calculating an evaluation function by {HEAD ~ means} of {TAIL ~ the gray level ratio} by repeating the gray level ratio calculating step in the specified area 
of
calculating an evaluation function by { HEAD ~ means } of { TAIL ~ the gray level ratio } by repeating the gray level ratio calculating step in the specified area
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH