as.mask

Pixel Image Approximation of a Window

Obtain a discrete (pixel image) approximation of a given window

Usage

as.mask(w, eps=NULL, dimyx=NULL, xy=NULL)

Details

This function generates a rectangular grid of locations in the plane, tests whether each of these locations lies inside the window w, and stores the results as a binary pixel image or `mask' (an object of class "owin", see owin.object).

The most common use of this function is to approximate the shape of another window 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 eps, dimyx and xy, which are mutually incompatible.

If eps is given, then it determines the grid spacing. If eps is a single number, then the grid spacing will be approximately eps in both the \(x\) and \(y\) directions. If eps is a vector of length 2, then the grid spacing will be approximately eps[1] in the \(x\) direction and eps[2] in the \(y\) direction.

If dimyx is given, then the pixel grid will be an \(m \times n\) rectangular grid where \(m, n\) are given by dimyx[2], dimyx[1] respectively. Warning: dimyx[1] is the number of pixels in the \(y\) direction, and dimyx[2] is the number in the \(x\) direction.

If xy is given, then this should be some kind of data specifing the coordinates of a pixel grid. It may be

• a list or structure containing elements x and y which are numeric vectors of equal length. These will be taken as \(x\) and y coordinates of the margins of the grid. The pixel coordinates will be generated from these two vectors.

• a pixel image (object of class "im").

• a window (object of class "owin") which is of type "mask" so that it contains pixel coordinates.

If xy is given, w may be omitted.

If neither eps nor dimyx nor xy is given, the pixel raster dimensions are obtained from spatstat.options("npixel").

There is no inverse of this function. However, the function as.polygonal will compute a polygonal approximation of a binary mask.

See Also

owin.object, as.rectangle, as.polygonal, spatstat.options

Aliases

• as.mask

Examples

# NOT RUN {
  w <- owin(c(0,10),c(0,10), poly=list(x=c(1,2,3,2,1), y=c(2,3,4,6,7)))
  
# }
# NOT RUN {
plot(w)
# }
# NOT RUN {
  m <- as.mask(w)
  
# }
# NOT RUN {
plot(m)
# }
# NOT RUN {
  x <- 1:9
  y <- seq(0.25, 9.75, by=0.5)
  m <- as.mask(w, xy=list(x=x, y=y))
# }