when the average value AV1 of {HEAD ~ the right area} is the same as the average value AV2 of {TAIL ~ the left area}, the gray level ratio is 1.0 
when the average value AV1 of { HEAD ~ the right area } is the same as the average value AV2 of { TAIL ~ the left area } , the gray level ratio is 1.0
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH