MASK

        [GREG2\]MASK [IN|OUT] [/BLANKING Bval]

    masks  all pixels of the Regular Grid array lying inside (if argument is
    IN) or outside (if argument is OUT or by DEFAULT)  the  current  polygon
    (see  command  POLYGON) by attributing them a blanking value. This value
    is defined by  command  SET BLANKING,  or  specified  using  the  option
    /BLANKING Bval.