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`

.