JP2007300472A - Image processing apparatus, image processing method, and printed matter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing apparatus, an image processing method, and printed matters which makes it possible to perform image creation which prevents deterioration of image quality and simultaneously reduces toner consumption. <P>SOLUTION: In an image processing unit 20, a screen processing unit 30 artificially creates an intermediate gradation image using mesh points which are a set of output dots for respective input image signals of respective color components carried out color separation. The screen processing unit 30 has a threshold value matrix for creating mesh points which are composed of predetermined dots corresponding to density of the input image signal, and a plurality of vacant profile tables for deleting a part of dots composing mesh points as substantial non-output dots, changes dot deletion information responding to color component information of the input image signal, and creates the image output signal which composes different mesh points for respective color component information. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image processing apparatus that generates an image signal using halftone dots when forming a color image.

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a printer using an electrophotographic method, an ink jet method, etc., an image expressing intermediate gradation by converting input multi-value image data into binary image data Processing is generally performed. As an image processing method for converting multi-valued image data into binary image data, halftone dots (colored dots) having a size corresponding to the inputted multi-valued image data are formed, and the size of the colored dots. There is known binarization processing (halftone processing) for reproducing the density of an intermediate gradation image in a pseudo manner.
For example, in a color image forming apparatus that forms a color image by electrophotography, four color toner images of yellow (Y), magenta (M), cyan (C), and black (K) are recorded on a sheet as a recording medium. A color image is created by sequentially overprinting the image. At that time, the density of each color toner image is reproduced as a set of many minute halftone dots by using the above-described binarization processing.

  By the way, in an electrophotographic color image forming apparatus, when each color toner image is transferred onto a recording sheet, generally one or two toners and an average of about 1.5 toners are stacked. At the time of fixing, the toner image is formed with a layer thickness of about several tens of micrometers. Such a toner amount is determined from the need to reproduce the maximum density. However, the toner amount set in this way tends to be excessive in the halftone dot that reproduces the intermediate gradation.

  Therefore, there is a problem that image quality deterioration such as image distortion is likely to occur in the image density region of the intermediate gradation during transfer. That is, at the time of transfer, as the toner layer becomes thicker, the toner is more likely to scatter, and image quality deterioration such as image distortion is likely to occur. In particular, when reproducing a color image, multiple color toner images are transferred, so that image deterioration is likely to occur during transfer.

Further, there is a problem that the color reproduction becomes insufficient in the image density region of the intermediate gradation due to excessive toner. That is, color reproduction by toner is caused by the toner fixed on the recording paper absorbing light of a predetermined wavelength. Therefore, in order to increase the light absorption efficiency, the color contained in the toner layer is formed as a thin layer. Although it is necessary to efficiently expose the material to light, in a halftone dot for intermediate gradation reproduction, since the toner layer becomes excessively thick as described above, color reproducibility tends to be insufficient.
Furthermore, excessive toner in the image density area of the intermediate gradation causes a problem that toner is unnecessarily consumed in the development process.
Therefore, when performing binarization processing, it is desired to suppress excessive toner in the image density region of the intermediate gradation while ensuring the maximum density.

Here, as a conventional technique for suppressing excessive toner in the image density region of the intermediate gradation, a low density conversion process for reducing the density of the input color image signal by thinning out a part of the input color image signal at a predetermined cycle is performed. In addition, when the density level of the input color image signal is determined for each pixel and it is determined that the density level of the input color image signal is smaller than a predetermined level, the image signal is thinned at a position. There is a technique for prohibiting the thinning process even if it exists (see, for example, Patent Document 1).
Further, there is a technique for printing halftone dots used for printing, in which the basic unit or minimum unit constituting the halftone dots has a hollow structure (see, for example, Patent Document 2).

Japanese Patent No. 3347411 JP 2005-26987 A

However, with the technique described in Patent Document 1 described above, it is possible to suppress toner consumption. However, since a part of the input color image signal is thinned out at a predetermined period, the reproducibility of image density and color tone is impaired. There is a problem.
On the other hand, in the technique described in Patent Document 2, by setting the halftone dot to a hollow structure, the spatial frequency is increased in a pseudo manner, and the gradation characteristic has a high gamma characteristic. As a result, there is a problem that it is difficult to express a color difference in a high-saturation multi-order color. In particular, the orange color is a color that is difficult to be identified even in a normal state, but becomes more difficult to identify.
As described above, in the conventional technique as described above, it is possible to suppress the excessive toner in the image density region of the intermediate gradation, but there is a problem that the image quality is deteriorated.

  The present invention has been made to solve the technical problems as described above, and an object of the present invention is to enable image formation with reduced toner consumption while preventing deterioration of image quality. To provide a processing device, an image processing method, and a printed matter.

  For this purpose, the image processing apparatus of the present invention artificially generates a halftone image using a halftone dot that is a set of output dots for each input image signal of each color component subjected to color separation. An image processing apparatus that omits halftone dot formation information for forming halftone dots consisting of predetermined dots corresponding to the density of the input image signal, and some dots forming halftone dots as virtually no output dots A halftone dot corresponding to the input image signal is formed using the halftone dot information storage unit for storing the dot omission information and the halftone dot formation information and the dot omission information stored in the halftone information storage unit. A screen processing unit that generates an image output signal to be stored, and the dot information storage unit stores a plurality of dot omission information having different dot omission amounts, and the screen processing unit stores color component information of the input image signal. According to multiple dot saving By switching the information, and generates an image output signal to form a different halftone dots for each color component information.

Here, the screen processing unit can switch a plurality of dot omission information according to the saturation of the color component information of the input image signal. For example, when the saturation of the color component information is low, dot omission information with few omissions can be used.
The screen processing unit may switch a plurality of dot omission information according to the hue of the color component information. For example, the screen processing unit can use dot omission information with less omission of dots when the color component information has a large hue angle separation from the primary color in the output image.

  Furthermore, the halftone dot formation information is a threshold value matrix including a threshold value that defines dots that form halftone dots according to the intensity of the input image signal to be processed, and the dot omission information is a threshold value determined in the threshold value matrix, This is a threshold conversion data table for correcting the dot inside the halftone dot in the direction to be a non-output dot, and the screen processing unit converts the threshold corresponding to the intensity of the input image signal to be processed based on the threshold matrix into threshold conversion. Referring to threshold conversion data corresponding to the intensity of the input image signal to be processed in the data table, a threshold conversion unit for converting to a second threshold for obtaining no output dot, and a second threshold corrected by the threshold conversion unit And a comparison operation unit for comparing the intensity of the input image signal to be processed.

In addition, the image processing apparatus further includes a color component information adding unit that adds hue and saturation color component identification information to the image signal, and the screen processing unit switches between a plurality of threshold conversion data tables based on the color component identification information. Can be characterized.
The screen processing unit also includes color component identification information attached to an input image signal corresponding to a specific dot forming a halftone dot, and color component identification information attached to an input image signal corresponding to a dot adjacent to the specific dot. In the boundary area of the threshold conversion data different from the above, the threshold threshold conversion data of the surrounding dots including the specific dot is weighted and averaged to form the boundary threshold conversion data, and the boundary threshold conversion data is used to generate the input image signal of the specific dot. A threshold value based on a corresponding threshold value matrix may be converted into a second threshold value.

  The image processing method according to the present invention generates a pseudo halftone image using halftone dots, which are a set of dots, for each input image signal of each color component that has undergone color separation. An image processing method for forming an air gap with virtually no output dots, the step of inputting an input image signal of each color component, and halftone dot formation information corresponding to the input image signal from a memory for storing halftone dot formation information Each of the color components based on the halftone dot formation information and the dot omission information, and the step of reading the dot omission information corresponding to the color component information of the input image signal from the memory storing a plurality of dot omission information. Generating an image output signal for forming halftone dots having different shapes.

Here, color component identification information is given to the input image signal, and image processing is performed by switching a plurality of dot omission information in accordance with the color component identification information.
In addition, switching of a plurality of dot omission information is performed according to the saturation of the color components of the input image signal. When the saturation is high, dot omission information in which many dots are omitted is used. It can be characterized by using dot omission information that is less omitted.

Furthermore, the switching of a plurality of dot omission information is performed in accordance with the hue of the color components of the input image signal, and the dot omission information in which the omission of the dots is often used for the color component of the hue close to the primary color in the output image. In a color component having a large hue angle that is separated from the color, dot omission information with less omission of dots can be used.
Furthermore, in the dot omission information boundary area where the dot omission information for a specific dot and the dot omission information for a dot adjacent to the dot are different, the dot omission information is calculated by weighted averaging the dot omission information of surrounding dots including the specific dot. It can be characterized as information.

The printed matter according to the present invention is a printed matter in which a halftone is expressed in a pseudo manner using a halftone dot which is a set of dots for each color component that has undergone color separation, and a halftone portion is drawn on the printed matter. The shape of the point is different for each color component.
Here, the amount of omission of the central dot for forming a halftone dot is different depending on each color component, so that the shape of the halftone dot is different.

  According to the present invention, it is possible to form an image while suppressing toner consumption while suppressing deterioration in image quality.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a diagram showing an overall configuration of an image forming apparatus 1 which is an electrophotographic multifunction machine to which the present embodiment is applied. FIG. 2 is a block diagram of the image processing unit.
An image forming apparatus 1 shown in FIG. 1 is an image reading apparatus (IIT: Image Input Terminal) 10 that reads an image, and an image processing apparatus that performs predetermined image processing on image data read by the image reading apparatus 10. An image processing unit (IPS) 20 and a marking engine 40 as an image forming unit which is a so-called tandem type digital color printer using an electrophotographic system. Further, an operation input device 60 such as a keyboard for inputting setting information and the like, and a control unit 70 that performs all drive control of the image forming apparatus 1 are provided. The image forming apparatus 1 can set the toner save mode by an operation via the operation input device 60.

The image reading device 10 is a so-called scanner that optically reads an image such as a document or a photograph with a line sensor or the like, and reads the read analog image information by R (red) including predetermined bits (for example, 8 to 10 bits). , G (green), B (blue), etc., are converted into digital image data for each color component and output to the image processing unit 20.
The image processing unit 20 includes a color conversion unit 21 that performs image processing such as color conversion on input image data, and a printer engine control unit 22.
The color conversion unit 21 includes a pre-processing unit 21A, a page memory 21B, and a post-processing unit 21C.

The preprocessing unit 21A performs shading correction / MTF correction on DEV-RGB image data, which is device-dependent RGB image information input from the image reading device 10, and then performs one-dimensional gradation correction on the input side. The image data is converted into ENL-RGB image data having an equivalent neutral brightness by the LUT. The input-side one-dimensional gradation correction LUT is determined in advance and stored in a memory (not shown), and a single color patch for colorimetry is read by the image reading device 10 so that the RGB value becomes gray with good balance. Is set to
The page memory 21B stores the converted ENL-RGB image data.

The post-processing unit 21C converts the image data based on ENL-RGB stored in the page memory 21B into L * a * b *, which is a device-independent color space, by matrix calculation (L * is the brightness, a *, Image data in which color is defined by expressing saturation and hue with b * (hereinafter referred to as L * a * b * image data), and image area separation and color processing.
The parameters of the color space conversion matrix are predetermined and stored in a memory (not shown). The colorimetric value (L * a * b *) of the single color patch and the RGB obtained by reading the single color patch with the image reading device 10 It is created as a scanner model from the values. The algorithm is not particularly limited. The coefficient of the parameter is determined based on the scanner model.

Image area separation separates pictograms using scaling. Here, a tag (D-Tag) as color component information is attached to the image data of L * a * b * according to the signal of a * b *. That is, the post-processing unit 21C that performs this processing is the color component information adding unit in the present embodiment. This D-Tag will be described in detail later.
On the other hand, in color processing, image data of L * a * b * is converted into image data of E-YMCK (Y: yellow, M: magenta, C: cyan, K: black) using a DLUT (direct lookup table). And filter processing / background removal.
The DLUT is a table in which E-YMCK data is stored in each discrete three-dimensional address data (L * a * b *), and is stored in a memory in advance.

  Here, the DLUT is formed as follows. That is, first, the gradation of the printer engine is corrected according to a desired purpose by the input side one-dimensional gradation correction LUT. As the correction method, the gray balance may be neutral (ENL method) or the color difference may be constant (E-Liner method), and the method is arbitrary. Then, by outputting a patch image through the input-side one-dimensional gradation correction LUT, a printer color model that associates L * a * b * with E-YMCK is created. As a color model creation algorithm, any of a neural network method, a multiple regression method, a Neugebauer method, or the like may be applied. Interpolation is performed on the signal between L * a * b * and E-YMCK data. The interpolation algorithm is arbitrary. The value of E-YMCK corresponding to the address data of L * a * b * is obtained by the color model thus formed. At that time, since a gap is generated between the L * a * b * space and the L * a * b * space that can be reproduced by the printer, the gap is compressed by a desired method (Gamut compression). To achieve a clear image quality, preserve the saturation, and to reproduce it faithfully, compress with minimum color difference. In addition, an inking (UCR) algorithm for determining E-YMCK from L * a * b * is optimal from the viewpoint of cost and image quality. The maximum amount of the inking amount is set to 100% by a predetermined algorithm, and the toner amount of each color is controlled according to the ratio. That is, the inking amount is also one element of parameter setting. In the DLUT, the correspondence relationship between L * a * b * thus obtained and E-YMCK is stored as a parameter.

  Further, the post-processing unit 21C performs gradation conversion processing on the E-YMCK image information into DEV-YMCK image data that matches the characteristics of the marking engine 40, based on the one-dimensional gradation correction LUT on the output side. . Normally, the parameter coefficient for colorimetric matching is used. However, when the toner save mode is selected, the gradation characteristics are different, and the corresponding parameter coefficient is used.

The printer engine control unit 22 includes a screen processing unit 30 that converts multivalued image data into binary image data, and a pulse width modulation unit 22A.
The screen processing unit 30 performs screen processing (binarization processing) on each of the multivalued image data for each color component input from the color conversion unit 21 by a halftone method which is one of the area gradation methods. To generate binarized data (1-bit image data). That is, a binarized recording signal that artificially represents the density of an intermediate gradation image is generated from image data, which is multi-value image information having density gradations, by the size of colored dots called halftone dots.
Here, in this embodiment, when the toner save mode is set, by omitting some of the central dots that form the halftone dots, toner consumption can be reduced while suppressing deterioration in image quality. It is designed to be illustrated. Such screen processing is performed based on a threshold value matrix that is halftone dot formation information and a gap profile table that is dot omission information, which are respectively stored in a pattern storage unit 33 as a halftone dot information storage unit described later. . This will be described in detail later.
The pulse width modulation unit 22A applies pulse width modulation to the image data (binarized recording signal) screen-processed by the screen processing unit 30 and supplies the image signal to the marking engine 40.

As shown in FIG. 1, the marking engine 40 includes four image forming units 46Y, 46M, 46C, and 46K that are arranged in parallel at regular intervals.
The image forming units 46Y, 46M, 46C, and 46K include a photosensitive drum 41 that forms an electrostatic latent image and carries a toner image, a charging roll 42 that uniformly charges the surface of the photosensitive drum 41 at a predetermined potential, The image forming apparatus includes a developing unit 43 that develops an electrostatic latent image formed on the body drum 41 and a drum cleaner 44 that cleans the surface of the photosensitive drum 41 after transfer. Further, corresponding to the image forming units 46Y, 46M, 46C, and 46K, laser exposure devices 45Y, 45M, 45C, and 45K that expose the photosensitive drum 41 are provided.

Here, the image forming units 46Y, 46M, 46C, and 46K are configured in substantially the same manner except for the toner stored in the developing device 43. The image forming units 46Y, 46M, 46C, and 46K form yellow (Y), magenta (M), cyan (C), and black (K) toner images, respectively.
Further, the marking engine 40 includes an intermediate transfer belt 50 on which the toner images of the respective colors formed on the photosensitive drums 41 of the image forming units 46Y, 46M, 46C, and 46K are transferred, and the image forming units 46Y, 46M, A primary transfer roll 47 that sequentially transfers (primary transfer) each color toner image of 46C and 46K to the intermediate transfer belt 50 at the primary transfer portion T1, and a superimposed toner image transferred onto the intermediate transfer belt 50 to the secondary transfer portion T2. And a secondary transfer roll 48 for batch transfer (secondary transfer) to the recording paper P, and a fixing device 54 for fixing the secondary transferred image on the recording paper P.

The marking engine 40 operates as follows to form a color image on the recording paper P with toner.
That is, when the binarized recording signal is supplied from the image processing unit 20 to each laser exposure device 45Y, 45M, 45C, 45K of the marking engine 40, each laser exposure device 45Y, 45M, 45C, 45K A modulated laser beam is generated based on the recording signal. In the yellow (Y) image forming unit 46Y, for example, the surface of the photosensitive drum 41 uniformly charged at a predetermined potential by the charging roll 42 is scanned and exposed with the laser light generated by the laser exposure device 45Y. Then, an electrostatic latent image is formed on the photosensitive drum 41. The formed electrostatic latent image is developed by the developing device 43, and a Y toner image is formed on the photosensitive drum 41. Similarly, in the image forming units 46M, 46C, and 46K, M, C, and K color toner images are formed.

  The color toner images formed by the image forming units 46Y, 46M, 46C, and 46K are sequentially electrostatically attracted by the primary transfer roll 47 onto the intermediate transfer belt 50 that rotates in the direction of arrow A in FIG. A toner image superimposed on the belt 50 is formed. As the intermediate transfer belt 50 moves, the superimposed toner image is conveyed to the secondary transfer portion T2 where the secondary transfer roll 48 is disposed. When the superimposed toner image is conveyed to the secondary transfer portion T2, the paper P is supplied from the paper cassette 53 to the secondary transfer portion T2 in accordance with the timing at which the toner image is conveyed to the secondary transfer portion T2. Then, the superimposed toner images are collectively electrostatically transferred onto the conveyed recording paper P by the transfer electric field formed by the secondary transfer roll 48 in the secondary transfer portion T2.

  Thereafter, the recording paper P on which the superimposed toner image has been electrostatically transferred is peeled from the intermediate transfer belt 50 and conveyed to the fixing device 54 by the conveying belts 51 and 52. The unfixed toner image on the paper P conveyed to the fixing device 54 is fixed on the recording paper P by being subjected to fixing processing by heat and pressure by the fixing device 54. Then, the recording paper P on which the fixed image is formed is conveyed to a paper discharge placement portion (not shown) provided in the discharge portion of the image forming apparatus.

Next, the screen processing unit 30 and its screen processing (binarization processing), which is a characteristic configuration in the present embodiment, particularly, generation of a binary recording signal forming a halftone dot in a hollow structure in the toner save mode. explain about.
As described above, the screen processing unit 30 simulates an intermediate gradation image from the multivalued image data having the density gradation for each color component input from the color conversion unit 21 according to the size of the colored dots called halftone dots. A binarized recording signal representing the density of the image is generated. At this time, in the toner save mode, the binarized recording signal according to the present embodiment forms a halftone dot having a hollow structure (ring shape) having a gap inside in the intermediate density region (intermediate gradation region). Is generated.

FIG. 3 is a block diagram illustrating a configuration of the screen processing unit 30 according to the present embodiment. 4 is a diagram showing halftone dots formed by the screen processing unit 30, FIG. 5 is a diagram showing a gap profile table stored in the gap profile storage unit 33P, and FIG. 6 is a threshold conversion unit 31 and a comparison processing unit 32. FIG.
As shown in FIG. 3, the screen processing unit 30 includes a threshold value conversion unit 31, a comparison processing unit 32, and a pattern storage unit 33 as a halftone dot information storage unit configured by a memory.
The pattern storage unit 33 includes a halftone dot profile storage unit 33M that stores a threshold matrix for forming a normal halftone dot image (an array of threshold values corresponding to each coordinate value of a pixel matrix in a unit halftone dot region), and an input image. A gap profile storage unit 33P that stores a gap profile table that defines a gap size corresponding to the density of the gap.

The threshold value conversion unit 31 has a threshold value (initial value as a first threshold value) corresponding to the intensity of the processing target image signal (multi-valued image data representing the input image density) in the threshold value matrix stored in the halftone dot profile storage unit 33M. The threshold value: th ₁ ) is corrected and converted into a threshold value (application threshold value: th ₂ as a second threshold value) that causes no output dots with reference to the gap size profile data BPT held in the gap profile storage unit 33P. .
The comparison processing unit 32 binarizes the multi-valued image data using the application threshold value th ₂ converted by the threshold value conversion unit 31 and forms a halftone image having a gap of a predetermined size. Is generated.

Hereinafter, each part will be described in more detail.
The threshold value matrix stored in the halftone dot storage unit 33M of the pattern storage unit 33 is profile data that defines the halftone dot size corresponding to the density of the input image, that is, the density of the input image for generating halftone dots. And a set of threshold data used for forming halftone dots.
Here, as shown in FIG. 4A, the threshold value matrix threshold data is from the center to the outer periphery in the unit dot area until the density of the input image reaches “0” from the transition point density Ccnt. The number of output dots gradually increases with a dot pattern similar to the conventional halftone dot growth, and in the density region above the transition point density Ccnt, the entire dot in the unit halftone dot region is set as the output dot. .

The air gap profile table BPT stored in the air gap profile storage unit 33P of the pattern storage unit 33 is air gap / non-air gap threshold data for forming air gaps in the halftone dots in a predetermined density area (intermediate density area). .
In the gap profile table BPT shown in FIG. 5, the horizontal axis represents the intensity of the image signal to be processed (multi-valued image data representing input image density: 256 gradations), and the vertical axis represents multi-valued image data (density). The threshold value of void / non-gap in FIG. 5 is taken as b (hereinafter referred to as void threshold value: b).

This gap profile table BPT indicates a gap threshold value: b with respect to the input image density. The concentration C1 is a concentration that gives a low-density side void formation start point, and the concentration C2 is a concentration that gives a high-density side void formation end point. That is, the input image density in the intermediate density region leading to C2 from C1, shown on the vertical axis gap threshold: with b, threshold matrix defined initial threshold: modifying the th _1. Further, the transition point concentration Ccnt is a concentration that gives the maximum value of the number of voids, in other words, a concentration at which the number of voids, which will be described later, changes from increasing to decreasing. The transition point density Ccnt indicates that when the intensity of multi-value image data representing an input image (corresponding to the density of the input image) is shifted from the low intensity side, all binary recording signals representing halftone dots are output dots (“1 ”) As the first value.

  The density C1 that gives the low-density side void formation start point is a white dot (no output) while maintaining the outline of the halftone dot formed by a set of colored dots (output dots) as colored dots (output dots). Set to place a dot). On the other hand, the density C2 which gives the high density side void formation end point is for arranging white dots (no output dots) in the halftone dots only in the intermediate density range. Note that this density C2 is a deterrent condition for preventing the formation of voids at higher density in consideration of image quality.

  In the present embodiment, as shown in FIG. 5, three types of gap profile tables (BPT1, BPT2, BPT3) that give different gap thresholds: b for the same image density are stored in the gap profile storage unit 33P. The threshold conversion unit 31 switches between these for use. The switching is performed according to the hue and brightness of the input image data. This will be described later.

As shown in FIG. 6, the threshold conversion unit 31 is stored in the halftone dot profile storage unit 33M using the gap threshold: b corresponding to the intensity of the image signal to be processed, defined by the gap profile table BPT. The initial threshold value th ₁ corresponding to the intensity of the image signal defined by the threshold value matrix is corrected and converted to the application threshold value th ₂ by an operation such as the following equation.

th ₁ <b → th ₂ = th _max
th ₁ ≧ b → th ₂ = th ₁ −b

Accordingly, the initial threshold by the threshold matrix: th ₁ is a gap threshold: If it is less than b, the applied threshold maximum value: th max _{(= 255)} is the thing becomes converted to the dot corresponding threshold non-output dot It becomes. That is, the dot at the center forming the halftone dot at low density is not output, and as a result, a binary recording signal for forming a donut-like halftone dot having a gap at the center is generated. The size of the gap (the number of non-output dots) corresponds to the size of the gap threshold: b.
Due to the voids in the data inside the halftone dots, in the output image, the colorant inside the halftone dots can be eliminated to form donut-like halftone dots as shown in FIG. Therefore, by appropriately changing the size of the void in the halftone dot by setting the void threshold: b, the colorant (toner) transfer property can be improved, the image quality can be improved, and the toner consumption can be reduced. It becomes possible to do.

The initial by the threshold matrix threshold: th ₁ is void threshold: If the above b is applied threshold: th ₂ is the initial threshold: th ₁ from the gap threshold: b becomes a value obtained by dividing, voids than the initial threshold value th ₁ Threshold: smaller by b. That is, the dots on the outer periphery of the halftone dot have a small threshold value, and these dots are easily converted to output dots. Therefore, the external shape of the halftone dot tends to be large.
As a result, the black dots can be corrected so as to adhere to the outside of the halftone dots instead of the gaps formed by the void formation process inside the halftone dot outline, and the area of the output / non-output pixels that changes due to the generation of the gaps The rate change can be suppressed. For this reason, when maintaining image quality and reducing toner consumption by forming voids in the halftone dots, the area coverage of the output image can be maintained, and the occurrence of abnormal density changes (tone jumps) can be suppressed.

Here, the threshold conversion unit 31 uses the air gap profile table BPT selected from the three types of air gap profile tables BPT (BPT1, BPT2, BPT3) stored in the air gap profile storage unit 33P to set the initial threshold value: th. Threshold correction conversion from ₁ to application threshold: th ₂ is performed. Note that there is an option that threshold correction conversion is not performed (initial threshold value: th ₁ = application threshold value: th ₂ ).
As shown in FIG. 5, the three types of gap profile tables BPT (BPT1, BPT2, BPT3) have different gap thresholds: b for the input image density, and the gap sizes in the halftone dots differ depending on the gap profile table BPT used. It is set to generate binary recording information.
FIG. 7 is a diagram illustrating an example of a halftone dot formed by a binarized recording signal in which threshold conversion is performed using the space (BPT = 0) and the space profile tables BPT1 to 3 that do not use the space profile table BPT. . In this example, the gap profile table BPT is not used. In (a), no gap is formed in the halftone dots, but by using the gap profile tables BPT1 to BPT3, as shown in (b) to (d). In the numerical order, larger voids are formed in the halftone dots (the amount of toner save increases).

Then, the threshold conversion unit 31 increases the amount of toner save as it is closer to the primary colors of toner used for drawing by the marking engine 40 (including Y, M, C, and K replaced with the primary color by inking). In addition, the gap profile table BPT having a larger toner save amount is selected as the saturation with a larger toner consumption amount is lower.
Specifically, in the post-processing unit 21C of the color conversion unit 21 described above, the gap profile table BPT is selected with reference to the D-Tag attached in accordance with the a * b * signal (hue and brightness). Do.

FIG. 8 is a diagram showing the relationship between the D-Tag given to the a * b * signal in the post-processing unit 21C and the color area. (A) is a hue ring, (b) shows the correspondence between the D-Tag and the gap profile table BPT for the color region shown in (a).
That is, D-Tag is “11” for a predetermined range of the lowest saturation, “10” for a region near the primary color (Y, M, C, K) in drawing, and the primary color. “00”, “10”, and “00” for the intermediate region of the most distant multi-order color regions (regions in the vicinity of R, G, B that are complementary to Y, M, C) 01 ”is attached.
The D-Tag “11” forms the largest void (the toner save amount is large), the void profile BPT3. Hereinafter, the D-Tag “10” includes the void profile BPT2, and the D-Tag “01” includes the void. The profiles BPT1 correspond to each other, and the smaller the number, the smaller the gap (the toner save amount decreases). In D-Tag “00”, the gap profile table BPT is not used (no gap is formed: toner saving is not performed).

As a result, as described above, a binarized recording signal is generated that forms a large void in the halftone dot as the color is closer to the primary color of the toner used for drawing or as the saturation is lower. On the other hand, the formation of voids in the halftone dots produces a binarized recording signal that makes the voids smaller or does not form in highly saturated multi-order colors, where graininess deteriorates and tone jumping is easily noticeable. It is something that has come to be.
Further, in the boundary region where the D-Tags of adjacent dots forming halftone dots are switched, the gap threshold value: b is set in consideration of the boundary conditions.
FIG. 9 is an explanatory view of an example thereof.
That is, the gap threshold value: b of a specific pixel is determined by weighted averaging of the gap threshold value: b of the surrounding pixels. For example, the gap threshold value: b of the center pixel (D-Tag “11”) shown in the figure is defined by the BPT3 corresponding to the gap threshold value: b (D-Tag “11”) of the pixel surrounding the pixel. The gap threshold: b × 3) defined by BPT2 corresponding to the gap threshold: b × 6, D-Tag “10” is used as a weighted average as the following equation.
b = (gap threshold value by 6 × BPT3 + gap threshold value by 3 × BPT2) / 9
Thereby, a tone jump in the boundary region can be prevented.
Note that the weighting method is not limited to the simple weighted average as in this example, and an optimum method for the system may be used.

Next, the operation of the image processing unit 20 will be described step by step.
FIG. 10 is a flowchart showing the flow of image processing executed by the image reading device 10, the image processing unit 20, and the marking engine 40. Steps 102 to 114 are processes executed in the image processing unit 20. Steps 111 to 113 are processes executed in the screen processing unit 30. In addition, the code | symbol of each action mechanism part in description is the above-mentioned FIG.
First, device-dependent DEV-RGB image data is input from the image reading apparatus 10 (step 101).
The pre-processing unit 21A performs shading correction / MTF correction on the input DEV-RGB (step 102), and the input side one-dimensional gradation correction LUT converts the image data to ENL-RGB having equivalent neutral brightness. Conversion is performed (step 103).
The ENL-RGB image data is temporarily stored in the page memory (step 104) and sent to the post-processing unit 21C.

Next, the post-processing unit 21C converts the ENL-RGB image data into L * a * b * image data, which is a device-independent color space, by an operation using a color space conversion matrix (step 105).
Thereafter, the image data of L * a * b * is branched into an image area separation pass and a color processing pass.
In the image area separation pass, image area separation is performed using enlargement / reduction (step 106), and D-Tag is added according to the a * b * signal (saturation and hue) (step 107). This D-Tag is sent to the printer engine control unit 22 at the subsequent stage as it is.
On the other hand, in the color processing pass, L * a * b * image data is converted to E-YMCK image data using DLUT (step 108).
Next, filter processing / background removal is performed on the E-YMCK image data (step 109). Further, gradation conversion processing is performed to DEV-YMCK image data that matches the characteristics of the marking engine 40 using the input-side one-dimensional gradation correction LUT (step 110).

The DEV-YMCK image data is sent to the screen processing unit 30 of the printer engine control unit 22 via, for example, an 8-bit multi-value interface (I / F).
The screen processing unit 30 reads the gap profile table BPT (BPT1 to 3) corresponding to the D-Tag attached to the input DEV-YMCK image data from the pattern storage unit 33 (step 111), and uses the threshold value matrix. The initial threshold value th1 determined by the threshold value table is corrected and converted to the application threshold value th2 (step 112), and screen processing (binarization processing) is performed on the DEV-YMCK image data using the application threshold value th2. Is converted into a binary recording signal (step 113).
Thereafter, the screen-processed image data (binary recording signal) is input to the pulse width modulation unit 22A of the printer engine control unit 22.
The pulse width modulation unit 22A modulates the image data as a pulse signal (step 114). The pulse-modulated image data is output to the marking engine 40 (step 115).
The marking engine 40 that has acquired the image data forms a color image on the recording paper P by each component as shown in FIG. 1 and prints it out (step 116).
Thereby, it is possible to obtain a printed matter of an image in which the halftone is simulated by a donut-shaped halftone dot having a gap in the center. The gap in the center of the halftone dot is larger as it is closer to the primary colors Y, M, C, and K of the toner, and as the saturation is lower. That is, toner consumption is suppressed in a primary color that is relatively easy to reproduce the color tone, and good color tone reproduction is possible in a multi-order color that is difficult to reproduce the color tone with a small gap.

  As described above in detail, in the present embodiment, the gap formed in the halftone dots is changed according to the saturation and hue of the image data. That is, the closer to the primary color of the toner used for drawing and the lower the saturation, the larger voids are formed in the halftone dots. On the other hand, the formation of voids in the halftone dots makes the voids small or not formed in high-saturation multi-colors where the deterioration of graininess and tone jumping are conspicuous. Thus, it is possible to rationally suppress the toner consumption while accurately expressing the color difference of the high-saturation multi-order color by suppressing the deterioration of the image quality.

The present invention is not limited to the above-described embodiment, and can be changed as appropriate.
For example, the above embodiment is an example in which the present invention is applied to processing of image data read by the image reading device 10, but the present invention is not limited to this, and PDL image data input from a personal computer or the like. It may be applied to the processing.
In the above embodiment, the threshold value matrix threshold value and the gap are formed by correcting and converting the threshold value of the threshold matrix based on the gap profile table stored in the pattern storage unit 33 and performing screen processing. And a threshold matrix to be formed, and may be applied to a configuration in which a gap is formed at a halftone dot by theoretically synthesizing an image formed by both.
Further, in the above embodiment, the halftone dot shape (gap shape) is changed based on the color component information (D-Tag) attached to the image data according to the hue and saturation at the time of color conversion. The YMCK signal may be buffered and the halftone dot shape may be changed according to the difference between the YMCK signals.

1 is a diagram illustrating an overall configuration of an image forming apparatus that is an electrophotographic multifunction peripheral to which the exemplary embodiment is applied. It is a block block diagram of an image processing part. It is a block diagram which shows the structure of a screen process part. It is a figure which shows the halftone dot formed by a screen process part. It is a figure which shows the space | gap profile table stored in the space | gap profile memory | storage part. It is a circuit block diagram of a threshold value conversion unit and a comparison processing unit. It is a figure which shows the halftone dot formed with a space | gap profile table. It is a figure which shows the relationship with the color area | region of D-Tag provided to an a * b * signal. It is explanatory drawing of an example of the boundary condition in which D-Tag switches. It is the flowchart which showed the flow of the image processing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... Image reading apparatus, 20 ... Image processing part (image processing apparatus), 21C ... Post-processing part (color component information provision part), 22 ... Printer engine control part, 30 ... Screen processing part, 31 ... Threshold conversion unit, 32 ... Comparison processing unit, 33 ... Pattern storage unit (halftone dot information storage unit), 40 ... Marking engine (image forming unit)

Claims (15)

An image processing apparatus that artificially generates a halftone image using a halftone dot that is a set of output dots for each input image signal of each color component subjected to color separation,
Dot omission to omit halftone dot formation information for forming halftone dots composed of predetermined dots corresponding to the density of the input image signal, and some dots forming the halftone dots as virtually no output dots A halftone dot information storage unit for storing information,
A screen processing unit that generates an image output signal for forming a halftone dot corresponding to the input image signal using the halftone dot formation information and the dot omission information stored in the halftone dot information storage unit. ,
The dot information storage unit stores a plurality of dot omission information with different dot omission amounts,
The screen processing unit switches the plurality of dot omission information according to color component information of the input image signal, and generates an image output signal that forms a different halftone dot for each color component information. Processing equipment.   The image processing apparatus according to claim 1, wherein the screen processing unit switches the plurality of dot omission information according to a saturation of the color component information of the input image signal.   The image processing apparatus according to claim 2, wherein the screen processing unit uses dot omission information with less omission of the dots when the saturation of the color component information is low.   The image processing apparatus according to claim 1, wherein the screen processing unit switches the plurality of dot omission information according to a hue of the color component information.   The image processing apparatus according to claim 4, wherein the screen processing unit uses dot omission information with less omission of dots when color component information having a large hue angle separation from a primary color in an output image is used. . The halftone dot formation information is a threshold value matrix that includes threshold values that define dots that form halftone dots according to the intensity of the input image signal to be processed;
The dot omission information is a threshold value conversion data table for correcting the threshold value set in the threshold value matrix in a direction in which the dot inside the halftone dot is set as a non-output dot.
The screen processing unit
The threshold corresponding to the intensity of the input image signal to be processed based on the threshold matrix is referred to the threshold conversion data corresponding to the intensity of the input image signal to be processed in the threshold conversion data table. A threshold value conversion unit for converting to a second threshold value to obtain
The image processing apparatus according to claim 1, further comprising: a comparison operation unit that compares the second threshold value corrected by the threshold value conversion unit with the intensity of the input image signal to be processed. A color component information adding unit for adding color component identification information of hue and saturation to the input image signal;
The image processing apparatus according to claim 6, wherein the screen processing unit switches between the plurality of threshold conversion data tables based on the color component identification information.   The screen processing unit includes color component identification information attached to an input image signal corresponding to a specific dot forming the halftone dot, and color component identification attached to an input image signal corresponding to a dot adjacent to the specific dot In the boundary area of the threshold conversion data different from the information, the threshold conversion data of the surrounding dots including the specific dot is weighted and averaged to form boundary threshold conversion data, and the boundary threshold conversion data is used to specify the specific The image processing apparatus according to claim 7, wherein a threshold value based on the threshold value matrix corresponding to an input image signal of dots is converted into the second threshold value. For each input image signal of each color component that has undergone color separation, a halftone image is generated in a pseudo manner using a halftone dot that is a set of dots, and a virtual non-output dot is included in the halftone dot. An image processing method for forming a void,
Inputting an input image signal of each color component;
Reading the halftone dot formation information corresponding to the input image signal from a memory storing halftone dot formation information;
Reading the dot omission information corresponding to the color component information of the input image signal from a memory storing a plurality of dot omission information;
Generating an image output signal for forming halftone dots having different gap shapes for each color component based on the halftone dot formation information and the dot omission information;
An image processing method comprising:   10. The input image signal is provided with color component identification information, and image processing is performed by switching the plurality of dot omission information according to the color component identification information. Image processing method. The switching of the plurality of dot omission information is performed according to the saturation of the color components of the input image signal,
The image processing method according to claim 9, wherein when the saturation is high, dot omission information in which the dot omission is often used is used, and when the saturation is low, dot omission information in which the dot omission is small is used. The switching of the plurality of dot omission information is performed according to the hue of the color components of the input image signal,
In the color component of the hue close to the primary color in the output image, using the dot omission information in which the dot omission is often performed,
The image processing method according to claim 9, wherein dot omission information with less omission of the dots is used for a color component having a large hue angle separated from the primary color.   In the dot omission information boundary area where the dot omission information in the specific dot is different from the dot omission information in the dot adjacent to the dot, the dot omission information of surrounding dots including the specific dot is weighted averaged. The image processing method according to claim 9, wherein the dot omission information is used. For each color component that has undergone color separation, a printed matter in which a halftone is simulated using a halftone dot that is a set of dots,
The printed matter, wherein a shape of the halftone dot that draws the halftone portion differs depending on each color component.   The printed matter according to claim 14, wherein the halftone dot has a different shape depending on each color component, and the halftone dot has a different shape.

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