This function computes the areas of the erosions of the window
w by each of the radii
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 $W$.
r should be a vector of positive numbers.
w should be a window (an object of class
owin.object for details)
or can be given in any format acceptable to
w is a rectangle, the computation is performed
using a pixel raster approximation.
To compute the eroded window itself, use