wherein after an average value AV1 of gray level data of each pixel in {HEAD ~ the right area} and an average value AV2 of gray level data of {TAIL ~ each pixel} in the left area are calculated, the gray level ratio of the average value AV1 to the average value AV2 is calculated by the following condition: 
wherein after an average value AV1 of gray level data of each pixel in { HEAD ~ the right area } and an average value AV2 of gray level data of { TAIL ~ each pixel } in the left area are calculated , the gray level ratio of the average value AV1 to the average value AV2 is calculated by the following condition :
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 OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH