def register
      each_corner { |c| @rm[c] = rm_centroid_on c }
      @rm.all? { |k,v| v[:status] == :ok }
    end
def register
      each_corner { |c| @rm[c] = rm_centroid_on c }
      @rm.all? { |k,v| v[:status] == :ok }
    end
find the XY coordinates of the 4 registration marks,
 plus the stdev of the search area as quality control