1. Field of the Invention
The present invention relates to a method of and an apparatus for converting colors to produce a print of desired designated densities using a printing machine that has been set to standard density conditions.
2. Description of the Related Art
Prints are produced by generating original film plates in various colors including cyan (C), magenta (M), yellow (Y), and black (K), for example, producing PS plates (presensitized plates) from the original film plates by exposure and development, mounting the PS plates on a printing press such as a rotary press or the like, and adjusting printing conditions including the ink film thickness, the dampening water, the temperature, etc.
Therefore, complex steps are involved in producing prints. In order to produce a print in desired colors, it has been customary prior to the production of the print to generate a proof using a simple output device such as a monitor, a color printer or the like, and adjust printing conditions in order for the proof to have the desired colors of the print to be produced.
Japanese Laid-Open Patent Publication No. 2007-208492, for example, discloses a method of confirming the colors of a print before the print is produced by a printing press. According to the disclosed method, if the colors of a proof fall in an allowable range with respect to the colors of the print, then the print is produced by the printing press without changing platemaking data for generating PS plates and target densities to be set as printing conditions in the printing press. On the other hand, if the colors of the proof deviate from the allowable range with respect to the colors of the print, then the platemaking data are changed or a target mixed-color halftone density or a target halftone dot area ratio which is related to the target density as the printing condition, and thereafter a proof is produced again, the process being repeated until the print having the desired colors is produced.
The colors of a print are normally adjusted by the operator who adjusts the ink keys to change the densities of the inks. The process of changing the densities of the proof by changing the target mixed-color halftone density or the target halftone dot area ratio, and the process of changing the densities of the print using the ink keys tend to cause the operator who makes adjustments to develop different sensations about the colors. Therefore, it is highly difficult to produce a print having desired colors which match the proof. Standard densities for prints that are desired by users may differ from user to user. For changing standard densities, it is necessary to adjust the ink keys and then print a color chart again to generate an ICC profile again.
However, such a process is highly time-consuming because a color chart made up of many color patches has to be printed depending on the new standard densities and measure the colorimetric values of the color chart. In addition, the operator needs to be highly experienced for changing standard densities using the ink keys or the like.
When the standard densities of a printing press are changed using ink keys, the standard densities are set uniformly with respect to one ink key. For example, it is assumed that, as shown in FIG. 10 of the accompanying drawings, images 6a, 6b are printed on a printing sheet 4 by a printing press whose standard densities have been adjusted by a plurality of ink keys 2 that are arrayed in a direction perpendicular to the printing direction, indicated by the arrow, of the printing sheet 4. Though standard densities for the images 6a, 6b can be adjusted in the direction perpendicular to the printing direction, they cannot be adjusted in the printing direction, but remain uniform by the corresponding ink keys 2. Accordingly, the densities of parts of the images 6a, 6b which correspond to the positions of the same ink keys 2 along the printing direction cannot be adjusted individually using the ink keys 2.