JPH09277590A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the reproducibility of an image density gradation to the PWM of an image to be formed. SOLUTION: A single fundamental color is set for input image data consisting of fundamental colors such as yellow, magenta, cyan and black (S21) and is divided into blocks of specified size in a main scan direction (S22). In addition, an average value is sought for each of the density values of these blocks (S23), and the density values are sorted in terms of level by the magnitude of the average value (S24). When it comes to an intermediate density, the gradation is converted using a rearrangement matrix with a maximum weighted position arranged at a different position per color (S25). Further, of a low density area and a high density area, the gradation is changed using the rearrangement matrix with the changed weighted position (S26, S27). This process is repeated in the entire auxiliary scan direction (S28) for all the fundamental colors (S30).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a laser printer or a copying machine, and more particularly to an image forming technique with improved gradation reproducibility of input image data.

[0002]

2. Description of the Related Art The outline of the process from development to printing in a color laser printer will be described with reference to FIG. In FIG. 12, a latent image is formed by raster-scanning the photosensitive drum 122 charged by the charger 121 first with a laser beam modulated by an image signal corresponding to a yellow toner image to form a latent image. 123 develop with yellow toner. Here, the image signal is a signal supplied from a color scanner (which is provided integrally with the apparatus in the case of a copying machine) or a computer, not shown, and is supplied as one screen data for each color of each toner. Then, the same cycle as described above is repeated for each of magenta, cyan, and black toners to form toner images of four colors on the photosensitive drum 122, and then the transfer electrode 124 collectively transfers the toner images to the transfer paper, and the separation electrode A full-color image can be obtained by separating with 125 and fixing with the fixing device 126.

Here, since the image signal is a multi-valued signal (8 bits = 256 gradations), it is necessary to form pixel dots on the photosensitive drum in a multi-valued manner. ) Is known.

[0004]

In such an image forming apparatus, when an image is formed by inputting a digital image signal having a high resolution, for example, a resolution of 600 dpi, FIG.
As shown in, the beam diameter of the laser is larger than the pixel dot pitch in the main scanning direction and the sub scanning direction, and each pixel dot overlaps with the adjacent pixel dot.

Therefore, as shown in FIG. 5, the relationship between the PWM for controlling the writing of pixel dots and the density of the image formed when the gradation conversion described later is not performed is about PWM.
At 25%, the image density has a high gamma characteristic that saturates from low density to high density all at once, and there is a problem that it is difficult to control gradation to linear characteristics. In order to improve the gradation characteristics, for example, it is conceivable that the diameter of the laser beam is narrowed by a lens or the like so that the pixel dots do not overlap each other, but this increases cost and is not desirable. Further, although the beam diameter can be narrowed by reducing the PWM value, a desired potential cannot be generated, the toner adhesion amount decreases, and the target gradation characteristic may not be reproduced. .

In view of the above-mentioned conventional problems, the present invention provides good gradation characteristics of image density while maintaining the resolution required for reproducibility of characters without using a costly method. It aims to improve the image quality.

[0007]

Therefore, according to the invention described in claim 1, the static exposure in one of the main scanning direction and the sub-scanning direction of the exposure means for image formation is performed with respect to the pixel pitch corresponding to the resolution. In an image forming apparatus having a large diameter, a block dividing unit that divides input image data into blocks composed of a plurality of pixels, an average value calculating unit that calculates an average value of density values of respective pixels in the block, and a pixel in the block Weighting distribution pattern setting means for setting a weighting distribution pattern for each, and correction means for correcting each pixel density value in the block based on the density average value in the block and the weighting distribution pattern, and the correction And an image forming unit that forms an image with the obtained density value.

Here, the pixel pitch corresponds to, for example, 42.3 μm in an image forming apparatus having a resolution of 600 dpi, and the static exposure diameter is the light diameter on the photosensitive member of the exposure means, for example, 1 of the peak output. It may be the light diameter of a region having / e ² or more. Incidentally, the static exposure diameter is such that when the exposure means is a laser, the polygon mirror and the photoconductor are in a stationary state,
It can be obtained by measuring the beam diameter on the surface of the photoconductor. Further, in the case of an LED array, it can be obtained by measuring the light diameter of a region having 1 / e ² or more of the peak output of one LED on the surface of the photoconductor while the photoconductor is stationary.

According to a second aspect of the present invention, the weighting distribution pattern setting means sets the pattern so that the weighting of a specific portion in the block is maximized and the weighting becomes smaller at a portion further away from the specific portion. I did it.
According to a third aspect of the present invention, the weighting distribution pattern setting means sets a pattern in which a weighting coefficient by which a density average value is multiplied is given to each pixel.

In the invention according to claim 4, the maximum value of the weighting coefficient is set to approximately 1. Claim 5
In the invention described in, the weighting distribution pattern setting means sets a pattern in which the weighting distribution is constant when the density average value in the block is within a predetermined density range, and when the density distribution is outside the predetermined range, the density distribution is set. The weighting distribution pattern is set so that the weighting distribution changes with respect to the change of the average value.

According to a sixth aspect of the present invention, the weighting distribution pattern is such that the weighting distribution is constant in the intermediate density range of the density mean value, and the lower the density mean value in the low density range of the density mean value, the more Has a large difference in weighting, and as the density average value increases, the difference in weighting in the block decreases and approaches the pattern of the intermediate density range, and the density average value increases in the high density range of the density average value. Accordingly, the pattern is set so as to change from the pattern close to the intermediate density range to the pattern in which the weighted area increases in the block area.

According to a seventh aspect of the present invention, the weighting distribution pattern setting means has a different weighting distribution for each average value level with respect to a low-density block whose average value is less than a predetermined value. A pattern in which a corrected density value is given is set for each pixel. According to an eighth aspect of the present invention, the weighting distribution pattern setting means is a function of a density average value in a block for each pixel with respect to a high density block in which the average value of the block is a predetermined value or more. The pattern which gave the function was set.

According to a ninth aspect of the present invention, the function sets the density value in proportion to the square of the average value. According to a tenth aspect of the present invention, the input image is a color image represented by a plurality of basic colors, and the weighting distribution pattern setting means has a pattern in which a weighting distribution is different for each pixel in a block for each basic color. Was set.

According to an eleventh aspect of the present invention, the maximum weighting portion of the weighting distribution pattern is arranged for each basic color with the center of the block composed of the matrix of N × N pixels as the center of rotation. It was arranged to rotate by 90 degrees each. According to a twelfth aspect of the present invention, the correction means sets a dither processing pattern having a determination value for dither processing for each block in units of a plurality of blocks, and an average value for each block is the dither. A means for performing dither processing by the processing pattern is included, and the density value is corrected based on the density value of each block obtained by the dither processing and the weighting distribution pattern.

According to the thirteenth aspect of the present invention, in the weighting distribution pattern, the density value of one pixel in the block is set as the density value obtained by the dither processing, and the other density values in the block are set as zero. I did it. According to a fourteenth aspect of the present invention, the input image is a color image represented by a plurality of basic colors, and dithered values are stored in different positions of the weighting distribution pattern for each basic color. .

According to a fifteenth aspect of the present invention, in the image forming apparatus in which the static exposure diameter in one of the main scanning direction and the sub scanning direction of the exposure means for image formation is large with respect to the pixel pitch corresponding to the resolution, Block dividing means for dividing the image data into blocks composed of a plurality of pixels, and weight distribution pattern setting means for setting a pixel weight distribution pattern in the block in which a maximum weight portion is arranged in the vicinity of an apex of the block. And, based on the density value of the specific pixel in the block and the pattern for weighting distribution,
It is configured to include a correction unit that corrects each pixel density value in the block and an image forming unit that forms an image with the corrected density value.

According to a sixteenth aspect of the present invention, in an image forming apparatus in which the static exposure diameter in one of the main scanning direction and the sub scanning direction of the image forming exposure means is large with respect to the pixel pitch corresponding to the resolution, Block dividing means for dividing the image data into blocks composed of a plurality of pixels, pattern setting means for weighting distribution for setting a pixel weighting distribution pattern in the block, and density value and weighting distribution for specific pixels in the block It is configured to include a correction unit that corrects each pixel density value in the block based on the pattern, and an image forming unit that forms an image with the corrected density value.

[0018]

According to the first aspect of the present invention, the beam diameter of the laser light is disadvantageous in terms of cost because the image is formed with the density value obtained by weighting the average value of the blocked input image data. The gradation characteristic in which the image density is rapidly saturated with respect to the change of the PWM can be made into the gradation characteristic which changes linearly without narrowing down by using the method such as the method. The image quality of the formed image can be further improved. In particular, the image quality can be remarkably improved in gradation reproducibility of an image having continuous gradation such as a photographic image.

According to the second aspect of the present invention, the weighting is gradually reduced as the distance from the maximum position of the weighting distribution is set, and the weighting distribution pattern is set. Since the toner adheres to the toner, stable image formation can be performed, and gradation characteristics can be stabilized. According to the third aspect of the invention, the weighting correction can be performed by a simple calculation by performing the weighting process of integrating the density average value of each block with the weighting coefficient.

According to the fourth aspect of the present invention, by setting the maximum value of the weighting coefficient to approximately 1, the saturation of the image density due to the increase of PWM is suppressed, and the weighting coefficient values of other pixels are set. By appropriately setting the and distribution, the gradation characteristic of the image density with respect to PWM can be made a good characteristic that linearly changes. According to the invention of claim 5, when the density average value of the block is within a predetermined density range, the weighting distribution pattern is set so that the weighting distribution changes in accordance with the change of the average value. For example, it is possible to obtain more appropriate gradation characteristics in a low density region or a high density region where the density of an image actually formed does not change linearly with respect to PWM.

According to the invention described in claim 6, the following effects can be obtained. In the low density region, there is a limit density value at which a dot image can be formed. Therefore, when an image is formed with the same pattern as the weighting distribution pattern that can obtain an appropriate gradation characteristic in the intermediate density region in the low density region, when the maximum density due to the maximum weighting falls below the limit density value, the image is formed. This is not done, and the density reproduction area on the low density side is narrowed. Therefore, by increasing the weighting difference in the extremely low density range so that the maximum density exceeds the limit density, and decreasing the weighting difference as the density average value increases to approach the pattern in the intermediate density range, Even in the region, it is possible to expand the region where good gradation characteristics can be obtained.

On the other hand, in the high density area, if weighting is performed with a pattern matched to the intermediate density area, even if the average density is increased, the proportion of the pixels with small weights becomes too large and the density becomes insufficient. The area may be narrowed and a powerful image may not be obtained. Therefore, by adopting a pattern in which the ratio of pixels with relatively high density in the block increases with the increase of the average density value, good gradation characteristics are maintained even in the high density region, and It is possible to secure the maximum density reproduction range.

According to the invention described in claim 7, the following effects can be obtained. As described above, there is a limit density value at which a dot image can be formed in the low density area. Therefore, in the low density region, accurate gradation characteristics can be obtained by setting the correction density value directly rather than setting the weighting as a coefficient by which the density average value is multiplied. Then, the image density can be changed linearly by changing and setting the pattern very finely for each average value level so that the corrected density value changes gently in accordance with the change of the density average value. Therefore, the reproduction range of gradation can be further expanded.

According to the invention described in claim 8, by setting the weighting for each pixel in the block by the function, the weighting can be gradually increased from the pattern of the intermediate density region to the entire region of the block. It is possible to improve the gradation characteristics in the high density region. According to the invention described in claim 9, by setting the pixel value in proportion to the square of the average value, it is possible to change the image density sensitively even to a minute change in the average value, and at the same time, set the maximum value. The density value and the intermediate density range can be connected smoothly, and the gradation reproducibility in the high density range can be further improved.

According to the tenth aspect of the present invention, by placing the maximum weighting portion for each basic color at a different position, the overlap of the colors of the images to be formed is reduced, and especially the coloring in the halftone portion is achieved. It is possible to further improve the sex. According to the eleventh aspect of the present invention, by arranging the maximum weighting portion for each basic color with respect to the image data in the same block at positions shifted in phase by 90 degrees, each color is arranged at different positions in an orderly manner. The color components can be arranged efficiently and evenly. Especially, in order to secure a large density reproduction range, Y, M, C,
Even when the K color is used as the basic color, the basic colors can be arranged at each corner of the block of N × N pixels to reduce color overlap and improve the color developability.

According to the twelfth aspect of the invention, even for an image of a type for which dithering is required, the average value of each block of the input image is subjected to dithering, and then weighted with a weighting pattern. By performing, it is possible to obtain an appropriate gradation having linearity. According to the thirteenth aspect of the present invention, by storing the dither processing result in only one specific pixel in the block, the saturation of the image density can be reduced and the gradation characteristic can be made linear.

According to the fourteenth aspect of the present invention, the dither processing results for the respective basic colors of the input image are stored at different positions in the block, so that the overlapping of the colors of the images to be formed is prevented, and especially the halftone is performed. It is possible to further improve the color developability in the area. According to the fifteenth aspect of the present invention, the static exposure of the exposure unit is performed by forming an image with the density value in which the maximum weighted portion is arranged near the apex in the block based on the density value of the blocked input image data. The gradation characteristic in which the image density is rapidly saturated with respect to the change of PWM can be changed to a linearly changing gradation characteristic without narrowing the diameter, thereby improving the gradation reproducibility at low cost. However, the quality of the formed image can be further improved. In particular, it is possible to easily improve the image quality in terms of gradation reproducibility of an image having continuous gradation such as a photographic image.

According to the sixteenth aspect of the present invention, the image is formed with the density value weighted on the basis of the density value of the blocked input image data, so that the static exposure diameter of the exposing means is not reduced and the PWM is performed. The gradation characteristic in which the image density is rapidly saturated with respect to the change of can be made into the gradation characteristic that linearly changes, so that the gradation reproducibility is improved at a low cost and The image quality can be further improved. In particular, the image quality can be easily improved in the gradation reproducibility of an image having continuous gradation such as a photographic image.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of a color laser printer according to the present invention. An outline of the configuration and a series of operations will be described with reference to FIG. 1. A photoconductor drum 10 as an image carrier having an OPC photosensitive layer coated on its surface is driven and rotated in one direction (clockwise direction in the figure), and a charge eliminator is used. After the charge is removed by 11 to remove the charge at the time of the previous printing, the peripheral surface is uniformly charged by the charger 12 to prepare for a new print.

After such uniform charging, the image exposure means 13
Image exposure is performed based on output image data described later.
The image exposure means 13 is a laser beam emitted from a laser light source.
Is rotated and scanned by the polygon mirror 131, and the fθ lens
The optical path is bent by the reflection mirror 133 after passing through 132 etc.
Therefore, it is projected on the peripheral surface of the charged photoconductor drum 10.
Edge of photoconductor drum 10 where latent image is formed on the drum surface
Yellow (Y), magenta (M), cyan (C),
Toner (paint) such as black (K) and carrier (magnetic material)
The developing device 14 filled with the developer composed of the mixture of
First, the development of the first color is installed inside the magnet.
The developing sleeve 141 that rotates while holding the stored developer
It is done. The developer is developed by the layer-forming rod.
It is transported to the development area after being regulated to a predetermined thickness on the sleeve 141.
You. AC between the photoconductor drum 10 and the developing sleeve 141
Bias V _ACAnd DC bias V_DCAnd are superimposed and applied
You. Here, the potential of the exposed portion of the photosensitive drum 10
(Ground potential) to V_L, Charged photosensitive layer other than exposed areas
Surface potential is V_HAnd the DC bias potential V_DCTo V_H>
V_DC> V_LBy setting so that
Bias V_ACTriggers you to leave your career
The given toner is V_DCHigher potential V_HPart of
Not attached, V_DCLower potential V_LAttached to the exposed part of
It is put on, visualized and developed.

After the development of the first color is completed in this way, the image forming process of the second color (for example, magenta) is started.
The photosensitive drum 10 is uniformly charged again, and a latent image based on the image data of the second color is formed by the image exposure means 13. For the third color (cyan) and the fourth color (black), the same image forming process as the second color is performed, and a total of four colors are developed on the peripheral surface of the photosensitive drum 10.

On the other hand, the recording paper P fed by the paper feed mechanism 22 by the paper feed cassette 21 is transferred to the photosensitive drum 10 and the transfer belt by the transfer belt device 30 in which the transfer belt 31 is stretched.
The multi-color image on the peripheral surface of the photoconductor drum 10 is fed all at once to the nip portion (transfer area) 35 formed between the recording paper P and the recording paper P.
Moved to Here, the upstream holding roller of the transfer belt 31
A high voltage is applied to the shaft 32a of 32, and the conductive brush 34 installed at a position facing the shaft 32a with the transfer belt 31 interposed therebetween is grounded, and the recording paper that has been fed is brushed by the brush 34a. And the transfer belt 31, and the brush 34 enters the transfer area while being attracted to the transfer belt 31 by the electric charge injected into the recording paper P. The recording paper P separated from the photoconductor drum 10 is a holding roller 33 on the downstream side on which the transfer belt 31 is stretched.
The shaft 33b of the transfer belt 31
Separate from The toner attached to the transfer belt 31 is removed by the cleaning blade 37. The transfer belt 31 is separated from the photoconductor drum 10 around the shaft 33b of the holding roller 33 on the downstream side as a rotation center during the formation of the multicolor image.

Recording paper P separated from the transfer belt device 30
Is conveyed to a fixing device 23 composed of two pressure-bonding rollers having a heater inside at least one roller, and heat and pressure are applied between the two pressure-bonding rollers to melt the adhered toner. After being fixed on the recording paper P, it is carried out of the apparatus. After the transfer, the toner remaining on the peripheral surface of the photosensitive drum 10 is discharged by the static eliminator 15 and then reaches the cleaning device 16 where it is scraped off into the cleaning device 16 by the cleaning blade 16a that is in contact with the photosensitive drum 10. After being carried out by a screw or the like, it is stored in the collection box. The photoconductor drum 10 from which the residual toner has been removed by the cleaning device 16 is exposed by the static eliminator 11 and then uniformly charged by the charger 12 to start the next image forming cycle. Further, if the recording paper P is wound around the photosensitive drum 10 without being separated from the transfer belt 31 and enters above the static eliminator 15, the cleaning blade 16a or the electrode wire may be damaged. A JAM sensor 36 for detecting is mounted near the static eliminator 15.

Pulse width modulation (PWM) is applied to the image signal input to the image exposure means of the color laser printer.
The first embodiment in which the reproducibility of the gradation of the image density with respect to is improved will be described. In the present embodiment, first, the input image data is divided into blocks of a predetermined number of pixels whose size is set according to the target resolution, and the pixel density is changed to the pixel corresponding to each pixel density in the block. Weighting correction of the density value is performed by the average value of the density and the preset weighting distribution pattern.

Specifically, the density average value is in the low density range (low P
WM) and intermediate density areas other than the high density area, the gradation characteristics of the image density for PWM are optimized by multiplying the coefficient of the preset rearrangement matrix by the density average value. As a result, a linear gradation characteristic is obtained by correcting the density value so that the weighting distribution differs depending on the average value.

In this embodiment, the beam diameter on the surface of the photoconductor is 63 μm in the main scanning direction and 75 μm in the sub scanning direction. The specific processing contents of the gradation characteristic optimization for this input image data are shown in the flowchart of FIG. Here, for example, with respect to color input image data represented by basic colors of Y, M, C, and K with 8-bit gradation, output image data for forming an image with a resolution of 600 dpi is processed in one processing line in the sub-scanning direction. Think about asking for each one. This gradation characteristic optimization process is performed with Y,
The process is performed for each color in the order of M, C, K.

Therefore, first, the input image data of Y is selected (step 21, hereinafter referred to as S21), and a block having 4 × 4 pixels as one unit is spread over the entire main scanning direction with respect to the input image data of Y. Set (S22). The pixel size of the block at this time is appropriately changed and set according to conditions such as resolution. For example, when the resolution is higher and the beam overlap is larger due to the shortened pixel dot pitch distance, the number of pixels in the block is increased.

FIG. 3A shows yellow image data as an example. It should be noted that the description will focus on the upper left one block in FIG. 3 (a). Next, the density average value of the pixels in the block is calculated for each set block (S23). The density average value for the block is "35", and similarly, the density average value is calculated for each block of one column of the input image data in the main scanning direction.

Then, it is determined which of the low-density area, the high-density area and the intermediate-density area described later the calculated average density value is (S24).
The obtained density average value is integrated with each weighting coefficient value of the rearrangement matrix having a different weighting distribution for each color shown in FIGS. 4 (a) to 4 (d) (S24), and the integration processing result is the optimization data for yellow color. And Since the input image data of FIG. 3 (a) is data for yellow color, integration processing with the average value "35" is performed using the rearrangement matrix of FIG. 4 (a). The result of this integration process is shown in Fig. 3 (b).

Each weighting coefficient of this rearrangement matrix is set by setting the maximum coefficient value to "1" for four pixels, and gradually decreasing as the distance from the setting position of the maximum coefficient value is set. Further, the weighting coefficient value for each basic color is rotated by 90 degrees around the center of the rearrangement matrix so that the arrangement position of the maximum coefficient value is different for each basic color.

With such an arrangement, as shown in FIG. 3B, the density value of the optimization data for the portion where the weighting coefficient is "1" becomes the largest, and the color image formed by superimposing each color described later is formed. In, the peak of the image density value of each basic color appears at the four corners of one block. That is, each basic color in each block is formed at a different position. As a result, the overlapping of colors is reduced, and in particular, it is possible to obtain an additive subtractive color mixture in which halftone can be expressed more clearly, and it is possible to reduce the degree of affecting the image density value of an adjacent block. Further, by setting the weighting coefficient to "1", it is possible to prevent the image density from suddenly becoming saturated with an increase in PWM.

It should be noted that in the present embodiment, FIG.
Although the weighting coefficient values shown in (d) were used, these weighting coefficient values should be appropriately changed and set according to various factors such as the output condition of the laser light used. By optimizing the input image data in this way, the gradation characteristic of the image density for PWM can be made the gradation characteristic shown in the gradation conversion result of FIG. 5, and the PWM in the area after the rise of the image density can be obtained. It is possible to reduce the change characteristic in which the image density suddenly becomes saturated with an increase in the, and a gradation characteristic in which the image density also gradually increases with an increase in PWM.

Next, a method for further optimizing the gradation characteristics in the low density region will be described. In the gradation characteristic of the image density with respect to the PWM, the image density has a characteristic that the image density sharply rises from 0 to a saturation state as the PWM increases in a region where the PWM value is about 25% or less. Therefore, the gradation characteristics are optimized in the low density region by performing the following gradation characteristics conversion processing (S26).

When the average value of the pixel values in a block is smaller than a predetermined value, even if the average value has a value larger than 0, a photosensitive member produced by irradiating a laser beam based on the average value The decrease in the potential of the electrostatic latent image on the drum surface is not so large that the toner actually adheres during the development stage, and as a result, pixel dots may not be formed in the formed image.

Therefore, an image in a low density region is formed by using the minimum density value (for example, "63" in 8-bit gradation) of output image data capable of forming a pixel dot as a nucleus. This is because toner is less likely to adhere to the photoconductor drum when a pixel having a density value in the eye field is isolated, but when there is a pixel having a density value equal to or higher than the limit value in the vicinity of the pixel, the toner has a limit. The toner is attached by a pixel having a density value equal to or higher than the density value, and the characteristic that the toner adheres is used with the pixel having the limit density value as a nucleus, and the intensity distribution of the pixels around the nucleus is finely changed. As a result, good gradation characteristics having linearity are obtained even in the low density region.

Specifically, for a low density area in which the average value of the density values of the blocks set in the input image data is smaller than 63, for example, the average value is 62 to 55, 54 to 47, 46 to 3
9, 38 to 31, 30 to 23, 22 to 15, 14 or less are classified into 7 stages and shown in Fig. 6 (a) to (g) corresponding to each stage.
The value of the matrix of × 4 is set as the density value of the output image data.

By performing the optimization process for the low density region, it is possible to obtain the gradation characteristic in which the image density gradually increases from 0 in the low density region as shown in FIG. The input image data can be reproduced more faithfully. Here, the minimum density value “63” capable of forming the pixel dots, each threshold value and the number of steps for classifying the average value of the block are appropriately changed according to changes in various factors such as laser output conditions. Shall be set.

Next, a method of further optimizing the gradation characteristics in the high density range will be described. In the high density area, if an image is formed using the same pattern as the weight distribution pattern that can obtain appropriate gradation characteristics in the intermediate density area, the average density increases and the proportion of pixels with low weighting becomes too large. As a result, the density of the image becomes insufficient. Therefore, the formed image is lightened, and the lack of density is particularly noticeable especially in the high density area. Therefore, the gradation characteristics are made more appropriate by performing the following gradation characteristics conversion processing for the high density region (S27).

Specifically, for a high density area in which the average value of pixel values in a block is equal to or more than a predetermined value described later, output image data is output using the rearrangement matrix for the high density area shown in FIG. Try to set it. Here, for each coefficient of the rearrangement matrix for high density, for “1”, the average value of the block of 4 × 4 pixels is directly input, and “A”, “B”,
For "C" and "D", for example, (1) to
It is determined by the function shown in the equation (4). A = 3 × (average value / 31) ² (1) B = 3 × (average value / 31) × (average value / 63) (2) C = 3 × (average value / 63) ² (3) D = 3 × (average value / 63) × (average value / 127) (4) When gradation conversion processing is performed using each coefficient in the above equation, the average value of the blocks set in the input image data is
When the average value is 171 or more, the rearrangement matrix shown in FIG. 7 is used to change the gradation characteristics because the density value tends to increase more than the image density before the gradation conversion process. To

As described above, by setting the pixel value using the function proportional to the square of the average value, the maximum image density of the formed image is further improved, and the high density area shown in FIG. Thus, the image density can be smoothly increased with an increase in the PWM value, the reproducibility in the high density region can be further improved, and the gradation reproduction width can be further expanded. By performing the above-described processing for the low density region and the high density region, the density distribution in the block becomes a form as shown in FIG. That is, in the low density region, while maintaining the maximum density at the limit density at which the toner adheres, the density of the skirt region that is continuous to the maximum density position gradually increases with the increase of the density average value and approaches the distribution form of the intermediate density region. To go. In the intermediate density range, the density distribution has a constant weighting pattern that increases greatly toward the maximum weighting position, and in the high density range, as the average density value increases, as a whole, especially in the vicinity of the maximum weighting position. The increase rate increases with
The overall concentration is increased.

Next, it is determined whether the above processing S22 to S26 have been completed for all the input image data for the basic color set in S21 (S28). If not completed, the processing line is set in the sub-scanning direction. Then, it is reset by advancing by the number of lines according to the block size (4 lines in this case) (S29), a series of processing is repeated for each processing line, and the gradation characteristics of the entire input image data are converted to be appropriate. The converted data. When the gradation characteristic conversion processing of the entire input image data is completed, the image data for all other basic colors are sequentially processed (S30). That is, since the first basic color is Y, the gradation characteristic conversion processing is similarly performed for M, C, and K in the same manner. With this, PWM
It is possible to obtain color output image data in which the gradation characteristic of the image density with respect to is optimized.

By inputting this output image data into the above-mentioned image exposure means, a color image can be output onto the recording paper. As described above, in the present embodiment, the image for PWM is not used, regardless of the technique that increases the cost of the device such as narrowing the beam diameter of the laser and newly adding a lens for narrowing the laser light. The reproduction range of density gradation is further expanded to ensure that pixel dots are formed in the low density area by using the minimum pixel value, while in the high density area On the other hand, by further increasing the image density, it is possible to further improve the reproducibility of gradation from the low density region to the high density region.

Further, by setting the rearrangement matrix so as to avoid the overlap of the respective basic colors as much as possible, the pixel dots of the respective colors are formed at different positions, and the coloring property particularly in the halftone portion can be further improved. . The gradation characteristic conversion processing of the input image data is
In the case of a printer separate from the scanner, the processing may be performed inside the printer, but the processed image data may be input to the printer after being processed by an external device (scanner or other signal processing device). In the case of a color copier with a built-in scanner, gradation processing is performed internally.

Next, a second embodiment in which the dither processing is performed using the average value for each block set for the input image data will be described. In the present embodiment, for example, dither processing is applied to an input image represented by Y, M, C, and K basic colors of 8-bit gradation, and based on the result of the dither processing, pixel dots of each color are Consider forming images with a resolution of 600 dpi while preventing overlapping.

The specific processing contents of this dither processing are shown in FIG.
It will be described based on the flowchart shown in FIG. This dithering process is performed for each basic color in the order of Y, M, C, K, similarly to the gradation characteristic converting process in the first embodiment. Therefore, first, the input image data of Y is selected (S91), and the image shown in FIG.
For the Y input image data shown as an example in (a), 8 ×
One processing line in the main scanning direction is divided into a plurality of blocks with 8 pixels as one block (S92). In addition, here, as shown in FIG. 10A, the description will focus on the upper left one block of the input image data. Further, in the present embodiment, the block size is set to 8 × 8 pixels, but the block size is appropriately changed and set according to conditions such as resolution.

Next, for each of the divided blocks, the block is divided into 2 × 2 pixel sub-blocks, and the average value of the pixel values in each sub-block is obtained (S
93). The result of this averaging process is shown in Fig. 10 (b). Then, for each average value obtained for each sub-block, dither processing is performed for each block as a unit (S94). The details of this dither processing will be described below.

Generally, there are various methods for the dither processing, but in the present embodiment, as shown in FIG.
Fat with a distribution in which each coefficient increases spirally from the center
The four-valued 4 × 4 pixel processing matrix called a tening pattern is used for quinarization. Specifically, 8
In the case of bit gradation, 0 to 255 density levels are divided into 5 density ranges at equal intervals. That is, "0" when the average value of the pixel values of the sub-block is smaller than the threshold value of the processing matrix 1, "63" when the average value of the processing matrix 1 or more and less than the processing matrix 2 is processed by the processing matrix 2 or more. If it is smaller than the matrix 3, it is "127", if it is more than the processing matrix 3 and smaller than the processing matrix 4, it is "191", and if it is more than the processing matrix 4, it is "255". .

Of the blocks shown in FIG. 10 (a),
Focusing on the upper left sub-block 101, the dither processing is performed. First, the average value of the sub-block 101 is compared with the corresponding position of each processing matrix shown in FIG. 12, that is, each coefficient at the upper left pixel positions 111 to 114 in the processing matrix. Then, the average value of this sub-block 101 is "6.
Since it is 2 ”, it can be seen that the coefficient“ 54 ”or more of the processing matrix 1 is smaller than the coefficient“ 118 ”of the processing matrix 2. Therefore, as a result of the dither processing,
Since the average value of the block 101 is between the processing matrix 1 and the processing matrix 2, it becomes "63".

The result obtained by such dither processing is stored only in the upper left position 102 for the corresponding sub-block as shown in FIG. 10 (c), and stored in the other three pixels 103, 104, 105. Stores 0 (S95). This process is performed for each sub-block and the dither process results are stored respectively. Then, it is determined whether S92 to S95 have been completed for the entire input image data for the basic color set in S91 (S96), and if not completed, the processing line is set in the sub-scanning direction according to the block size. It is reset by advancing by the number of lines (in this case, 8 lines) (S97), a series of processing is repeated for each processing line, and dither processing is performed over the entire input image data.

When the dither processing for the entire input image data is completed, the input image data for all other basic colors are sequentially processed (S98). Then, the dither processing result for each basic color is stored in the corresponding sub-block shown in FIG. 10 (c), for example, for the upper left Y, the upper right M, the lower right C, and the lower left K. To do so.

By storing in different positions for each basic color in this way, it is possible to prevent the color components from overlapping and to obtain additive subtractive color mixture. Further, it is possible to realize more proper gradation expression without narrowing the beam diameter of the laser or changing the prescription of the photoconductor. Although the five-valued dithering process is performed in the present embodiment, the multi-valued number is appropriately changed depending on the input image data and various conditions of the image forming apparatus.

The dither processing may be performed inside the printer, or may be processed outside the printer and then the processed image data may be input to the printer. In the above embodiment, since the density average value for each block is weighted, a smooth density change between blocks is guaranteed. However, as a simple method, instead of the density average value, the density value of a specific pixel is changed. May be weighted for each pixel.

Further, instead of the density average value, the density value of each pixel may be weighted as it is. In this case, the low density region and the high density region are determined by using the density of the specific pixel.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a color laser printer.

FIG. 2 is a flowchart of a gradation characteristic changing process according to the first embodiment.

FIG. 3 is a diagram illustrating an integration process on an input image.

FIG. 4 is a diagram showing coefficient values of a rearrangement matrix set for each color.

FIG. 5 is a diagram showing characteristics of image density for improved PWM.

FIG. 6 is a diagram showing each coefficient value of a weighting distribution pattern for a low density region.

FIG. 7 is a diagram showing a rearrangement matrix for a high density region.

FIG. 8 is a conceptual diagram showing a form of change of a weighting coefficient with respect to a distance from a maximum weighting coefficient position in a block with an increase in the density average value.

FIG. 9 is a flowchart of dither processing according to the second embodiment.

FIG. 10 is a diagram for explaining specific contents of dither processing.

FIG. 11 is a diagram showing a processing matrix for quinarization in dither processing.

FIG. 12 is a configuration diagram of an image forming apparatus.

FIG. 13 is a diagram showing a relationship between a pixel dot pitch and a beam diameter of laser light.

[Explanation of symbols]

 111,112,113,114 Processing Matrix

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Claims (16)

[Claims] 1. An image forming apparatus in which an exposure means for image formation has a large static exposure diameter in one of a main scanning direction and a sub-scanning direction with respect to a pixel pitch corresponding to a resolution, and input image data is input from a plurality of pixels. Block dividing means for dividing into blocks, average value calculating means for calculating an average value of pixel density values in the block, and weight distribution pattern setting means for setting a weight distribution pattern for each pixel in the block Correcting means for correcting each pixel density value in the block based on the density average value in the block and the weighting distribution pattern, and image forming means for forming an image with the corrected density value,
An image forming apparatus comprising: 2. The weighting distribution pattern setting means,
The image forming apparatus according to claim 1, wherein the weighting of a specific portion in the block is maximized, and the pattern is set such that the weighting becomes smaller at a portion farther from the specific portion. 3. The pattern setting means for weighting distribution,
3. A pattern in which a weighting coefficient by which an average density value is multiplied is set for each pixel is set.
An image forming apparatus according to claim 1. 4. The image forming apparatus according to claim 3, wherein a maximum value of the weighting coefficient is set to approximately 1. 5. The weighting distribution pattern setting means,
When the density average value in the block is within a predetermined density range, a weighting distribution is set to a fixed pattern, and when it is outside the predetermined range, weighting is performed so that the weighting distribution changes with respect to the change of the density average value. The image forming apparatus according to any one of claims 1 to 4, wherein a distribution pattern is set. 6. The weighting distribution pattern is a pattern in which the weighting distribution is constant in an intermediate density range of density average values,
In the low density area of the density average value, the lower the density average value, the larger the difference in weighting within the block, and as the density average value increases, the difference in weighting within the block decreases and the pattern approaches the pattern in the intermediate density area. 6. In the high density area of the density average value, the pattern is set so that as the density average value increases, the pattern closer to the intermediate density area changes to the pattern in which the area in the block where the weighting increases increases. Image forming device. 7. The weighting distribution pattern setting means,
7. A pattern, in which a correction density value is given to each pixel, is set so that a low-density block whose average value is equal to or less than a predetermined value has a different weighting distribution for each average value level. The image forming apparatus described. 8. The weighting distribution pattern setting means,
8. A pattern in which a weighting function, which is a function of the average density value in a block, is set for each pixel for a high-density block whose average value is equal to or larger than a predetermined value is set. The image forming apparatus according to item 1. 9. The image forming apparatus according to claim 8, wherein the function sets the density value in proportion to the square of the average value. 10. The input image is a color image represented by a plurality of basic colors, and the weighting distribution pattern setting means sets a pattern in which a weighting distribution is different for each pixel in a block for each basic color. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. 11. The arrangement of the maximum weighting portion of the weighting distribution pattern is performed by rotating each basic color by 90 degrees with the center of the block composed of the matrix of N × N pixels as the center of rotation. Claim 1 which is arranged
The image forming apparatus according to item 0. 12. The correction means sets a dither processing pattern having a judgment value for dither processing for each block in units of a plurality of blocks, and an average value for each block is set by the dither processing pattern. The image forming apparatus according to claim 1, further comprising a dither processing unit, which corrects the density value based on the density value of each block obtained by the dither processing and the weighting distribution pattern. 13. The weighting distribution pattern sets a density value of one pixel in a block as a density value obtained by dithering and sets another density value in the block as 0.
An image forming apparatus according to claim 1. 14. The input image is a color image represented by a plurality of basic colors, and dithered values are stored in different positions of the weighting distribution pattern for each basic color. The image forming apparatus according to claim 13. 15. A pixel pitch corresponding to a resolution,
In an image forming apparatus having a large static exposure diameter in one of a main scanning direction and a sub scanning direction of an exposure unit for image formation, a block dividing unit that divides input image data into blocks composed of a plurality of pixels, and a vertex in the block. Based on the density distribution pattern setting means for setting the pixel weighting distribution pattern in the block, in which the maximum weighting portion is arranged in the vicinity, and the density value and the weighting distribution pattern of the specific pixel in the block, the block A correction means for correcting each pixel density value in the inside, and an image forming means for forming an image with the corrected density value,
An image forming apparatus comprising: 16. A pixel pitch corresponding to a resolution,
In an image forming apparatus having a large static exposure diameter in one of a main scanning direction and a sub scanning direction of an exposure unit for image formation, a block dividing unit that divides input image data into blocks composed of a plurality of pixels, and pixels in the blocks. A weighting distribution pattern setting means for setting a weighting distribution pattern; a correction means for correcting each pixel density value in the block based on the density value of a specific pixel in the block and the weighting distribution pattern; An image forming means for forming an image with the density value thus determined,
An image forming apparatus comprising: