1. Technical Field
The present invention generally relates to an electrophotographic image output device such as an image forming apparatus that is a copier, a printer, a facsimile machine, a plotter, or a multifunction peripheral (MFP) having at least two of coping, printing, facsimile transmission, plotting, and scanning capabilities.
2. Description of the Background Art
In electrophotographic image formation, generally color stability is lower compared with offset printing, and it is preferred to enhance color stability. One of features of electrophotography is image processing, which includes parallel line processing (e.g., line screen or parallel line pattern). The number and the screen angle (i.e., inclination) of lines, by which characteristics of line screen are defined, are determined for each color considering factors such as moire, jaggy, banding, and the like that degrade image quality.
In conventional line screens, the screen angle of zero (0) degree or close to it is often allocated to yellow, which is less recognizable even when image quality is degraded. However, in the case of yellow parallel lines whose screen angle is adjacent to zero, toner images tend to be disturbed due to scavenging (described later) or the like. Further, it is possible that color fluctuations increase in the secondary colors of yellow, namely, red and green, in addition to yellow.
For example, JP-2007-156394-A proposes allocating the number (line per inch or lpi) and the screen angle of the respective color lines as follows using parallel line dithering in all of yellow, cyan, magenta, and black to improve moire with combination of dithering matrix different from conventional combinations: yellow: 200.0 lpi and 0.0 degrees, cyan: 180.3 lpi and 146.3 degrees, magenta: 180.3 lpi and 33.7 degrees, and black: 200.0 lpi and 90.0 degrees. This approach is to improve image quality, with the occurrence of moire and banding effects reduced, by optimizing the respective color line screen angles.