The image X will be expanded by a margin of n
pixels, or extended to fill the window W,
with new pixel values set to value. The argument value should be a single value (a vector of length
1), normally a value of the same type as the pixel values of X.
It may be NA. Alternatively if X is a
factor-valued image, value can be one of the levels of
X.
If n is given, it may be a single number, specifying the
width of the border in pixels. Alternatively it may be a vector
of length 2 or 4. It will be replicated to length 4, and these numbers
will be interpreted as the border widths for the
(left, right, top, bottom) margins respectively.
Alternatively if W is given, the image will be extended to
the window W.