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