JPH05304594A - Image forming method - Google Patents

Abstract

PURPOSE:To form an image having a screen angle without causing unevenness of tilt stripes by forming dots so that three dot centers close to each other formed in a same mirror face satisfy a specific condition. CONSTITUTION:In the image forming method provided with a scanning optical system employing a rotary polygon mirror, an optional point A is selected, a 1st close point B closest to the point A among points formed by the same mirror face with the point A is extracted, and a point closest in common to both the points A, B is extracted as a 2nd close point C. A point D is a closest point to the point A and a distance between the points A, D is equivalent to a substantial resolution. Then a difference of the distance among the three points being the optional dot center A on a scanning line formed by an optional mirror face, the 1st closest dot center B formed closest to the dot center A by the same mirror face other than on the scanning line, and the 2nd closest dot center C is set smaller than the closest dot distance (substantial resolution).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method suitable for 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 this specification, the center of "dot" means the center of fatting when the dither method or the density pattern method is used.

[0003]

2. Description of the Related Art Conventionally, a dither method, a 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 the setting method of the threshold matrix, and further, in reproducing a color image performed by superposing multicolor ink or multicolor toner. Is particularly advantageous for obtaining an average color uniformity that does not depend on color moire fringes, misregistration, and the like.

By the way, the applicant of the present invention, apart from this,
We have already proposed a method for improving gradation while maintaining high resolution with a relatively simple device configuration. Such a method is applied to an apparatus which scans a photosensitive drum with a light beam modulated by an image signal and forms an image using an electrophotographic method, as disclosed in Japanese Patent Laid-Open No. 60-258103. .

FIG. 5 is a block diagram showing a circuit configuration for generating a signal for realizing such a method.

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 the reference clock signal 24 generated by the oscillator 33 by the timing signal generation circuit 34 (in this case, 1/2
A pattern signal 26 (for example, a triangular wave) having the screen clock 25 obtained by counting down the period as a synchronization signal is generated by the pattern signal generator 35.

Analog image signal 22 and pattern signal 26
Is compared with a comparator 36, and the analog image signal 22
The binarized image data 27 is created with "0" when it is larger and "1" when it is smaller.

In such an image processing method, a digital image signal is once converted into an analog image signal and then compared with a triangular wave having a predetermined period to perform almost continuous pulse width modulation, thereby providing a high gradation property. Image output with high image quality can be obtained.

In such a pulse width modulation circuit, a shift register 37 is provided between the timing signal generating circuit 34 and the pattern signal generator 35 to delay the phase of the pattern signal 26 for each scanning line. Thus, a halftone dot image having an image forming angle can be output.

FIG. 6 is a waveform diagram showing an example of delay generation of a pattern signal for each scanning line by this delay circuit. Further, FIG. 7 is a schematic diagram showing a pattern of an output image of each color by this pattern signal.

Here, the recording density is 80 dp in the main scanning direction.
i, 400 dpi in the sub-scanning direction, and the delay amount for each line of the pattern signal was 2/5 pixels (image forming angle 26.6 °). At this time, a polygon mirror having eight faces was used.

[0013]

However, in the image forming apparatus using the scanning optical system (for example, a laser printer using a polygon mirror) as in the above-mentioned conventional example, image deterioration such as pitch unevenness may occur. know. Therefore, in the conventional configuration, when an image having a screen angle is formed, the writing position periodically shifts in each direction of the main scanning and the sub-scanning for each scanning line, and as shown in FIG. In particular, there is a drawback that diagonal stripes are generated especially in the highlight and halftone portions.

This is because the scanning beam forming each dot has a light amount distribution as shown in FIG. 9, so that when the beam is scanned at a position deviated from the position to be originally formed, it is shown in FIG. As described above, the positions of the dots arranged at substantially equal intervals with the surrounding pixels are displaced, and as shown in FIG.
This is caused by uneven interference between the dots (thus, the higher the resolution, the more this defect tends to increase).

This problem is common to all image forming apparatuses that form an image having a screen angle by using the above-mentioned scanning optical system.

[0016]

According to the present invention, in an image forming method provided with a scanning optical system using a rotatable polygon mirror, an arbitrary scanning line formed on an arbitrary mirror surface can be used. Differences in respective distances between the dot center and the three points of the center of the first proximity dot and the center of the second proximity dot formed near the dot center by the same mirror surface other than on the scanning line are By satisfying the condition that the distance between the closest dots (substantial resolution) is smaller, it is possible to form an image having a screen angle with no oblique stripes.

With respect to the above-mentioned problems, the inventor of the present invention has proposed a conventional image forming apparatus using the PWM method.
As a result of a study, it is necessary to reduce the difference between the distances of three adjacent points formed by the same polygon mirror surface as shown in FIG. 1 in order to eliminate the above-mentioned oblique stripes. Understood, the present invention was completed.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a drawing which best represents the features of the present invention. In the drawing, points A, B, and C are three adjacent points formed by the same mirror surface. Further, “◯” represents the center of the dot. It can be seen from the figure that the difference between the distances of the three adjacent points is small, as seen from an arbitrary point A,
Close points B, C, formed by the same mirror surface
It can be seen that the distances are almost the same at any points of E, F and G. In other words, since they are formed by the same mirror surface, even if there is a deviation due to this mirror surface, all these points will be displaced in the same amount and in the same direction, so the actual image will be uneven streaks in a specific direction. It was found that (diagonal stripes) did not occur.

Three adjacent points formed by the same mirror surface
Description: In FIG. 1, an arbitrary point A is selected. The first proximity point B is extracted as the point closest to the point A among the points formed by the same mirror surface as the point A. Although the point F is also close, the same result as selecting the point B is obtained. Therefore, the point B is selected here.

Next, when a point that is common to both points A and B is extracted, the second proximity point C is selected (for the same reason as point E, point F is not selected here). The point D is the closest point of the point A, and the distance between A and D corresponds to the actual resolution.

As described above, three adjacent points formed by the same mirror surface are selected.

Relationship between Occurrence of Oblique Streaks 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 lines. Table 1 shows the results of studying how small the size needs to be.

As can be seen from the table, the state of occurrence of diagonal stripes is

[0024]

[Outer 1] It has been found that good results are obtained when the above conditions are satisfied.

Description of three adjacent points where diagonal stripes occur As shown in the conventional example, the main scanning resolution is 80 dpi,
When the sub-scan is 400 dpi, the image forming angle is 26.6 °, and the number of surfaces of the polygon mirrors used is eight, diagonal stripes occur.

FIG. 2 shows the above-mentioned three adjacent points based on the above conditions. No. of Table 1 As you can see from 1,

[0027]

[Table 1]

[0028]

[Outside 2] That is, what is outside the conditions found by the present invention are:
It can be confirmed that diagonal stripes are generated.

(Other Examples) As can be seen from Table 1,
The image pattern is No. If the conditions such as 2, 5 and the like are satisfied, the effects of the present invention can be obtained.

Further, although the above-mentioned embodiment has the same configuration as the device 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.

An embodiment using the dither method according to the pattern satisfying the conditions according to the present invention is shown in FIGS.
It is shown in (b). The respective states are shown in Table 2.

[0033]

[Table 2]

[0034]

As described above, according to the present invention, when the centers of three adjacent dots formed by the same mirror surface satisfy the above condition, the slanted stripe shape as shown in the above problem is obtained. There is an effect that an image having a screen angle can be formed without causing the unevenness.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of an image forming pattern that 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 in which the present invention is applied to a dither method.

FIG. 5 is a block diagram showing a pulse width modulation circuit in the printer.

6 is a waveform diagram showing an example of delay of a pattern signal in the pulse width modulation circuit shown in FIG.

FIG. 7 is a schematic diagram showing a pattern of an output image of each color by the same pattern signal shown in FIG.

FIG. 8 is a diagram showing a case where images of respective colors are duplicated in the printer,
It is a schematic diagram which shows a mode that the diagonal stripe was generated.

FIG. 9 is a schematic diagram showing 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 where interference has occurred between dots in a conventional image.

Claims (1)

[Claims] 1. An image forming method for forming an image using a polygon mirror, wherein two dots are provided for each of three adjacent dots that can be formed on mutually different scanning lines by a light beam reflected by one mirror surface. An image forming method characterized in that dots are formed so that all the differences in distance between the centers of the dots are smaller than the distance between the closest dots.