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