JP2004260790A - Halftone screening method, computer program, recording medium, image processing device, and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To more improve picture quality in digital halftone screening. <P>SOLUTION: The multi-level digital halftone screening method for reproducing multi-stage concentrations with dots is set with a first dot growth step of growing the dots in a single color portion and a second dot growth step of growing the dots in a mixed color portion, and the dots are grown by selectively using the first and second dot growth steps for the single color portion and the mixed color portion. In this case, the dots are grown to be expanded and to eliminate a blank paper portion in the first dot growth step, and the dots are grown to reduce the overlapping of dots as much as possible in the second dot growth step. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention uses a dot forming method (growing method) suitable for the color of a drawing object and the type of image, and performs a halftone screening method of multi-valued digital image data for improving print image quality. For executing a halftone screening method for multi-valued digital image data, a recording medium storing the computer program, and an image forming apparatus such as a printer, a copying machine, a facsimile equipped with the image processing apparatus About.

  In a halftone process for digitally obtaining a halftone image in a print image, a masking process using a dither matrix is performed. In the masking processing, a dither matrix prepared in advance and an input image are compared for each pixel, and the pixel is assigned to one of black and white values based on the comparison result. This masking processing is also called dither processing or threshold processing. This processing is known, as disclosed in Patent Document 2, for example. However, the description of Patent Document 1 will be cited and the basic points will be described.

  Here, for simplicity, the masking process will be described using a dither matrix of 4 × 4 pixels (4 rows × 4 columns pixels). FIG. 11 shows an example of a dither matrix pattern expressing a maximum of 16 gradations (conventional example). FIG. 11A shows a spiral dither matrix pattern in which dots are concentrated, and FIG. 11B shows a Bayer dither matrix pattern in which dots are dispersed. A halftone image (pseudo gray image) can be obtained by applying such a dither matrix to a gray image and performing a masking process.

  FIG. 12 is a functional block diagram showing the masking process. In FIG. 12, 10 is a dither mask, 20 is a threshold processing (halftone processing), 100 is a gray image, and 200 is a pseudo gray image. By performing the masking process based on the dither mask 10 on the grayscale image 100 by the threshold value process 20, a pseudo grayscale image 200 can be obtained. Note that the threshold processing 20 in FIG. 12 is a mask of coordinates given by a remainder operation (imodK, jmodL) using the dither mask size (K, L) for the pixel value of the coordinates (i, j) of the grayscale image. This is a process in which the process of associating the pixel values is cyclically performed on the entire grayscale image 20 (all coordinates).

  An example of this threshold processing is shown in FIG. 13 as an explanatory diagram. FIG. 13A is a diagram showing an array of pixel values in a specific portion having the same size as the dither mask in the grayscale image. FIG. 11B is a diagram showing an array (pattern) of pixel values in a mask of 4 × 4 pixels. FIG. 11C is a diagram showing an array of pixel values (binary) in a pseudo grayscale image in a black and white pattern. For each of the pixels from the first row, first column (1, 1) to the 16th row, 16th column (16, 16) in the specific portion of the grayscale image, the pixel values of the corresponding dither mask pixels in the array are compared with pixel values Perform If the pixel value of the grayscale image has not reached the pixel value of the dither mask, the result of the threshold processing is “0”, that is, “black”. If the pixel value of the grayscale image has reached the pixel value of the dither mask, the threshold value has been reached. The result of the processing is “1”, that is, “white”.

  Since the grayscale pixel value in the first row and first column (1, 1) is 35 and the dither mask pixel value is 60, the pixel value of the pseudo grayscale image is “0”, that is, “black”. Since the grayscale pixel value of the first row and second column (1, 2) is 43 and the dither mask pixel value is 14, the pixel value of the pseudo grayscale image is “1”, that is, “white”. Since the grayscale pixel value of the first row and the third column (1, 3) is 48 and the dither mask pixel value is 53, the pixel value of the pseudo grayscale image is “0”, that is, “black”. The same applies hereinafter. Here, the description is made in black and white, but in the case of color, the density is set in this way for each of YMCK.

  As a conventional technique related to multi-level digital halftone screening using dots capable of reproducing multi-level densities, for example, as disclosed in Patent Document 2, dots are generated on the highlight side for a single element color in a multi-level halftone. 2. Description of the Related Art There is known an invention in which pseudo contours are suppressed by lowering the ratio to improve image quality.

  According to the present invention, there is provided a medium in which a halftone processing program for executing halftone processing based on image data in which each pixel constituting an image is represented by a plurality of element colors in multiple gradations is recorded, and A halftone processing function capable of performing a plurality of different halftone processings based on each other, and a halftone processing selection function of selecting and executing the halftone processing corresponding to each element color by the halftone processing function are executed. The configuration is such that The halftone processing function can execute a plurality of different halftone processings based on image data. Here, the halftone processing selection function causes the halftone processing function to select and execute halftone processing corresponding to each element color. That is, the halftone processing selection function appropriately selects halftone processing suitable for each element color, and the halftone processing function performs halftone processing according to this selection. Then, halftone image data is generated. In the halftone process, a process of determining a dot generation rate for reproducing the tone value based on the tone value expressing each element color is executed.

At this time, the halftone process executes a process of lowering the dot generation rate on the highlight side with respect to the content of one halftone process among the plurality of halftone processes with respect to the content of the other halftone processes. Thus, by lowering the dot generation rate on the highlight side, it is possible to suppress the lightness on the highlight side.
JP-A-2002-232715 JP 2001-339614 A

  Digital halftone screens that can express multiple gradations by changing the density of dots can reduce the change in density when the next dot is painted by combining dots with different densities, resulting in smooth gradation expression and granularity. However, in reality, the image quality has not reached the required level. Therefore, in digital screening, a method of improving image quality by performing edge correction by changing the width of a pulse for realizing a dot on an output device has been proposed, but it still reaches the required level of image quality improvement. I didn't.

  Further, in the above-described known example, the false contour is suppressed by lowering the dot generation rate on the highlight side for the elementary color of the single color in the multi-value halftone, and the image quality is improved. This is only processing, and no consideration is given to processing on the color mixing portion side, and it is hardly sufficient to improve image quality. Furthermore, no particular consideration has been given to changing the dot growth method according to the type of image to be drawn, such as characters, fine lines, photographs and graphics, and there is no room for improving the image quality. Was.

  The present invention has been made in view of such a situation of the related art, and an object of the present invention is to make it possible to further improve image quality in digital halftone screening.

  A first means for achieving the above object is a multi-value digital halftone screening method for reproducing multi-level densities with dots, a first dot growing step of growing dots in a single color portion, and a multi-color digital halftone screening method. A second dot growth step for growing dots is set, and the dots are grown by using the first and second dot growth steps selectively in the single color portion and the mixed color portion.

  The second means is characterized in that, in the first means, the dots are grown in the first dot growing step so that the dots are enlarged and the white background is eliminated.

  The third means is characterized in that in the first means, the dots are grown in the second dot growth step so as to minimize overlap between the dots.

  According to a fourth aspect, in the first to third aspects, the dot is grown by modulating a pulse width.

  A fifth means is the fourth means, wherein the dots are deformed so that the dot areas before and after the dot growth correction become equal.

  According to a sixth aspect, in the first to third aspects, in the first growth step, the halftone dot is corrected to a shape closer to a rectangle.

  A seventh means is characterized in that, in the first to third means, in the second growth step, the halftone dots of other colors are corrected to have a shape such that overlapping is reduced.

  Eighth means is a multi-valued digital halftone screening method using dots capable of reproducing multi-level densities, a third dot growing step of growing dots in a binary image portion including a character portion or a thin line portion, and a photographic portion. Alternatively, a fourth dot growth step of growing dots in a halftone image portion including a graphics portion is set, and the third and fourth growth processes are selectively used in the binary image portion or the halftone image portion. The method is characterized by growing dots.

  A ninth means is the liquid crystal display device according to the eighth means, wherein the third dot growing step comprises first growing a dot pattern having a low density level.

  According to a tenth aspect, in the eighth aspect, in the fourth dot growing step, a dot shape difference between successive densities is reduced.

  According to an eleventh aspect, in the tenth aspect, in the fourth dot growing step, the dot growth order is the same for each density level, but the relation between the density levels is different.

  In a twelfth aspect, in the tenth or eleventh aspect, the fourth dot growing step includes a step of stopping applying another dot until a certain dot reaches a maximum level.

  According to a thirteenth aspect, in the twelfth aspect, the fourth dot growth step is applied to an engine having low density dot reproducibility.

  According to a fourteenth aspect, in the tenth or eleventh aspect, the fourth dot growing step includes a step of shifting to painting on another dot before a certain dot reaches the highest level. .

  According to a fifteenth aspect, in the fourteenth aspect, the fourth dot growing step is applied to an engine having a low dot density and high reproducibility.

  A sixteenth means is characterized in that the computer program is configured to include a procedure for executing each step of the halftone screening method according to the first to fifteenth means by a computer program.

  A seventeenth means is characterized in that the computer program according to the sixteenth means is read by a computer and is recorded on a recording medium in an executable manner.

  The eighteenth means is characterized in that the image processing apparatus includes a control means for executing each step of the halftone screening method according to the first to fifteenth means based on a computer program.

  A nineteenth means is an image processing apparatus for performing multi-level digital halftone screening for reproducing multi-level densities with dots, wherein a first dot growing means for growing dots in a single color portion and a dot in a mixed color portion are provided. A second dot growing means for growing; and a control means for switching the first and second dot growing means between the single color portion and the mixed color portion, so that different processing is performed between the single color portion and the mixed color portion. And growing the dots.

  A twentieth means is a third dot growing means for growing dots in a binary image portion including a character portion or a thin line portion in an image processing apparatus for performing multi-level digital halftone screening for reproducing multi-level densities with dots. A fourth dot growing means for growing dots in a halftone image portion including a photograph portion or a graphics portion; and the third and fourth growing steps by the character / thin line portion and the photograph / graphics portion. A switching control means is provided, wherein different processing is performed in the binary image section and the halftone image section to grow dots.

  A twenty-first means includes an image processing apparatus according to the eighteenth to twentieth means, and an image forming apparatus for forming a visible image on a recording medium based on image data processed by the image processing apparatus. The feature is that it is configured.

  In the following embodiments, each of the steps or the first to fourth means is constituted by a program, and the program is stored in a large-capacity storage means such as the ROM 69 or a hard disk drive (not shown). This program is transferred from a server (not shown) via a network, a public line, a LAN, or the like, or is read from a recording medium such as a CD-ROM mounted on a CD-ROM drive (not shown) and downloaded, or is upgraded. Can be

  The image processing apparatus is supported by the IPU 49, and the control of the image forming apparatus is controlled by a CPU (not shown) in the main controller, and the control of image processing (apparatus) is controlled by the CPU 68 in the IPU 49.

  According to the present invention, the halftone screening method is changed between a single color portion and a mixed color portion, or a character / thin line portion and a photograph / graphics portion, so that halftone screening according to the characteristics of the image can be performed. Image quality can be improved in digital halftone screening.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram schematically illustrating an overall configuration of an image forming system according to an embodiment of the present invention. In FIG. 1, an image forming system according to the present embodiment includes an image forming apparatus PR having a scanner and a printer function, host computers PC1, PC2, and PC3 such as PCs, a LAN and a parallel I / F (PI / F). The print data from the host computers PC1, PC2, and PC3 is transmitted to the image forming apparatus PR via a LAN and a parallel I / F (PI / F) so that the image can be output from the image forming apparatus PR. Is configured.

  FIG. 2 is a schematic configuration diagram illustrating an outline of an internal configuration of the image forming apparatus PR according to the embodiment of the present invention.

  In FIG. 1, an image forming apparatus PR includes an automatic document feeder (hereinafter, referred to as an ADF) 1, a reading unit 50, a writing unit 57, an image forming unit 19, a sheet feeding unit 29, a finisher 100, and a duplex sheet feeding unit 111. It is configured.

  The ADF 1 has a function of automatically feeding a document to be read to the reading position of the reading unit 50, reading the document, and then discharging the document to a predetermined position. The reading unit 50 optically converts the document sent from the ADF 1 to the reading position. The reading and writing unit 57 optically modulates the image data read by the reading unit 50 to write on the photoconductor (image forming medium) of the image forming unit 19 to form a latent image, and the sheet feeding unit 29 A toner image obtained by developing the latent image with toner is transferred to a transfer paper (recording medium) sent from the printer, fixed, and output.

  More specifically, when the start key 34 on the operation unit 30 is pressed, the document bundle placed on the document table 2 of the ADF 1 with the image side of the document facing upward is fed from the lowermost document to the feeding roller 3, The sheet is fed to a predetermined position on the contact glass 6 by the feeding belt 4. After the reading unit 50 reads the image data of the document on the contact glass 6, the document that has been read is discharged by the feed belt 4 and the discharge roller 5. Further, when the document set detection sensor 7 detects that the next document is present on the document table 2, the next document is fed onto the contact glass 6 in the same manner as the previous document. The ADF 1 is provided with a feed roller 3, a feed belt 4, and a discharge roller 5, and these rollers 3, 4, and 5 are driven by a motor.

  The paper feeding unit 29 includes a first tray 8, a second tray 9, a third tray 10, a first paper feeding device 11 for picking up transfer paper from the first to third trays 8, 9, 10, and a second paper feeding unit. The image forming apparatus includes a paper device 12, a third paper feeding device 13, and a vertical transport device 14 that transports the transfer paper picked up by the first to third paper feeding devices 11, 12, 13 to the image forming unit 19 side. In the paper feeding unit 29, the transfer papers stacked on the first tray 8, the second tray 9, and the third tray 10 are respectively transferred by the first paper feeding device 11, the second paper feeding device 12, and the third paper feeding device 13. The paper is fed and transported by the vertical transport unit 14 to a position where it contacts the photoconductor 15.

  The reading unit 50 includes an exposure lamp 51, first, second, and third mirrors 52, 55, and 56, an imaging lens 59, and a CCD image sensor 54. The reflected light of the original on the contact glass 6 exposed by the exposure lamp 51 is guided to the image forming lens 59 via the first to third mirrors 52, 55, 56, and the CCD image sensor 54 is formed by the image forming lens 59. And is read by the CCD image sensor 54.

  The writing unit 57 includes a laser output device 58 including a polygon mirror, an imaging lens 59 formed of an fθ lens, and a mirror 60. Laser light emitted from a laser diode of the laser output device 58 is reflected by the polygon mirror, Optical writing is performed by scanning the photoconductor 15 of the image forming unit 19 in the main scanning direction.

  That is, the image data read by the reading unit 50 is written on the photoconductor 15 by the laser beam from the writing unit 57, and is developed by the developing device 27 with toner, so that a toner image is formed on the photoconductor 15. The toner image on the photoconductor 15 is transferred onto the transfer paper while the transfer paper is being conveyed by the conveyance belt 16 at the same speed as the rotation of the photoconductor 15. Thereafter, the image is fixed by the fixing unit 17, and the image is discharged to the finisher 100 of the post-processing device by the sheet discharging unit 18.

  The finisher 100 of the post-processing apparatus performs predetermined post-processing on the transfer paper on which an image has been formed, and has a normal discharge roller 102 direction and a staple 106 for transferring the transfer paper conveyed by the paper discharge roller 19 of the main body. It can be guided toward the staple processing section. This switching is performed by the branch deflecting plate 101. By switching the branch deflecting plate 101 upward, the sheet can be discharged to the normal sheet discharge tray 104 via the transport roller 103. Further, by switching the branch deflecting plate 101 downward, the sheet can be discharged to the staple table 108 via the transport rollers 105 and 107. The transfer paper stacked on the staple table 108 is aligned with the jogger 109 for paper alignment every time one sheet is discharged, and the end faces of the transfer paper parallel to the paper conveyance direction are aligned. Bound. The transfer paper group bound by the stapler 106 is stored in the stapling completion paper discharge tray 110 by its own weight.

  The normal discharge tray 104 is a discharge tray having a so-called shift function that can move in a direction orthogonal to the transfer sheet transport direction. The discharge tray 104 having a shift function reciprocates in a direction orthogonal to the transfer paper transport direction for each original or for each copy unit sorted by the image memory, and sorts out the discharged transfer paper in a simple manner. Things.

  When images are formed on both sides of the transfer paper, the transfer paper fed from each of the paper feed trays 8 to 10 and guided is not guided to the paper ejection tray 104 side, but the branching claw 112 for path switching is used. By setting it on the upper side, it is once stocked in the duplex sheet feeding unit 111. Thereafter, the transfer paper stocked in the duplex paper feed unit 111 is fed again from the duplex paper feed unit 111 in order to transfer the toner image formed on the photoconductor 15 again. The re-feeded transfer paper is fed to the position of the conveyor belt 16, the image is transferred to the back side as if it was transferred to the front side, and after being fixed, the branch claw 112 is set on the lower side, and the back side is set. The transfer paper on which the image has been transferred is guided to the paper discharge tray 104. The double-sided paper supply unit 111 is used when an image is created on both sides of the transfer paper as described above.

  FIG. 3 is a block diagram showing a control system of the image forming apparatus PR according to the embodiment of the present invention, focusing on the main controller.

  This control system mainly includes a main controller 30, an operation unit 30, an image processing unit (IPU) 49, an ADF 1, and a printer controller 26. The main controller 20 controls the entire image forming apparatus PR. The main controller 20 includes an operation unit 30 for performing display to an operator, input of function setting from the operator, control of a scanner, control of writing a document image in an image memory, control of image formation from an image memory, and the like. A distributed control device such as a unit (IPU) 49 and an automatic document feeder (ADF) 1 is connected. Each decentralized control device and the main controller 20 exchange machine status and operation commands as needed. Further, a main motor 25 and various clutches 21 to 24 required for paper conveyance and the like are also connected. That is, the photoconductor 15, the transport belt 16, the fixing unit 17, the paper discharging unit 18, and the developing unit 27 are driven by the main motor 25, and the paper feeding devices 11 to 13 drive the main motor 25 by the paper feeding clutches 22 to 24. The vertical transport unit 14 is driven to transmit the drive of the main motor 25 by the intermediate clutch 21.

  The printer controller 26 analyzes an external image and a command to instruct printing, develops a bitmap into a printable state as image data, analyzes the print mode from the command, and determines an operation. The LAN control unit 261 and the parallel I / F 262 function to receive and operate the image and the command via the LAN and the parallel I / F. The operation unit 30 is connected to a liquid crystal display 31 and key input units 32, 33, 34, and 35, and performs predetermined processing in accordance with the input from the key input units 32, 33, 34, and 35. The result is output to the liquid crystal display 31 and displayed.

  The reading operation of the reading unit 50 of the image forming apparatus having the above-described configuration and the operation of the image forming unit 19 until a latent image is formed on a recording surface will be described in more detail. In the following description, a latent image is a potential distribution generated by converting an image into optical information and irradiating the image on the surface of the photoconductor.

  The reading unit 50 includes a contact glass 6 on which a document is placed and an optical scanning system. The optical scanning system includes an exposure lamp 51, a first mirror 52, a lens 53, a CCD image sensor 54, and the like. I have. The exposure lamp 51 and the first mirror 52 are fixed on a first carriage (not shown), and the second mirror 55 and the third mirror 56 are fixed on a first carriage (not shown). When reading a document image, the first carriage and the second carriage are mechanically scanned at a relative speed of 2: 1 so that the optical path length does not change. This optical scanning system is driven by a scanner drive motor (not shown).

  The document image is read by the CCD image sensor 54, converted into an electric signal, and processed. By moving the lens 53 and the CCD image sensor 54 in the left-right direction in FIG. 2, the image magnification changes. That is, the positions of the lens 53 and the CCD image sensor 54 in the left-right direction are set corresponding to the designated magnification.

  The writing unit 57 includes a laser output unit 58, an imaging lens 59, and a mirror 60. Inside the laser output unit 58, a rotating polygon mirror (polygon mirror) that rotates at a high speed at a constant speed by a laser diode as a laser light source and a motor. ) Is provided. The laser light emitted from the laser output unit 58 is polarized by a polygon mirror that rotates at a constant speed, passes through an imaging lens 59, is turned back by a mirror 60, and is condensed and imaged on the surface of the photoconductor 15. The deflected laser light is exposed and scanned in a direction (main scanning direction) perpendicular to the direction in which the photoconductor 15 rotates, and performs recording of the image signal output from the selector 64 of the IPU 49 in line units. An image (electrostatic latent image) is formed on the surface of the photoconductor 15 by repeating main scanning at a predetermined cycle corresponding to the rotation speed and the recording density of the photoconductor 15.

  As described above, the laser light output from the writing unit 57 is applied to the photoconductor 15 of the image forming system. Although not shown, a beam sensor that generates a main scanning synchronization signal is disposed at a position near one end of the photoconductor 15 where the laser beam is irradiated. Based on the main scanning synchronization signal, control of image recording start timing in the main scanning direction and generation of a control signal for inputting and outputting an image signal described later are performed.

  FIG. 4 is a block diagram illustrating details of the image processing unit (IPU) 49 of the image forming apparatus PR according to the present embodiment. The IPU 49 includes an image reading unit 50 and an image writing unit 57 in this embodiment.

  The light emitted from the exposure lamp 51 irradiates the original surface, and the reflected light from the original surface forms an image on the imaging surface of the CCD image sensor 54 by the imaging lens 53. The CCD image sensor 54 photoelectrically converts reflected light from the document received on the image forming surface, and converts the reflected light into a digital signal by the A / D converter 61. The image signal converted into the digital signal is subjected to shading correction by the shading correction unit 62, and then input to the MTF and γ correction unit 63, where predetermined image processing such as MTF correction and γ correction is performed. The image signal subjected to the image processing further passes through a first print synthesizing section (print synthesizing I) 72, and then the destination of the image signal is switched by the first selector 64. One of the destinations is the scaling unit 71, and the other is the image memory controller 65. When the scaling unit 71 is selected, the image signal that has passed through the scaling unit 71 is enlarged or reduced in accordance with the scaling factor, sent to the second selector 76, and the destination of the image signal is set to the second print synthesis unit. 73 or the image memory controller 65. When the image signal is sent to the second print synthesizing unit 73, the image signal is sent to the writing unit 57 as it is. The image memory controller 65 and the first selector 64 and the image memory controller 65 and the second selector 76 are configured to be capable of bidirectionally inputting and outputting image signals. Further, the print image data 74 is input to the first print synthesizing section 72 and the second selector 76, and the input of image data output from a data processing device such as a personal computer is input to the first selector 64. An external image input / output I / F 75 for outputting to an external device such as another printer connected to the LAN is provided, and the first selector 64 operates to input or output to the external image input / output I / F 75 Can also be switched.

  In the scaling process, image data can be stored in the image memory 66 without scaling, and can be read from the image memory 66 and executed in the scaling process before being sent to the writing unit 57. It is also possible to scale the image from the image memory 66 and return it to the image memory 66 again.

  The IPU 49 receives print image data (for example, data output from a data processing device such as a personal computer) 74 supplied from the outside in addition to the image data input from the reading unit 50 and an external image input / output I / F 75. The first selector 64 and the second selector 76 have a function of selecting input / output of a plurality of data so that image data input / output by the first selector 64 and the second selector 76 can be processed.

  Further, the IPU 49 is provided with a CPU 68, a ROM 69, a RAM 70, an image memory controller 65 mentioned above, and an image memory 66. The CPU 68 performs settings for the image memory controller 65 and the like, and controls the reading unit 50 and the writing unit 57. The ROM 69 stores a program to be executed by the CPU 68, and the RAM 70 stores data used by the CPU 68 when executing the program. In addition, it functions as a work area when the CPU 68 executes the program. Further, the CPU 68 can write and read data in the image memory 66 via the memory controller 65. Further, an I / O port 67 is provided for connection with an external device.

  In such an image forming apparatus, when used as a printer, image data from a personal computer connected via a network or locally is used, and when used as a copier, image data read by a scanner is used as a facsimile. The image data transferred using the communication line is temporarily stored in a memory, and the image forming apparatus reads out the image data stored in the memory, performs image processing, and converts the image data into an image processed image data. An image is formed based on the image. In this image processing, shading correction, γ correction, MTF correction, pseudo halftone processing, and the like are performed.

  In digital halftone screens that can express multiple gradations by changing the density of one dot, by successfully combining dots with different densities, the density change when the next dot is painted is reduced, and smooth gradation expression is achieved. Can be realized. FIGS. 3A and 3B are process diagrams showing the processing steps of a digital halftone screen when using a collective dither. In the case of using the collective dither, as shown in FIG. 3A, a wide area is first painted with thin dots 1 and then dots 2 of the next density are overlaid, or as shown in FIG. 3B. There is also a method of applying a thin dot 1 and then superimposing a dark dot 2. Here, a 3 × 3 matrix is taken as an example.

  Normally, one method is adopted.However, in the case of drawing in a single color such as cyan and magenta used in business graphics, dark dots are superimposed after painting a wide area with thin dots so as to make paper white as inconspicuous as possible. Painting can improve color reproducibility. Conversely, in the case of mixed-color drawing, in order to suppress color bleeding due to overlapping of the color printing dots, a method of forming dots in a concentrated manner so as not to overlap with each other rather than expanding dots is adopted. Also, pulse width modulation can be combined with the above method.

  In this way, when dots are enlarged in a single-color portion and dots are grown so that there is no white background, the dots are enlarged in a single-color portion as described in the conventional example, and dots are removed so that there is no white background. The process is performed as shown in FIG. 3A using the dither pattern to be grown. In other words, in the process of creating a halftone dither held in the printer in advance, a dither pattern in which dots grow as shown in FIG. 3A is created as a screen used for monochromatic drawing, and the dither pattern is created during monochromatic drawing. Use to grow the dots. As described above, by adopting the dot shape growth method suitable for each of the single color portion and the mixed color portion, it is possible to improve the image quality suitable for the drawing object. In addition, in drawing a single color portion, by expanding from a thin dot and growing it so as to eliminate a white paper portion, color reproducibility of a drawing target in a single color can be improved.

  FIGS. 4A, 4B, and 4C are diagrams showing specific examples of pulse width modulation according to the present embodiment. Here, the diameter of each dot is changed in four steps by changing the width of pulse generation as shown in FIG. 4B with respect to the basic pulse width shown in FIG. Can be. Further, as shown in FIG. 4C, the dot generation position can be changed by changing the timing of generating the pulse.

  If the shape correction of the halftone dot is performed using this, the halftone dot can be made as close to a rectangular shape as possible for smooth solid drawing. Normally, a halftone dot having a shape far from a rectangle must be formed from the shape of one dot, but in the case of a dot as shown in FIG. 5A, pulse width modulation may be applied as follows. .

  That is, considering the upper and lower dots in the cell separately, in this example, it is assumed that the minimum dot diameter is one-fourth of a pixel by pulse width modulation, and there is a difference in size of four upper and lower dots (one pixel). is there. In order to eliminate the difference, the lower right dot is reduced by two and the upper dot is expanded. As a result, the dot shape becomes rectangular as shown in FIG. 5B, and the smoothness of the halftone screen can be improved. Further, in this method, since the area of the dot does not change before and after the shape correction, the change in density can be reduced. Further, by performing such processing, the shape can be corrected so as to avoid overlapping of dots during mixed color drawing, so that the color reproducibility can be more effectively improved.

  That is, as shown in FIGS. 4A, 4B, and 4C, the process of changing the dot width (dot shape) by pulse width modulation and the dot forming process performed using a dither pattern are performed. By combining them as shown in FIGS. 5A and 5B, dots having a uniform dot shape can be created. At this time, as shown in FIG. 5A, three cells connected in an L-shape are regarded as one dot, but even the same three cells are formed as 2 × 1.5 rectangular dots. . As a result, although the dot area is unchanged, printing can be performed with a pattern having a uniform dot shape.

  By correcting the dot shape by modulating the pulse width in this way, the occurrence of a visual pattern can be suppressed, and the image quality can be improved. In addition, by making the dot areas before and after correction as equal as possible, it is possible to suppress the occurrence of density changes before and after correction. Further, by correcting the halftone dots to a shape closer to a rectangle, smooth screening can be realized.

  In the mixed-color portion, unlike the single-color portion, processing is performed as shown in FIG. 3B using a dither pattern for growing dots so as to minimize overlap between dots. In other words, in the process of creating a halftone dither held in the printer in advance, a dither pattern in which dots grow as shown in FIG. 3A is created as a screen used for monochromatic drawing, and the dither pattern is created during monochromatic drawing. Use to grow the dots. Other processes are the same as those for the single color portion. As described above, by growing the dots so that the overlap between the dots in the drawing of the mixed color portion is minimized, the color reproducibility of the drawing target by the mixed color can be improved. Further, by modifying the halftone dots of other colors into a shape that reduces the overlap, screening with improved color reproducibility can be realized.

  In this embodiment, the printer driver mounted on each of the personal computers PC1 to PC3 shown in FIG. 1 sends information on whether the color portion is a single color portion or a mixed color portion to the printer PR side. The dither pattern is selected by the CPU 68 of the IPU 49 on the PR side. Here, only the processing shown in FIGS. 3A and 3B is illustrated, but a large number of dither patterns are prepared according to the type of image and the processing.

  Therefore, the CPU 68 controls the growth of dots by using the first and second dot growth processes for the single color portion and the mixed color portion, and the procedure for performing such processes is performed by a ROM or a hard disk drive (not shown). In accordance with the program stored in the large-capacity storage means. This program loads the CD-ROM storing the program via the LAN or into a CD-ROM drive (not shown) and downloads the program data read from the CD-ROM to a large-capacity storage device such as a hard disk device. Used or upgraded.

  In the first embodiment, the processing is changed by changing the dither pattern depending on the color state, that is, whether the color is a single color or a mixed color. However, the processing may be changed in accordance with a difference in image type. There is. For example, in many cases, an image area is divided into a character or line drawing area and an area represented by a halftone dot such as a photograph, a picture, or graphics, in other words, a binary image area and a halftone dot area are divided into different image areas to change the processing. An example according to this example will be described as a second embodiment. Note that the configuration of the image forming apparatus, the image forming operation, and the like are the same as those in the first embodiment, and therefore only different points will be described, and redundant description will be omitted.

  On the other hand, digital screening has a problem in that non-solid characters and thin lines are easily cut off due to binarization restrictions. For this reason, in a binary output device, the probability of drawing dots by using dispersed dither (FIG. 8A) is increased, and reproducibility is improved. However, the dispersion dither has a drawback that it is easily affected by the dot gain and the gradation is impaired. Further, when the reproducibility of one dot is poor, there is a possibility that the drawing performance of the low-density portion is significantly reduced. For this reason, in the multi-level device of this embodiment, the number (area) of dots to be drawn is increased by preferentially growing dots having a low density level, so that the set dither (FIG. 8B) is increased. The use of color characters and thin lines is suppressed. It can be seen from the figure that even with the same density, the total number of non-white dots is larger in the multi-tone set dither, so that the probability that the drawing object will hit the dot to be painted increases and the reproducibility also improves. In this way, by growing the halftone dots for characters and thin lines first from the dot pattern of the low density level, it is possible to suppress interruption of the line drawing.

  On the other hand, dither for photography and graphics aims to improve graininess, so it is better to reduce the difference in dot shape at continuous density. Further, by changing the dot growth order at each density level, variations in the growth method can be provided, and the dot growth method most suitable for the engine characteristics can be selected from these. For example, in the case of an output device whose dot reproducibility at a low density level is not very good as an engine characteristic, a method of spreading the dots to other dots after the dot reaches the maximum density level is used. This state is shown in FIG. This makes it possible to supplement the reproducibility of a dot having a low density level in a highlight portion.

  Conversely, when low-density dot reproducibility is good, high-density dot growth is suppressed and low-medium-density dot growth is prioritized, as shown in FIG. 9B. As a result, it is possible to realize the dot reproduction in which the influence of dust and dot gain is suppressed and the dot shape difference between successive densities is suppressed.

  Therefore, in this embodiment, a dot pattern of a light density level is first grown in a binary image portion composed of characters, thin lines, and the like. On the other hand, in a halftone dot image portion composed of a photograph, a picture, and graphics, the difference in dot shape between successive densities is reduced. As a result, output quality can be improved regardless of the type of image.

  To reduce the dot shape difference between successive densities, for example, when the density is represented by 8 bits (256 gradations), for example, a dithering is performed such that there is no significant difference between the dot shapes of the density 100 dot and the density 101 dot. It means to process with a pattern. As a result, the difference between the drawn dot shapes formed in each dither pattern is set so as not to be large. Thus, even when the dot reproducibility, dust, dot gain, and the like differ due to the characteristics of the printer engine of the image forming apparatus, the difference in dot shape is prevented from increasing. The fact that the dot shape difference does not increase means that the graininess does not worsen, and it is possible to prevent the image quality of the halftone dot image from deteriorating.

  In a halftone dot such as a photograph portion and a graphics portion, the dot growth order is the same for each density level, but the dots are grown so that the relationship between the density levels is different. That is, the image forming apparatus according to the present embodiment includes a multi-value printer in which one dot can express a plurality of densities (for example, 8 bits), and each dot can express a different level of densities. In order to make the association between the respective density levels different, a pattern is set in which different growth orders in the cell are set for each density level as shown in FIG. 10, and the density is set according to this pattern. I do. This makes it possible to realize half-toning corresponding to various engine situations.

  At that time, the application of another dot is stopped until one dot reaches the highest level. That is, a dither pattern is prepared in the image forming apparatus for processing to stop applying another dot until a certain dot reaches the highest level, and the dither pattern is held, and the dot formation in the halftone image portion is performed. Used when This makes it possible to improve the output quality of an engine with low density dot reproducibility. Therefore, it is desirable to apply dot formation using such a dither pattern to an engine with low density dot reproducibility.

  Alternatively, instead of stopping painting other dots until a certain dot reaches the highest level, a transition to painting another dot may be made before a certain dot reaches the highest level. In this case, too, a dither pattern is prepared on the image forming apparatus side so that a certain dot reaches the highest level so as to shift to painting on another dot, the dither pattern is held, and the halftone dot image is stored. Used when forming dots in a part. As a result, it is possible to improve the granularity of an engine having a high dot reproducibility at a low density. Therefore, it is desirable that the dot formation using such a dither pattern is applied to an engine having low dot reproducibility and high dot reproducibility.

  Other components that are not particularly described are configured and function the same as in the first embodiment.

FIG. 1 is a diagram schematically illustrating an overall configuration of an image forming system according to an embodiment of the present invention. 1 is a schematic configuration diagram illustrating an outline of an internal configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating a control system of the image forming apparatus according to the embodiment of the present invention, mainly focusing on a main controller. FIG. 2 is a block diagram illustrating details of an image processing unit (IPU) of the image forming apparatus according to the embodiment of the present invention. FIG. 4 is a diagram showing a growth process of a digital halftone screen when using a collective dither. FIG. 4 is a diagram illustrating a growth step by pulse width modulation in an example of the present invention. FIG. 9 is a diagram illustrating a state in which a halftone dot shape correction is performed by combining a digital halftone screen growth step and a pulse width modulation growth step. FIG. 9 is a diagram showing a comparison of thin line reproducibility using a dispersed dither pattern (a) and a multi-valued set dither pattern (b). It is a figure showing a growth process of a multivalued dot. FIG. 9 is a diagram illustrating an example in which a pattern in which different growth orders in a cell are set for each density level is created, and the density is set according to the pattern. FIG. 9 is a diagram showing an example of a dither matrix pattern expressing a maximum of 16 gradations in a conventional example. FIG. 9 is a functional block diagram showing a masking process performed conventionally. FIG. 9 is a diagram illustrating an example of a threshold processing that has been conventionally performed.

Explanation of reference numerals

20 Main controller 49 IPU
68 CPU
69 ROM
70 RAM
PC1 to PC3 Personal Computer PR Image Forming Apparatus

Claims (21)

In a multi-level digital halftone screening method for reproducing multi-step density with dots,
A first dot growing step of growing dots in a single color portion;
A second dot growing step of growing dots in the mixed color portion;
Is set,
A halftone screening method, characterized in that dots are grown by using the first and second dot growth steps selectively for the single color portion and the mixed color portion.   2. The halftone screening method according to claim 1, wherein in the first dot growing step, the dots are grown so that the dots are enlarged and the white paper portion is eliminated.   2. The halftone screening method according to claim 1, wherein in the second dot growing step, the dots are grown so that the overlap between the dots is minimized.   4. The halftone screening method according to claim 1, wherein the dot growth is performed by modulating a pulse width.   5. The halftone screening method according to claim 4, wherein the dots are deformed so that the dot areas before and after the dot growth correction become equal.   4. The halftone screening method according to claim 1, wherein in the first growing step, the halftone dots are corrected to have a shape closer to a rectangle.   4. The halftone screening method according to claim 1, wherein in the second growth step, halftone dots of other colors are corrected to have a shape that reduces overlap. 5. . In a multi-level digital halftone screening method using dots capable of reproducing multi-level densities,
A third dot growing step of growing dots in a binary image portion including a character portion or a thin line portion;
A fourth dot growing step of growing dots in a halftone image portion including a photographic portion or a graphics portion;
Is set,
A halftone screening method characterized in that dots are grown in the binary image portion or the halftone dot image portion by selectively using the third and fourth growth steps.   9. The halftone screening method according to claim 8, wherein in the third dot growing step, a dot pattern having a low density level is grown first.   9. The halftone screening method according to claim 8, wherein the fourth dot growing step reduces a dot shape difference between successive densities.   11. The halftone screening method according to claim 10, wherein, in the fourth dot growth step, the dot growth order is the same for each density level, but the relationship between the density levels is different.   12. The halftone screening method according to claim 10, wherein the fourth dot growing step includes a step of stopping applying another dot until a certain dot reaches a maximum level.   13. The halftone screening method according to claim 12, wherein the fourth dot growing step is applied to an engine having low dot reproducibility at low density.   12. The halftone screening method according to claim 10, wherein the fourth dot growing step includes a step of shifting to applying to another dot before a certain dot reaches the highest level.   15. The halftone screening method according to claim 14, wherein the fourth dot growing step is applied to an engine having low density dot reproducibility.   A computer program comprising a procedure for executing each step of the halftone screening method according to any one of claims 1 to 15 on a computer.   17. A recording medium, wherein the computer program according to claim 16 is read by a computer and recorded so as to be executable.   An image processing apparatus comprising: a control unit configured to execute each step of the halftone screening method according to any one of claims 1 to 15 based on a computer program. In an image processing apparatus that performs multi-level digital halftone screening to reproduce multi-step density with dots,
First dot growing means for growing dots in a single color portion;
Second dot growing means for growing dots in the mixed color portion;
Control means for switching the first and second dot growth means between the single color portion and the mixed color portion;
An image processing apparatus for growing dots by performing different processes in the single color portion and the mixed color portion. In an image processing apparatus that performs multi-level digital halftone screening that reproduces multi-step densities with dots,
Third dot growing means for growing dots in a binary image portion including a character portion or a thin line portion;
Fourth dot growing means for growing dots in a halftone image portion including a photograph portion or a graphics portion;
Control means for switching the third and fourth growth steps between the character / thin line portion and the photograph / graphics portion;
An image processing apparatus comprising: performing different processing on the binary image section and the halftone image section to grow dots. An image processing apparatus according to any one of claims 18 to 20,
Image forming means for forming a visible image on a recording medium based on the image data processed by the image processing device,
An image forming apparatus comprising:

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