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.