This function computes the areas of the erosions of the window
w by each of the radii r[i].
The morphological erosion of a set \(W\) by a distance \(r > 0\)
is the subset
consisting of all points \(x \in W\) such that the
distance from \(x\) to the boundary of \(W\) is greater than
or equal to \(r\). In other words it is the result of trimming
a margin of width \(r\) off the set \(W\).
The argument r should be a vector of positive numbers.
The argument w should be a window (an object of class
"owin", see owin.object for details)
or can be given in any format acceptable to as.owin().
Unless w is a rectangle, the computation is performed
using a pixel raster approximation.
To compute the eroded window itself, use erosion.