JP2008167241A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing the density of an area of which the density is specified by a user from being sharply deviated from the density expected by the user when half-tone processing is applied to the area. <P>SOLUTION: A theoretical dot-number calculation part 124 generates a half-tone image HG3 of maximum density by performing half-tone processing of a noticed image element CE and calculates the number of theoretical dots of the noticed image element CE from the half-tone image HG3 of the maximum density. A real dot-number calculation part 125 generates a half-tone image of the noticed image element CE by using a screen pattern of density specified by density specification information in the noticed image element CE and calculates the number of real dots from the generated half-tone image. A density setting part 126 sets the density of the noticed image element CE so that the number of real dots calculated by the real dot-number calculation part 125 is included within an allowable range predetermined based on the theoretical dot number dots calculated by the theoretical dot-number calculation part 124. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a printer, a copier, or a multifunction peripheral.

  In an image forming apparatus such as a printer or a copier, halftone processing is performed on an original image consisting of a multivalued image to generate a halftone image that reproduces the multivalued image with 1-bit data in a pseudo manner. It is widely performed to reproduce an original image on a recording paper by forming a halftone image on the recording paper.

  For example, the user can set the density for a certain character included in the original image. When the density is set by the user, the image forming apparatus causes the density of the certain character to be the density specified by the user. Screen processing (halftone processing) is performed to reproduce a certain character at a density different from that of other characters included in the original image.

  By the way, when halftone processing is performed on a multi-value image, for example, if a character set at a density of 80% is halftone processed, depending on the shape of the character, the density may only be seen at about 50%.

  Further, when the character size is reduced, the shape of the character is lost, and the character may not be discriminated depending on the designated density. Therefore, in Patent Document 1, in order to prevent an image from being lost when a small-size character is subjected to halftone processing at a relatively low density of 10% to 50% (paragraph [0005]), a predetermined size or larger is used. Are processed with a halftone threshold matrix having a predetermined first threshold matrix size. For pixel elements having a predetermined size or larger, a predetermined first value smaller than the first threshold matrix size is used. A technique for halftoning with a halftone threshold matrix with a threshold matrix size of 2 is disclosed (paragraph [0008]).

Also, in Patent Document 2, in order to prevent character collapse when a small size character is halftone processed at a high density (paragraph [0005]), if the character size is a predetermined point or less, A technique is disclosed in which a normal size screen process is executed when a size screen process is executed and a predetermined point is exceeded (paragraph [0033]).
JP-A-9-214761 JP 2004-306555 A

  However, the methods shown in Patent Documents 1 and 2 are feed-forward methods in which halftone processing is switched according to the size of the image element to be subjected to halftone processing. It has not been confirmed whether the density of the image matches the density specified by the user, and there has been a problem that an image having a density as expected by the user cannot be obtained.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of preventing the density of the area from becoming a density far from the density expected by the user as a result of performing the halftone process on the area designated by the user. Is to provide.

  An image forming apparatus according to the present invention includes an image acquisition unit that acquires an original image composed of a plurality of image elements including multi-value images, and an acquisition unit that acquires density designation information for designating the density of each image element. , The original image is divided into a plurality of areas, area setting means for sequentially setting the divided areas as attention areas, and a screen for storing a screen pattern predetermined according to the density designated by the density designation information A pattern storage means and each image element included in the attention area are sequentially identified as the attention image element, and the identified attention image element is halftone processed using a screen pattern having the highest density to generate the highest density halftone image. Calculating the number of dots of pixels constituting the halftone dots included in the generated maximum density halftone image, and calculating the number of dots and the attention Based on the density designated by the density designation information for the image element, the theoretical dot number calculating means for calculating the theoretical dot number of the target image element, and the density designation information designated for the target image element A halftone image is generated by halftoning the region of interest using a density screen pattern, and an actual dot number that is the number of dots of pixels constituting a halftone dot included in the generated halftone image is calculated. A dot number calculating unit; and a density setting unit that sets a density of the image element of interest so that the actual dot number falls within a predetermined allowable range centering on the theoretical dot number. .

  According to this configuration, the original image is divided into a plurality of areas, the divided areas are sequentially set as attention areas, and each image element included in the attention area is sequentially identified as attention image elements, and the identified attention The image element is halftone processed using the highest density screen pattern to generate the highest density halftone image, and the number of dots of the pixels constituting the halftone dot of the generated highest density halftone image is calculated and calculated. The theoretical number of dots of the target image element is calculated based on the number of dots and the density designated by the density designation information for the target image element. In addition, the image element of interest is halftone processed using a screen pattern having a density specified by the density specification information to generate a halftone image, and the number of dots of pixels constituting the halftone dot of the generated halftone image A certain number of real dots is calculated. Then, the density of the image element of interest is set so that the actual number of dots falls within a predetermined allowable range centering on the number of theoretical dots.

  That is, the theoretical number of dots, which is the theoretical number of dots from the density specified by the user, and the number of dots of the image obtained by halftone processing using the screen pattern defined for the density specified by the user. Compared with a certain number of actual dots, the density of the target image element is set and halftone processed so that the actual number of dots approaches the theoretical number of dots, so the density of the target image element is far from the density expected by the user. As a result, it is possible to prevent the original image from being reproduced at a density far from the density expected by the user.

  Further, the theoretical dot number calculating means performs a halftone process on the image element of interest in a screen pattern having the highest density among the image elements included in the area of interest, and sets image elements other than the image element of interest to the density designation information. To generate a first halftone image by halftone processing at a density specified by the above, and to halftone process the image element of interest with a screen pattern of the lowest density, and to specify image elements other than the image element of interest with the density specification A halftone process is performed at a density specified by information to generate a second halftone image, and a differential image of the first and second halftone images is generated to generate the highest density halftone image. It is preferable.

  According to this configuration, in the attention area, the target image element is halftone processed with the screen pattern having the highest density, and the image elements other than the target image element are halftone processed with the density designated by the density designation information. A halftone image is generated, and in the region of interest, the image element of interest is halftone processed with the screen pattern having the lowest density, and the image elements other than the image element of interest are halftone processed with the density specified by the density designation information. Since the second halftone image is generated, and the difference image between the first and second halftone images is generated as the highest density halftone image, the image elements other than the target image element are canceled out. A maximum density halftone image in which elements are extracted with high accuracy can be generated.

  Further, the density setting means is a case where the first target area for which the density is set is adjacent to the second target area where the previous density is set, and each image included in the first target area If the density specified by the density setting information for the element is the same as the density specified by the density setting information for each image element included in the second region of interest, the theoretical dot number calculating means and the actual Without calculating the theoretical dot number and the actual dot number in the dot number calculation means, the density of each image element set in the second attention area is determined for each image element included in the first attention area. It is preferable to set the concentration.

  According to this configuration, the first region of interest whose density is to be set is adjacent to the second region of interest for which the previous concentration has been set, and for each image element included in the first region of interest. When the density specified by the density setting information is the same as the density specified by the density setting information for each image element included in the second region of interest, the theoretical dot number calculating means and the actual dot number calculating means The calculation process of the theoretical dot number and the actual dot number is not executed, and the density of each image element set in the second attention area is set as the density of each image element included in the first attention area. Therefore, the processing speed can be increased.

  Further, it is preferable that the multi-value image is a color image composed of a plurality of color component images, and the region setting means sets the attention region for each color component image.

  According to this configuration, the attention area is set in each color component image, and the process of setting the density is executed so that the density of each image element becomes the density expected by the user in the set attention area. Even in a color image, it is possible to prevent the density of the attention area from becoming a density far from the density expected by the user.

  Further, the density setting means extracts the edge of the target image element included in the target area before halftone processing, and designates the angle of the extracted edge with respect to the reference straight line and the target image element by the density designation information It is preferable that a difference between the screen pattern having the density and the screen angle with respect to the reference line is calculated, and the density of the target image element is corrected according to the calculated difference.

  According to this configuration, the edge of the target image element included in the target region before the halftone process is extracted, the angle of the extracted edge with respect to the reference straight line, and the density screen set in the target image element Since the density set in the target image element is corrected in accordance with the difference from the screen angle of the pattern, the screen angle and the edge angle of the target image element are separated, so that the density is lower than expected by the user. It is possible to prevent the original image from being reproduced with the density.

  Further, it is preferable that the density setting unit does not execute the process of setting the density of the target image element when the size of the target image element is larger than a predetermined value and occupies most of the target area.

  According to this configuration, when the target image element occupies most of the target region, the process of setting the density for the target image element is not executed, so that the processing speed can be increased. That is, when the target image element occupies most of the target region, the target image element is likely to be a background image, and the user often does not request a severe density for the background image. The density expected by the user can be reproduced for images other than the background image while speeding up.

  According to the present invention, it is possible to prevent the original image from being reproduced at a density far from the density expected by the user.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described. In addition, in each figure, what attached | subjected the same code | symbol shall represent the same thing.

(Embodiment 1)
FIG. 1 is a diagram schematically showing an internal configuration of an image forming apparatus according to an embodiment of the present invention. In this figure, an image forming apparatus 10 constitutes a tandem type color printer, and is disposed above a main body 12 for printing a color image on a recording paper (transfer paper). The recording paper discharging unit 14 is configured to discharge recording paper on which a color image is printed by the unit 12.

  The main body 12 includes a paper feed cassette 20 disposed in the lower portion of the housing 18, an image forming portion 22 disposed in the upper portion of the housing 18, and a lower portion of the image forming portion 22 in the housing 18. Are disposed between the sheet feeding cassette 20 and the transfer conveyance unit 24, the fixing unit 26 disposed on the downstream side of the transfer conveyance unit 24 in the housing 18, and the transfer conveyance unit 24. A first transport path 28 and a second transport path 30 disposed between the fixing unit 26 and the recording paper discharge unit 14 are provided.

  The paper feed cassette 20 is configured such that the recording paper can be replenished by being pulled out of the housing 18, and the recording paper accumulated inside is fed one by one by a paper feed roller (not shown). 1 is fed out to the conveyance path 28 side. Note that a predetermined number of the paper feeding cassettes 20 are arranged corresponding to the size of the recording paper.

  The image forming unit 22 is configured to multiplexly form a plurality of toner images on a recording sheet, and includes a first image forming unit 221 that forms a magenta toner image, and a second image forming that forms a cyan toner image. A unit 222, a third image forming unit 223 for forming a yellow toner image, and a fourth image forming unit 224 for forming a black toner image are arranged at predetermined intervals along the conveyance direction of the recording paper. It will be.

  Each of the image forming units 221 to 224 includes a photosensitive drum 225, a charging unit 226 disposed to face the peripheral surface of the photosensitive drum 225, and a downstream side of the charging unit 226 on the periphery of the photosensitive drum 225. An exposure unit 227 comprising an LED print head (for example, having a number of pixels of 7168 in the line direction) disposed opposite to the surface, and on the peripheral surface of the photosensitive drum 225 on the downstream side of the exposure unit 227. A developing device 228 disposed opposite to the developing device 228 and a cleaning unit 229 disposed downstream of the developing device 228 and opposed to the peripheral surface of the photosensitive drum 225 are provided. Further, the transfer unit 230 is formed by disposing a transfer roller 244 described later between the developing device 228 and the cleaning unit 229 on the peripheral surface of the photosensitive drum 225.

  The photosensitive drum 225 of each of the image forming units 221 to 224 is rotated in the clockwise direction shown in the figure by a drive motor (not shown). Further, the developing devices 228 of the first to fourth image forming units 221 to 224 are each provided with a toner box. The chromatic color magenta toner is supplied to the toner box of the first image forming unit 221, and the chromatic color cyan toner is supplied to the toner box of the second image forming unit 222. The 223 toner box stores a chromatic yellow toner, and the fourth image forming unit 224 stores an achromatic black toner.

  The transfer conveyance unit 24 includes a driven roller 241 disposed near the first image forming unit 221, a driving roller 242 disposed near the fourth image forming unit 224, and a driven roller 241. A transfer belt 243 that is an endless image carrier disposed across the driving roller 242 and a position on the downstream side of the developing device 228 of the photosensitive drum 225 of each of the image forming units 221 to 224 via the transfer belt 243. Four transfer rollers 244 arranged so as to be able to be pressed against each other, a reflection type photosensor 245 arranged below the driving roller 242 in the vicinity of the transfer belt 243, and a transfer on the downstream side of the reflection type photosensor 245. And a blade 246 disposed at a position in contact with the belt 243.

  In the transfer conveyance unit 24, the recording paper conveyed from the first conveyance path 28 is electrostatically adsorbed onto a transfer belt 243 that rotates counterclockwise by a drive motor (not shown) and conveyed downstream. At the same time, the toner image is transferred to the recording paper at the position of the transfer unit 230 of each of the image forming units 221 to 224. The transfer belt 243 is made of a synthetic resin material having heat resistance such as a polyimide resin whose surface is coated with, for example, silicone.

  In addition, the reflection type photosensors 245 provided in the transfer conveyance unit 24 are disposed at both ends in the width direction (main scanning direction) orthogonal to the transfer direction of the transfer belt 243 and are formed on the transfer belt 243. Detect the density of the image.

  The reflection type photosensor 245 includes a light emitting unit configured by a light emitting diode that transmits light toward the transfer belt 243, and a light receiving unit configured by a photodiode that receives reflected light reflected by the transfer belt 243. And a detection circuit unit that converts the amount of reflected light received by the light receiving unit into a voltage value.

  The blade 246 is for scraping off deposits such as toner on the transfer belt 243, and is formed to have a length substantially the same as the width direction dimension of the transfer belt 243, and its tip is always the surface of the transfer belt 243. It is arrange | positioned in the state contact | abutted to. The blade 246 is kept away from the transfer belt 243 when it is not necessary to perform the scraping operation of the adhered matter, and the surface of the transfer belt 243 only when it is necessary to perform the scraping operation of the adhered matter. It is good also as a structure made to contact | abut.

  The fixing unit 26 performs a fixing process by heating the recording paper onto which the toner images formed on the surface of the photosensitive drum 225 of the image forming unit 22 are transferred, and includes a heat shield box 261 and a heat shield. A fixing roller 262 provided in the upper part of the box 261 and including a heater; a pressure roller 263 provided in pressure contact with the fixing roller 262 in the lower part of the heat shielding box 261; and the heat shielding box 261. A front conveyance path 264 that is disposed in front of the fixing roller 262 and the pressure roller 263 and guides the recording paper conveyed from the transfer conveyance unit 24 between the fixing roller 262 and the pressure roller 263, and a heat shielding box. A rear conveyance path 265 that is disposed behind the fixing roller 262 and the pressure roller 263 in the H.261 and guides the recording paper subjected to the fixing process to the second conveyance path 30. It is equipped with a.

  The first conveyance path 28 conveys the recording paper fed from the paper feed cassette 20 to the transfer conveyance unit 24 side, and includes a plurality of conveyance roller pairs 281 disposed at predetermined positions, and a transfer conveyance unit. 24, and a registration roller pair 282 for taking the timing of the image forming operation and the paper feeding operation of the image forming unit 22 is provided. The plurality of transport roller pairs 281 and the registration roller pairs 282 are each driven to rotate by an unillustrated drive motor via an electromagnetic clutch. A registration sensor 283 made of a photo interrupter or the like is disposed in front of the registration roller pair 282. When the leading edge of the recording paper is conveyed to the registration roller pair 282, an output signal from the registration sensor 283 is output. Based on this, the conveyance of the recording paper is temporarily stopped.

  The second transport path 30 transports the recording paper fixed by the fixing unit 26 to the recording paper discharge unit 14. A plurality of transport roller pairs 301 are disposed at predetermined positions, and are disposed on the exit side. A discharge roller pair 302 is disposed. The transport roller pair 301 and the discharge roller pair 302 are driven to rotate by an unillustrated drive motor via an electromagnetic clutch.

  The recording paper discharge unit 14 is formed integrally with the main body 12 on the upper surface of the casing 18 constituting the main body 12, and the recording paper that has been transported from the second transport path 30 and has undergone the fixing process is used. The images are sequentially accumulated so that the image-formed surface is on the back side.

  The image forming apparatus 10 configured as described above operates as follows. That is, in each photosensitive drum 225 of the image forming unit 22, an electrostatic region is formed on the surface by the charging unit 226, and the electrostatic region is exposed by the output light from the exposure unit 227, thereby static electricity based on the image data. An electrostatic latent image is formed, and then a toner image is formed by the developing device 228. Further, the fixing roller 262 of the fixing unit 26 is energized by applying a voltage to the built-in heater by a voltage supply unit (not shown), and is heated and controlled so that the surface of the fixing roller 262 reaches a fixing possible temperature.

  On the other hand, a recording paper of a specified size is fed out from the paper feed cassette, conveyed to the front of the registration roller pair 282 by the first conveyance path 28, and temporarily stopped. Then, the recording paper conveyed to the front of the registration roller pair 282 is conveyed to the transfer conveying unit 24 after timing with the image forming operation of the image forming unit 22, and each of the image forming units 221 to 224. Thus, the toner images are sequentially transferred onto the recording paper. In other words, the image is transferred in a state where the magenta toner, the cyan toner, the yellow toner, and the black toner are superimposed on each other on the recording paper.

  Then, the recording sheet on which the toner image is transferred is conveyed into the fixing unit 26, is heated by the fixing roller 262, is sandwiched between the fixing roller 262 and the pressure roller 263, and is conveyed to the downstream side. The paper is discharged to the recording paper discharge unit 14 through the conveyance path 30. The toner remaining on the surface of each photosensitive drum 225 having the toner image transferred to the recording paper is removed by the cleaning unit 229. This operation is sequentially repeated to execute printing on a predetermined number of recording sheets.

  FIG. 2 is a block diagram of the image forming apparatus 10. The image forming apparatus 10 includes a LAN communication unit 110, an image processing unit 120, storage units 130 and 140, a screen pattern storage unit 150, an image forming unit 160, and a control unit 170. Each block is connected via a bus line so that various data can be transmitted and received.

  The LAN communication unit 110 includes a LAN (Local Area Network) interface, receives print data transmitted from a terminal device connected via the LAN according to the LAN communication protocol, and stores the print data in the print data storage unit 141. Here, the print data is composed of data described in accordance with PDL (Page Description Language). PDL is a language for describing a print image in units of pages, and indicates a character constituting one page, a background image as a background of characters displayed around the character, and an abstract image such as a triangle or a circle. A vector image and a bitmap image representing a photograph or the like are handled as one unit object. In this embodiment, the print data includes only a character object and a background image object. And The character object includes density designation information for designating the density of each character constituting the character object, position information indicating the position where each character is arranged, color information indicating the color of each character, and the like. The background image object includes density designation information for designating the density of the background image, position information indicating the position where the background image is arranged, color information indicating the color of the background image, and the like. 1 or a plurality of characters designated with the same density as one image element, and a group of background images designated with the same density as one image element. The color information is composed of numerical values representing the red (R), green (G), and blue (B) color components in 0 to 255 gradations, and the position information is composed of data representing the position in two dimensions. The

  The print data is created by the terminal device based on data created by the user using word processor software installed in a terminal device including a personal computer connected to the image forming apparatus 10 or application software such as spreadsheet software. Data. Then, the user sets density designation information, color information, and position information for each of the character image element and the background image element using the application software.

  The image processing unit 120 includes, for example, an ASIC (Application Specific Integrated Circuit), an image acquisition unit 121, an area setting unit 122, a density designation information acquisition unit 123, a theoretical dot number calculation unit 124, an actual dot number calculation unit 125, and A density setting unit 126 is provided.

  The image acquisition unit 121 reads the print data received by the LAN communication unit 110 from the print data storage unit 141, and converts the print data into a bitmap image using color information and position information included in the read print data. The original image is acquired by developing and generating an original image composed of multi-valued images, and stored in the original image storage unit 142. Here, the original image is composed of color component images of cyan (C), magenta (M), yellow (Y), and black (K). Each color component image is composed of pixel data arranged in a matrix with P (P is a positive number) row Q (Q is a positive number) column, and each pixel data is represented by a numerical value of 0 to 255 gradations, for example. The

  The area setting unit 122 divides each color component image into a plurality of areas, and sequentially sets the divided areas as attention areas. The region of interest has a rectangular shape composed of pixels of p (p is a positive number and smaller than P) rows × q (q is a positive number and smaller than Q) columns. A predetermined value is adopted. Here, the region setting unit 122 sequentially sets the region of interest so as to raster scan the region located in the lower right from the region located in the upper left of the color component image divided into a plurality of regions. Note that the size of the area defined by the area setting unit 122 is smaller than the size of one character included in the original image.

  The density designation information acquisition unit 123 acquires density designation information for designating the density of each image element included in each color component image of C, M, Y, and K from the print data stored in the print data storage unit 141. And stored in the original image storage unit 142.

  Here, the density designation information acquisition unit 123 identifies, from the print data, the density designation information set for each image element included in the target area in the target area set in the area setting unit 122. Density designation information is acquired as density designation information for each image element. Here, the density designation information is information for designating the density at the time of halftone processing of the original image, and is expressed by a numerical value from 0% indicating the minimum density to 100% indicating the maximum density, and 100 indicating the maximum density. In%, the original image is solid and halftoned. In the print data, 100% indicating the highest density is set as the density designation information in the character and background image in which the density designation information is not set by the user.

  The theoretical dot number calculation unit 124 sequentially identifies each image element included in the attention area set by the area setting unit 122 as the attention image element, and performs the halftone process using the identified attention image element using the screen pattern with the highest density. To generate the highest density halftone image, calculate the number of dots of the pixels that make up the halftone dots included in the generated highest density halftone image, and specify density information for the calculated number of dots and the image element of interest. The number of theoretical dots of the image element of interest is calculated based on the density specified by.

  Here, the theoretical dot number calculation unit 124 performs halftone processing on the target image element among the image elements included in the target region with the screen pattern having the highest density, and image elements other than the target image element are designated by the density designation information. The halftone process is performed at a density to generate a first halftone image, and the target image element is halftone processed with the screen pattern having the lowest density, and the image elements other than the target image element are specified by the density designation information. A halftone process is performed to generate a second halftone image, and a difference image between the first and second halftone images is generated to generate the highest density halftone image.

  Note that a halftone image is a binary image that reproduces the shade of a multi-value image in a pseudo manner using fine halftone dots. For example, the pixels constituting the halftone dots are represented by a value of “1”, and constitute a background. The pixel to be represented is represented by a value of “0”. Further, the number of dots of a halftone image indicates the number of pixels constituting a halftone dot.

  FIGS. 3A and 3B are explanatory diagrams of the highest density halftone image. FIG. 3A shows the attention area CD before the halftone processing, and FIG. 3B shows the highest when the character image element is the attention image element. It is explanatory drawing of the calculation process of a density | concentration halftone image, (c) is explanatory drawing of the calculation process of the highest density | concentration halftone image when the image element of a background image is made into an attention image element. The attention area CD shown in FIG. 3A includes a character image element E1 indicating a part of the character “A” and an image element E2 of a background image indicating a part of the background image “A”. Thus, a density of 60% is designated by the density designation information for the image element E1, and a density of 25% is designated by the density designation information for the image element E2.

  First, as shown in FIG. 3B, the theoretical dot number calculation unit 124 identifies the image element E1 included in the attention area CD as the attention image element CE, and sets the attention image element CE to the maximum density of 100%. The image element E2, which is an image element other than the target image element CE, is subjected to halftone processing at a density (= 25%) designated by the density designation information with respect to the image element E2, and the first tone is processed. The halftone image HG1 is generated, and the target image element CE is halftone processed with the screen pattern having the lowest density, and the image element E2 other than the target image element CE is density designated by the density designation information with respect to the image element E2. (= 25%) to generate a second halftone image HG2 by halftone processing, and generate a difference image between the first and second halftone images HG1 and HG2. Doing, to produce the highest density half-tone image HG3 of when the image element E1 and the image-of-interest element CE. As a result, the image element E2 is canceled out. As a result, in the attention area CD, the area of the image element E1 is expressed with a density of 100%, and the other areas are expressed with pixels of gradation of “0”. A halftone image HG3 can be obtained.

  Then, the theoretical dot number calculation unit 124 calculates the number of dots of the pixels constituting the halftone dots included in the maximum density halftone image HG3, and the density designated by the density designation information for the image element E1 is set to this dot number. Is multiplied by “60% (= 0.6)” to calculate the theoretical dot number of the target image element CE when the image element E1 is the target image element CE.

  Next, as shown in FIG. 3C, the theoretical dot number calculation unit 124 identifies the image element E2 included in the attention area CD as the attention image element CE, and sets the attention image element CE to the maximum density of 100%. The image element E1, which is an image element other than the target image element CE, is subjected to halftone processing at a density (= 60%) designated by the density designation information with respect to the image element E1. The halftone image HG1 is generated, and the target image element CE is halftone processed with the screen pattern having the lowest density, and the image element E1 other than the target image element CE is designated by the density designation information with respect to the image element E1. Halftone processing is performed at a density (= 60%) to generate a second halftone image HG2, and a difference image between the first and second halftone images HG1 and HG2 is generated. Doing, to produce the highest density half-tone image HG3 of when the image element E2 and the image-of-interest element CE. As a result, the image element E1 is canceled out. As a result, in the attention area CD, the area of the image element E2 is represented by 100% density, and the other areas are represented by pixels of gradation of “0”. A halftone image HG3 can be obtained.

  Then, the theoretical dot number calculation unit 124 calculates the number of dots of the pixels constituting the halftone dots included in the maximum density halftone image HG3, and the density designated by the density designation information with respect to the image element E2 as the number of dots. Is multiplied by “25% (= 0.25)” to calculate the theoretical number of dots when the image element E2 is the target image element CE.

  The actual dot number calculation unit 125 shown in FIG. 2 has a screen pattern having a density designated by the density designation information for the attention image element CE included in the attention area CD for which the theoretical dot number calculation unit 124 has calculated the theoretical dot number. A halftone image is generated by performing halftone processing using, and the actual number of dots, which is the number of dots of pixels constituting a halftone dot included in the generated halftone image, is calculated. Here, the actual dot number calculation unit 125 specifies the screen pattern having the density indicated by the density designation information set for the target image element CE from the screen pattern storage unit 150.

  The density setting unit 126 pays attention so that the actual dot number calculated by the actual dot number calculation unit 125 falls within a predetermined allowable range centered on the theoretical dot number calculated by the theoretical dot number calculation unit 124. The density of the image element CE is set. That is, when the actual number of dots in the target image element CE is larger than the upper limit value of the allowable range, the density setting unit 126 decreases the density designation information set in the target image element CE by a predetermined value, and the theoretical dot number The calculation unit 124 and the actual dot number calculation unit 125 are made to calculate the theoretical dot number and the actual dot number again. On the other hand, when the actual number of dots in the target image element CE is smaller than the lower limit value of the allowable range, the density setting unit 126 increases the density designation information set in the target image element CE by a predetermined value, and the theoretical dot number The calculation unit 124 and the actual dot number calculation unit 125 are made to calculate the theoretical dot number and the actual dot number again. Then, the density setting unit 126 repeatedly executes the process of setting the density until the actual number of dots falls within the allowable range.

  Here, the density setting unit 126 is a case where the first region of interest for which density is to be set is adjacent to the second region of interest for which the previous density has been set, and is included in the first region of interest. When the density specified by the density setting information for each image element is the same as the density specified by the density setting information for each image element included in the second region of interest, the theoretical dot number calculation unit 124. The actual dot number calculation unit 125 does not perform the calculation process of the theoretical dot number and the actual dot number, and the density of each image element set in the second attention area is set to each image element included in the first attention area. Set as the density of.

  The storage unit 130 includes an SRAM (Static Random Access Memory) and is used as a work memory for the image processing unit 120. The storage unit 140 includes a DRAM such as an SDRAM (Synchronous DRAM) and is used as a work memory for the image processing unit 120 and the control unit 170, and includes a print data storage unit 141, an original image storage unit 142, and a halftone image storage unit. 143. The print data storage unit 141 stores the print data received by the LAN communication unit 110. The original image storage unit 142 stores the original image acquired by the image acquisition unit 121. The halftone image storage unit 143 stores the halftone image generated by the actual dot number calculation unit 125.

  The screen pattern storage unit 150 stores a screen pattern used when the theoretical dot number calculation unit 124 and the actual dot number calculation unit 125 perform halftone processing. Here, the screen pattern storage unit 150 is a predetermined screen pattern for each color of C, M, Y, and K, and a predetermined screen pattern for each density designated by the density designation information. Remember. The screen pattern is composed of a matrix of p rows × q columns having the same size as the region set by the region setting unit 122, and threshold data is stored in each element constituting the matrix. Then, the theoretical dot number calculation unit 124 and the actual dot number calculation unit 125 apply a screen pattern to the region of interest, set a value of “1” to a pixel having a value larger than the threshold data, and smaller than the threshold data. A halftone process is performed by setting a value of “0” to a pixel having a value. In this embodiment, the screen angle of the screen pattern differs for each color of C, M, Y, and K, and also for each density designated by the density designation information.

  The image forming unit 160 includes the image forming unit 22 shown in FIG. 1 and forms a halftone image stored in the halftone image storage unit 143 on a recording sheet under the control of the control unit 170.

  The control unit 170 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), and the like, and the CPU executes a control program stored in the ROM, whereby the image forming apparatus 10 overall control.

  Next, the operation of the image forming apparatus 10 will be described. FIG. 4 is a flowchart showing the operation of the image forming apparatus 10. First, in step S1, when print data is received by the LAN communication unit 110 and stored in the print data storage unit 141, the image acquisition unit 121 reads the print data from the print data storage unit 141 and is included in the print data. By using the color information and position information, etc., the print data is converted into C, M, Y, and K color component images to obtain an original image composed of C, M, Y, and K color component images. Stored in the original image storage unit 142.

  Next, the area setting unit 122 divides any one of the C, M, Y, and K color component images into an area composed of pixels of p rows and q columns, and sets a plurality of divided areas. One of the regions is set as a region of interest (step S2). FIG. 5 is an explanatory diagram of a region of interest. In FIG. 5, the character image element E1 is designated with a density of “60%” by the density designation information, and the background image image element E2 is designated with a density of “25%” by the density designation information. And As shown in FIG. 5, the region setting unit 122 identifies any one of the C, M, Y, and K color component images CG, MG, YG, and KG constituting one original image. Then, the identified color component image is divided into areas of p rows × q columns, and the attention area CD is sequentially set so as to perform raster scanning from the upper left area toward the lower right area.

  Here, the region setting unit 122 first specifies the color component image CG, sequentially sets the attention region CD for the color component image CG, and when the processing for all the attention regions CD is completed, the color component image is next processed. When MG is set and the processing for all the attention areas CD is completed, the color component image YG is set, and when the processing for all the attention areas CD is completed, the color component image KG is set. Are set sequentially. The order of specifying the color component images CG, MG, YG, and KG by the region setting unit 122 is not limited to the above, and may be specified in any order.

  Next, the theoretical dot number calculation unit 124 sequentially specifies the image elements E1 and E2 included in the attention area CD set by the area setting unit 122 as the attention image element CE as shown in FIG. 5 (step S3). Here, when the attention area CD includes the character image element E1 and the background image image element E2 as shown in FIG. 5, the theoretical dot number calculation unit 124 first pays attention to the character image element E1. The image element CE may be specified, and then the background image element E2 may be specified as the attention image element CE. First, the background image element E2 may be specified as the attention image element CE, and then the character image element. You may specify E1 as an attention image element. In addition, when the attention area CD includes a plurality of character image elements, the theoretical dot number calculation unit 124 performs the raster scanning of the image element located at the lower right from the image element located at the upper left. If the image element of the background image is included, the image element of the background image located at the upper left is sequentially identified as the target image element CE so as to be raster scanned. May be.

  Next, when the density designation information in the target image element CE identified in step S3 indicates 100% density (YES in step S4), the actual dot number calculation unit 125 determines the color component image to which the target image element CE belongs. A screen pattern having a density of 100% in color is specified from the screen pattern storage unit 150, and the target image element CE is halftone processed using the specified 100% screen pattern to generate a halftone image (step S11). The generated halftone image is output to the halftone image storage unit 143.

  On the other hand, when the density designation information in the target image element CE specified in step S3 does not indicate 100% density (NO in step S4), the theoretical dot number calculation unit 124 adds the target area CD1 to the target area CD1, as shown in FIG. When the included image element E1 is specified as the target image element CE, the highest density halftone image HG3 is generated by the above-described processing shown in FIG. 3B, and halftone dots included in the generated highest density halftone image are displayed. The number of dots of the constituent pixels is calculated, and the calculated number of dots is multiplied by “60% (= 0.6)” that is the density designated by the density designation information with respect to the image element E1, thereby obtaining the image element E1. Calculate the number of theoretical dots.

  Then, when the density setting process for the image element E1 is completed by the density setting unit 126, the theoretical dot number calculation unit 124 identifies the image element E2 included in the attention area CD1 as the attention image element CE, and identifies the identified attention. The highest density halftone image HG3 is generated by the above-described processing shown in FIG. 3C for the image element CE, and the number of dots constituting the halftone dots included in the generated highest density halftone image HG3 is calculated. The theoretical number of dots of the image element E1 is calculated by multiplying the number of dots by “25% (= 0.25)” which is the density designated by the density designation information for the image element E2.

  Next, the actual dot number calculation unit 125 has the density designated by the density designation information for the target image element CE in the color of the color component image to which the target image element CE for which the theoretical dot number has been calculated in step S5 belongs. The number of pixels constituting the halftone image of the generated halftone image by specifying the screen pattern from the screen pattern storage unit 150 and generating the halftone image by halftoning the target image element CE using the specified screen pattern. And the actual number of dots is calculated (step S6).

  Next, the density setting unit 126 determines that the actual dot number calculated in step S6 is larger than the upper limit value of the predetermined allowable range centered on the theoretical dot number calculated in step S5 (YES in step S7). ), The density set in the target image element CE is decreased by a predetermined value, and the density of the target image element CE is set (step S12). On the other hand, the density setting unit 126 determines that the actual dot number calculated in step S6 is smaller than the lower limit value of the predetermined allowable range centered on the theoretical dot number calculated in step S5 (NO in step S7). And the density set in the target image element CE is increased by a predetermined value, and the density of the target image element CE is set (step S12).

  Here, as the upper limit value of the allowable range, a value obtained by multiplying the theoretical dot number by the dot number obtained by multiplying the theoretical dot number by a predetermined ratio (for example, 5%) is adopted. As the lower limit, a value obtained by subtracting the number of dots obtained by multiplying the number of theoretical dots by a predetermined ratio (for example, 5%) from the number of theoretical dots is employed. In setting the density of the target image element CE, the density setting unit 126 may increase or decrease the density value uniformly by 10% to simplify the process, or set the density of the target area CD. As the number of times increases, the density value to be increased or decreased may be reduced to increase the processing speed.

  Next, the actual dot number calculation unit 125 specifies the screen pattern having the density set by the density setting unit 126 from the screen pattern storage unit 150, and uses the specified screen pattern to perform halftone processing on the target image element CE. A halftone image is generated (step S13), the actual number of dots in the generated halftone image is calculated (step S14), and the process returns to step S7.

  As described above, the density setting unit 126 increases or decreases the density in the target image element CE until the actual number of dots of the half pattern image obtained by performing the halftone process on the target image element CE falls within the allowable range.

  On the other hand, the actual number of dots calculated in step S6 is less than or equal to the upper limit value of the allowable range determined in advance around the theoretical number of dots calculated in step S5 (NO in step S7), and the lower limit value of the allowable range. In the above case (NO in step S8), that is, when the actual number of dots is within the allowable range, the density setting unit 126 outputs the halftone image generated in step S6 to the halftone image storage unit 143. (Step S <b> 9), the halftone image storage unit 143 is stored.

  Next, when all the image elements included in the attention area CD are set as the attention image elements CE (YES in step S10), the theoretical dot number calculation unit 124 proceeds with the process to step S15, and in the attention area CD. If all the image elements are not set as the attention image element CE (NO in step S10), the process returns to step S3, and the next image element is specified as the attention image element CE.

  Next, the area setting unit 122 sets all areas partitioned in all the color component images CG, MG, YG, and KG of C, M, Y, and K as the attention area CD (YES in step S15). When the process is finished and there is an area that is not set as the attention area CD among the areas divided in the color component images CG, MG, YG, and KG (NO in step S15), the process returns to step S2, and the next The attention area CD is set.

  As described above, according to the image forming apparatus 10, the color component images CG, MG, YG, and KG constituting the original image are partitioned into a plurality of regions, and each partitioned region is sequentially set as the attention region CD, The original image is partitioned into a plurality of regions, each partitioned region is sequentially set as the attention region CD, each image element included in the attention region CD is sequentially identified as the attention image element CE, and the identified attention image element CE Is halftone processed using the highest density screen pattern to generate the highest density halftone image HG3, and the number of dots of pixels constituting the halftone dot of the generated highest density halftone image HG3 is calculated and calculated. The theoretical dot number of the target image element CE is calculated by multiplying the number of dots and the target image element CE by the density specified by the density specification information. Further, the target image element CE is halftone processed using the screen pattern having the density designated by the density designation information to generate a halftone image, and the number of dots of pixels constituting the halftone dot of the generated halftone image The actual number of dots is calculated. Then, the density of the target image element CE is set so that the actual number of dots falls within a predetermined allowable range centering on the number of theoretical dots.

  That is, a dot of a halftone image obtained by halftone processing using a theoretical dot number that is the theoretical number of dots from a density specified by the user and a screen pattern determined for the density specified by the user Since the density of the target image element CE is set and the halftone process is performed so that the actual dot number approaches the theoretical dot number, the density of the target image element CE is set to the user's expectation. As a result, it is possible to prevent the original image from being reproduced at a density far from the density expected by the user.

  In the attention area CD, the attention image element CE is halftone processed with the screen pattern having the highest density, and each of the image elements other than the attention image element CE is halftone processed with the density designated by the density designation information. After the halftone image HG1 is generated, the target image element CE is halftone processed with the screen pattern having the lowest density, and the image elements other than the target image element CE are halftone processed with the density designated by the density designation information. Since the halftone image HG2 is generated and the difference image between the halftone images HG1 and HG2 is generated as the highest density halftone image, the image elements other than the target image element CE are canceled out, and the target image element is accurately obtained. The extracted highest density halftone image HG3 can be generated.

(Embodiment 2)
Next, an image forming apparatus 10a according to Embodiment 2 of the present invention will be described. The image forming apparatus 10a according to the second embodiment is characterized in that the density of the target image element CE is set in consideration of the screen angle of the screen pattern, and the ratio of the size of the target image element CE in the target region CD. The processing for setting the density of the target image element CE is omitted or executed according to the above.

  In the image forming apparatus 10a according to the second embodiment, the internal configuration and the block configuration are the same as those in the first embodiment, and therefore FIG. 1 and FIG. 2 are used. In the second embodiment, the description of the same elements as those in the first embodiment is omitted.

  When the ratio of the size of the attention image element CE in the attention area CD is larger than the specified value and occupies most of the attention area CD, the density setting section 126 applies the theoretical dot number calculation section 124 and the actual dot number calculation section 125 to For the target image element CE, the process for calculating the theoretical dot number and the actual dot number is not executed, and the density setting process is not executed. Here, as the specified value, experimentally obtained values such as 80%, 90%, and 95% can be appropriately employed.

  Further, the density setting unit 126 extracts the edge of the image included in the target image element CE before the halftone process is performed, and the angle of the extracted edge with respect to the reference line and the screen having the density set in the target image element CE. A difference from the screen angle with respect to the reference straight line of the pattern is calculated, and the density of the target image element CE is corrected by a value corresponding to the calculated difference.

  FIG. 6 is a flowchart showing the operation of the image forming apparatus 10a. The processing from step S21 to S23 is the same as the processing from step S1 to S3 shown in FIG.

  In step S50, the density setting unit 126 calculates a ratio of the size of the attention image element CE in the attention area CD, determines whether or not this ratio is larger than the specified value, and if larger than the specified value (step S50). YES), the target image element CE is halftone processed using the screen pattern having the density designated by the density designation information to generate a halftone image (step S51), and the generated halftone image is stored in the halftone image. To the unit 143.

  On the other hand, when the ratio of the attention image element CE in the attention area CD is equal to or less than the specified value (NO in step S50), the density setting unit 126 executes processing for setting the density for the attention image element CE. The process proceeds to step S24.

  Next, the density setting unit 126 executes the density correction process shown in step S40 after the actual number of dots in the target image element CE falls within the allowable range by the processes in steps S24 to S28. In addition, since the process of step S24-S28 is the same as step S4-S8 shown in FIG. 3, description is abbreviate | omitted. Moreover, since the process of step S29-S35 is also the same as the process of step S9-S15 shown in FIG. 3, description is abbreviate | omitted.

  FIG. 7 is a flowchart showing a detailed flow of the density correction processing in step S40. First, the density setting unit 126 reads the target image element CE before the halftone process from the original image storage unit 142, and executes a process of extracting an edge of an image included in the target image element CE. Here, the density setting unit 126 extracts edges of the image included in the target image element CE by performing filter processing using the edge extraction filter on the target image element CE (step S41).

Next, the density setting unit 126 approximates the extracted edges with straight lines, obtains each straight line as a straight line segment, sets the horizontal side or vertical side of the outer periphery of the region of interest CD as a reference straight line, and the reference straight line. While calculating | requiring the angle of each straight line segment, calculating | requiring the length of each straight line segment, calculating | requiring the average value of the angle of the straight line segment in attention image element CE from the calculated | required straight line segment angle and length, and paying attention to the calculated | required average value. The angle of the edge of the image included in the image element CE is calculated (step S42). Here, the average value θ of the straight line segments is θ _i , the length of the i-th straight line segment, the length l _i , the total length of the straight line segments included in the attention image element CE, and the attention area CD. When the number of included straight line segments is n, it is calculated using, for example, equation (1).

θ = (l ₁ / L) · θ ₁ + (l ₂ / L) · θ ₂ +... + (l _n / L) · θ _n (1)
Next, the density setting unit 126 determines the screen angle of the screen pattern with respect to the density of the target image element CE set in the allowable range in steps S27 and S28 of FIG. 6, and the edge angle calculated in step S43. The absolute value of the difference is calculated (step S43).

  Next, the density setting unit 126 corrects the density of the target image element CE using a value determined in advance according to the absolute value of the difference calculated in step S43. Here, if the absolute value of the difference calculated in step S43 is equal to or less than a predetermined threshold, the density setting unit 126 does not correct the density and if the absolute value of the difference exceeds the threshold, The density is corrected by adding a predetermined value according to the absolute value of the difference to the currently set density. If the absolute value of the difference exceeds the threshold value, the density setting unit 126 may correct the density so that it increases as it moves away from the threshold value, or uniformly adds the same value to the currently set density. The density may be corrected accordingly.

  Next, the density setting unit 126 generates a halftone image by performing the halftone process on the target image element CE before the halftone process is performed using the corrected density screen pattern (step S45). The process returns to step S29 shown in FIG.

  As described above, according to the image forming apparatus 10a according to the second embodiment, the edge of the target image element CE is extracted, the angle of the extracted edge with respect to the reference straight line, and the density set in the target image element CE. Since the density set in the target image element CE is corrected in accordance with the difference between the screen angle of the screen pattern and the angle of the edge of the image included in the target image element CE is separated, It is possible to prevent the original image from being reproduced at a density lower than the density expected by the user.

  In addition, when the attention image element CE occupies most of the attention area CD, the processing for setting the density for the attention image element CE is not executed, so that the processing speed can be increased. That is, when the attention image element CE occupies most of the attention area CD, the attention image element CE is likely to be a background image, and the user often does not request a severe density for the background image. The density expected by the user can be reproduced for images other than the background while increasing the processing speed.

1 is a diagram schematically showing an internal configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a block diagram of an image forming apparatus according to an embodiment of the present invention. It is explanatory drawing of a highest density halftone image, (a) shows the attention area | region before performing a halftone process, (b) shows the highest density halftone image when a character image element is made into an attention image element. It is explanatory drawing of a calculation process, (c) is explanatory drawing of the calculation process of the highest density halftone image when the image element of a background image is made into an attention image element. 3 is a flowchart illustrating an operation of the image forming apparatus according to the first embodiment of the present invention. It is explanatory drawing of an attention image element. 6 is a flowchart illustrating an operation of the image forming apparatus according to the second embodiment of the present invention. It is a flowchart which shows the detailed flow of a density correction process.

Explanation of symbols

10, 10a Image forming apparatus 110 LAN communication unit 120 Image processing unit 121 Image acquisition unit 122 Area setting unit 123 Density designation information acquisition unit 124 Theoretical dot number calculation unit 125 Actual dot number calculation unit 126 Density setting units 130 and 140 Storage unit 141 Print data storage unit 142 Original image storage unit 143 Halftone image storage unit 150 Screen pattern storage unit 160 Image forming unit 170 Control unit

Claims (6)

Image acquisition means for acquiring an original image composed of a plurality of image elements consisting of multi-valued images;
Acquisition means for acquiring density designation information for designating the density of each image element;
A region setting means for partitioning the original image into a plurality of regions, and sequentially setting each partitioned region as a region of interest;
Screen pattern storage means for storing a predetermined screen pattern according to the density designated by the density designation information;
Each image element included in the attention area is sequentially identified as the attention image element, and the highest density halftone image is generated by halftone processing the identified attention image element using the screen pattern of the highest density, and the highest generated The number of dots of pixels constituting a halftone dot included in the density halftone image is calculated, and based on the calculated number of dots and the density designated by the density designation information for the target image element, the target image element A theoretical dot number calculating means for calculating the theoretical dot number;
A halftone image is generated by halftoning the region of interest using a screen pattern having a density specified by density specification information for the image element of interest, and halftone dots included in the generated halftone image are configured. An actual dot number calculating means for calculating an actual dot number that is the number of dots of a pixel to be
An image forming apparatus comprising: a density setting unit configured to set a density of the image element of interest so that the actual number of dots falls within a predetermined allowable range centering on the number of theoretical dots.   The theoretical dot number calculating means halftone-processes the target image element among the image elements included in the target region with a screen pattern having the highest density, and designates an image element other than the target image element by the density designation information The first halftone image is generated by halftoning at the density obtained, and the target image element is halftone processed with the screen pattern having the lowest density, and the image elements other than the target image element are determined by the density designation information. Generating the highest density halftone image by generating a second halftone image by halftoning at a specified density and generating a difference image of the first and second halftone images; The image forming apparatus according to claim 1, wherein:   The density setting means is a case where the first target area for which density is to be set is adjacent to the second target area for which the previous density has been set, and for each image element included in the first target area. On the other hand, when the density designated by the density setting information is the same as the density designated by the density setting information for each image element included in the second region of interest, the theoretical dot number calculating means and the actual Without calculating the theoretical dot number and the actual dot number in the dot number calculation means, the density of each image element set in the second attention area is determined for each image element included in the first attention area. The image forming apparatus according to claim 1, wherein the image forming apparatus is set as a density. The multi-valued image is a color image composed of a plurality of color component images,
The image forming apparatus according to claim 1, wherein the area setting unit sets the attention area for each color component image.   The density setting unit extracts an edge of the target image element included in the target area before halftone processing, and specifies the angle of the extracted edge with respect to a reference straight line and the target image element specified by the density specification information. 5. The image according to claim 1, wherein a difference between a screen angle of a density screen pattern and a screen angle with respect to the reference line is calculated, and the density of the image element of interest is corrected according to the calculated difference. Forming equipment.   The density setting means does not execute the process of setting the density of the target image element when the size of the target image element is larger than a predetermined value and occupies most of the target region. The image forming apparatus according to any one of 1 to 5.

Images