,2,1), y=c(2,3,4,6,7)))
\dontrun{plot(w)}
m <- as.mask(w)
\dontrun{plot(m)}
x <- 1:9
y <- seq(0.25, 9.75, by=0.5)
m <- as.mask(w, xy=list(x=x, y=y))
}
\author{Adrian Baddeley
\email{adrian@maths.uwa.edu.au}
\url{http://www.maths.uwa.edu.au/~adrian/}
and Rolf Turner
\email{r.turner@auckland.ac.nz}
}
\keyword{spatial}
\keyword{manip}