when the average value {HEAD ~ AV1} of the right area is smaller than {TAIL ~ the average value AV2} of the left area, the gray level ratio is AV2/AV1; and 
is smaller than
when the average value { HEAD ~ AV1 } of the right area is smaller than { TAIL ~ the average value AV2 } of the left area , the gray level ratio is AV2 / AV1 ; and
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH