If the argument X is a pixel image, the result is a point
pattern, consisting of the centre of every pixel whose pixel value
is not NA. If X is a window which is a binary mask, the result is a point
pattern consisting of the centre of every pixel inside the window
(i.e. every pixel for which the mask value is TRUE).
Otherwise, X is first converted to a window, then
converted to a mask using as.mask, then handled as
above.