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