 maintaining, by the motion coordinate calculator, {TAIL ~ the reference image} previously set in {HEAD ~ a Y channel reference unit} when it is determined at the step that the Y axis motion vector has a value of zero; and 
maintaining , by the motion coordinate calculator , { TAIL ~ the reference image } previously set in { HEAD ~ a Y channel reference unit } when it is determined at the step that the Y axis motion vector has a value of zero ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH