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