Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-04-21
2001-08-28
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S240000
Reexamination Certificate
active
06281924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming images in an image forming device such as a printer.
2. Description of Related Art
There has been proposed a laser printer of a type as disclosed by U.S. Pat. Nos. 5,640,654 and 5,724,633. In this type of laser printer, a latent electrostatic image is first formed on a photosensitive member according to image light exposed thereon. The latent image is then developed by toner and is transferred onto an image recording mediums such as a sheet of paper, that is being conveyed by a predetermined sheet conveying mechanism. Thus, dots of toner are selectively attached on the surface of the sheet while the sheet is being conveyed in a predetermined sheet conveying direction.
SUMMARY OF THE INVENTION
It is noted, however, that while the sheet of paper is being conveyed by the sheet conveying mechanism, mechanical oscillation possibly occurs onto the sheet conveying mechanism or onto the image recording medium, per se. Conveyance of the image recording medium is therefore irregulated periodically. As a result, undesired stripe images or bandings are formed on the sheet surface. The stripe images extend perpendicularly to the sheet conveying direction, and deteriorates the resultant image.
This problem arises not only in laser printers but also in other various types of printers such as an ink jet printer that selectively attaches dots of ink to the surface of a sheet while conveying the sheet in the sheet conveying direction.
It is therefore an objective of the present invention to overcome the above-described problems and to provide an improved image forming method for forming images while preventing occurrence of jitter.
In order to attain the above object and other objects, the present invention provides a method for forming an image, comprising the steps of: receiving an input image density value for each of a plurality of pixel positions, the plurality of pixel positions being defined two-dimensionally in first and second predetermined directions which are perpendicular to each other; and converting the input image density value for each pixel position into output data for indicating whether to turn ON or OFF a pixel dot on the corresponding pixel position, by repeatedly locating a dither matrix along the first and second predetermined directions, the dither matrix including a plurality of threshold values which are arranged two-dimensionally in the first and second predetermined directions in a manner that pixel dots will be turned ON in a predetermined order when the input image density value changes uniformly successively, the predetermined order being defined so that as the input image density gradually increases, a linear pattern is first generated to extend parallel to the first direction and then grows along the second direction, comparing the input image density value at each pixel position with a threshold value at a corresponding location of the dither matrix, and producing, based on the compared result, the output data for each pixel location. The method may further comprise the step of selectively forming, based on the output data, a pixel dot at each pixel position on an image recording medium, while conveying the image recording medium in the first predetermined direction.
The dither matrix may have a plurality of cells that are arranged in several rows and in several columns. Each row extends along the second direction, and each column extends along the first direction. The plurality of threshold values are allocated in the plurality of cells, respectively. The dither matrix serves to selectively turn ON dots at its constituent cells in a particular pattern that corresponds to the input image density value inputted to pixel positions corresponding to those cells.
The plurality of threshold values may be arranged in the dither matrix so that the amount of the threshold value will increase first in the first predetermined direction, and then increase in the second predetermined direction.
Thus, according to the present invention, the threshold values are set in the dither matrix so that as the input image density increases, a linear pattern will first grow in the first direction, in which the image recording medium such as a sheet of paper is conveyed. The linear pattern is formed from turned-ON pixel dots arranged in the first direction. For example, when each pixel dot is produced by black color, the linear pattern appears a black line on a black-and-white image. Accordingly, even when the image recording medium or a mechanism for conveying the image recording medium generates oscillation, the amount of the area of the image pattern, produced by the turned-ON dots, will change very little because the image pattern extends in the image recording medium conveying direction. Accordingly, the jitter problem can be properly avoided.
This is contrary to the case where a comparative dither matrix is used. In the comparative dither matrix, threshold values are determined one by one spirally around the center location of the dither matrix so that the amount of the threshold value will increase spirally outwardly from the center location. In the comparative dither matrix, threshold values may be determined one by one from some corner-edge location so that the amount of the threshold value will increase away from that corner-edge. According to the comparative dither matrix, as the input image density increases, an image pattern will grow from the center of the dither matrix spirally outwardly, or will gradually spread from some corner-edge. Accordingly, when the image recording medium oscillates in its conveying direction, undesirable bandings or stripes will be formed in the image pattern perpendicularly to the conveying direction.
It is noted that a resolution drops if the size of the dither matrix is increased. Accordingly, the size of the dither matrix has to be limited to some value. Because the dither matrix size is thus limited, the threshold values can not be arranged to grow infinitely. Accordingly, threshold values are arranged so that when the linear image pattern grows to reach both edges of the dither matrix in the first direction, the linear image pattern will then grow in the direction perpendicular to the first direction. That is, the threshold values are arranged go that after the threshold value increases in the first direction in the entire length of the dither matrix, then the threshold value will increase in the second direction. With this method, it is possible to restrain occurrence of jitter successfully to a great degree that even when undesirable bandings or stripes are produced, they will not be visually perceived.
The dither matrix may have a length in the first direction and a width in the second direction, the length being greater than the width. In comparison with a square-shaped comparative dither matrix whose length is equal to the width, the rectangular-shaped dither matrix, whose length is longer than the width, can reproduce a higher input image density by the linear pattern. In other words, the first direction-elongated dither matrix can reproduce halftone images of a higher density without being effected by the jitter problem, in comparison with the square-shaped dither matrix. Occurrence of jitter can therefore be avoided more effectively.
The predetermined order may be defined to turn ON a first pair of two dots, after completely producing the linear pattern over an entire length of the dither matrix in the first direction, so that a line connecting the first pair of two dots will form a predetermined angle with respect to the second direction, the predetermined angle being substantially equal to an angle that is formed between the second direction and a direction, in which the dither matrix is repeatedly located along the second direction.
The threshold values may be arranged in the dither matrix in the predetermined order so that the threshold values, of a predetermined minimum value to a first predetermined middle value greater
Brother Kogyo Kabushiki Kaisha
Le N.
Oliff & Berridg,e PLC
Pham Hai C.
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