JPH10257336A - Gradation reproducing method and image-forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gradation reproducing method and an image-forming device, with which a color image or multicolor image having less conspicuous textures can be formed, while suppressing the deviation or nonuniformity of color. SOLUTION: As a gradation pattern corresponding to one color component, a 1st gradation pattern of pattern to vertically and linearly grow a dot with the fine increase of gradation is used and as a gradation pattern corresponding to the other color component, a 2nd gradation pattern of pattern to be thickened in the state of clad with the increase of gradation is used. At the low-density section of color image at least, a linear gradation pattern image PSFM due to the 1st gradation pattern and a clad-state gradation pattern image PSFK due to the 2nd gradation pattern are overlapped.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of reproducing gradation using a pattern whose dot area ratio increases or decreases in accordance with the gradation, for each image data of a color component of a color image, and its gradation reproduction. The present invention relates to an image forming apparatus to which the method is applied.

[0002]

2. Description of the Related Art With the spread of personal computers, electrophotographic full-color printers have become increasingly popular.
It is beginning to spread not only for business use but also for home use. In an electrophotographic printer, a dither method is generally used as a method of expressing gradation.

FIG. 13 is a block diagram for explaining the operation of the dither method. In FIG. 13A, the density value of each pixel of the input image information 81 and the threshold value of each element of the threshold value matrix MT11 as a gradation pattern are one-to-one.
Are compared. When the density value of each pixel is larger than the threshold value of each element, the value of the position corresponding to that element becomes “1”. Conversely, when the density value of each pixel is smaller than the threshold value of each element, , The value of the position corresponding to the element is “0”. As a result, a gradation pattern image 83 is obtained. In the example of FIG. 13A, the density value (level) of the image information 81 is set to “4”.

[0004] Further, in the threshold value matrix MT11 shown in FIG. 13 (A), it is possible to reproduce an image by gradation of 16 gradations (levels). However, as shown in FIG. By dividing and controlling the laser lighting time, the gradation can be increased.

In addition to the dither method described above, FIG.
As shown in FIGS. 14A to 14H, an output pattern corresponding to each density value is created in advance as a look-up table (LUT), and when image information is input,
Refer to the lookup table according to the density value,
A method of outputting an image is also used.

These tone reproduction methods are applied to color image formation, and for example, as shown in FIGS. 15A and 15B, two color images of black and magenta are superimposed using the same threshold matrix MT. In this case, two-color dots completely overlap as shown in FIG. 15 (C) due to unevenness in the distance between output dots due to the influence of pitch unevenness due to rotation unevenness of the photoreceptor or variation in the printing position. On the contrary, they may be juxtaposed as shown in FIG. At this time, when the dots completely overlap as shown in FIG. 15C, the color becomes blackish, and when the dots are juxtaposed as shown in FIG. 15D, the color becomes reddish. Therefore, even if an image is originally uniform in density, when the image is printed, color unevenness occurs due to unevenness in the way dots overlap.

Therefore, in order to disperse how dots overlap, a gradation pattern (also called a density pattern or an exposure pattern) is prepared for each color component, and the screen angle of the gradation pattern is made different for each color component. In order to prevent the occurrence of color unevenness, some measures have been taken.

For example, in the dither method, FIG.
And the unit threshold matrix MT13 is shifted in the vertical direction and the horizontal direction, respectively.
And set its screen angle.
Gradation pattern PSG13 shown in FIG. 16 (B) is, a pixel unit threshold matrix MT13 to the right, four pixels in this example, b pixels in the downward direction, since the arranged by shifting 1 pixel in this example, tan .theta ₁ ≒ 1/4 and has a screen angle θ ₁ of about 14 degrees.

The gray scale pattern PSG shown in FIG.
13 means that the unit threshold matrix MT13 of FIG. 16A is shifted by one pixel to the left and four pixels downward, so that tan θ = 4/1, and the screen of about 104 degrees it can also be said to have an angle θ _2.

That is, the gradation pattern P shown in FIG.
SG13 has two screen angles θ different from each other by 90 degrees.
₁ and θ ₂ . The resolution (number of lines) of an image output using the gradation pattern PSG13 shown in FIG.
Is in inverse proportion to the length c (unit is pixel) of a line connecting the centers of the images, and the resolution of the image is set to be substantially the same for each color. Therefore, in this specification, the resolution is represented by c. If the difference between the screen angles of any two gradation patterns corresponding to each color is small, the texture (rosette) becomes conspicuous as shown in FIG. 17, for example. The difference must be set to be as large as possible.

[0011]

However, in a conventional color or multicolor image forming apparatus using the dither method,
Since the number of elements constituting the threshold matrix as a unit is limited to a certain number or less, the screen angle that can be set is limited to a finite number of discrete values. When the resolution is to be increased, the number of elements is further limited, and the possible screen angle value is further limited.

Further, as described above, in the conventional dither method, since the gradation pattern PSG has two screen angles θ ₁ and θ ₂ different from each other by 90 degrees, the gradation pattern PSG corresponding to each color component is provided. The screen angle that can be set for is further limited.

In a conventional color or multicolor image forming apparatus, even if different screen angles are set for each gradation pattern PSG corresponding to each color component, a large difference between the screen angles cannot be set. . Usually, in order to suppress the occurrence of color unevenness, each gradation pattern PSG
It is desirable to provide a screen angle difference of 20 degrees or more for each, but as in recent years, the Y, M, C, and K
When full color is reproduced by color, the angle is insufficient. For this reason, there has been a problem that the occurrence of color shift or color unevenness cannot be sufficiently suppressed, or a ring-shaped texture (rosette) is periodically generated, thereby deteriorating the image quality.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and is capable of suppressing the occurrence of color misregistration or color unevenness and forming a color image or a multicolor image with less noticeable texture. It is an object to provide a reproduction method and an image forming apparatus.

[0015]

According to a first aspect of the present invention, there is provided a method of reproducing gradation by using a pattern of increasing or decreasing a dot area ratio according to a gradation for each image data of a color component of a color image. Wherein the pattern corresponding to one color component is a pattern in which dots grow linearly in the vertical direction with a slight increase in gradation.
The second pattern, which is a pattern that becomes bulky as the gradation increases, is used as the pattern corresponding to the other one color component, and at least a low density portion of the color image is used. The linear pattern image of the first pattern and the block pattern image of the second pattern are overlapped.

According to a second aspect of the present invention, there is provided a method of reproducing gradations for each image data of four color components of a color image by using a pattern whose dot area ratio increases or decreases in accordance with the gradation. In addition, as the pattern corresponding to one color component, a first pattern in which dots grow linearly in the vertical direction with a slight increase in gradation is used, and the first pattern corresponding to another color component is used. The second pattern having a screen angle substantially equal to that of the first pattern is used as the second pattern having the same screen angle as the first pattern. As the pattern, a second pattern having a screen angle that is substantially the same as that of the first pattern is used, and the other two patterns are used.
The third pattern and the fourth pattern having different screen angles from the first pattern and the second pattern and having different screen angles from each other are used as the patterns corresponding to the three color components, and In the low density portion, the linear pattern image of the first pattern and the block pattern image of the second pattern are overlapped.

According to a third aspect of the present invention, there is provided an image forming apparatus for performing tone reproduction on each image data of four color components of a color image by using a pattern whose dot area ratio increases or decreases according to the gradation. A first pattern that is used in correspondence with one color component and is a pattern in which dots grow linearly in the vertical direction with a slight increase in gradation, and another one color component. The second pattern is used in correspondence with the second pattern having a screen angle that is substantially the same as that of the first pattern and the other two color components. And a third pattern and a fourth pattern having different screen angles from the first pattern and the second pattern and having different screen angles from each other.

Next, a gradation reproducing method according to the present invention will be described with reference to the drawings. Of the four gradation patterns PSG shown in FIGS. 4 to 7, the two gradation patterns PSGK and PSGM shown in FIGS. 4 and 5 both have a screen angle of 0 degrees. One of them, the gradation pattern PSG of FIG.
K is a pattern that becomes bulky from the center to the periphery as the gradation increases. That is, a block-shaped gradation pattern image PSFK as shown in FIG. 8 is obtained by the gradation pattern PSGK. The gradation pattern PSGM shown in FIG. 5, which is another one, is a pattern in which dots grow linearly in the vertical direction with a slight increase in gradation. With the gradation pattern PSGM, when the image information has a low density, a linear gradation pattern image PSFM as shown in FIG. 9 is obtained. A linear gradation pattern image PSFM is obtained in a relatively low density area as can be understood from FIG. Therefore, the present invention has a great effect in a portion where the image has a low density, that is, a region where a substantially linear gradation pattern image PSFM is obtained.

When the gradation pattern image PSFK shown in FIG. 8 and the gradation pattern image PSFM shown in FIG.
The image shown in FIG. 2 is obtained. FIG. 12A shows an image when the positions of both tone pattern images PSFK and PSFM are exactly the same, and FIGS. 12B and 12C show that the positions of both tone pattern images PSFK and PSFM are top and bottom. 5 shows images when they are displaced from each other in the directions.

As can be understood from FIG. 12, there is no change in the overlapping area regardless of whether the positions of the two gradation pattern images PSFK and PSFM are correct or they are shifted from each other in the vertical direction. That is, color shift or color unevenness does not occur depending on the position shift in the vertical direction. Most of the displacement of the gradation pattern image PSF occurs in the sub-scanning direction in which the driving force acts, that is, in the vertical direction.
Since it hardly occurs in the main scanning direction, it is sufficient for practical use if positional deviation in the vertical direction is considered.

[0021]

FIG. 1 is a block diagram showing the overall configuration of a full-color image forming apparatus 1 according to the present invention, and FIG. 2 is a sectional view showing the configuration of an image forming section 10.

In FIG. 1, an image forming apparatus 1 includes an image forming section 1 for forming a full-color image by an electrophotographic method.
0, a CPU 21 for overall control, a ROM 22, a RAM 23 used as a working area, a memory 24 for temporarily storing input image data, and an input unit 25.

The ROM 22 stores a control program, a threshold matrix MT for binarization by the dither method, and the like. The threshold matrix MT is provided for each of four color components of Y (yellow), M (magenta), C (cyan), and K (black). The threshold matrices MTY, MTM, MT
It may be described as C or MTK. These threshold matrices MTY, MTM, MTC, and MTK are also gradation patterns, and correspond to patterns in the present invention.

In FIG. 2, the image forming section 10 is provided with a photoconductor 11 as an image carrier having an organic photoconductive material (OPC) applied on its surface, which is rotatable in the direction of arrow A. Around the photoreceptor 11, along the rotation direction thereof, a charging brush 12, a laser exposure device 13, a developing device 14,
An intermediate transfer member 15 and a cleaner unit 16 are provided.

The discharge of the charging brush 12 causes the photosensitive member 11
Is charged to a constant potential, and a laser beam is applied to the surface of the photoreceptor 11 by the laser exposure device 13 in accordance with image information, and an electrostatic latent image is formed in the charged area.
The electrostatic latent image is visualized as a toner image by the developing device 14.

The developing device 14 has four colors of Y, M, C, and K.
Developing units 14Y, 14M, 14C having respective color toners
14K. These four developing units 14Y, 14
Although not shown, M, 14C, and 14K are switchable. The electrostatic latent images formed on the surface of the photoreceptor 11 are the developing units 14Y, 14M, 14C, 1 of these colors.
The development is performed sequentially at 4K, and the toner image is transferred to the intermediate transfer member 15 each time the development of each color is performed. Therefore, the four color toner images are superimposed on the intermediate transfer member 15, and a full color toner image is formed.

The toner image formed by the intermediate transfer member 15 is transferred onto a sheet (not shown), and is then thermally fixed by a fixing device (not shown). As a result, a full-color image is formed on the sheet. The toner remaining on the photoconductor 11 without being transferred to the intermediate transfer member 15 is collected by the cleaner unit 16.

FIG. 3 is a flowchart showing the control operation by the CPU 21. In FIG. 3, after the entire initial setting is performed (# 11), image information corresponding to one color component is read out of the image data input to the memory 24 (# 12). This image information is image information having gradation such as 64 gradations or 256 gradations. The read image information is compared with the threshold matrix MT corresponding to the color component of the threshold matrix MT stored in the ROM 22, and the image information is binarized by the dither method (# 13).

The laser exposure device 13 is controlled based on the binarized image information (gradation pattern image), and an image is formed (# 14). Thereby, the image forming unit 1
At 0, a toner image corresponding to one color is formed. Then, the processes from step # 12 onward are repeated for each of the four color components (# 15).

As described above, toner images are formed for all four color components, and these are combined to form a full-color image. Next, the threshold matrices MTY, MTM, MTC, MTK stored in the ROM 22
Will be described.

It is assumed that these threshold matrices MTY, M
When the same threshold matrix MTK, MTM is used for each of the TM, MTC, and MTK, for example, the screen angles are the same.
The color misregistration as shown in FIG.

Further, in order to disperse the state of how dots overlap, threshold matrices MTY, MTM, MTC, MT
The screen angle is made different for each K, so that the occurrence of color shift can be suppressed. However, in this case, if the difference between the screen angles of the two colors of interest becomes small, the texture (rosette) becomes conspicuous, so that the difference between the screen angles of the two colors is preferably 20 degrees or more.

For example, the screen angle based on the threshold matrix in which the value of the resolution c is near “4” shown in FIG. 16 is approximately as shown in (a) to (h) of Table 1, The screen angle is selected from among them.

[0034]

[Table 1]

However, when the gradation pattern images corresponding to the four colors of Y, M, C, and K are all formed by the conventional method, any one of (a) to (h) in Table 1 is used. Even if the screen angle for four colors is selected in such a combination,
The difference in screen angle between colors becomes small. This is due to the above-described two-directional screen angle. Therefore, according to the conventional method, the screen angles of the four-color gradation pattern images cannot be set to be different from each other by 20 degrees or more, and it is inevitable that color shift or color unevenness occurs.

Therefore, in this embodiment, K, M,
Threshold matrices MTK, MT corresponding to four colors of Y and C
The gradation patterns PSGK, PSGM, PSGY, and PSGC shown in FIGS. 4 to 7 are used as M, MTY, and MTC. These gradation patterns PSGK, PSGM, PSG
When the density value (level) of the image information is, for example, “8” by Y and PSGC, tone pattern images PSFK, PSFM, PSFY, and PSFC shown in FIGS. 8 to 11 are obtained.

Of the four tone patterns PSG, two tone patterns PSGK and PSGM shown in FIGS.
Has a screen angle of 0 degree. One of them, the gradation pattern PSGK of FIG. 4, is a pattern that becomes thicker from the center toward the periphery as the gradation increases. That is, by the gradation pattern PSGK,
A massive tone pattern image PSFK as shown in FIG. 8 is obtained. Focusing on the fact that the gradation pattern image PSFK has a block shape, it may be referred to as a block-shaped gradation pattern image PSFK.

Another gradation pattern PSG shown in FIG.
M indicates that the dots are linear in the vertical direction (also referred to as linear or linear) as the gray level increases slightly, that is, as the gray level increases in each of 8 gray levels out of 32 gray levels. ). With the gradation pattern PSGM, when the image information has a low density, a linear gradation pattern image PSFM as shown in FIG. 9 is obtained. Focusing on the fact that the gradation pattern image PSFM is linear, it may be referred to as a linear gradation pattern image PSFM.

When the gradation pattern image PSFK shown in FIG. 8 and the gradation pattern image PSFM shown in FIG.
The image shown in FIG. 2 is obtained. FIG. 12A shows an image when the positions of both tone pattern images PSFK and PSFM are exactly the same, and FIGS. 12B and 12C show that the positions of both tone pattern images PSFK and PSFM are top and bottom. 5 shows images when they are displaced from each other in the directions.

As can be understood from FIG. 12, the overlapping area does not change even when the positions of the two tone pattern images PSFK and PSFM are correct or when they are shifted from each other in the vertical direction. That is, color shift or color unevenness does not occur depending on the position shift in the vertical direction. As described above, the two gradation patterns PS each having the screen angle of 0 degree.
Even if GK and PSGM are used, no color shift or color unevenness occurs.

It should be noted that the positional deviation of the gradation pattern image PSF mostly occurs in the sub-scanning direction (paper running direction) where the driving force acts, that is, in the vertical direction, and hardly occurs in the main scanning direction. It is practically sufficient if the positional deviation in the vertical direction is considered.

Since the screen angles of the two gradation patterns PSGK and PSGM are both 0 degrees, the two gradation patterns PSGK and PSGM have a screen angle of 0 degree and the other two gradation patterns PSGY and PSGC and the other two gradation patterns PSGY and PSGC have the same screen angle.
It is possible to increase the difference in screen angle between the two gradation patterns PSGY and PSGC.

That is, the gradation pattern PSG shown in FIG.
Y has a screen angle of about 30 degrees, whereby a gradation pattern image PSFY shown in FIG. 10 is obtained. FIG.
The gray scale pattern PSGC shown in FIG.
In this way, the gradation pattern image PSFC shown in FIG. 11 is obtained.

These four gradation patterns PSGK, PS
By using GM, PSGY, and PSGC, the difference between the screen angles is about 30 degrees, color shift or color unevenness can be suppressed, and a color image with less noticeable texture can be formed.

In the above embodiment, the gradation pattern P
SGM and PSGK are the first pattern in the present invention,
The gradation patterns PSGY, PSG correspond to the second pattern.
GC corresponds to the third pattern or the fourth pattern in the present invention.

In the above-described embodiment, the four gradation patterns PSGK, PSGM, PSGY, and PSGC may be used for other color components. When a full-color image is formed by three colors, one of the two
The screen angles of the gradation patterns PSG for the colors may both be 0 degrees, and the screen angles of the gradation patterns PSG for the other colors may be different. When the image to be formed is not full color, for example, when the image is composed of two color components, the block-shaped gradation pattern PSG and the linear gradation pattern PSG may be used.

In the above-described embodiment, the case where the gradation pattern PSG is the threshold matrix MT has been described. However, a look-up table or a density pattern used for a density pattern method may be used. Further, the size of the gradation pattern PSG, the arrangement of the thresholds, and the like can be appropriately changed in addition to the above. Further, the structure, shape, number, material, and the like of the entire image forming unit 10 or the image forming apparatus 1 or each unit may be various other than those described above.

[0048]

According to the first to third aspects of the present invention,
The occurrence of color shift or color unevenness can be suppressed, and a color image or a multicolor image with less noticeable texture can be formed.

According to the invention of claims 2 and 3, 4
Depending on the color components of the colors, a full-color image with no color shift or color unevenness and less noticeable texture can be formed.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an overall configuration of a full-color image forming apparatus according to the present invention.

FIG. 2 is a cross-sectional view illustrating a configuration of an image forming unit.

FIG. 3 is a flowchart showing a control operation by a CPU.

FIG. 4 is a diagram showing an example of a gradation pattern.

FIG. 5 is a diagram illustrating an example of a gradation pattern.

FIG. 6 is a diagram showing an example of a gradation pattern.

FIG. 7 is a diagram illustrating an example of a gradation pattern.

FIG. 8 is a diagram illustrating an example of a gradation pattern image.

FIG. 9 is a diagram illustrating an example of a gradation pattern image.

FIG. 10 is a diagram illustrating an example of a gradation pattern image.

FIG. 11 is a diagram illustrating an example of a gradation pattern image.

FIG. 12 is a diagram showing an image when a linear gradation pattern image and a block gradation pattern image are superimposed.

FIG. 13 is a block diagram for explaining the operation of the dither method.

FIG. 14 is a diagram illustrating an example of a lookup table.

FIG. 15 is a diagram illustrating an example of an overlapping state of two-color images.

FIG. 16 is a diagram for explaining a screen angle of a gradation pattern.

FIG. 17 is a diagram illustrating an example of a texture.

[Explanation of symbols]

1 Image forming apparatus PSGK tone pattern (pattern, second pattern) PSGM tone pattern (pattern, first pattern) PSGY tone pattern (pattern, third pattern,
Fourth pattern) PSGC gradation pattern (pattern, fourth pattern,
Third pattern)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04N 1/60 H04N 1/40 D

Claims (3)

[Claims] 1. A method for reproducing gradation using a pattern in which a dot area ratio increases or decreases in accordance with a gradation, for each image data of a color component of a color image, the method corresponding to one color component. As the pattern, a first pattern, which is a pattern in which dots grow linearly in the vertical direction with a slight increase in gradation, is used. As the pattern corresponding to another one color component, the first pattern is used. A second pattern, which is a pattern that thickens together in a lump, is used, and at least a low-density portion of the color image includes a linear pattern image of the first pattern and the second pattern.
And a block pattern image of the above pattern is overlapped. 2. A method of reproducing tone using a pattern in which a dot area ratio increases or decreases in accordance with a tone, for each image data of four color components of a color image. As the corresponding pattern, the first pattern, which is a pattern in which dots grow linearly in the vertical direction with a slight increase in gradation, is used. As the pattern corresponding to another color component, The second pattern, which is a pattern that becomes thicker in a lump with an increase and has a screen angle substantially equal to that of the first pattern.
And a third pattern and a fourth pattern having different screen angles from the first pattern and the second pattern and different screen angles from each other as the patterns corresponding to the other two color components. In at least a low density portion of the color image, a linear pattern image of the first pattern and the second
And a block pattern image of the above pattern is overlapped. 3. An image forming apparatus for performing tone reproduction on each of image data of four color components of a color image by using a pattern whose dot area ratio increases or decreases in accordance with the gradation. The first pattern, which is a pattern in which dots grow linearly in the vertical direction with a slight increase in gradation, and the one pattern used in correspondence with another color component, Is a pattern that becomes thicker in a lump as the number of
A second pattern having a screen angle substantially the same as that of the second pattern is used in correspondence with the other two color components. The first pattern and the second pattern have different screen angles and are different in screen angle from each other. An image forming apparatus comprising: a third pattern and a fourth pattern which are different from each other.