Usage: WlzGreyCrossing [-o< out object >] [-c #] [-h] [< in object >]
      Options:
      -o Output object file name.
      -c grey crossing value, set to 0.
      -h Help, prints this usage message.
      Computes a grey value crossing image from a 2D domain object with
      integral grey values.
      Objects are read from stdin and written to stdout unless the filenames
      are given.
      Example: WlzGreyCrossing -o crossing.wlz myobj.wlz
      The input Woolz object is read from myobj.wlz, filtered and written
      to crossing.wlz.