spatstat (version 1.55-0)

distfun: Distance Map as a Function

Description

Compute the distance function of an object, and return it as a function.

Usage

distfun(X, …)
  # S3 method for ppp
distfun(X, …, k=1, undef=Inf)
  # S3 method for psp
distfun(X, …)
  # S3 method for owin
distfun(X, …, invert=FALSE)

Arguments

X

Any suitable dataset representing a two-dimensional object, such as a point pattern (object of class "ppp"), a window (object of class "owin") or a line segment pattern (object of class "psp").

…

Extra arguments are ignored.

k

An integer. The distance to the kth nearest point will be computed.

undef

The value that should be returned if the distance is undefined (that is, if X contains fewer than k points).

invert

If TRUE, compute the distance transform of the complement of X.

Value

A function with arguments x,y. The function also belongs to the class "distfun" which has a method for print. It also belongs to the class "funxy" which has methods for plot, contour and persp.

Details

The “distance function” of a set of points \(A\) is the mathematical function \(f\) such that, for any two-dimensional spatial location \((x,y)\), the function value f(x,y) is the shortest distance from \((x,y)\) to \(A\).

The command f <- distfun(X) returns a function in the R language, with arguments x,y, that represents the distance function of X. Evaluating the function f in the form v <- f(x,y), where x and y are any numeric vectors of equal length containing coordinates of spatial locations, yields the values of the distance function at these locations. Alternatively x can be a point pattern (object of class "ppp" or "lpp") of locations at which the distance function should be computed (and then y should be missing).

This should be contrasted with the related command distmap which computes the distance function of X on a grid of locations, and returns the distance values in the form of a pixel image.

The result of f <- distfun(X) also belongs to the class "funxy" and to the special class "distfun". It can be printed and plotted immediately as shown in the Examples.

A distfun object can be converted to a pixel image using as.im.

See Also

distmap, plot.funxy

Examples

# NOT RUN {
   data(letterR)
   f <- distfun(letterR)
   f
   plot(f)
   f(0.2, 0.3)

   plot(distfun(letterR, invert=TRUE), eps=0.1)

   d <- distfun(cells)
   d2 <- distfun(cells, k=2)
   d(0.5, 0.5)
   d2(0.5, 0.5)

   z <- d(japanesepines)
# }