alpha-shape calculation

Calculate a window containing all points of a point pattern. The window is not convex but as close as possible to the points.

alphahull(X, alpha = NULL, CheckArguments = TRUE)

Arguments

X

A planar point pattern (spatstat.geom::ppp.object).

alpha

A smoothing parameter to delimit concave polygons.

CheckArguments

If TRUE (default), the function arguments are verified. Should be set to FALSE to save time in simulations for example, when the arguments have been checked elsewhere.

Value

A window, i.e. a spatstat.geom::owin.object.

Details

The typical use of this function is to define a narrow window around a point pattern that has been created with a default, rectangle window.

The window is built by the alphahull::ashape() function and then transformed into a spatstat.geom::owin.object. The alpha parameter determines the smallest size of zones excluded from the window. If it is not specified, a first attempt is 1/256 of the diameter of the existing window of X. If the shape cannot be calculated, alpha is doubled and a new attempt is made.

Examples

# Simulate a point pattern
if (require(spatstat.random)) {
  X <- rpoispp(10)
  plot(X)
  # Calculate its border
  X$window <- alphahull(X)
  plot(X)
}