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