(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a digital color copying machine and a color facsimile machine, which reproduces a full-color image according to image data obtained by performing color-separation on an original image.
(2) Related Art
In an image forming apparatus, such as a digital color copying machine, digital image data of red (R), green (G), and blue (B) obtained by performing color separation on an original are converted into density data of cyan (C), magenta (M), yellow (Y), and black (K) used for color reproduction by an image signal processing unit, and a printing unit forms a reproduction image by transferring the toner of each color onto a copying paper according to the values of the density data.
In such image forming apparatus, the image signal processing unit performs correction on the image data to avoid image defects caused by low scanning accuracy, poor color reproducibility, and other factors. After this correction, the printing unit forms a reproduction image.
In a full-color reproduction image, degradation in saturation is likely to catch the user's attention. When saturation degradation in the reproduction image is anticipated due to the granular characteristics of the toners, saturation correction needs to be performed on the image data by the image processing unit. In a conventional digital color copying machine, saturation correction is performed in accordance with the same saturation corrective condition regardless of the type of the original image.
In the case where the saturation corrective condition is determined invariably as above, however, when copying the reproduced image as an original, there are problems that a decrease in saturation is noticeable, and that the image defects get worse as the generation of a copy becomes newer.
Compared with a silver print, the input and output resolution of a conventional digital color copying machine is limited. The reproducible ranges of color and density are narrow, and the diameter of a toner particle in the printing unit is large. When inspected closely, the reproduction dots of the toner particles have different shapes, which results in uneven density and minute noise in the reproduction image. So, it is imperative that the reproduction image (hereinafter referred to as "hard copy") show a deterioration in saturation. When copying such hard copy as an original (a copy made from a hard copy is called a "generation copy" in the sense that a newer copy is made one generation after another), the density unevenness, the minute noise, and the impurity are increased even more, which causes a further decrease in saturation.
Taking the above problems into consideration, when making a generation copy, it is necessary to set a variety of saturation corrective conditions which can compensate for the image deterioration of the hard copy.