when the average value {HEAD ~ AV1} of {TAIL ~ the right area} is larger than the average value AV2 of the left area, the gray level ratio is AV1/AV2 
when the average value { HEAD ~ AV1 } of { TAIL ~ the right area } is larger than the average value AV2 of the left area , the gray level ratio is AV1 / AV2
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