JP2014011723A - Image processing apparatus and control method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form more appropriate images by correcting image data according to a spot shape of a light source on an image carrier and a pattern of the image data in an image forming apparatus using an electrophotographic method.SOLUTION: An image processing apparatus is for an image forming apparatus forming images by exposing an image carrier by an exposure unit, and has generation means for generating control signals for controlling the exposure unit on the basis of input image data. The control signals are generated, for target pixels constituting the input image data, according to a pattern in an area including the target pixels and similarity to a shape of a light intensity distribution when the target pixels are exposed on the image carrier.

Description

  The present invention relates to an image processing apparatus for performing image formation by electrophotography and a control method thereof.

  Conventionally, in an electrophotographic image forming apparatus, as an exposure unit for forming an electrostatic latent image on an image carrier, there is a method in which laser light from a semiconductor laser is deflected and scanned to form an image on the image carrier. Used. The exposure means in this system generally includes a light source unit that emits laser light by a semiconductor laser, a deflection scanning unit that deflects and scans laser light from the light source unit by a polygon mirror, and guides laser light from the light source unit to the deflection scanning unit. It comprises an optical unit that focuses light on an image carrier. In such an image forming apparatus, a method for calculating an output density characteristic based on a light intensity distribution of laser light by an exposure unit is known.

  Patent Document 1 discloses a method for correcting image data in accordance with a light intensity distribution calculated from laser light specifications (such as a beam diameter).

Japanese Patent Laid-Open No. 9-331452

  However, the shape of the light intensity distribution of the light beam (hereinafter referred to as spot shape) may have anisotropy. Therefore, the output density characteristics differ depending on the similarity between the spot shape and the pattern of the output image data. Therefore, in the method described in Patent Document 1, the image data is corrected without considering the pattern of the image data.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to form a more suitable image by correcting image data in accordance with the spot shape of the light source on the image carrier and the pattern of the image data in an image forming apparatus using an electrophotographic system. And

  In order to solve the above problems, the present invention provides an image processing apparatus of an image forming apparatus that forms an image by exposing an image carrier with an exposure unit, and controls the exposure unit based on input image data The control signal includes, for a target pixel constituting the input image data, a pattern in a region including the target pixel, and light when the target pixel is exposed on the image carrier. It is generated according to the similarity with the shape of the intensity distribution.

  According to the present invention, in an image forming apparatus using an electrophotographic system, it is possible to correct image data in accordance with the spot shape of the light source on the image carrier and the pattern of the image data, thereby forming a more suitable image.

Block diagram showing the configuration of the image forming system Flow chart showing correction processing Example of spot shape stored by the spot shape storage unit The block diagram which shows the detailed structure of the image formation part 107 Diagram showing correction table The figure which shows the example of the similarity degree of a spot shape and the pattern of image data A diagram schematically showing the difference in density characteristics due to the similarity between the spot shape and the image data pattern The figure which shows the structure of an image forming apparatus

  Hereinafter, the present invention will be described in detail according to preferred embodiments with reference to the accompanying drawings. In the following embodiments, a case where the present invention is applied will be described taking a tandem electrophotographic image forming apparatus as an example. The above configuration is an example of the present invention and is not limited to the illustrated configuration.

<Example 1>
FIG. 1A is a block diagram illustrating an image forming system applicable to the first embodiment. The image forming system includes an image forming apparatus 1 and an external device 2. The external apparatus 2 is a host computer (PC) or the like and outputs image data to be printed to the image forming apparatus 1.

  FIG. 1B is a block diagram illustrating a configuration of the image forming apparatus 1. The image forming apparatus 1 includes a controller unit 11 and an engine unit 10. The controller unit 11 includes an image processing unit 13, an image data storage unit 14, a spot shape storage unit 15, a similarity calculation unit 16, and a PWM processing unit 17. The engine unit 10 includes an image forming unit 18.

  The controller unit 11 temporarily stores image data to be printed input from the external device 2 in an image input buffer (not shown) or the like, and outputs the image data to the image processing unit 502. The image processing unit 13 performs resolution conversion processing, color separation processing, gradation correction processing, halftone processing, and the like on the input image data, and converts the input image data into image data that can be output by the engine unit 10. Convert. In an image forming apparatus using an electrophotographic system, it is desirable that image data to be formed is dot-concentrated and stable image data. Therefore, the halftone processing unit performs halftone processing for converting the image data into an AM screen.

  The pattern acquisition unit 14 acquires pattern information indicating what pattern the region including the target pixel has in the image data output from the image processing unit 18. In this embodiment, a region centered on the target pixel is cut out and acquired as pattern information. The spot shape storage unit 15 holds spot shape information indicating the spot shape (light intensity distribution) on the image carrier of the light source exposed by the image forming unit 18 described later. The spot shape varies depending on the position in the longitudinal direction on the image carrier. Therefore, the spot shape storage unit 15 stores a plurality of pieces of spot shape information in association with positions in the longitudinal direction on the image carrier. Here, image data representing the spot shape is stored.

  The similarity calculation unit 16 receives the pattern information of the target pixel from the pattern acquisition unit 14. In addition, the similarity calculation unit 16 acquires spot shape information on the image carrier corresponding to the position of the target pixel in the image data from the spot shape storage unit 15. Based on the acquired pattern information and spot shape information, the similarity calculation unit 16 calculates the similarity and outputs the similarity to the PWM processing unit 17.

  The PWM processing unit 17 performs PWM processing on the image data in accordance with the similarity received from the similarity calculation unit 16 and calculates a control signal for driving the exposure unit in the image forming unit 13. Here, first, the conventional PWM processing is performed on the image data, and then the PWM width is corrected according to the similarity.

  FIG. 4 is a diagram illustrating a detailed configuration of the image forming unit 18 in the engine unit 10. The engine unit 10 follows the intermediate transfer belt 101 from the upstream side in the rotation direction R1 of the intermediate transfer belt 101, CMYK image forming units 100a, 100b, 100c, and 100d, a secondary transfer device 102, and an intermediate transfer belt cleaning device. 104. A fixing device 103 is disposed on the downstream side of the secondary transfer device 102. The image forming units 100a, 100b, 100c, and 100d for each color of cyan (C), magenta (M), yellow (Y), and black (K) perform the same processing. The image forming unit 100a includes a photosensitive drum 1001a, a charging device 1002a, an exposure device 1003a, a developing device 1004a, a primary transfer device 1005a, and a cleaning device 1006a. The same applies to the image forming units 100b, 100c, and 100d.

  Hereinafter, the image forming process in the engine unit 10 will be described in detail.

  The image forming units 100 a, 100 b, 100 c, and 100 d form toner images on the photoconductors that are the respective image carriers using the respective color toners, and sequentially primary transfer them onto the intermediate transfer belt 101. In this embodiment, the image forming unit 100a uses C toner, the image forming unit 100b uses M toner, the image forming unit 100c uses Y toner, and the image forming unit 100d uses K toner. Note that the image forming unit and the color material to be used are not limited to four types. For example, in addition to the four colors of CMYK, there may be a light color toner or a clear toner, or four or less colors. Further, the order of the image forming units for each color is not limited to this embodiment, and an arbitrary order is conceivable.

  The toner images are formed in parallel at different timings in the order of the image forming units 100a, 100b, 100c, and 100d.

  First, the photosensitive drum 1001a included in the image forming unit 100a has an organic photoconductor layer having a negative polarity on the outer peripheral surface, and rotates in the direction of arrow R3.

(Charging)
The charging device 1002a is charged with a negative voltage and irradiates the surface of the photosensitive drum 1001a with charged particles, thereby charging the surface of the photosensitive drum 1001a to a uniform negative potential. The charged photosensitive drum 1001a rotates in the direction of arrow R3.

(exposure)
The exposure apparatus 1003a performs optical writing on the photosensitive drum 1001a based on a control signal acquired from the controller 11. Thereby, an electrostatic latent image is formed on the surface of the charged photosensitive drum 1001a. At this time, the light shape (spot shape) on the photosensitive drum 1001a differs depending on the position in the longitudinal direction. Details of the spot shape that differs depending on the position will be described later.

(developing)
The developing device 1004a supplies a negatively charged toner to the photosensitive drum 1001a using a developing roller that rotates at a substantially constant speed. Thus, the toner is attached to the electrostatic latent image on the photosensitive drum 1001a, and the electrostatic latent image is developed.

(Primary transfer)
The primary transfer device 1005a primarily transfers the toner image carried on the negatively charged photosensitive drum 1001a to the intermediate transfer belt 101 using a transfer roller to which a positive charge is applied.

(cleaning)
The cleaning device 1006a removes the residual toner image remaining on the photosensitive drum 1001a that has passed through the primary transfer device 1005a.

  Up to this point, the cyan image forming unit 100a has been described, but the same applies to the image forming units 100b, 100c, and 100d. In the case of forming a color image, the charging, exposure, development, temporary transfer, and cleaning steps so far are sequentially performed in the image forming units 100a, 100b, 100c, and 100d for each color. As a result, an image in which toner images of CMYK four colors are superimposed is formed on the intermediate transfer belt.

(Secondary transfer)
The secondary transfer device 102 secondarily transfers the toner image carried on the intermediate transfer belt 101 to the recording medium P that moves in the arrow R2 direction.

(Fixing)
The fixing device 103 performs processing such as pressure heating on the recording medium P onto which the toner image has been secondarily transferred, and fixes the image.

(Belt cleaning)
The intermediate transfer belt cleaning device 104 removes residual toner remaining on the intermediate transfer belt 101 that has passed through the secondary transfer device 102.

  The engine unit 10 forms an image on a recording medium by the process described above.

  Here, the principle of different density characteristics depending on the spot shape and the image data pattern will be described. In this embodiment, the spot shape differs depending on the position where the laser beam scans on the image carrier. FIG. 3A shows the relationship between the position in the longitudinal direction on the image carrier and the spot shape. In this case, an isotropic spot shape is formed at the center position, and the spot shape is distorted toward both ends. At the left end, the spot shape is distorted in the diagonally upward right direction, and at the right end, the spot shape is distorted in the diagonally upward left direction. In the example of FIG. 3, the coordinates are determined by normalizing the center coordinate of the image carrier as 0 and the length of the image forming region in the longitudinal direction as 2. FIG. 3B is a diagram for explaining spot shape information and a target pixel. Here, FIG. 3B shows image data composed of 8 × 8 blocks. The central 4 × 4 block corresponds to the target pixel in the input image data. FIG. 3B shows an isotropic spot shape. However, it can be seen that the light intensity distribution also exists around the pixel of interest. Here, consider the density characteristics of the spot shapes and image patterns at positions -1.0, 0, and 1.0.

  FIG. 6 shows the correlation between the spot shape and the pattern. As shown in FIG. 6A, when the direction in which the spot shape extends and the direction of the pattern forming the dots are opposite, the degree of similarity is low and the correlation is weak. Further, as shown in FIG. 6C, when the direction in which the spot shape extends and the direction of the pattern forming the dots are the same, the degree of similarity is high and the correlation is strong.

  FIG. 7 is a diagram schematically showing the difference in electrostatic latent image (potential distribution) on the photosensitive drum formed by the correlation between the spot shape and the pattern. When the potential exceeds the threshold value, the toner adheres sufficiently and an image can be formed. The dotted line indicates the potential distribution when the spot shape is isotropic (correlation between the spot shape and the pattern is medium).

  When the correlation between the spot shape and the pattern is strong, when the exposure is performed based on the pattern, a narrow and deep potential distribution is formed as compared with the case where the spot shape is isotropic. As a result, the area where the potential exceeds the threshold and the toner adheres is reduced. As a result, it can be said that the output density tends to decrease in the entire area of the image.

  When the correlation between the spot shape and the pattern is weak, when the exposure is performed based on the pattern, a broad and shallow potential distribution is formed as compared with the case where the spot shape is isotropic. As a result, when the periphery of the target pixel has a low gradation, the potential does not reach the threshold value, and the toner hardly adheres. On the other hand, when the periphery of the pixel of interest has high gradation, the area where the toner adheres with the potential exceeding the threshold value increases. As a result, it can be said that the output density tends to decrease at low gradations and the output density tends to increase at high gradations.

  As described above, the electrostatic latent image (potential distribution) formed on the photosensitive drum varies depending on the degree of similarity between the spot shape on the image carrier and the pattern in the image data, and the output density characteristics differ. . Therefore, in this embodiment, the width of the PWM signal is corrected according to the similarity between the spot shape and the pattern of the image data.

  FIG. 2 is a flowchart of a PWM signal calculation process in the image forming apparatus applicable to the first embodiment. The following processing is repeated for all the pixels of the image data. In the case of forming a color image, the following processing is individually performed on the image data of each color.

  In step S21, the pattern acquisition unit 14 acquires pattern information indicating a pattern around the target pixel. In this embodiment, a region centered on the target pixel is cut out as a pattern 501 shown in FIG. 5 and acquired as pattern information. Here, the image data serving as the pattern information is cut out with the same size and the same resolution as the image data stored in the spot shape storage unit 14 described later.

  In step S <b> 22, the similarity calculation unit 16 acquires spot shape information from the spot shape storage unit 15. The spot shape varies depending on the position in the longitudinal direction of the image carrier. Therefore, spot shape information corresponding to the target pixel is acquired from a position where the target pixel in the image data is exposed on the image carrier. FIG. 3 shows the spot shape information held by the spot shape storage unit 15. The spot shape storage unit 15 stores five pieces of image data representing the spot shape to be formed in correspondence with the longitudinal position (coordinates) on the image carrier as spot shape information. In the example of FIG. 3, the coordinates are determined by normalizing the center coordinate of the image carrier as 0 and the length of the image forming region in the longitudinal direction as 2. Here, spot shape information corresponding to the position closest to the exposure position of the target pixel is acquired from the five image data stored in the spot shape storage unit 15.

  In step S23, the similarity calculation unit 16 calculates the similarity between the pattern near the target pixel and the corresponding spot shape. The similarity calculation unit 16 calculates the similarity based on the pattern information acquired in step S21 and the spot shape information acquired in step S22. The similarity is an index indicating how similar the pattern and the spot shape are. As described above, in the example shown in FIG. 6, the similarity increases in the order shown in (1), (2), and (3). In this embodiment, the similarity between the image data indicating the pattern and the image data indicating the spot shape is defined as the mean square error of the pixel value difference representing each pixel.

  In step S24, the PWM processing unit 17 performs PWM processing on the target pixel according to the similarity. In this embodiment, first, normal PWM processing is performed on the target pixel. Thereafter, a correction table corresponding to the similarity is selected. FIG. 5 shows three correction tables according to the degree of similarity. When the similarity is larger than the first threshold, the correction table (c) is used. When the similarity is equal to or less than the first threshold and greater than the second threshold, the correction table (b) is used. When the similarity is not more than the second threshold value, the correction table (a) is used. In addition, in order to clarify each difference, the dotted lines in the correction tables (a) and (c) indicate the correction table (b). The correction table is created in advance by patch measurement or simulation of each similarity. The PWM processing unit 17 selects a correction table according to the similarity from the correction table shown in FIG. 6 according to the similarity, and corrects the width of the PWM signal using the selected correction table. The corrected PWM signal is output as a control signal for controlling the exposure unit in the image forming unit 17.

  Thus, the PWM signal calculation process in the present embodiment is completed.

  In this embodiment, the similarity between the pattern of the image data and the spot shape is calculated, and a control signal (PWM signal) for controlling the exposure unit of the image forming apparatus 18 is generated according to the similarity. Thereby, it can correct | amend appropriately according to the output density characteristic which changes with the anisotropy of spot shape. As a result, a more suitable image can be formed regardless of the similarity between the spot shape and the image data pattern.

<Example 2>
In the first embodiment, the method for correcting the width of the PWM signal in accordance with the similarity between the image pattern and the laser beam has been described. In the second embodiment, a description will be given of a configuration in which tone correction according to similarity is performed before halftone processing.

  FIG. 8 is a block diagram illustrating a configuration of an image forming apparatus applicable to the second embodiment. In addition, the same code | symbol is attached | subjected about the structure similar to Example 1, and description is abbreviate | omitted.

  The image processing unit 18 includes a pattern acquisition unit 14, a spot shape storage unit 15, a similarity calculation unit 16, a gradation correction unit 19, a gradation correction table storage unit 20, and a halftone processing unit 21.

  The input image data (hereinafter, input image data) is subjected to halftone processing by the halftone processing unit 21 after the gradation correction unit 19 has been subjected to gradation correction. The halftone processing unit 21 converts the input image data into image data that can be output by the image forming unit 18. From the image data obtained from the halftone processing unit, the PWM processing unit 17 generates a control signal for controlling the image forming unit 18 by PWM processing and outputs it to the engine 10.

  The gradation correction table storage unit 20 holds a plurality of gradation correction tables. The gradation correction unit 19 selects one appropriate gradation correction table from the gradation correction table storage unit 20 according to the similarity calculated by the similarity calculation unit 16. Based on the selected gradation correction table, gradation correction of the input image data is performed. Accordingly, an appropriate image can be formed according to the similarity between the pattern of the image data and the spot shape.

<Other examples>
In the above-described embodiment, the similarity calculation unit 16 is configured to select and acquire one of the spot shape information stored in the spot shape storage unit 102, but is not limited thereto. A configuration is also possible in which a plurality of pieces of spot shape information are selected from the spot shape storage unit 102 and are combined to obtain spot shape information corresponding to the pixel of interest. The spot shape information indicating the spot shape is not limited to image data. When the spot shape has directionality, information indicating the direction in which the spot shape extends may be used. In that case, it is necessary to prepare similar information for the pattern of the image data.

  In the above-described embodiment, the similarity is calculated based on the mean square error in the region. However, the definition of the similarity is not limited to the illustrated example. In addition, the area indicating the pattern can be divided into several blocks and defined as the mean square error of the representative value of each block. As representative values at this time, for example, the difference in the average pixel value of the pixels in the block, the difference in the maximum pixel value, the distance of the center of gravity position, and the like can be considered. Another possible method is to calculate the similarity after performing pre-processing such as resolution conversion, size change, smoothing, or the like on both or one of the image data in the vicinity of the target pixel indicating the pattern and the image data indicating the spot shape.

  In the above-described embodiment, the table is used to perform correction according to the similarity, but the present invention is not limited to this method. A configuration using a function for obtaining a corrected PWM width in accordance with the similarity is also conceivable.

  In the above-described embodiment, the PWM width is changed in order to perform correction according to the degree of similarity. However, the present invention is not limited to this method. A configuration in which the driving current of the light source is changed according to the similarity is also conceivable.

  In the above-described embodiment, the image forming apparatus using laser light as the exposure unit has been described as an example. As another exposure means, there is a method of performing exposure using a light source such as a plurality of LEDs arranged on a line. In this method as well, the same effect can be obtained by controlling the exposure according to the spot shape of the exposed light source and the pattern of the image data, as in the previous embodiment.

The present invention can also be realized by supplying a storage medium storing software program codes for realizing the functions of the above-described embodiments to a system or apparatus. In this case, the function of the above-described embodiment is realized by the computer (or CPU or MPU) of the system or apparatus reading and executing the program code stored in the storage medium so that the computer can read it.

Claims (14)

An image processing apparatus of an image forming apparatus that forms an image by exposing an image carrier with an exposure unit,
Generating means for generating a control signal for controlling the exposure unit based on input image data;
The control signal has a similarity between a pattern in a region including the target pixel and a shape of a light intensity distribution when the target pixel is exposed on the image carrier with respect to the target pixel constituting the input image data. An image processing apparatus generated according to the method.   The image processing apparatus according to claim 1, wherein the similarity indicates how close the direction of the pattern is to the longitudinal direction of the shape of the light intensity distribution. Further, for a target pixel constituting the input image data, pattern information acquisition means for acquiring pattern information indicating a pattern in a region including the target pixel;
Shape information acquisition means for acquiring shape information indicating the shape of the light intensity distribution on the image carrier corresponding to the target pixel;
The image processing apparatus according to claim 1, wherein the generation unit generates a control signal for the pixel of interest according to the pattern information and the shape information. Furthermore, it has a calculation means for calculating a similarity indicating the similarity from the pattern information and the shape information,
The image processing apparatus according to claim 3, wherein the generation unit generates a control signal for the target pixel in accordance with the similarity calculated by the calculation unit.   The generation unit converts the input image data into image data that can be output by the image forming apparatus, and then performs PWM processing on the converted image data in accordance with the similarity. 5. The image processing apparatus according to 4.   The generation unit performs gradation correction according to the similarity on the input image data, and then converts the gradation-corrected image data into image data that can be output by the image forming apparatus. The image processing apparatus according to claim 4. The shape information is first image data representing the spot shape,
The pattern information is second image data representing an image obtained by cutting out a region including the target pixel,
The similarity is calculated from an average square error of a pixel value difference representing each pixel in the first image data and the second image data. The image processing apparatus according to item. The shape information is first image data representing the spot shape,
The pattern information is second image data representing an image obtained by cutting out a region including the target pixel,
5. The similarity is calculated by dividing each of the first image data and the second image data into a plurality of blocks and calculating a mean square error of representative values of each block. The image processing device according to claim 6.   The image processing apparatus according to claim 1, wherein the control signal is generated based on an output density characteristic of the image forming apparatus corresponding to the similarity.   The generation unit may generate more isotropic than the spot shape when the similarity of the pixel of interest is high, or when the pixel of interest has a low gradation and the similarity of the pixel of interest is low. The image processing apparatus according to claim 1, wherein a control signal that increases an exposure amount representing the target pixel is generated.   The generating unit performs control such that when the pixel of interest has a high gradation and the similarity of the pixel of interest is high, the amount of exposure representing the pixel of interest is smaller than when the spot shape is isotropic. The image processing apparatus according to claim 1, wherein a signal is generated.   The image processing apparatus according to claim 1, wherein the image processing apparatus is built in the image forming apparatus.   A computer program that causes a computer to function as the image processing apparatus according to any one of claims 1 to 12 by being read and executed by a computer. A method of controlling an image processing apparatus of an image forming apparatus that forms an image by exposing an image carrier by an exposure unit,
The image processing apparatus includes a generation unit,
The generation means generates a control signal for controlling the exposure unit based on input image data,
The control signal has a similarity between a pattern in a region including the target pixel and a shape of a light intensity distribution when the target pixel is exposed on the image carrier with respect to the target pixel constituting the input image data. A control method generated according to the method.

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