A characteristic curve is defined on a coordinate plane where a visually equal scale (by normalizing 1/3 square of any one of three stimulus values of an object color) is used as a first axis and a gradation value of original data is used as a second axis, the boundary value of a level of error diffusion is decided through the characteristic curve. 視覚的に均等な尺度(物体色の三刺激値のいずれかの1/3乗を正規化したもの)を第1の軸とし、元データの階調値を第2の軸とする座標平面上に特性曲線を定義し、当該特性曲線を通じて誤差拡散のレベルの境界値を定める。 - 特許庁