An image scanner that reads an original document as electronic image data includes a line sensor such as a CCD (Charge Coupled Device) or CIS (Contact Image Sensor), in which a plurality of light-receiving elements are linearly arrayed. The image scanner reads the original document image while moving the line sensor in a direction (sub-scanning direction) orthogonal to the array direction (main scanning direction) of the light-receiving elements. Alternately, there is an image scanner configured such that the line sensor is fixed and the original document is moved and the original document image is read.
However, there is a problem in that when an original document image is read by moving the line sensor or the original document, color misalignment occurs due to an effect from mechanical vibration that occurs in a driving mechanism for moving the line sensor or the original document in the sub-scanning direction, or feed accuracy. An example of the reading method is a line sequential reading method in which light sources of at least three colors, such as R (red), G (green) and B (blue), for example, are sequentially caused to emit light while the line sensor is moved, thereby reading the original document image with the colored light of these light sources. In the line-sequential reading method, generally, the line sensor or an original document to be read is continuously moved during the reading operation, and thus the read positions of the R, G, and B light at the emission timing shift in the sub-scanning direction as described above, causing color misalignment to occur when an original document including characters, line images or the like is read.
Even in a case where the sensors covering the main scanning direction separately for the colors R (red), G (green) and B (blue) are disposed so as to be displaced from each other in the sub-scanning direction such that the colors are read simultaneously, color misalignment occurs due to fluctuation in the moving speed in the sub-scanning direction. This is because, with respect to image data read at a certain point in time, the positions of the sensors are different for each color, and thus it is necessary to correct the positions of the sensors in the sub-scanning direction according to the moving speed and the spacing between the sensors in the sub-scanning direction. However, the estimated read position and the actual read position are displaced from each other due to fluctuation in the moving speed in the sub-scanning direction, and as a result, color misalignment occurs.
Various image reading methods are proposed in order to correct or reduce color misalignment. A method for correcting color misalignment is known in which image data for each read line is replaced by the weighted average of the image data of the read line and the image data of the read line of the same color immediately preceding or subsequent thereto. In Japanese Patent Laid-Open No. 2000-224375, a technique is proposed where the emission order of the R, G and B light is controlled in order to further improve the positional accuracy of R, G and B in the lines after averaging. Also, in Japanese Patent Laid-Open No. 2002-142122, a technique is proposed where an original document is read at double resolution in the sub-scanning direction, and after calculating the weighted average with the lines immediately preceding or subsequent thereto, the resultant image data is thinned.
As another image reading method, for example, in Japanese Patent Laid-Open No. 04-11458, a technique is proposed where the image information read for each emission color is converted to luminance information and color-difference information, and spatial frequency band restriction is applied to only the color-difference information such that color misalignment is reduced while maintaining the perceived resolution. Also, in Japanese Patent Laid-Open No. 2003-259135, a technique is proposed in which after the image information is converted to luminance information and saturation information, a pixel having the lowest luminance in the vicinity of a target pixel is set as a determination pixel, and if the target pixel is determined to be an achromatic color based on the saturation information in the vicinity of the determination pixel, the data of the target pixel is replaced to the achromatic color.
However, the conventional techniques have problems described below. For example, with the image reading method disclosed in Japanese Patent Laid-Open No. 2000-224375, not only the pixels affected by color misalignment but the resolution of the image, which is a fundamental performance criterion, deteriorates, which is not preferable. Also, although the degree of color misalignment is reduced as a result of the weighted average processing, it is not completely eliminated. Also, with the image reading method disclosed in Japanese Patent Laid-Open No. 2002-142122, although deterioration of the resolution can be avoided, since the reading resolution is doubled, the size of the pixel data to be processed is doubled. Therefore, if the operation speed of the color misalignment correction circuit is the same, double the reading time is required. Of course, the method can be realized without extending the reading time by increasing the operation speed of the color misalignment correction circuit two fold, but the apparatus cost will increase if a circuit configuration that supports such high-speed operation is realized, which is not preferable.
Also, with the image reading method disclosed in Japanese Patent Laid-Open No. 04-11458, although the effect on the perceived resolution, reading time and cost is small, the color misalignment is only reduced to some extent due to the spatial frequency band restriction, and is not completely eliminated. Furthermore, with the image reading method disclosed in Japanese Patent Laid-Open No. 2003-259135, although the method has a sufficient effect for black characters, no processing is performed with respect to the color misalignment in the contour portions of colored characters, the color misalignment in black characters on a chromatic color background, or the like.