1. Technical Field
The present invention relates to a color conversion table creation method, a non-transitory computer readable recording medium stored with a color conversion table creation program, and a color conversion table creating apparatus, for creating a color conversion table.
2. Description of Related Art
To create a color conversion table necessary for color management, an image forming apparatus outputs a plurality of patch images corresponding to the respective combinations of C (cyan), M (magenta)), Y (yellow), and K (black), and then the CMYK values of each patch image and corresponding measured color values (L*a*b* values, XYZ values, or the like) are associated with each other. Since it is difficult to output patch images corresponding to all the respective combinations of C, M, Y, and, K and measure the colors thereof, a general way is to output approximately several hundred patch images, associate the CMYK values and the measured color values with each other, and then perform interpolation calculation, to create a color conversion table of a predetermined size.
In the electrophotographic or ink-jet image forming apparatus, printing using 100% of the amount of color material of all of C, M, Y, and K normally results in failure in toner fixation, spreading of ink, or the like. To avoid such problems, the total use amount of color material is limited in general.
Attempts have been made to improve accuracy of a color profile for image forming apparatus with limitation of total use amount of color material. For example, Japanese Patent Application Laid-Open No. 2004-147257 proposes a technique for reducing deterioration of accuracy attributable to a process for limiting the total use amount of color material. In this technique, output color values at the perimeter of a color gamut when the total use amount of color material is not limited are adjusted so that the output color values would also be at the perimeter of the color gamut after the limitation of the total use amount of color material is imposed. However, this technique has a problem that an interpolation error attributable to the process for limiting the total use amount of color material occurs at the time of performing interpolation calculation for a color conversion table, which makes it difficult to obtain an accurate color-conversion table.
Meanwhile, Japanese Patent Application Laid-Open No. 2012-249182 proposes a technique for improving color accuracy in a low-lightness area (i.e., area with limitation of total use amount of color material) by increasing the number of patch images in a color chart. With this technique, it is possible to prevent an interpolation error attributable to a process for limiting the total use amount of color material. However, this technique is not preferable since long time is required for the operation of measuring color of patch images and data processing.
In the image forming apparatus with limitation of total use amount of color material, a certain amount is set as a set amount at which the total use amount of color material is started to be limited, and a process for gradually reducing the input CMYK values having the total use amount of color material exceeding the set amount is performed, in order to prevent gradation characteristics from being deteriorated by a process for limiting the total use amount of color material. Hence, it is desired to create a highly-accurate color conversion table for an image forming apparatus which selectively performs, on input CMYK values having the total use amount of color material exceeding the set amount, a process for reducing the total use amount of color material, without increasing the number of patch images in a color chart.