The morphological closing (Serra, 1982)
of a set $W$ by a distance $r > 0$
is the set of all points that cannot be
separated from $W$ by any circle of radius $r$.
That is, a point $x$ belongs to the closing $W*$
if it is impossible to draw any circle of radius $r$ that
has $x$ on the inside and $W$ on the outside.
The closing $W*$ contains the original set $W$. For a small radius $r$, the closing operation
has the effect of smoothing out irregularities in the boundary of
$W$. For larger radii, the closing operation smooths out
concave features in the boundary. For very large radii,
the closed set $W*$ becomes more and more convex.
This function computes the closing of the window w
as a binary pixel mask. If w is not already a mask, it is first
converted to a mask by as.mask. The arguments
"..." determine the pixel resolution. There is a sensible
default.
The algorithm simply applies dilate.owin followed by
erode.owin.