1. Field of the Invention
The present invention relates to an image processing technique, and particularly relates to a color-shift correction at the time of serial transfer of multiple color images formed by developer units provided for the respective colors.
2. Description of the Related Art
In recent years, among electrophotographic color image forming apparatuses, an increasing number of color image forming apparatuses include as many developers and photoconductors as color materials and employ a system in which different color images are serially transferred onto an image conveyor belt and a recording medium, so that image forming can be speeded up.
While use of this system (tandem system) can considerably shorten the throughput time, there arises a problem attributable to defects such as non-uniformity or poor attachment position accuracy of a lens of a deflection scanner, and poor assembling position accuracy of the deflection scanner itself to the image forming apparatus. Specifically, such defects make scan lines inclined or curved, and the degree of the inclination or the curve differs among the colors. This causes a problem of a color shift due to displacements of the color images on paper on which the images are transferred. Consequently, it is difficult to obtain a high quality color image.
To cope with the color shift, for example, Japanese Patent Laid-Open No. 2002-116399 describes a method in which, in an assembling process of a deflection scanner, the degree of the curving of a scan line is measured by using an optical sensor, and the lens is mechanically turned to adjust the curving of the scan line and then is fixed.
Japanese Patent Laid-Open No. 2003-241131 describes a method in which, in a process of assembling a deflection scanner to an image forming apparatus, the degree of the curving of a scan line is measured by using an optical sensor, and the deflection scanner is mechanically tilted to adjust the curving of the scan line, and then is assembled to the image forming apparatus.
In this respect, in order to correct an optical path of an optical system, it is necessary to adjust a position of a test toner image by mechanically operating an optical error correction system including a light source and an f-θ lens, a mirror on the optical path, and the like. Accordingly, the methods described in Japanese Patent Laid-Open Nos. 2002-116394 and 2003-241131 require a highly accurate movable member, thus leading to a high cost.
Furthermore, it takes a long time to complete a correction of an optical path in an optical system, and thus frequent correction is impossible. Besides, the shift in the optical path length changes due to an influence by a temperature rise or the like of the machine. For this reason, even though a correction is performed at a certain time point, the influence by the temperature rise of the machine cannot be eliminated. Thus, it is difficult to prevent the color shift by correcting the optical path of the optical system.
Meanwhile, Japanese Patent Laid-Open No. 2004-170755 describes a method in which the degree of the inclination and curving of a scan line is measured by using an optical sensor, bitmap image data is corrected so that the inclination and curving can be cancelled out, and then an image is formed from the corrected image data. In the method, since image data is processed for an electrical correction, neither a mechanical adjustment member nor an adjusting step in assembling process is required. In this respect, it is possible to cope with a color shift at a lower cost in this method than in the methods described in Patent Laid-Open Nos. 2002-116394 and 2003-241131.
The electrical color-shift correction is divided into a one-pixel-basis correction and a less-than-one-pixel-basis correction. In the one-pixel-basis correction, a pixel is offset (crosses over) in a sub scanning direction on a one-pixel basis in accordance with a correction amount for an inclination and a curve. In the less-than-one-pixel-basis correction, tone values of preceding and succeeding pixels in the sub scanning direction are corrected. Implementation of the less-than-one-pixel-basis correction solves the problem of an unnatural step at an offset boundary occurring due to the one-pixel-basis correction, thus achieving smoothing out of an image.
However, as one of adverse effects of the aforementioned electrical color-shift correction, there can be cited non-uniformity (irregularity) in density of a fine image involved with the less-than-one-pixel-basis correction.
FIG. 1 is a diagram illustrating non-uniformity in density of a fine image. In FIG. 1, an input image 101 is a thin line having a constant tone value. When an image 102 is actually formed by performing a color-shift correction on the input image 101, an outputted image after the color-shift correction is a thin line image having non-uniform density, even though the input image 101 has the constant image tone value. This is because an electrophotographic image forming apparatus is generally poor at forming an isolated pixel while maintaining a proportional relationship between an image tone value and an actual image density value. A fine image formed of such a thin line remarkably exhibits influence of the poorness as non-uniformity in density.
One of remedies for the non-uniformity in density of a fine image is to avoid performing the less-than-one-pixel-basis correction on the fine image. Specifically, the image is binarized, and the binarized image is compared with a smoothing judgment pattern stored in advance. When the image matches the pattern, the less-than-one-pixel-basis correction is not performed. When the image does not match the pattern, the less-than-one-pixel-basis correction is performed.
However, when the less-than-one-pixel-basis correction is not performed, there occurs an unnecessary step (bump) at the offset boundary due to the one-pixel-basis correction as described above. Such a step generated by the offset is noticeable particularly in a fine image in which characters in small point size are drawn or a fine pattern is repeatedly drawn.