black when four colors are used for calibration light cyan and light magenta are added when six colors are used alternatively special colors including orange and green may be added in addition to the four colors that correspond to a printing material ink or toner of the printer 17 the operator uses the operation unit to select an item u201c print chart u201d of the operation menu to perform printing at step s 25 the process causes the measuring device 18 to measure the patch density of each of the charts printed at step s 24 and the measured patch density is stored in the measured density storage unit 15 at step s 26 on the basis of the target curve stored in the target curve storage unit 16 and the measured densities of the charts stored in the measured density storage unit 15 the process generates the calibration curve which is stored in the calibration curve storage unit 14 the method of generating the calibration curve is as follows the inverse function of a measured density function is computed so that a function obtained by combining a calibration curve function input pre compensated cmyk data values output post compensated cmyk data values and the measured density function input cmyk data values output density values corresponds to the target curve function input cmyk data values output density values this computed inverse function is the calibration curve function which is applied to an image to be processed to realize the most accurate.