JP2012195857A - Image processing system, image forming device, control method of image processing system, and control program of image processing system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing system which prevents occurrence of wood pattern-like noise, can perform an image processing by an error diffusion method with comparatively high image quality and can speedily perform printing without performing costly light quantity adjustment.SOLUTION: The image processing system outputs output data with which an image is formed by an image forming device performing an error diffusing processing on input data, reducing a value and forming a latent image in an image carrier by using two light sources, for example. In the image processing system, a binarized result storing section 63 stores a processing result on pixels in a prescribed range, which are at the periphery of a target pixel, and a dot counting section 65 obtains a count value on the number of dots for respective regions corresponding to two light sources based on the processing result. A reference value calculating section 61 obtains a reference value in accordance with density of the target pixel, and an inversion processing section 55 compulsorily inverts the processing result of the error diffusion processing on the target pixel based on the count value and the reference value, and outputs the output data.

Description

  The present invention relates to an image processing apparatus, an image forming apparatus, a control method for the image processing apparatus, and a control program for the image processing apparatus, and more particularly to an image processing apparatus, an image forming apparatus, and an image processing apparatus for performing error diffusion processing of image data. The present invention relates to a control method and a control program for an image processing apparatus.

  In electrophotographic image forming apparatuses (MFP (Multi Function Peripheral), facsimile apparatus, copier, printer, etc.) having a scanner function, a facsimile function, a copying function, a function as a printer, a data communication function, and a server function. In order to faithfully reproduce a halftone original image such as a photographic image, a low-value processing method using an error diffusion method may be performed.

  Error diffusion processing is one type of image processing that lowers the value of a halftone image such as a photograph while smoothly expressing gradation. In error diffusion processing, the gradation level of an image is lowered by a constant threshold value, and the error between the density value of the pixel of interest and the density value of the corresponding lowered data becomes multiple peripheral areas within a certain range. The pixels are weighted and distributed. According to the error diffusion method, the density of the image is maintained, so that it is possible to obtain a low-value image that is relatively faithful to the image before processing.

  By the way, particularly in an image forming apparatus that employs an electrophotographic process that forms a latent image on an image carrier with a plurality of light sources, noise may occur in the formed image in the following cases. That is, when there is a difference in the amount of light among a plurality of light sources, and in the image subjected to error diffusion processing, the average density is locally different between the print areas corresponding to each of the plurality of light sources. It is.

  Such noise is prominently generated particularly in a halftone solid area (filled area) close to a highlight portion. Such noise may look like wood grain.

  Hereinafter, for example, in the image forming apparatus that scans alternate lines with light from two light sources, a mechanism in which the above-described woodgrain noise is generated will be described.

  FIG. 8 is a first diagram illustrating a mechanism for generating noise.

  The image shown in FIG. 8 shows the result after binarization by performing error diffusion processing on an intermediate density image close to highlight. In FIG. 8, the image is an enlargement of the bitmap data, and each square area surrounded by a grid represents one pixel. After binarization, the image has, for example, white pixels and black pixels (dots; colored gray in FIG. 8). The black pixels are arranged in various places according to the density of the image before processing.

  FIG. 9 is a second diagram for explaining the mechanism by which noise occurs.

  FIG. 9 shows the pixels of the bitmap image shown in FIG. 8 separated from those included in the odd lines and those included in the even lines. In other words, FIG. 9 shows an area printed with an odd-numbered light source and an even-numbered line when image formation (printing) is performed in the electrophotographic process using two light sources for the bitmap image shown in FIG. It is divided into areas printed with a light source for use.

  In the example shown in FIG. 9, the number of dots included in odd lines is relatively small, and the number of dots included in even lines is relatively large. As described above, in the conventional error diffusion process, the distribution of dots may be locally uneven between the odd line area and the even line area.

  FIG. 10 is a table for explaining the relationship between the distribution of dots in an image subjected to error diffusion processing and the appearance of the image when there is a light amount difference between two light sources.

  In FIG. 10, it is assumed that the light amount of the light source corresponding to the odd line is relatively low and the light amount of the light source corresponding to the even line is relatively high. In this case, when the number of odd-line dots is relatively large (when the even-line dots are relatively small), the appearance of the image formed is relatively thin (so that the image density appears to be light). Become).

  On the other hand, when the number of odd-line dots is relatively small (when the even-numbered dots are relatively large), the appearance of the image formed is relatively dark (the image density appears to be dark). .

  If the odd-numbered dots and the even-numbered dots are approximately equal to each other, even if there is a difference between the light amount of the light source corresponding to the odd line and the light amount of the light source corresponding to the even line, the image formed Appearance of “normal” (normal density). In other words, if the dot distribution is substantially uniform between the odd lines and the even lines, the density of the image formed looks almost the same as the density of the original image.

  The woodgrain noise is generated when three states as shown in FIG. 10 exist locally and randomly.

  Conventionally, in order to reduce the occurrence of such noise, for example, a countermeasure method of adjusting the light amounts of a plurality of lasers by measuring the result of outputting a test image has been taken.

  For example, in Patent Document 1 below, an image pattern for adjustment is printed by changing the amount of light, the density of each color of the printed pattern is detected, and the amount of light is such that the difference in density among a plurality of light sources becomes small. An image forming apparatus that emits light from a light source is disclosed.

  A method is known in which a countermeasure against the occurrence of a geometric interference pattern generated in the error diffusion process is performed by periodically changing a feedback coefficient used in the error diffusion process.

  For example, Patent Document 2 below discloses an image processing apparatus in which an error feedback coefficient is periodically changed in the main scanning direction and the phase is shifted between even / odd lines. That is, in this image processing apparatus, dots can be uniformed in a zigzag pattern at the highlight portion of the image.

  There is also known a method of making it difficult to form isolated dots by adjusting an integration coefficient of error diffusion.

  For example, Patent Document 3 below discloses an image processing apparatus that performs processing to suppress the generation of isolated dots and increase the size of dots by making the diffusion coefficient of error diffusion negative for adjacent pixels of the target pixel. Has been. Patent Document 3 discloses that when forming an image by the multi-beam method, the dots are easily grown in the sub-scanning direction so that the dots are grown so as to average the density of even / odd lines. Has been.

  Patent Document 4 below discloses an image forming apparatus in which dots are enlarged by increasing the value of a matrix coefficient for error diffusion for a pixel far from the pixel of interest to be quantized.

  In Patent Document 5 below, a dot monitoring area corresponding to the input gradation is provided, and the binarization result for the target pixel is inverted according to the comparison result between the dot count value and the reference value. Discloses an image processing method for preventing worm noise.

JP 2008-134326 A Japanese Patent Laid-Open No. 9-23335 JP 2007-81657 A JP 2002-218239 A JP 2004-274181 A

  However, the conventional techniques as described above have the following problems.

  That is, for example, in the configuration described in Patent Document 1, there is a problem that adjustment time and adjustment cost for adjusting the light amounts of a plurality of light sources are required.

  For example, in the configuration described in Patent Document 2, there is another problem that gradation reproducibility is reduced when the error feedback coefficient is reduced. In addition, in the control of the error feedback coefficient, there is a problem that it is difficult to distribute data to lines corresponding to a plurality of light sources, for example, to distribute data equally to even lines and odd lines.

  In configurations such as those described in Patent Documents 3 and 4, isolated dots are difficult to form and the dots become large, and the graininess of the processed image may deteriorate. In particular, when many light sources such as four light sources are used, such a configuration cannot be adopted because the dots may become too large and the image may become so large that the feeling of roughness of the image cannot be overlooked. In addition, as described above, it is difficult to evenly distribute data to lines corresponding to a plurality of light sources.

  Note that the image processing method described in Patent Document 5 performs processing by separating the dot monitoring region, but it cannot be said that a plurality of light sources can perform printing with an equal amount of light to prevent noise generation. There is a problem.

  The present invention has been made in order to solve such problems, and can prevent the generation of wood-tone noise, can perform image processing by the error diffusion method with a relatively high image quality, and is costly. An object of the present invention is to provide an image processing apparatus, an image forming apparatus, a control method for the image processing apparatus, and a control program for the image processing apparatus that can execute printing promptly without adjusting the light amount.

  In order to achieve the above object, according to one aspect of the present invention, an image is formed by an image forming apparatus that performs error diffusion processing on input data to lower the value and forms a latent image on an image carrier using a plurality of light sources. The image processing apparatus that outputs the output data to be processed includes a storage unit that stores processing results of error diffusion processing for pixels in a predetermined range around the target pixel that is a processing target in the input data, and a plurality of light sources Counting means for obtaining a count value relating to the number of dots based on the processing result stored in the storage means for each area corresponding to each of the above, an acquisition means for acquiring a reference value according to the density of the target pixel, and a counting means Inverting means for forcibly inverting the processing result of the error diffusion processing for the pixel of interest based on the count value obtained by the above and the reference value acquired by the acquiring means. That.

  Preferably, the storage unit stores the processing result for an area set so that the sub-scanning direction of the input data is longer than the main scanning direction.

  Preferably, the reference value is set so that dots are more evenly distributed in the highlight area of the input data.

  Preferably, the plurality of light sources are two light sources, and two regions corresponding to each of the two light sources are regions that are odd-numbered lines in the sub-scanning direction for input data and sub-scanning directions for input data And an area that is an even-numbered line.

  According to another aspect of the present invention, an image forming apparatus includes: an image forming unit that forms an image on a sheet by forming a latent image on an image carrier with a plurality of light sources; The image forming unit performs image formation based on output data output from the image processing apparatus.

  According to yet another aspect of the present invention, an error diffusion process is performed on input data to lower the value, and an image is formed by an image forming apparatus that forms a latent image on an image carrier using a plurality of light sources. The control method of the image processing apparatus that outputs data includes the processing result of error diffusion processing for pixels in a predetermined range around the target pixel of the input data for each area corresponding to a plurality of light sources. A storage step for storing, a count step for obtaining a count value related to the number of dots based on the processing result stored in the storage step for each region corresponding to a plurality of light sources, and a reference value corresponding to the density of the pixel of interest Based on the acquisition step, the count value obtained in the count step, and the reference value obtained in the acquisition step. And a reversing step for forcibly inverting the result.

  According to yet another aspect of the present invention, an error diffusion process is performed on input data to lower the value, and an image is formed by an image forming apparatus that forms a latent image on an image carrier using a plurality of light sources. The control program of the image processing apparatus that outputs data displays the processing result of the error diffusion processing for pixels in a predetermined range around the target pixel of the input data for each region corresponding to a plurality of light sources. A storage step for storing, a count step for obtaining a count value related to the number of dots based on the processing result stored in the storage step for each region corresponding to a plurality of light sources, and a reference value corresponding to the density of the pixel of interest Based on the acquisition step, the count value obtained in the count step, and the reference value obtained in the acquisition step. An inverting step of inverting the processing result forcibly executed by a computer.

  According to these inventions, the processing result of the error diffusion processing is forcibly inverted according to the count value counted for each region corresponding to each of the plurality of light sources and the reference value. Accordingly, an image processing apparatus that can prevent the occurrence of wood-tone noise, perform image processing by the error diffusion method with relatively high image quality, and can execute printing promptly without performing costly light amount adjustment. An image forming apparatus, an image processing apparatus control method, and an image processing apparatus control program can be provided.

1 is a front view showing an image forming apparatus having an image processing apparatus in an example of an embodiment of the present invention. 2 is a block diagram illustrating a configuration of a control circuit of the image forming apparatus. FIG. It is a block diagram which shows the structure of an error diffusion process part. It is a flowchart which shows an example of an error diffusion process. It is a figure which shows an example of the area | region where memory | storage of a binarization result is performed by the binarization result memory | storage part regarding an attention pixel. It is a figure which shows another example of the area | region where binarization result memory | storage is performed by the binarization result memory | storage part regarding an attention pixel. It is a graph explaining the example of a setting of the reference value by a reference value calculation part. It is a 1st figure explaining the mechanism in which noise generate | occur | produces. It is a 2nd figure explaining the mechanism in which noise generate | occur | produces. It is a table | surface explaining the relationship between the distribution of the dot in the image in which the error diffusion process was performed, and the appearance of the image when there is a light amount difference between two light sources.

  Hereinafter, an example of an image forming apparatus (an example of an image processing apparatus) according to an embodiment of the present invention will be described.

  [Overview]

  The image forming apparatus is an MFP (Multi Function Peripheral) having a scanner function, a copying function, a printer function, a facsimile function, a data communication function, and a server function. In the scanner function, an image of a set original is read and stored in an HDD (Hard Disk Drive) or the like. In the copying function, it is further printed (printed) on paper or the like. In the function as a printer, when a print instruction is received from an external terminal such as a PC (personal computer), printing is performed on a sheet based on the instruction. In the facsimile function, facsimile data is received from an external facsimile machine and stored in an HDD or the like. In the data communication function, data is transmitted / received to / from a connected external device. In the server function, a plurality of users can share data stored in the HDD or the like.

  The image forming apparatus forms an image on a sheet by an electrophotographic process. The image forming apparatus forms an image by forming a latent image on an image carrier using two light sources, for example.

  The image forming apparatus functions as an image processing apparatus that performs image processing on image data. For example, the image forming apparatus performs image processing on image data to be processed such as an image read by a scanner function or an image to be printed. The image forming apparatus performs error diffusion processing that binarizes image data by an error diffusion method.

  In the present embodiment, when performing an error diffusion process on image data, the image forming apparatus performs image data for each of two areas corresponding to two light sources when image formation is performed based on the image data. The result of binarizing is stored. Further, the image forming apparatus performs the reversal processing of the binarized result by comparing the number of dots in each region.

  As a result, it is possible to prevent the occurrence of woodgrain noise when an image is formed on an image after error diffusion processing. Effective countermeasures against noise generation due to image processing without adjusting the amount of light from multiple light sources (laser light sources, etc.) to be uniform as in the past, that is, without requiring time and cost for the adjustment process Can be applied.

  [Embodiment]

  First, the configuration of the image forming apparatus in an example of the present embodiment will be described.

  [Configuration of Image Forming Apparatus]

  FIG. 1 is a front view showing an image forming apparatus having an image processing apparatus in an example of an embodiment of the present invention.

  Referring to the drawing, the image forming apparatus 1 includes an image forming unit 3, an image reading unit 5, an image processing unit 21, a paper feeding unit 30, and a paper discharging unit 31.

  FIG. 2 is a block diagram illustrating a configuration of the control circuit of the image forming apparatus 1.

  Referring to the figure, various functions of image forming apparatus 1 are realized by the operation of each module under the control of CPU (central processing unit) 11 that controls the entire system of image forming apparatus 1. In addition to the above-described units, the image forming apparatus 1 includes a display unit 7, a panel operation unit 9, a storage unit 13, a ROM 15, a RAM 17, a nonvolatile memory 19, an image output unit 23, a facsimile control unit 25, a network connection unit 27, and the like. I have.

  The CPU 11 includes a determination unit 11a, a deletion unit 11b, a control unit 11c, and a notification unit 11d. The CPU 11 uses the determination unit 11a, the deletion unit 11b, the control unit 11c, the notification unit 11d, and the like to perform communication and signal transmission / reception with each unit of the image forming apparatus 1 and perform various determinations and information deletion. Or the like to control the entire system of the image forming apparatus 1.

  The image forming unit 3 includes, for example, a toner image forming unit (not shown), a paper transport unit (not shown), and a fixing device (not shown), and forms an image on a paper by an electrophotographic method. To do. The sheet is conveyed from the sheet feeding unit 30 to the toner image forming unit by the sheet conveying unit. The paper on which the image is formed by the toner image forming unit and the fixing device is discharged to the paper discharge unit 31 by the paper transport unit. The image forming unit 3 forms an image on a sheet based on the image subjected to the image processing by the image processing unit 21. The image forming unit 3 is configured to be able to form a color image on a sheet by synthesizing four color images by a so-called tandem method.

  The image reading unit 5 is disposed on the upper part of the housing of the image forming apparatus 1. The image reading unit 5 includes an ADF (Auto Document Feeder) 5a. The image reading unit 5 performs the above-described scanner function. The image reading unit 5 scans a document placed on a transparent document table with a contact image sensor and reads it as image data. Further, the image reading unit 5 reads the image data by the contact image sensor while sequentially taking in a plurality of documents set on the document tray by the ADF 5a. Image data read by the image reading unit 5 is converted into data of a predetermined data format by the CPU 11 and stored in the storage unit 13 or the like.

  The display unit 7 is, for example, an LCD (Liquid Crystal Display) that displays an image. Various images are displayed on the display unit 7 under the control of the CPU 11 such as an image showing the state of the image forming apparatus 1 and an operation guide image. The display unit 7 may also serve as the panel operation unit 9.

  The panel operation unit 9 is, for example, an LCD provided with a touch panel. The panel operation unit 9 displays a guidance screen for the user or displays an operation button to accept a touch operation from the user. The panel operation unit 9 performs display under the control of the CPU 11. When the user inputs an operation, the panel operation unit 9 transmits an operation signal or a predetermined command corresponding to the operation to the CPU 11. That is, the user can cause the image forming apparatus 1 to execute various operations by operating the panel operation unit 9.

  The storage unit 13 is, for example, an HDD (Hard Disk Drive). The storage unit 13 stores job (JOB) data sent from the outside via the network connection unit 27, image data read by the image reading unit 5, and the like. The storage unit 13 stores setting information of the image forming apparatus 1 and a control program 13a for performing various operations of the image forming apparatus 1. The storage unit 13 can store a plurality of jobs transmitted from one client PC or a plurality of client PCs. The storage unit 13 may also serve as the ROM 15 and the nonvolatile memory 19.

  The ROM 15 is, for example, a flash ROM (Flash Memory). The ROM 15 stores data used for operating the image forming apparatus 1. Similar to the storage unit 13, the ROM 15 may store various control programs, function setting data of the image forming apparatus 1, and the like. The CPU 11 reads data from the ROM 15 and writes data to the ROM 15 by performing predetermined processing. The ROM 15 may be non-rewritable.

  The RAM 17 is a main memory of the CPU 11. The RAM 17 is used to store data necessary when the CPU 11 executes the control program 13a as will be described later.

  The nonvolatile memory 19 is, for example, a flash ROM (Flash Memory). The nonvolatile memory 19 may store various control programs, function setting data of the image forming apparatus 1, and the like, like the ROM 15 and the storage unit 13. The CPU 11 reads data from the nonvolatile memory 19 and writes data to the nonvolatile memory 19 when necessary to control the image forming apparatus 1.

  The image processing unit 21 has a function of performing various image processing under the control of the CPU 11. Image processing includes, for example, processing for converting image data to be printed into CMYK format data, and processing for correcting image data according to the characteristics of the image data.

  For example, the image output unit 23 can transmit the image data stored in the storage unit 13 or the like to an external PC or the like via the network connection unit 27 or the like. The image output unit 23 can output an image by various communication protocols such as e-mail and FTP (File Transfer Protocol).

  The facsimile control unit 25 controls the above-described facsimile function and performs facsimile communication with an external device. The facsimile control unit 25 includes a receiving unit 25a that receives data by facsimile communication.

  The network connection unit 27 is configured by combining, for example, a hardware unit such as a NIC (Network Interface Card) and a software unit that performs communication using a predetermined communication protocol. The network connection unit 27 connects the image forming apparatus 1 to an external network such as a LAN. As a result, the image forming apparatus 1 can communicate with an external apparatus such as a client PC connected to the external network. When the image forming apparatus 1 is connected to an external network to which a PC or the like is connected, the image forming apparatus 1 can receive a print job from the PC or the like. Further, the image forming apparatus 1 may transmit the image data read by the image reading unit 5 to the PC by the above-described image output unit 23 or the like, or may be transmitted by E-mail via a mail server or the like. it can. The network connection unit 27 may be configured to be connectable to an external network by wireless communication.

  The CPU 11 controls various operations of the image forming apparatus 1 by executing a control program 13 a or the like stored in the ROM 15, the RAM 17, or the storage unit 13. When an operation signal is sent from the panel operation unit 9 or an operation command is transmitted from a PC or the like that can communicate via the network connection unit 27, the CPU 11 executes a predetermined control program 13a in response thereto. Accordingly, a predetermined function of the image forming apparatus 1 is executed in accordance with an operation input or the like applied to the panel operation unit 9 by the user.

  In the present embodiment, the image forming unit 3 includes, for example, two laser light sources 4a and 4b (hereinafter, these may be collectively referred to as a laser light source 4), and forms an image by electrophotography. In other words, the image forming unit 3 irradiates light emitted from two light sources onto the image carrier and scans the image carrier in the main scanning direction, thereby forming a latent image on the image carrier. The image formed on the image carrier is transferred and fixed on a sheet, and an image is formed on the sheet. By performing image formation using two light sources in this way, the resolution in the sub-scanning direction of the image to be formed can be further increased.

  The image processing unit 21 is provided with an error diffusion processing unit 21a that performs error diffusion processing on input data, lowers the value, and outputs the result. The operation of the error diffusion processing unit 21a will be described later.

  [Description of error diffusion processing]

  In the present embodiment, the image forming apparatus 1 performs an error diffusion process that lowers the input data, which is image data, and outputs it as an image process. In the error diffusion process, for example, binarization is performed. The error diffusion processing is performed based on the control of the CPU 11 by an error diffusion processing unit 21a provided in the image processing unit 21, for example. For example, the CPU 11 controls the error diffusion process by executing the control program 13 a of the storage unit 13.

  FIG. 3 is a block diagram showing a configuration of the error diffusion processing unit 21a.

  Referring to FIG. 3, when an image input signal is input as input data, error diffusion processing unit 21a performs processing on the input data, and outputs output data that is processed image data as an image output signal. The output data can be an object of image formation by the image forming apparatus.

  The error diffusion processing unit 21a includes an addition unit 51, a comparison unit 53, an inversion processing unit (an example of an inversion unit) 55, a subtraction unit 57, an error integration unit 59, and a reference value calculation unit (an example of an acquisition unit). 61, a binarization result storage unit (an example of a storage unit) 63, and a dot count unit (an example of a count unit) 65.

  The image input signal has a gradation reproducibility of, for example, 256 gradations (8 bits). When the image input signal is input to the error diffusion processing unit 21 a, the image input signal is input to the addition unit 51 and the reference value calculation unit 61.

  In the adder 51, the image input signal and the error integration result (error data) that is the output from the error integrator 59 are added. The added result is output to the comparison unit 53 and the subtraction unit 57.

  In the comparison unit 53, the input addition result is compared with the binarized threshold value. If the addition result is larger, the dot is turned on (outputs an on signal) for the pixel. If the addition result is smaller, the dot is turned off. Here, “on” means that the value is 1 (for example, black), and “off” means that the value is 0 (for example, white or colorless). The value is sent to the inversion processing unit 55. In other words, the comparison result is sent from the comparison unit 53 to the inversion processing unit 55.

  The inversion processing unit 55 includes a determination unit 67 and a forcible processing unit 69. The output from the comparison unit 53 is input to the forcible processing unit 69. The forcible processing unit 69 performs processing for forcibly inverting the binarization result in a predetermined case as will be described later. The processing result of the forcible processing unit 69 is output from the error diffusion processing unit 21a as an image output signal (output data). The processing result of the forcible processing unit 69 is output to the subtraction unit 57 and the binarization result storage unit 63.

  The subtraction unit 57 receives the addition result output from the adder 51 and the processing result of the forcible processing unit 69, that is, the image output signal. In the subtraction unit 57, an error between the image output signal and the addition result is calculated. At this time, since the image output signal is binarized data and the addition result is data before binarization, the image output signal is multiplied by an appropriate multiple (for example, 255 times). The error is calculated. The calculated error is output to the error integration unit 59.

  The error integrating unit 59 accumulates the error input from the subtracting unit 57, and outputs the accumulated error data to the adding unit 51 so as to be distributed to peripheral pixels of the target pixel. Note that normal error diffusion processing (without compulsory inversion processing) by the adding unit 51, the comparing unit 53, the subtracting unit 57, and the error integrating unit 59 is a known one, and various methods can be used. It is.

  The reference value calculation unit 61 receives the image input signal input to the error diffusion processing unit 21a. The reference value calculation unit 61 calculates a reference value according to the density (gradation) of the image input signal, that is, the density of the target pixel. The calculated reference value is output to the determination unit 67.

  The binarization result storage unit 63 receives the image output signal output from the forcible processing unit 69. Based on the input image output signal, the binarization result storage unit 63 binarizes the binarization result (binarized) for pixels within a predetermined range around the target pixel to be processed in the input data. Result, that is, the result of error diffusion processing). The area that is the storage target of the binarization result will be described later.

  The dot count unit 65 counts the binarization result stored in the binarization result storage unit 63. That is, the dot count unit 65 obtains a count value related to the number of dots (a count value related to the number of dots on or off) based on the processing result of the error diffusion processing stored in the binarization result storage unit 63. ,get. The count value is sent to the determination unit 67.

  Based on the count value obtained by the dot count unit 65 and the reference value obtained by the reference value calculation unit 61, the inversion processing unit 55 obtains the processing result of the error diffusion processing for the target pixel, that is, the binarization result. Forces inversion processing.

  That is, the determination unit 67 performs determination based on the reference value input from the reference value calculation unit 61 and the count value input from the dot count unit 65. The determination is made as to whether or not the binarization result should be forcibly inverted for the target pixel by comparing the reference value and the count value.

  The forcible processing unit 69 performs processing for forcibly inverting the binarization result based on the determination result by the determination unit 67. That is, when the determination unit 67 determines that the binarization result for the target pixel should be inverted, the binarization result for the target pixel is inverted and output.

  Here, in the present embodiment, the error diffusion processing unit 21 a has 2 for each of the two regions corresponding to the two laser light sources 4 when image formation is performed on the input data based on the image data. Based on the number of dots in each area, the binarization result is stored, and the binarization result for the target pixel that is the processing target in the input data is inverted.

  That is, the binarization result storage unit 63 stores the processing result of the error diffusion processing for pixels in a predetermined range around the target pixel for each of the two areas corresponding to the two laser light sources 4. In other words, the target region for storing the processing result of the error diffusion processing is a region obtained by dividing a region composed of pixels in a predetermined range around the target pixel into two regions corresponding to the two laser light sources 4 respectively. Is set.

  The dot count unit 65 obtains a count value related to the number of dots based on the processing result stored by the binarization result storage unit 63 for each of the two areas. In other words, the dot count unit 65 counts the binarization result for each of the two regions that the two laser light sources 4 will print, and obtains a count value.

  Whether the determination unit 67 should invert the pixel of interest based on the first count value for the first region, the second count value for the second region, and the reference value of the two regions. Or make a decision. In the present embodiment, for example, when the first count value is larger than the value obtained by subtracting the reference value from the second count value and smaller than the value obtained by adding the reference value to the second count value, inversion should be performed. It is determined that it is not. An operation example of the determination unit 67 will be described later.

  FIG. 4 is a flowchart illustrating an example of error diffusion processing.

  Hereinafter, the error diffusion processing that the CPU 11 causes the error diffusion processing unit 21a and the like to execute will be described with the CPU 11 as a processing subject. The process described below is performed for each target pixel to be processed in the input data.

  In step S <b> 101, the CPU 11 performs input processing for input data of the target pixel. The input pixel is input to the adding unit 51 and the like.

  In step S103, the CPU 11 performs a process of binarizing (lowering the value) for the target pixel.

  In step S <b> 105, the CPU 11 performs a process of determining two regions corresponding to the two laser light sources 4 for the input data. The region is determined for each pixel of interest. The determination is performed by, for example, the binarization result storage unit 63 or the dot count unit 65.

  In step S107, the CPU 11 performs processing for storing the binarization result (lowering result storing processing). The storage is performed using the binarization result storage unit 63.

  In step S109, the CPU 11 performs a process of counting values related to dots based on the stored data. This process is performed using the dot count unit 65. At this time, the dot count unit 65 acquires a count value for the target pixel for each of the two areas determined in step S105.

  In step S111, the CPU 11 performs a process for calculating a reference value. The reference value is calculated by the reference value calculation unit 61.

  In step S113, the CPU 11 performs a process of comparing the reference value with the count value.

  In step S115, the CPU 11 determines whether or not it is necessary to invert the binarization result for the target pixel based on the comparison result. The determination is performed using the determination unit 67.

  When it is necessary to invert in step S115, in step S117, the CPU 11 executes a forced inversion process. The forcible processing unit 69 inverts the binarization result for the target pixel.

  When the forced inversion process is performed in step S117 or when it is not necessary to invert in step S115, in step S119, the CPU 11 outputs the binarization result for the target pixel as an image output signal. That is, output data is output.

  [Description of a region composed of pixels in a predetermined range around the target pixel]

  FIG. 5 is a diagram illustrating an example of an area where the binarization result is stored by the binarization result storage unit 63 regarding the pixel of interest.

  The two areas corresponding to the two laser light sources 4 are an area that is an odd-numbered line (odd line) in the sub-scanning direction for input data and an even-numbered position in the sub-scanning direction for input data. Some areas are lines (even lines). Here, in the above description, the sub-scanning direction of the input data is the paper feeding direction, that is, the scanning direction of the light from the laser light source 4 when the image forming unit 3 performs image formation based on the input data (main scanning direction). The direction orthogonal to the (scanning direction). In FIG. 5, the sub-scanning direction is the vertical direction of the paper surface, and the main scanning direction is the horizontal direction of the paper surface.

  Referring to FIG. 5, when target pixel 101 is a processing target, region 110 considered for target pixel 101 in the input data, that is, region 110 in which the binarization result storage unit 63 stores the binarization result A rectangular region composed of a plurality of predetermined pixels in both vertical and horizontal directions (main scanning direction and sub-scanning direction).

  In the present embodiment, the dot count unit 65 divides the region 110 into two regions that correspond to the two laser light sources 4 and acquires count values for each of the two regions. That is, the two laser light sources 4 respectively correspond to the pixels of the odd lines and the pixels of the even lines in the image data. Among the pixels in the area 110, the count values for the pixels of the odd lines (in FIG. 5) “Total A”) and the count value (“Total B” in FIG. 5) for pixels in even lines are acquired. In other words, among the pixels constituting the region 110, the pixels of the odd lines shown in white in FIG. 5 are distinguished from the pixels of the even lines shown in gray, and the count is performed for each region constituted by each pixel. The value is obtained.

  Note that the region 110 is not limited to one in which the main scanning direction is shorter than the sub-scanning direction as shown in FIG.

  FIG. 6 is a diagram illustrating another example of a region in which the binarization result is stored by the binarization result storage unit 63 with respect to the target pixel.

  Referring to FIG. 6, the binarization result storage unit 63 stores the binarization result for the area 210 of the input data that is set so that the sub-scanning direction is longer than the main scanning direction. It may be. That is, the area 210 may be set so that the number of pixels arranged in the sub-scanning direction is larger than the number of pixels arranged in the main scanning direction. Also in this case, similarly, the dot count unit 65 acquires the count value for each of the two regions distinguished by the odd-numbered line and the even-numbered line, regarding the region 210 for the target pixel 101.

  When the region 110 as shown in FIG. 5 is set for the target pixel, in the region shown in white in FIG. 5 (total A), the dots are on the right side, and in the region shown in gray (total B), the dots are on the left side. In this way, even if the dots are biased to the left and right (in the main scanning direction), if the count values of the respective regions are the same, the determination unit 67 does not issue a forced reversal instruction for the binarization result. Therefore, there is a possibility that noise such as a high-frequency grain is generated locally (within a narrow area). However, by setting a so-called vertically long area 210 in the sub-scanning direction that is longer than the main scanning direction as shown in FIG. Therefore, the occurrence of noise in the local area as described above can be effectively prevented.

  [Description of reference value]

  In the present embodiment, the reference value is set according to the density of the pixel of interest so that the dots are more evenly distributed in the highlight area of the input data.

  FIG. 7 is a graph for explaining an example of setting a reference value by the reference value calculation unit 61.

  In FIG. 7, the horizontal axis of the graph indicates the gradation (input gradation) of the image input signal, and the vertical axis of the graph indicates the reference value, that is, the number of dots.

  In the formed image, woodgrain noise tends to occur in the highlight portion. For this reason, the reference value is set to be relatively small (strict) on the highlight side (left side in FIG. 7) and relatively large (loose) on the shadow side (right side in FIG. 7). Thereby, dots are more evenly distributed in the highlight area.

  Thus, the following effects can be obtained by setting the reference value relatively small in the predetermined input gradation region. That is, for example, in an image having a tone of 256 gray scales, the degree of freedom of dot arrangement is very low in an intermediate density region near 128 LSB (the 128th tone from black). If the forced inversion process as described above is performed in this state, a repeated pattern or a geometric pattern may easily occur in the image. For example, when error diffusion processing is performed on a gradation image, a density region in which randomly arranged dots exist as shown in a normal error diffusion processing result without forced inversion processing, and forced inversion processing as described above. If the region subjected to is mixed, the appearance of the entire image may be deteriorated. However, in the present embodiment, the reference value remains relatively large in the density region where the grain-like noise is unlikely to occur, and the forced inversion process is difficult to be performed, and the normal error diffusion process without the forced inversion process is performed. Is easier to be done. Therefore, it is possible to prevent the appearance of the entire image from deteriorating.

  Note that a highlight region (density region) for which the reference value is relatively small can be set as appropriate. For example, the highlight area may be set as an area where the density is 0% to 10%, or a different density area such as an area where the density is 0% to 50% may be set. Further, the reference value may be changed stepwise or sequentially in accordance with the input gradation.

  [Operation Example of Determination Unit 67]

  Hereinafter, an example of the determination operation of the determination unit 67 for the target pixel 101 when the count value is counted for the region 110 as illustrated in FIG. 5 will be described. Here, the count value (“total A” in FIG. 5) for the pixels on the odd lines in the region 110 is referred to as a first count value, and the count value (“total B” in FIG. 5) for the pixels on the even lines. Called 2 count value.

  As described above, the determination unit 67 determines that the forced inversion process should be performed based on the first count value, the second count value, and the reference value when the following expression is not satisfied.

  [(Second count value) − (reference value)] <(first count value) <[(second count value) + (reference value)]

  For example, when the area 110 is the highlight portion and the first count value is larger than the value obtained by adding the reference value to the second count value, the above relationship is not satisfied. In this case, since the determination unit 67 determines that the forced inversion process should be performed, even if the dot of the pixel of interest 101 belonging to the odd line is determined to be on (dot ON), the result is canceled and the dot Is replaced with OFF (dot OFF). As a result, dot formation is induced to pixels on the side belonging to the even lines one line downstream in the sub-scanning direction.

  More specifically, since the binarization result is canceled by the forced inversion process, the error of the pixels in the vicinity of the target pixel 101 is sufficiently accumulated. In this state, if the target pixel shifts to a pixel downstream by one line, it becomes easy to hit a dot on the shifted target pixel. Further, since the pixel of interest is shifted by one line, the region that has been an odd line and the region that has been an even line until then are reversed, so that after the pixel of interest is shifted, the first count value becomes the second count value. There is a high possibility that the value will be smaller than the value obtained by adding the reference value to. Therefore, the process of canceling the target pixel, that is, the forced inversion process is difficult to execute.

  On the other hand, even when the area 110 is a shadow part and the first count value is smaller than the value obtained by subtracting the reference value from the second count value, the above relationship is not satisfied. In this case, since the determination unit 67 determines that the forced inversion process should be performed, even if the dot of the target pixel 101 belonging to the odd line is determined to be off, the result is canceled and the dot is switched on. Is done. This induces the dots to be turned off in the pixels belonging to the even lines (formation of formation of off dots).

  More specifically, since the dot is replaced with ON by the forced inversion process, the error of the pixel in the vicinity of the target pixel 101 is sufficiently eliminated, and if the target pixel is shifted downstream by one line, the shift is performed. A state in which dots are difficult to strike occurs for the subsequent pixel of interest. In addition, by shifting one line, the area that has been an odd line and the area that has been an even line until then are reversed. Therefore, after the pixel of interest is shifted, the first count value is changed from the second count value to the reference value. It is larger than the subtracted value, and the forced inversion process is difficult to execute.

  [Effects of the embodiment]

  In the image forming apparatus configured as described above, the binarization result, which is the error diffusion processing result, can be reflected almost equally on the two areas corresponding to the two laser light sources 4. Since the dots are evenly distributed to the areas to which the respective laser light sources 4 correspond, even when there is a light amount difference between the two laser light sources 4, image formation is performed with a uniform density as a whole. Become. With a simple configuration, it is possible to suppress the occurrence of woodgrain noise, and it is possible to perform image processing by the error diffusion method with a good appearance and a relatively high image quality. In the image forming apparatus 1, it is possible to perform printing promptly without performing costly light amount adjustment or the like.

  [Others]

  The error diffusion process may be realized by a software process performed by the CPU operating based on a control program stored in the storage unit. In that case, the CPU may perform processing according to the flow shown in FIG.

  The binarization result storage unit collectively stores the binarization result for a region of a predetermined range with respect to the pixel of interest without distinguishing the two laser light sources, and the dot count unit is a storage target of the binarization result. The area may be divided into two areas corresponding to the two laser light sources, and the count value may be obtained for each area.

  The light source used for image formation in the image forming unit may not be a laser light source. For example, the image forming unit may perform image formation with a linear light source in which a plurality of LEDs (light emitting diodes) are connected.

  The image forming unit may be capable of forming an image by an electrophotographic process using, for example, three or more light sources. In this case, it is only necessary that the dot count is performed for the region corresponding to each of the plurality of light sources and the comparison with the reference value is performed.

  The reference value is not limited to the one set as described above. The reference value may be set, for example, as one of two values according to the density of the pixel of interest, that is, the input gradation, or calculated according to the input gradation using a predetermined calculation formula. You may be made to do.

  The image forming apparatus may be a monochrome / color copying machine, a printer, a facsimile machine, or a multifunction machine (MFP) thereof.

  The hardware configuration of the image forming apparatus is not limited to the above, and image processing may be performed by various control circuits.

  Further, the image processing apparatus according to the present invention is not limited to that used in the image forming apparatus. For example, the present invention can be applied to image processing apparatuses used in various apparatuses such as a scanner apparatus that reads image data, an imaging apparatus, and an image data transmission / reception apparatus.

  The processing in the above embodiment may be performed by software or by using a hardware circuit.

  A program for executing the processing in the above-described embodiment can be provided, or the program can be recorded on a recording medium such as a CD-ROM, a flexible disk, a hard disk, a ROM, a RAM, or a memory card and provided to the user. It may be. The program may be downloaded to the apparatus via a communication line such as the Internet. The processing described in the text in the above flowchart is executed by the CPU according to the program.

  The above embodiment should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 3 Image forming part 4, 4a, 4b Laser light source 11 CPU
13a Control program 21 Image processing unit 21a Error diffusion processing unit 51 Addition unit 53 Comparison unit 55 Inversion processing unit (an example of inversion means)
57 subtraction unit 59 error integration unit 61 reference value calculation unit (an example of an acquisition unit)
63 Binarization result storage unit (an example of storage means)
65 dot count section (an example of counting means)
67 judgment part 69 compulsory processing part

Claims (7)

An image processing device that performs output diffusion processing on an input data and lowers the value to output image data that is formed by an image forming device that forms a latent image on an image carrier using a plurality of light sources. ,
Storage means for storing a processing result of the error diffusion processing for pixels in a predetermined range around a target pixel to be processed in the input data;
Count means for obtaining a count value relating to the number of dots based on the processing result stored by the storage means for each region corresponding to each of the plurality of light sources;
Obtaining means for obtaining a reference value according to the density of the target pixel;
Image processing comprising: reversing means for forcibly reversing the processing result of the error diffusion processing for the target pixel based on the count value obtained by the counting means and the reference value obtained by the obtaining means apparatus.   The image processing apparatus according to claim 1, wherein the storage unit stores the processing result for an area set so that a sub-scanning direction of the input data is longer than a main scanning direction.   The image processing apparatus according to claim 1, wherein the reference value is set so that dots are more evenly distributed in a highlight area of the input data. The plurality of light sources are two light sources,
Two regions corresponding to each of the two light sources are regions that are odd-numbered lines in the sub-scanning direction for the input data and regions that are even-numbered lines in the sub-scanning direction for the input data. The image processing apparatus according to claim 1, wherein: An image forming unit that forms an image on a sheet by forming a latent image on an image carrier with the plurality of light sources;
An image processing apparatus according to any one of claims 1 to 4,
The image forming apparatus, wherein the image forming unit forms the image based on output data output from the image processing apparatus. Method of controlling image processing apparatus for outputting output data for image formation by image forming apparatus for lowering value by performing error diffusion process on input data and forming latent image on image carrier using a plurality of light sources Because
A storage step of storing the processing result of the error diffusion processing for pixels in a predetermined range around the target pixel of the input data for each region corresponding to the plurality of light sources;
For each region corresponding to the plurality of light sources, a count step for obtaining a count value related to the number of dots based on the processing result stored in the storage step;
An acquisition step of acquiring a reference value according to the density of the target pixel;
Image processing comprising: an inversion step for forcibly inverting the processing result of the error diffusion processing for the target pixel based on the count value obtained in the counting step and the reference value obtained in the obtaining step Control method of the device. Control program for an image processing apparatus that outputs output data that is subjected to image formation by an image forming apparatus that forms a latent image on an image carrier using a plurality of light sources by lowering the value by performing error diffusion processing on the input data Because
A storage step of storing the processing result of the error diffusion processing for pixels in a predetermined range around the target pixel of the input data for each region corresponding to the plurality of light sources;
For each region corresponding to the plurality of light sources, a count step for obtaining a count value related to the number of dots based on the processing result stored in the storage step;
An acquisition step of acquiring a reference value according to the density of the target pixel;
Based on the count value obtained in the counting step and the reference value obtained in the obtaining step, the computer is caused to execute an inversion step for forcibly inverting the processing result of the error diffusion processing for the target pixel. Control program for image processing apparatus.