1. Technical Field
The present invention relates to inkjet printing apparatuses performing printing by discharging ink to a print medium from an inkjet head.
2. Background Art
Inkjet printing apparatuses forming an image by discharging ink droplets from a nozzle of an inkjet head and landing them to a sheet are being widely used.
Of the inkjet printing apparatuses, an inkjet printing apparatus of a line type performing printing by discharging the ink droplets from the fixed, long inkjet head while transferring the sheet has drawn attention in recent years in terms of speed-up.
Some inkjet printing apparatuses can perform full-color printing using black (K), cyan (C), magenta (M), and yellow (Y) inks (refer to, for example, Patent Literature 1).
When such an inkjet printing apparatus capable of performing the full-color printing forms a gray image having medium to low density, normally, it does not use the K ink but the C, M, Y inks to form the gray image. There are two reasons for that.
As a first reason, if the gray image having the low density is formed only with the K ink or the K, C, M, Y inks, many pixels are thinned out to form the gray image with less pixels (dots). Then, graininess is deteriorated.
As a second reason, with respect to a K dot, if a position of other color dot is deviated due to landing deviation, the gray image may have other color.
With the above-described two reasons, the inkjet printing apparatus capable of performing the full-color printing normally forms the gray image having the medium to low density using the three color inks of C, M, Y without using the K ink, but, a problem may occur even with such a method. For example, the positions of the C, M, Y ink dots are deviated from one another and, as a result, the gray image may have other color.
Thus, it can be considered to use only the K ink, raise resolution higher than set resolution (set to higher resolution), and decrease an ink discharge amount per one pixel (set to a smaller ink discharge amount), so as to form the gray image.
The graininess is described as below. “High graininess” or “good graininess” indicates a state in which, since the pixel included in the printed image is small, it is hardly visible. Thus, when the graininess is high, the printed image looks smooth. On the other hand, “low graininess” or “bad graininess” indicates a state in which, since the pixel included in the printed image is large, the pixel is easily visible. Thus, when the graininess is low, the printed image looks variable in grain and rough.
The graininess is an indication for evaluating the printed image by an impression when it is viewed with human eyes. Whether the graininess is high or low can be determined by mechanical measurement such as a size of the pixel included in the printed image, a level of arrangement of the pixels, and so on. The smaller the pixel is, or the more uniform the arrangement of the pixels is, the higher the graininess becomes. On the other hand, the larger the pixel is, or the less uniform the arrangement of the pixels is, the lower the graininess becomes.
With the higher resolution and the smaller ink discharge amount, the grain of the image can be reduced in size and, thus, deterioration of the graininess in the printed image can be reduced. Further, since only the K ink is used, the gray image would not have the other colors.