This function finds the smallest rectangle (with sides parallel to
  the coordinate axes) that contains all the given objects.
For a window (object of class "owin"), the bounding box
  is the smallest rectangle that contains all the vertices of the
  window (this is generally smaller than the enclosing frame,
  which is returned by as.rectangle).
For a point pattern (object of class "ppp" or "lpp"),
  the bounding box
  is the smallest rectangle that contains all the points of the pattern.
  This is usually smaller than the bounding box of the window of the
  point pattern.
For a line segment pattern (object of class "psp")
  or a linear network (object of class "linnet"), the
  bounding box is the smallest rectangle that contains all endpoints
  of line segments.
  
For a pixel image (object of class "im"), the image will
  be converted to a window using as.owin,
  and the bounding box of this window is obtained.
  
If the argument is a list of several objects, then
  this function finds the smallest rectangle that contains
  all the bounding boxes of the objects.