JP3530538B2 - Image forming method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitably applied to an image forming apparatus using a scanning optical system, and more particularly to an image forming method for forming an image having a screen angle. In the present specification, the center of a "dot" means the center of fatting when a dither method or a density pattern method is used. Conventionally, dither method, density pattern method and the like are well known as methods for expressing halftones. According to these, it is possible to easily form an image having a screen angle by setting a threshold matrix, and to reproduce a color image formed by superimposing multicolor inks or multicolor toners. Is particularly advantageous for removing average moire fringes and obtaining an average color uniformity regardless of misregistration. By the way, the applicant of the present application separately stated that
A method of improving the gradation with a relatively simple apparatus configuration while maintaining high resolution has already been proposed. Such a method is applied to an apparatus that scans a photosensitive drum with a light beam modulated by an image signal and forms an image using an electrophotographic method as disclosed in, for example, JP-A-60-258103. . FIG. 5 is a block diagram showing a circuit configuration for generating a signal for realizing such a method. [0006] The digital data 21 from the digital data output device 31 is converted into an analog image signal 22 by a D / A converter 32. On the other hand, the image data transfer pixel clock 23
Is divided by a timing signal generation circuit 34 by a reference clock signal 24 generated by an oscillator 33 (here, 1/2).
A pattern signal 26 (for example, a triangular wave) is generated by a pattern signal generator 35 using a screen clock 25 obtained by counting down to a cycle) as a synchronization signal. The analog image signal 22 and the pattern signal 26
Is compared with the analog image signal 22 by the comparator 36.
The binary image data 27 is created as “0” when is larger and “1” when smaller. In such an image processing method, a digital image signal is converted into an analog image signal once, and then compared with a triangular wave having a predetermined period, thereby performing almost continuous pulse width modulation, thereby achieving high gradation with high gradation. High quality image output is obtained. In such a pulse width modulation circuit, a shift register 37 is provided between the timing signal generator 34 and the pattern signal generator 35 to delay the phase of the pattern signal 26 for each scanning line. Thus, a halftone image having an image forming angle can be output. FIG. 6 is a waveform diagram showing an example of occurrence of a delay of a pattern signal for each scanning line by the delay circuit. FIG. 7 is a schematic diagram showing a pattern of an output image of each color according to the pattern signal. The recording density is 80 dp in the main scanning direction.
i, 400 dpi in the sub-scanning direction, and the amount of delay of the pattern signal for each line was set to 2/5 pixel (image formation angle: 26.6 °). At this time, eight polygon mirrors were used. However, in an image forming apparatus using a scanning optical system (for example, a laser printer using a polygon mirror), image deterioration such as pitch unevenness occurs as in the above-mentioned conventional example. I know I will. For this reason, in the conventional configuration, when an image having a screen angle is formed, the writing position is periodically shifted in the main scanning direction and the sub-scanning direction for each scanning line. In addition, there is a disadvantage that an oblique line is generated particularly in a highlight and a halftone portion. This is because the scanning beam forming each dot has a light amount distribution as shown in FIG. 9, and when the beam is scanned at a position shifted from the position where it should be formed, the scanning beam shown in FIG. As shown in FIG. 11, the positions of the dots arranged at substantially equal intervals with the surrounding pixels are shifted.
This is because uneven interference occurs between the dots (therefore, the higher the resolution, the more the disadvantage tends to increase). This problem is common to all image forming apparatuses that form an image having a screen angle using the above-described scanning optical system. According to the present invention, there is provided an image processing method for forming an image using a polygon mirror on a main scanning line.
The distance between dots that can be formed, the sub-scanning resolution, and the image
Based on the image formation angle, for three adjacent dots that can be formed on different scan lines by the light beam reflected by one mirror surface, the difference between the centers of each two dots is all the closest. It is characterized in that it is set to be smaller than the distance between dots. The present inventor has solved the above-mentioned problem in an image forming apparatus using the PWM method shown in the conventional example.
Investigations have shown that in order to eliminate the diagonal streaks as described above, it is necessary to reduce the difference in distance between each of three adjacent points formed by the same polygon mirror surface as shown in FIG. Understand, the present invention has been completed. FIG. 1 is a drawing that best illustrates the features of the present invention. In the drawing, points A, B, and C are three adjacent points formed by the same mirror surface. “O” indicates the center of the dot. The fact that the difference between the distances of the three adjacent points is small means that, from the arbitrary point A,
Close points B, C, formed by the same mirror surface
It can be seen that the distances are almost the same for all points E, F, and G. In other words, even if the mirror surface is misaligned, all these points are misaligned in the same direction in the same direction because they are formed by the same mirror surface. It was found that (oblique streaks) did not occur. Three adjacent points formed by the same mirror surface
Description In FIG. 1, an arbitrary point A is selected. A first proximity point B is extracted as a point formed by the same mirror surface as the point A and closest to the point A. Point F is close, but the result is the same as that of selecting point B, so point B is selected here. Next, when a point close to both points A and B is extracted in common, a second proximity point C is selected (for the same reason as point E, point F is not selected here). Point D is the closest point of point A, and the distance between A and D corresponds to the substantial resolution. As described above, three adjacent points formed by the same mirror surface are selected. Relationship Between Oblique Streak Generation and Three Points As described above, it is necessary to reduce the difference in distance between any three adjacent points formed by the same mirror surface in order to prevent the occurrence of oblique streaks. Table 1 shows the result of examining how much the size needs to be reduced. As can be seen from the table, the generation state of the oblique streak is as follows. It was found that good results were obtained when Description of Three Proximate Points where Oblique Streaks Occur As described in the conventional example, the main scanning resolution is 80 dpi,
When the number of surfaces of the polygon mirror used is 8 at the sub-scanning direction of 400 dpi and the image forming angle of 26.6 °, an oblique streak occurs. FIG. 2 shows the above three adjacent points based on the above conditions. No. 1 in Table 1. As can be seen from FIG. [Outside 2] That is, what is outside the conditions found by the present invention is:
It can be confirmed that oblique streaks are generated. (Other Embodiments) As can be seen from Table 1,
The image pattern is designated as No. If the above conditions are satisfied, such as 2, 5, etc., the effect of the present invention can be obtained. In the above-described embodiment, the configuration is the same as that of the apparatus using the PWM method shown in the conventional example.
The present invention is not limited to this. For example, the effect of the present invention can be obtained even if the image forming pattern shown in FIG. 1 is output by a dither method using a dot matrix as shown in FIG. 3 at a resolution of 600 dpi in both main scanning and sub-scanning. Needless to say. FIGS. 4A and 4B show an embodiment using a dither method by a pattern satisfying the conditions according to the present invention.
(B). Table 2 shows the respective states. [Table 2] As described above, according to the present invention, the center of three adjacent dots formed by the same mirror surface satisfies the above condition, thereby achieving the above-mentioned problem. There is an effect that an image having a screen angle can be formed without generating oblique streak-like unevenness.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an embodiment of an image forming pattern, which best represents the present invention. FIG. 2 is a diagram showing a conventional image forming pattern. FIG. 3 is a diagram showing a dot matrix, which is an embodiment in which the present invention is applied to a dither method. FIG. 4 is a diagram showing a pattern obtained by applying the present invention to a dither method. FIG. 5 is a block diagram showing a pulse width modulation circuit in the printer. 6 is a waveform chart showing an example of delay of a pattern signal in the pulse width modulation circuit shown in FIG. 7 is a schematic diagram showing a pattern of an output image of each color according to the same pattern signal shown in FIG. FIG. 8 illustrates a case where each color image is overlapped in the printer.
It is a schematic diagram which shows a mode that a diagonal streak occurred. FIG. 9 is a schematic diagram illustrating a light amount distribution in a laser beam printer. FIG. 10 is a schematic diagram showing a dot arrangement in a conventional image. FIG. 11 is a schematic diagram showing a state in which interference has occurred between dots in a conventional image.

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Claims (1)

(57) [Claim 1] In an image forming method for forming an image using a polygon mirror, a distance between dots that can be formed on a main scanning line and a sub-scanning solution
Difference in distance between centers of two adjacent dots for three adjacent dots that can be formed on mutually different scanning lines by a light beam reflected by one mirror surface based on an image forming angle and an image forming angle. Are set to be smaller than the distance between the closest dots.