JP3658024B2 - Image forming apparatus and method - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to an image forming apparatus and method, and more particularly to an image forming apparatus and method capable of forming a full-color multi-value image.
[0002]
[Prior art]
In a conventional image forming apparatus capable of forming a full-color multi-value image, for example, a process of transferring a recording image formed on an image carrier by charging, exposure, and development onto the recording paper is repeated a plurality of times, whereby Various types have been developed and put into practical use for forming a color image by forming a plurality of color superimposed images.
[0003]
A conventional image forming apparatus as described above will be described below with reference to the accompanying drawings. FIG. 6 shows a longitudinal sectional view of a conventional image forming apparatus capable of forming a color image.
[0004]
In FIG. 6, an image carrier 3 called an electrophotographic photosensitive drum (hereinafter referred to as a photosensitive drum 3) rotates in the direction of the arrow in the figure and is uniformly charged by a charger 10, and a laser diode (not shown), An electrostatic latent image is formed on the photosensitive drum 3 by irradiating a light image with the exposure unit 11 constituted by a rotary polygon mirror, a lens and the like that are rotationally driven by a high-speed motor. This latent image is transferred to the developing device 1 composed of each color developing device 1a, 1b, 1c, 1d containing a color developer such as yellow (Y), magenta (M), cyan (C), black (BK). And developed as a visible image, that is, a toner image.
[0005]
On the other hand, the transfer material 7 is conveyed by being fixed to the surface of a transfer device (transfer drum) 2 having a drum shape, for example, by a gripper 5, electrostatically adsorbed by an adsorber 8, and held on the transfer drum 2. The toner image on the photosensitive drum 3 is transferred onto the transfer material 7 wound around the transfer drum 2 in an overlapping manner.
[0006]
More specifically, after the electrostatic latent image formed on the photosensitive drum 3 is first visualized by the developing device 1a containing a yellow (Y) developer by the exposure based on the image signal of the first color such as yellow. Then, the image is transferred to the transfer material 7 held on the transfer drum 2. Subsequently, after the residual developer on the photosensitive drum 3 is removed by the cleaner 12, an electrostatic latent image of the second color is formed on the photosensitive drum 3 by exposure based on an image signal of the second color, for example, magenta. M) After being visualized by the developing device 1b having a developer, the yellow visible image of the first color is transferred onto the transfer material 7 on the transferred transfer drum 2 in an overlapping manner. Next, the same method as described above is repeated to transfer, for example, cyan (C) as the third color and black (BK) as the fourth color onto the transfer material 7 on the transfer drum 2. Thereafter, the transfer material 7 is separated from the transfer drum 2 by the separator 6 and fixed by the fixing device 4 to obtain a permanent image.
[0007]
The transfer drum 2 from which the transfer material 7 has been separated is removed from the developer adhering to the surface thereof by a transfer body cleaner (not shown), further neutralized by the static eliminator 9, and electrically initialized.
[0008]
When expressing a multi-valued image in the color image forming apparatus as described above, a method of expressing multi-value gradation by a matrix of a plurality of pixels in which one-pixel expression is a binary expression only by turning on / off the laser (Dither) and pulse width modulation (PWM) to express gradation by changing the basic pixel size in multiple stages by changing the laser lighting time, and also changing the laser light quantity Thus, there is a technique such as luminance modulation that expresses gradation by changing the exposure portion potential of the latent image of the basic pixel in a plurality of stages.
[0009]
In general, it is known that gradation expression by pulse width modulation (PWM) or luminance modulation is superior in gradation characteristics compared to gradation expression by dither. However, even in the gradation expression by PWM, the area of the latent image of the basic pixel formed in the highlight portion of the image becomes small, so the reproducibility is lowered. Also in the gradation expression by luminance modulation, the potential contrast of the latent image of the basic pixel formed in the highlight portion of the image becomes small, so the reproducibility is also lowered. As a method for preventing this deterioration in reproducibility, a method of expressing gradation by concentrating the area of basic pixels in PWM and the amount of light of basic pixels in luminance modulation by setting a plurality of basic pixels as one unit. Is used. This method is hereinafter referred to as blend γ.
[0010]
Hereinafter, the blend γ will be described in detail using PWM as an example.
[0011]
FIG. 7 is a diagram showing the arrangement of basic pixels when the blend γ is not applied in PWM. In FIG. 7, one rectangular frame represents one basic pixel and 6 × 3 pixels in total. A colored area having a predetermined width at the center of each pixel represents a pixel width that is actually reproduced by PWM. According to FIG. 7, especially in the highlight portion, the reproducibility deteriorates because the area (pixel width) reproduced in each basic pixel is small.
[0012]
Here, FIG. 8 shows an arrangement of basic pixels when blend γ is applied in PWM. Although the arrangement of 6 × 3 pixels is shown in the same manner as in FIG. 7 described above, in the case where the blend γ is applied, the reproduction area (pixel width) for all three pixels is set in units of 3 × 1 pixels. By distributing to two pixels, stable gradation can be obtained without changing the optical density. In other words, compared to the case where blend γ is not applied, images with the same optical density can be expressed with the basic pixels concentrated so that stable gradation expression is possible. It is possible to reproduce the tonal characteristics.
[0013]
As described above, the blend γ is a method suitable for stably expressing gradation in a state where basic pixels are concentrated, that is, with a plurality of basic pixels as one unit.
[0014]
Hereinafter, as an example of a method for realizing the blend γ described above, a lookup table (LUT) for correcting the input multi-value image signal so that the optical density of the output multi-value image is linear. A method of preparing a plurality of items will be described.
[0015]
As shown in the block diagram of FIG. 9, blending γ can be realized by preparing two LUTa, b having different input / output characteristics and outputting each LUT to the PWM circuit L1 while switching. The input / output characteristics of LUTa, b shown in FIG. 9 are shown in FIG. As can be seen from FIG. 11, the input / output characteristics of each LUTa, b are set so that the difference between the respective output signals obtained with respect to the same input signal becomes large in the highlight portion. The difference between the output signals is set so as to decrease as the distance from the highlight portion increases. That is, in the highlight portion, basic pixels that are widely reproduced are obtained when the LUTa is referred to, and basic pixels that are reproduced narrowly are obtained when the LUTb is referenced.
[0016]
Here, if the LUT is repeatedly referred to in the order of LUTa, LUTa, and LUTb with respect to the input image signal, for example, when the signal I is input, the PWM circuit L1 receives the signals Oa, Oa, Output signals are sent in the order of Ob. Since the output signal satisfies Oa> Ob, the signal sent to the PWM circuit L1 is an image in a state where basic pixels having a large reproduction width are concentrated in the sub-scanning direction, that is, as shown in FIG. Can be output.
[0017]
In addition, as shown in the block diagram of FIG. 10, a single LUTc is used, and a plurality of PWM circuits L2 and L3 having different input / output characteristics are prepared and output while switching each PWM circuit. It is possible to realize.
[0018]
As described above, the case of PWM has been described as an example, but this is the same in the case of luminance modulation. By applying blend γ, gradation can be expressed using basic pixels having a large potential contrast. Therefore, the gradation reproducibility is improved even in the highlight portion. As in the case of PWM, there are a method of preparing a plurality of LUTs and a method of preparing a plurality of luminance modulation circuits.
[0019]
By the way, in the color image forming apparatus as described above, the moiré frequency is shifted to the high frequency side by setting the screen angle to be different for each color, thereby suppressing the occurrence of moiré fringes in colors that are easily noticeable at low frequencies. ing. Further, when the mechanical vibrations are different during the sequential development of the four colors, color misregistration occurs. However, by using this method, the color misregistration is made inconspicuous.
[0020]
[Problems to be solved by the invention]
However, in the conventional color image forming apparatus, if the screen angle is given when the blend γ is applied to PWM, luminance modulation, etc., the concentrated basic pixels are dispersed as shown in FIG. . Accordingly, there is a problem that the effect of improving the reproducibility of the highlight portion by the blend γ is lowered.
[0021]
The present invention has been made to solve the above-described problem. To solve the above-described problem, when a multi-value image signal is input, gradation is expressed not only in a basic pixel unit but also in a plurality of pixel units. An object of the present invention is to provide an image forming apparatus and method capable of stably expressing a highlight portion even when a screen angle is provided.
[0022]
[Means for Solving the Problems]
In order to achieve the above-described object, the present invention has the following configuration.
[0023]
That is, an image forming apparatus that forms an image by comparing a multi-value image signal and a reference signal, wherein the phase of the reference signal is changed in units of expression pixels represented by a plurality of basic pixels. In the highlight unit of the change means, the second change means for changing the phase of the reference signal in units of the basic pixels, and the highlight portion of the multi-value image signal, the multi-value image signal is switched to the first change means. Other than the highlight portion, the switching means for switching to the second changing means, the reference signal whose phase has been changed by the first or second changing means, and the multi-value image signal are compared, Reproduction means for reproducing gradation in units of the expression pixels .
[0024]
Further, the reproducing means selectively uses the reference signal whose phase has been changed by the first or second changing means and the multi-value image signal by using a plurality of lookup tables having different input / output characteristics. The gradation is reproduced based on the signal obtained by the conversion .
[0025]
Further, the reproduction means performs a pulse width modulation by comparing the reference signal whose phase has been changed by the first or second changing means and the multilevel image signal, and a plurality of pulses having different input / output characteristics. The gradation is reproduced by selectively using a width modulation circuit .
[0026]
Further, the phase of the reference signal changed by the first or second changing unit is different for each color constituting the multi-value image signal .
[0027]
Further, the reference signal is a triangular wave .
[0028]
Further, based on the image formed by comparing the multi-value image signal and the reference signal, a recorded image is formed on the image carrier by charging, exposing and developing, and transferred to the transfer paper using the recorded image. It is characterized by doing.
[0029]
[Action]
With the configuration described above, gradation expression can be performed with respect to a multi-value image signal in a highlight portion in units of a plurality of pixels, and stable gradation expression can be achieved even when a screen angle is provided. Furthermore, by switching so that the screen angle is expressed in units of basic pixels in areas other than the highlight area, it is possible to obtain a stable gradation expression in the highlight area and a smooth screen angle in the other areas. Therefore, it is possible to output each color image with different screen angles for each color, and it is possible to output the highlight portion image stably while preventing moire and color shift. Is obtained.
[0030]
【Example】
Hereinafter, an embodiment according to the present invention will be described in detail with reference to the drawings.
[0031]
<First embodiment>
In this embodiment, an example in which a blend γ is applied to PWM will be described. Since blend γ has been described in the above-described conventional example, detailed description thereof is omitted here. The apparatus configuration to which the present embodiment is applied is also the same as that of the above-described conventional example, and thus the description thereof is omitted.
[0032]
In this embodiment, the screen angle is given by shifting the PWM pulse appearance synchronization. Hereinafter, generation of a screen angle in the present embodiment will be described with reference to FIG.
[0033]
Inside the PWM circuit, the pulse signal P is obtained by comparing the level of the input image signal I with the reference pattern signal S (triangular wave in this embodiment), and the emission pulse time T of a laser diode (not shown) is obtained. I have control. In the present embodiment, the screen angle is given by shifting the appearance synchronization of the pulse signal P. That is, the appearance synchronization of the pulse signal P is shifted by shifting the phase of the triangular wave.
[0034]
Next, the screen angle and blend γ in this embodiment will be described. For example, as described above with reference to the related art, a blend γ that uses two LUTa and LUTb shown in FIG. 11 and expresses gradation with three basic pixels as one unit will be described as an example.
[0035]
The input image signal refers to each LUT by repeating LUTa, LUTa, and LUTb. As shown in FIG. 2, the phases of the triangular waves S1, S2, S3, and S4 are shifted by the phase difference P for every three basic pixels that express gradation. That is, the triangular waves S1, S2, S3 are in phase, and the triangular wave S4 is out of phase by P with respect to the triangular waves S1, S2, S3.
[0036]
As described above, this embodiment can have the screen angle A shown in FIG. Accordingly, the basic pixels of the output image are not discrete in the sub-scanning direction as shown in FIG. 12, but are adjacent and concentrated in the sub-scanning direction as shown in FIG.
[0037]
As described above, according to the present embodiment, even if a screen angle is given, the gradation reproduction of the highlight portion can be stably performed without reducing the effect of the basic pixel concentration by the blend γ.
[0038]
Further, by outputting the image of each color with a different screen angle for each color, it is possible to stably output the image of the highlight portion while preventing moiré and color misregistration.
[0039]
In this embodiment, two LUTs are used and an example of blend γ that expresses a gradation with three basic pixels as one unit in the sub-scanning direction has been described. However, this embodiment is limited to this example. It is not something. For example, two or more LUTs may be prepared, and the method of combining the concentration of basic pixels is not limited to this.
[0040]
Further, an example in which a plurality of LUTs are prepared has been described. However, as described in the conventional example, the same effect can be obtained by preparing a plurality of PWM circuits. Of course, gradation expression by luminance modulation may be performed instead of PWM.
[0041]
<Second embodiment>
Hereinafter, a second embodiment according to the present invention will be described in detail.
[0042]
In the first embodiment described above, an example is described in which the triangular wave serving as the PWM reference signal is shifted by the phase difference P for each of the three basic pixels expressing the gradation in order to have an appropriate screen angle in the blend γ. did. That is, as shown in FIG. 2, the triangular waves S1, S2, and S3 have the same phase, and the triangular wave S4 is shifted in phase by P with respect to the triangular waves S1, S2, and S3. In contrast, in the second embodiment, a method of shifting the triangular wave for each basic pixel will be described.
[0043]
The apparatus configuration to which the second embodiment is applied is also the same as that of the first embodiment described above, and a description thereof will be omitted. Also in the second embodiment, as in the first embodiment described above, the two γTa and LUTb shown in FIG. 11 are used, and a blend γ that expresses gradation with three basic pixels as one unit will be described as an example. .
[0044]
In the second embodiment, when an image signal representing a highlight portion is input, the phase of the triangular wave is shifted by the phase difference P every three basic pixels as shown in FIG. 2 as in the first embodiment described above. Therefore, the screen angle A is given. When an image signal representing an area other than the highlight portion is input, the screen angle A is obtained by shifting the phase of each triangular wave by a phase difference P3 for each basic pixel as shown in FIG. Make it. At this time, since the relationship of P3 = P / 3 is established, the angle of the screen angle is the same for the highlight portion and the portion that is not.
[0045]
Note that the optimum boundary value may be determined experimentally so that the optimum output image can be obtained by the apparatus in order to distinguish the highlighted portion of the image signal from the non-lighted portion in the second embodiment.
[0046]
In the second embodiment, the screen angle is switched between the highlight portion and other than the highlight portion as described above, whereby the basic pixels are concentrated in the highlight portion as shown in FIG. Since the screen angle is expressed, the highlight portion can be reproduced stably. On the other hand, in the area other than the highlight portion, the screen angle is formed in units of basic pixels having a higher resolution as shown in FIG. 5, so that a smooth image with less jaggy (jaggedness) can be formed.
[0047]
In the second embodiment, as described above, it has been experimentally known that an optical density of about 0.3 is appropriate as the boundary value for switching how the screen angle is given.
[0048]
As described above, according to the second embodiment, since the highlight portion has a screen angle in units of a plurality of pixels, stable reproduction can be performed. In areas other than the highlight portion, the screen angle can be set in units of basic pixels. Therefore, a smooth image with little jaggy can be obtained.
[0049]
The present invention may be applied to a system composed of a plurality of devices or an apparatus composed of a single device. Needless to say, the present invention can also be applied to a case where the present invention is achieved by supplying a program to a system or apparatus.
[0050]
【The invention's effect】
As described above, according to the present invention, for a multi-value image signal, gradation expression is performed by a plurality of pixels in a highlight portion, and stable gradation expression is possible even when a screen angle is given. Become.
[0051]
Furthermore, by switching so that the screen angle is expressed in units of basic pixels in areas other than the highlight area, it is possible to obtain a stable gradation expression in the highlight area and a smooth screen angle in the other areas.
[0052]
Accordingly, it is possible to output images of each color with different screen angles for each color, and it is possible to stably output an image of a highlight portion while preventing moiré and color shift.
[0053]
[Brief description of the drawings]
FIG. 1 is a diagram showing a basic concept of PWM in an embodiment according to the present invention.
FIG. 2 is a diagram illustrating a phase of a PWM reference wave in the present embodiment.
FIG. 3 shows a basic pixel group of a highlight portion having a screen angle in the present embodiment.
FIG. 4 is a diagram showing a phase of a PWM reference wave in the second embodiment according to the invention.
FIG. 5 is a diagram illustrating a basic pixel group other than a highlight portion having a screen angle in the second embodiment.
FIG. 6 is a cross-sectional view of a conventional color image forming apparatus.
FIG. 7 is a diagram showing a basic pixel group by conventional PWM.
FIG. 8 is a diagram illustrating a basic pixel group based on a conventional blend γ.
FIG. 9 is a block diagram showing a conventional blend γ.
FIG. 10 is a block diagram showing a conventional blend γ.
FIG. 11 is a diagram illustrating an example of input / output characteristics of a conventional LUT.
FIG. 12 is a diagram showing a basic pixel group when a screen angle is given in a conventional blend γ.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Developing device 2 Transfer drum 3 Photosensitive drum 4 Fixing device 5 Gripper 6 Separator 7 Transfer material 8 Adsorber 9 Charger 10 Charger 11 Exposure unit 12 Cleaner

Claims (12)

An image forming apparatus that forms an image by comparing a multi-value image signal and a reference signal,
First changing means for changing the phase of the reference signal in units of expression pixels represented by a plurality of basic pixels ;
Second changing means for changing the phase of the reference signal in units of the basic pixels;
Switching means for switching to the first changing means in the highlight portion of the multi-valued image signal, and switching to the second changing means in other than the highlight portion of the multi-valued image signal;
Reproduction means for comparing the reference signal, the phase of which has been changed by the first or second changing means, with the multi-valued image signal, and reproducing gradation in units of the expression pixels. An image forming apparatus. The reproduction means includes
Obtained by selectively converting the reference signal whose phase has been changed by the first or second changing means and the multi-valued image signal by using a plurality of lookup tables having different input / output characteristics. The image forming apparatus according to claim 1, wherein the gradation is reproduced based on the signal . The reproduction means includes
A plurality of pulse width modulation circuits having different input / output characteristics for selectively performing pulse width modulation by comparing the reference signal whose phase has been changed by the first or second changing means and the multilevel image signal. by using the image forming apparatus according to claim 1, wherein the reproducing the gradation. 2. The image forming apparatus according to claim 1 , wherein the phase of the reference signal changed by the first or second changing unit is different for each color constituting the multi-value image signal . The reference signal is an image forming apparatus according to claim 1, characterized in that it is a triangular wave. Based on the image formed by comparing the multi-value image signal and the reference signal, a recorded image is formed on the image carrier by charging, exposing, and developing, and transferred to the transfer paper using the recorded image. The image forming apparatus according to claim 1, wherein: An image forming method for forming an image by comparing a multi-value image signal and a reference signal,
A first changing step of changing the phase of the reference signal in units of expression pixels represented by a plurality of basic pixels;
A second changing step of changing the phase of the reference signal in units of the basic pixels;
In the highlight portion of the multi-valued image signal, switching to the first changing step, and in a portion other than the highlight portion of the multi-valued image signal, switching step to switch to the second changing step;
Comparing the reference signal whose phase has been changed in the first or second changing step with the multi-valued image signal, a reproduction step for reproducing gradation in units of the expression pixels An image forming method. The reproduction process includes
Obtained by selectively converting the reference signal whose phase has been changed in the first or second changing step and the multi-value image signal by using a plurality of lookup tables having different input / output characteristics. The image forming method according to claim 7, wherein the gradation is reproduced based on the signal . The reproduction process includes
A plurality of pulse width modulation circuits having different input / output characteristics for selectively performing pulse width modulation by comparing the reference signal whose phase has been changed by the first or second changing step and the multi-value image signal. 8. The image forming method according to claim 7 , wherein the gradation is reproduced by use. The position phase of the first or the second of said reference signal is modified by the changing step, the image forming method according to claim 7, wherein different for each color constituting the multi-value image signal. The image forming method according to claim 7 , wherein the reference signal is a triangular wave . Based on the image formed by comparing the multi-value image signal and the reference signal, a recorded image is formed on the image carrier by charging, exposing, and developing, and transferred to the transfer paper using the recorded image. The image forming method according to claim 7, wherein:

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