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.