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