1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and a printing apparatus, and more particularly, to an image forming apparatus and an image forming method for creating image data to record onto a recording medium (paper), and to a printing apparatus which prints the image data onto a recording medium.
2. Description of the Related Art
Japanese Patent Application Publication No. 2003-291325 discloses an image recording apparatus employing a line type recording head having a plurality of recording elements configured following a prescribed direction, in which correction data is created for correcting the characteristics (recording divergence, density non-uniformities) of recording elements in two-dimensional positions on a recording medium, and an input image signal supplied to the recording elements in two-dimensional positions is adjusted on the basis of the correction data thus created.
Japanese Patent Application Publication No. 08-123940 discloses an image area extraction apparatus which sets upper and lower threshold values for image parameters on the basis of the average values of image parameters (brightness, luminance, saturation, hue, chromaticity) within a first area that is within a second area specified in an image, and which extracts only those pixels within the second area which have image parameter values in the range designated by the upper and lower threshold values. The technology described in Japanese Patent Application Publication No. 08-123940 performs color correction and tone correction in respect of the area extracted on the basis of the image parameters.
Japanese Patent Application Publication No. 2004-090648 discloses a color control apparatus which controls printing of a polychromatic image by controlling the respective quantities of inks used to print an image on the basis of the respective color densities of the inks determined in a printed image.
In general, in an offset printing apparatus, fine adjustment of color is performed simply by using ink keys in the printing step, as described in Japanese Patent Application Publication No. 2004-090648. If it is not possible to correct colors adequately by means of simple fine adjustment of color in the printing step, then specialized knowledge and complicated work is required in the color correction process and this is difficult to achieve at the level of skill of normal print operators. Therefore, it becomes necessary to return to the plate manufacturing step and carrying out the coloring process again for each image, before manufacturing the printing plate again and carrying out printing.
In a printing system used in commercial printing, it is often the case that the station where the output image data is created (plate manufacturing department) and the station where that image data is actually output (printed) onto a recording medium (hereinafter referred to as “paper”) are situated in different locations. For example, there may be a printing system in which a plate manufacturing department situated in a town which is readily accessible to a user requesting a print is connected by high-velocity communications line with a printing department which is situated in a suburb or out-of-town location. Consequently, returning work to the plate manufacturing step in order to perform a color correction process produces a marked decline in the productivity of the printing system. In order to carry out color correction such as that described above in a printing step, it is necessary to provide a high-specification color conversion function in the printing department. Therefore, the costs of the printing apparatuses (printers) in the printing department are increased and the work involved in the printing step becomes complicated.
In the conventional analogue offset printing apparatus, as shown in FIGS. 18 and 19, ink keys (K1, K2, . . . ) corresponding to respective areas A1, A2, . . . , in the direction (hereinafter referred to as “breadthways direction of printing”) perpendicular to the direction of conveyance of the paper P1 are provided, and if there is a difference in the finished image when the results of test printing are compared with a desired printed item (proof), then in the printing step the operator of the printing apparatus is able to adjust the density of the respective color plates (for example, a C (Cyan) plate, an M (Magenta) plate, a Y (Yellow) plate and a B (Black) plate), by operating the ink keys corresponding to the areas in the breadthways direction of printing. By adjusting the amount of ink output for each color plate, individually and respectively, as described above, it is possible to absorb differences in conditions in each printing apparatus (environment, differences between individual printing apparatuses, temporal change).
In a digital printing apparatus which uses an electrophotographic method or an inkjet method, the finish is kept more uniform than in an analogue offset printing apparatus, through the use of feedbacks and controls of various types, but as described above, in the printing step, it is necessary to adjust the tone and color reproduction if the results of test printing show a different finish to the desired printed item.
Furthermore, in the conventional offset printing apparatus, it has been possible to adjust the color and tone in each one of areas aligned in the breadthways direction of printing, by adjusting the ink keys provided for the respective colors, but it has not been possible to change the color and tone in respect of areas aligned in the conveyance direction of the paper. For example, as shown in FIG. 20, it is supposed that four A4-size images (A, B, C, D) are arranged on a sheet of paper P2 of half Kiku size (636 mm×469 mm), and that as a result of test printing, there have been the following requests: “reduce red in skin tone of human subjects in image A” and “increase redness of sunset in image B”. In this case, in image A, the ink output volume of the M (Magenta) plate is reduced and in image B, the ink output volume of the M plate is increased. However, in the case of the conventional offset printing apparatus, in the printing department it is not possible to perform adjustment which changes the ink output volume in the paper conveyance direction, and therefore the images are returned to the plate manufacturing department and the image faces (A, B, C, D) must be changed or the image data itself must be subjected to halftone conversion processing or color conversion processing again.