1. A method of operating a computation device to process an image using a threshold value matrix for quantization of an n-value signal in each pixel of the image into an m-value (n>m>1) using a dither process, the method comprising: (i) creating the threshold value matrix by: executing a first process of creating an initial dot pattern of a predetermined dot rate “a” and arranging a gradation value corresponding to the dot rate “a” at a pixel position of the threshold value matrix, the pixel position corresponding to the initial dot pattern, as a threshold value; executing a second process so as to generate a dot pattern of dot rate “b′” having a gradation value g+δg based on a reference pattern of dot rate “b” having a gradation value g, said second process comprising the steps of: a) calculating a total obtained by adding peripheral error to an input value of the gradation value of g+δg; b) determining whether or not a dot has already occurred at a position of a pixel of interest in the dot pattern of dot rate “b”; c) when it is determined that a dot has already occurred in step b), adding a dot to a pixel of interest in the dot pattern of dot rate “b′”, substituting a first value as an evaluation value, finding an error between the total and the evaluation value, and diffusing the error to an unprocessed peripheral pixel; d) when it is determined that no dot has occurred in step b), comparing the total with a threshold value, adding a dot to the pixel of interest in the dot pattern of dot rate “b′” when the total is greater as a result of the comparison, substituting the first value as an evaluation value, turning off a dot at the pixel of interest in the dot pattern of dot rate “b′” when the total is not greater as a result of the comparison, substituting a second value as a result of the comparison, calculating an error between the evaluation value and the total, and diffusing the error to an unprocessed peripheral pixel; and e) replacing the threshold value at a position of a pixel of a newly produced dot by gradation value g+δg that corresponds to dot rate dot rate “b′”; (ii) processing the image using the created threshold value matrix.