JP2011015241A - Method and apparatus for processing image, program, recording medium and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the degradation in image quality in composite type halftone processing for switching error diffusion processing and dither processing corresponding to a gradation part.SOLUTION: Whether an original image (input image) is a dot original or a continuous tone original is determined. When it is the dot original, the threshold matrix set of a first size is selected from the number of dots and an output resolution. When it is the continuous tone original, the threshold matrix set of a second size is selected from the output resolution. Whether or not a gradation is a dither processing part is discriminated. When it is the dither processing part, a dither processing threshold in the selected matrix set is used to perform the dither processing. When it is an error diffusion processing part, an error diffusion processing threshold in the selected matrix set is used to perform the error diffusion processing.

Description

  The present invention relates to an image processing method, an image processing apparatus, a program, a recording medium, and an image forming apparatus.

  As an image forming apparatus such as a printer, a facsimile machine, a copying apparatus, and a multifunction machine of these, for example, a liquid ejection apparatus including a recording head composed of a liquid ejection head (droplet ejection head) that ejects liquid droplets of a recording liquid (liquid) As a liquid while transporting a medium (hereinafter also referred to as “paper”, but the material is not limited, and a recording medium, a recording medium, a transfer material, recording paper, etc. are also used synonymously). The recording liquid (hereinafter referred to as ink) is attached to a sheet to form an image (recording, printing, printing, and printing are also used synonymously). Some image forming apparatuses perform image formation using an electrophotographic process.

  The liquid discharge type image forming apparatus means an apparatus for forming an image by discharging liquid onto a medium such as paper, thread, fiber, fabric, leather, metal, plastic, glass, wood, ceramics, etc. “Image formation” means not only giving an image having a meaning such as a character or a figure to a medium, but also giving an image having no meaning such as a pattern to the medium. It also includes a device for forming metal wiring. The liquid is not particularly limited as long as it is a liquid that can form an image.

  In such an image forming apparatus, dither processing or error diffusion processing is used as halftone processing for converting a multi-tone image into output data having a multi-value number (three or more values) smaller than input data using a threshold value. It has been.

  In this case, when a copy process is performed in an apparatus equipped with an image reading device that reads an original image, all input image data is captured as image data. Therefore, in order to obtain a high-quality output image, the image is once processed. The image area is separated and correction processing is performed according to each element (whether it is a character, continuous tone, or halftone dot) of the input document (Patent Documents 1 to 3).

  However, in a copier (copying apparatus) that is supposed to be used stand-alone, image processing must be performed by a controller installed in the copying apparatus, and thus the weight of error diffusion processing is a problem that cannot be ignored. In order to solve this problem, there is known a method of switching to distributed dither processing in a shadow portion where the granularity is not a problem (Patent Document 4).

Japanese Patent No. 3164400 Japanese Patent Laid-Open No. 06-176144 JP 2006-157331 A JP 2008-162197 A

  However, as described above, when composite halftone processing combining error diffusion processing and dispersion dither is performed, depending on the processed document image (halftone or continuous tone), the halftone processing of the document interferes with halftone processing. It has been confirmed that moiré occurs and the problem that the graininess of the copy image deteriorates occurs.

  The present invention has been made in view of the above problems, and reduces image quality degradation such as moire and blurring in composite halftone processing while reducing the computation load caused by switching halftone processing. With the goal.

In order to solve the above problems, an image processing method according to the present invention includes:
In a halftone processing method for converting a multi-tone image into multi-valued output data of three or more values smaller than input data using a threshold value,
For the highlight portion expressed by only the minimum output value of the multi-value number and the shadow portion expressed only by the maximum output value of the multi-value number, a pair comparison of the input value and the threshold matrix is performed. Apply dithering to quantize,
A quantization error around the pixel of interest is added to the middle portion that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value. Applying an error diffusion process that performs quantization,
The size of the dispersion threshold matrix used for error diffusion processing and dither processing is switched according to the halftone dot pitch of the input image or the presence or absence of halftone dots.

Here, an image determination process for determining whether or not the input image is a halftone image is performed on the input image,
If the input image is a halftone dot image as a result of the determination process, the next first-size distributed threshold matrix is displayed. If the input image is not a halftone image, the second second size distributed threshold matrix is displayed. Using the matrix as the error diffusion and dither threshold respectively
The first size = an integer closest to (halftone pitch / output resolution pitch),
The second size = an integer closest to (1 mm / output resolution pitch),
It can be set as this.

  The image determination process may be determined based on an external input or a frequency characteristic of the input image.

  The threshold matrix used for quantization determination in the error diffusion processing has a size of g × h (both g and h are integers of 2 or more), the threshold matrix size and threshold arrangement order, and the threshold matrix in dither processing. The size and the threshold arrangement order can be made common.

Also,
When the input value is 0 or outside the gradation range where error diffusion processing is performed, the quantization error value of the pixel of interest can be set to 0 regardless of the result of dither processing.

An image processing apparatus according to the present invention includes:
In an image processing apparatus that performs halftone processing for converting a multi-tone image into multi-valued output data of three or more values smaller than input data using a threshold value,
For the highlight portion expressed by only the minimum output value of the multi-value number and the shadow portion expressed only by the maximum output value of the multi-value number, a pair comparison of the input value and the threshold matrix is performed. Apply dithering to quantize,
A quantization error around the pixel of interest is added to the middle portion that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value. Applying an error diffusion process that performs quantization,
A configuration is provided that includes means for switching the size of a dispersion threshold matrix used for error diffusion processing and dither processing according to the halftone dot pitch of the input image or the presence or absence of halftone dots.

The program according to the present invention is:
In a program for causing a computer to perform halftone processing for converting a multi-tone image into multi-valued output data of three or more values smaller than input data using a threshold value,
For the highlight portion expressed by only the minimum output value of the multi-value number and the shadow portion expressed only by the maximum output value of the multi-value number, a pair comparison of the input value and the threshold matrix is performed. Apply dithering to quantize,
A quantization error around the pixel of interest is added to the middle portion that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value. Applying an error diffusion process that performs quantization,
The computer is configured to perform processing for switching the size of the dispersion threshold value matrix used for error diffusion processing and dither processing according to the halftone dot pitch of the input image or the presence or absence of halftone dots.

  The recording medium according to the present invention records the program according to the present invention.

An image forming apparatus according to the present invention includes:
In an image forming apparatus that forms an image by performing halftone processing that converts a multi-tone image into multi-valued output data that is three or more values smaller than input data using a threshold value,
For the highlight portion expressed by only the minimum output value of the multi-value number and the shadow portion expressed only by the maximum output value of the multi-value number, a pair comparison of the input value and the threshold matrix is performed. Apply dithering to quantize,
A quantization error around the pixel of interest is added to the middle portion that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value. Applying an error diffusion process that performs quantization,
A configuration is provided that includes means for switching the size of a dispersion threshold matrix used for error diffusion processing and dither processing according to the halftone dot pitch of the input image or the presence or absence of halftone dots.

  According to the image processing method, the image processing apparatus, the program, the storage medium, and the image forming apparatus according to the present invention, the highlight portion expressed only by the minimum output value among the multi-value numbers and the maximum output among the multi-value numbers. For the shadow part expressed only by the value, a dithering process that performs quantization by pairwise comparison of the input value and the threshold matrix is applied, and the output value is one or more steps larger than the minimum output value, and the maximum For the middle part expressed by an output value that is smaller by one or more steps than the output value of the input pixel, an error diffusion process is performed in which quantization is performed by adding a quantization error around the pixel of interest, and the halftone dot pitch or halftone Since the configuration is such that the size of the dispersion threshold matrix used for error diffusion processing and dither processing is switched according to the presence or absence of points, the calculation load by switching halftone processing is reduced, and combined halftone processing is performed. It is possible to reduce the deterioration of the kick image quality.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an external perspective view illustrating an example of an image forming apparatus according to the present invention including an image processing apparatus according to the present invention and a program for performing an image processing method according to the present invention. It is side surface explanatory drawing of the mechanism part of the apparatus. It is a plane explanatory drawing similarly. It is a block diagram which shows the outline | summary of the control part of the apparatus. It is explanatory drawing with which it uses for description of the flow of the image processing in a general copy mode. It is explanatory drawing with which it uses for description of a general dither process. It is explanatory drawing with which it uses for description of generation | occurrence | production of a moire when the basics of a halftone image and a dither matrix correspond. It is explanatory drawing with which it uses for description of a general error diffusion process. It is explanatory drawing which shows an example of the output dot pattern of a middle gradation part. It is explanatory drawing with which it uses for description of multi-value dither. It is explanatory drawing with which it uses for description of the input image in a highlight part, the image after an error diffusion process, and the image after a dither process. It is explanatory drawing with which it uses for description of the switching of the gradation area and halftone process in this invention. It is explanatory drawing with which it uses for description of the pattern tendency of a threshold value matrix and image degradation in an error diffusion process. It is an enlarged explanatory view of a halftone image. It is explanatory drawing with which it uses for description of the number of lines in a gradation expression, and a horizontal pixel pitch. It is explanatory drawing with which it uses for description of the image when the dispersibility by a threshold value matrix is improved, and when it is insufficient. It is a flowchart with which it uses for description of the whole flow of the halftone process which concerns on this invention.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. An example of an image processing apparatus according to the present invention for performing an image processing method according to the present invention and an image forming apparatus according to the present invention including a program will be described with reference to FIGS. 1 is an external perspective view of the image forming apparatus, FIG. 2 is a side view of the mechanism of the apparatus, and FIG. 3 is a plan view of the same.

  As shown in FIG. 1, the image forming apparatus includes an image reading unit (scanner unit) 2 that reads an image on an upper part of an apparatus main body 1 that performs image formation. The apparatus main body 1 is detachably mounted with a paper feed cassette 3 for stocking paper to be fed to the mechanism unit, and a paper discharge tray for stocking paper to be discharged after an image is formed above the paper feed cassette 3. 4 is installed. Further, on the front side of the apparatus main body 1, there is a cartridge mounting portion 5 for mounting an ink cartridge, and an operation / display portion (operation panel) 6 for inputting various operation signals and displaying display information is arranged.

  As shown in FIGS. 2 and 3, the main body 1 has a carriage 33 with main and slave guide rods 31 and 32 which are guide members horizontally mounted on the left and right side plates 21 </ b> A and 21 </ b> B of the main body 1. It is slidably held in the scanning direction, and is moved and scanned in the direction indicated by the arrow (carriage main scanning direction) via a timing belt by a main scanning motor (not shown).

  The carriage 33 is provided with recording heads 34a and 34b composed of liquid ejection heads for ejecting ink droplets of yellow (Y), cyan (C), magenta (M), and black (K). The “recording head 34” is arranged in a sub-scanning direction orthogonal to the main scanning direction, and the ink droplet ejection direction is directed downward.

  Each of the recording heads 34 has two nozzle rows. One nozzle row of the recording head 34a has black (K) droplets, the other nozzle row has cyan (C) droplets, and the recording head 34b has one nozzle row. One nozzle row ejects magenta (M) droplets, and the other nozzle row ejects yellow (Y) droplets. As the recording head 34, a recording head having a nozzle row of each color in which a plurality of nozzles are arranged on one nozzle surface can be used.

  Further, the sub tanks 35a and 35b, which are sub tanks as the second ink supply unit for supplying ink of each color corresponding to the nozzle row of the recording head 34, are referred to as the “sub tank 35” when they are not distinguished. ) Is installed. In the sub tank 35, ink cartridges (main tanks) 10y, 10m, 10c, and 10k of various colors that are detachably attached to the cartridge loading unit 4 are supplied from the ink supply tubes 36 of the respective colors by the supply pump unit 24. The recording liquid is replenished and supplied.

  On the other hand, as a paper feeding unit for feeding the papers 42 stacked on the paper stacking unit (pressure plate) 41 of the paper feeding tray 2, a half-moon roller (feeding) that separates and feeds the papers 42 one by one from the paper stacking unit 41. A separation pad 44 made of a material having a large friction coefficient is provided facing the paper roller 43) and the paper feed roller 43, and the separation pad 44 is urged toward the paper feed roller 43 side.

  In order to feed the paper 42 fed from the paper feeding unit to the lower side of the recording head 34, a guide member 45 for guiding the paper 42, a counter roller 46, a transport guide member 47, and a tip pressure roller. And a conveying belt 51 that electrostatically attracts the fed paper 42 and conveys it at a position facing the recording head 34.

  The transport belt 51 is an endless belt, and is configured to wrap around the transport roller 52 and the tension roller 53 and circulate in the belt transport direction (sub-scanning direction). Further, a charging roller 56 that is a charging unit for charging the surface of the transport belt 51 is provided. The charging roller 56 is disposed so as to come into contact with the surface layer of the transport belt 51 and to rotate following the rotation of the transport belt 51. The transport belt 51 rotates in the belt transport direction when the transport roller 52 is rotationally driven through timing by a sub-scanning motor (not shown).

  Further, as a paper discharge unit for discharging the paper 42 recorded by the recording head 34, a separation claw 61 for separating the paper 42 from the conveying belt 51, a paper discharge roller 62, and a spur 63 that is a paper discharge roller. And a paper discharge tray 3 below the paper discharge roller 62.

  A duplex unit 71 is detachably mounted on the back surface of the apparatus body 1. The duplex unit 71 takes in the paper 42 returned by the reverse rotation of the conveyance belt 51, reverses it, and feeds it again between the counter roller 46 and the conveyance belt 51.

  Further, a maintenance / recovery mechanism 81 for maintaining and recovering the nozzle state of the recording head 34 is disposed in the non-printing area on one side of the carriage 33 in the scanning direction. The maintenance / recovery mechanism 81 includes cap members (hereinafter referred to as “caps”) 82a and 82b (hereinafter referred to as “caps 82” when not distinguished from each other) for capping the nozzle surfaces of the recording head 34, and nozzle surfaces. A wiper member (wiper blade) 83 for wiping the recording medium, an empty discharge receiver 84 for receiving liquid droplets when performing empty discharge for discharging liquid droplets that do not contribute to recording in order to discharge the thickened recording liquid, and a carriage And a carriage lock 87 for locking 33. A waste liquid tank 100 for storing waste liquid generated by the maintenance recovery operation is mounted on the lower side of the head recovery mechanism 81 in a replaceable manner with respect to the apparatus main body.

  Further, in the non-printing area on the other side of the carriage 33 in the scanning direction, there is an empty space for receiving liquid droplets when performing empty discharge for discharging liquid droplets that do not contribute to recording in order to discharge the recording liquid thickened during recording or the like. A discharge receiver 88 is disposed, and the idle discharge receiver 88 includes an opening 89 along the nozzle row direction of the recording head 34.

  In this image forming apparatus configured as described above, the sheets 42 are separated and fed one by one from the sheet feed tray 2, and the sheet 42 fed substantially vertically upward is guided by the guide 45, and includes the transport belt 51 and the counter. It is sandwiched between the rollers 46 and conveyed, and the leading end is guided by the conveying guide 37 and pressed against the conveying belt 51 by the leading end pressing roller 49, and the conveying direction is changed by approximately 90 °. Then, the sheet 42 is attracted to the charged conveyance belt 51, and the sheet 42 is conveyed in the sub-scanning direction by the circular movement of the conveyance belt 51.

  Therefore, by driving the recording head 34 according to the image signal while moving the carriage 33, ink droplets are ejected onto the stopped paper 42 to record one line, and after the paper 42 is conveyed by a predetermined amount, Record the next line. Upon receiving a recording end signal or a signal that the trailing edge of the paper 42 has reached the recording area, the recording operation is finished and the paper 42 is discharged onto the paper discharge tray 3.

  When performing the maintenance and recovery of the nozzles of the recording head 34, the nozzle 33 performs the suction from the nozzles by moving the carriage 33 to a position facing the maintenance and recovery mechanism 81 which is the home position and performing capping by the cap member 82. By performing a maintenance and recovery operation such as idle ejection for ejecting droplets that do not contribute to image formation, image formation by stable droplet ejection can be performed.

Next, an outline of the control unit of the image forming apparatus will be described with reference to FIG. This figure is an overall block diagram of the control unit.
The control unit 500 includes a CPU 511 that also serves as a unit that performs control related to halftone processing according to the present invention that controls the entire apparatus, and a program that includes a program that performs halftone processing according to the present invention executed by the CPU 511, ROM 502 for storing other fixed data, RAM 503 for temporarily storing image data and the like, rewritable nonvolatile memory 504 for retaining data even while the apparatus is powered off, and various signals for image data ASIC 505 for processing input / output signals for controlling image processing for processing, rearrangement, etc. and other control of the entire apparatus, and scanner control unit 516 for controlling scanner means 2 are provided.

  Further, a print control unit 508 including a data transfer unit for driving and controlling the recording head 34 and a driving signal generating unit, a head driver (driver IC) 509 for driving the recording head 34 provided on the carriage 33 side, An AC bias is applied to the charging roller 56, a main scanning motor 554 that moves and scans the carriage 33, a sub-scanning motor 555 that rotates the conveyance belt 51, a motor drive unit 510 that drives the maintenance / recovery motor 556 of the maintenance / recovery mechanism 81. An AC bias supply unit 511 and the like are provided.

  The control unit 500 is connected to an operation panel 514 for inputting and displaying information necessary for the apparatus.

  The control unit 500 has an I / F 506 for transmitting and receiving data and signals to and from the host side. From the host 600 side such as an information processing device such as a personal computer and an imaging device such as a digital camera, a cable or The data is received by the I / F 506 via the network. Further, reading information of the original image from the scanner 2 is taken in via the scanner control unit 516.

  The CPU 501 of the control unit 500 reads and analyzes the print data in the reception buffer included in the I / F 506 for the reception data from the host side, and performs necessary image processing, data rearrangement processing, and the like in the ASIC 505. The image data is transferred to the print control unit 508. Note that generation of dot pattern data for image output is performed by the printer driver 601 on the host 600 side. As for the read data of the original image from the scanner 2, necessary input correction processing and output processing are performed as will be described later, image data is generated, and transferred to the print control unit 508.

  The print control unit 508 transfers the above-described image data to the head driver 509 as serial data, and outputs a transfer clock, a latch signal, a control signal, and the like necessary for transferring the image data and confirming the transfer to the head driver 509. In addition to the above, it includes a D / A converter that performs D / A conversion on the drive pulse pattern data stored in the ROM 502, and a drive signal generation unit including a voltage amplifier, a current amplifier, and the like. A drive signal composed of a plurality of drive pulses is output to the head driver 509.

  The head driver 509 selectively selects a drive pulse that constitutes a drive signal provided from the print control unit 508 based on image data corresponding to one line of the print head 34 that is input serially, and drops droplets on the print head 34. The recording head 7 is driven by applying it to a driving element (for example, a piezoelectric element) that generates energy to be discharged. At this time, by selecting a driving pulse constituting the driving signal, for example, dots having different sizes such as a large droplet, a medium droplet, and a small droplet can be sorted.

  The I / O unit 513 acquires information from various sensor groups 515 mounted on the apparatus, extracts information necessary for controlling the printer, and print control unit 508, motor control unit 510, AC bias supply unit Used to control 511. The sensor group 515 includes an optical sensor for detecting the position of the paper, a thermistor for monitoring the temperature in the machine, a sensor for monitoring the voltage of the charging belt, an interlock switch for detecting opening and closing of the cover, and the like. The I / O unit 513 can process various sensor information.

Next, an image processing method according to the present invention in this image forming apparatus will be described.
First, generally, the flow of image processing from reading an original image with an image reading device (scanner) to outputting it with an image forming device (printer unit) is as shown in FIG. On the other hand, in the input correction processing unit 401, the scanner γ correction unit 411, the RGB → YUV conversion unit 412, the smoothing unit 413, the edge enhancement unit 414, the YUV → RGB conversion unit 415, the background removal unit 416, and the edge enhancement unit 417. Then, the input document image data (input image) is corrected.

  In other words, in the case of a copy in which a manuscript image is copied and printed out, all input image data is captured as “image” data. At this time, the scanner performance, the state of the input manuscript (recording quality, dirt and damage, etc.) , Surface gloss) and the like, the data is captured in a state of deterioration from the original. Therefore, the input correction processing unit 401 decomposes the image data captured from the scanner into object elements by image area separation, and applies various correction processes such as edge enhancement, background removal, and color correction. The object element indicates whether the input image data is classified as “character”, “thin line”, “image”, or “graphics”.

  In the output processing unit 402 for inputting the corrected image data output from the input correction processing unit 401, a CMM processing unit 421 that performs conversion into a color space (RGB color system → CMY color system), BG / UCR processing unit 422 that performs black generation / under color removal from values, scaling processing unit 423 that performs enlargement processing according to resolution, and printer γ correction unit that performs input / output correction reflecting device characteristics and user preferences 424, a necessary process is performed by the halftone processing unit 425 including a dither matrix that replaces the image data with the dot pattern arrangement ejected from the image forming apparatus. Although not shown, the print image data is obtained by the halftone process. A raster pattern that divides the dot pattern data into data for each scan and further develops the data in accordance with the position of each nozzle for recording. Performing grayed, and sends the print control unit 508 (printer output).

  In the output processing unit 402, the B / UCR processing unit 422, the printer γ correction unit 423, and the halftone processing unit 425 perform processing according to the separation result of the image area separation unit 218.

  By the way, the result of the image area separation by the image area separation unit 418 becomes important information in performing the processing of the input correction processing unit 401 and the output processing unit 402 described above. As a result, an abnormal image is generated, so that a high processing capability and performance are required for the image area separation portion.

  Whether image separation can be performed accurately at high speed depends on the performance of the image processing controller. When an image processing controller that does not have sufficient processing capability is used, pattern matching, frequency analysis, etc. The image area separation calculation that makes full use of the image processing is heavy and results in a significant reduction in throughput. On the other hand, having an image processing controller with sufficient processing capability is costly for image forming apparatuses and image processing apparatuses. Cause the problem of high.

  In view of this, an image forming apparatus designed to reduce costs employs a configuration in which image area separation is omitted and only minimum correction processing (weak smoothing and background removal) is applied. The tone processing unit 425 also compensates for low input correction processing by error diffusion processing as halftone processing.

  As shown in FIG. 6, the dithering process compares the input multi-value image data shown in FIG. 6A with the threshold value of the dither matrix shown in FIG. This is a process of replacing with a dot pattern arrangement. Since the dither processing determines the ON / OFF of the dots by comparing the dither matrix with the input data, the processing is very light and a small amount of memory is required for the calculation.

  However, when dither processing is employed in the copy mode, moire occurs due to interference with the halftone dot pattern of the document. That is, as shown in FIG. 6, no matter how much data exists on the document image, no dot is generated unless the threshold value of the dither matrix is exceeded. When the data distribution is configured in a regular arrangement like a halftone document, the dot loss due to the dither processing may have regularity and appear as moire. For example, when halftone dot original data as shown in FIG. 7A is dithered with a dither matrix having a line tone, as shown in FIG. Moire appears in the output image.

  On the other hand, the error diffusion process performs, for example, the process shown in FIG. 8, and the calculation is performed by reflecting the peripheral quantization error in the ON / OFF determination of the dots. Even if the difference occurs, the difference is reflected as the ease of occurrence of dots in the surrounding pixels. As a result, missing dots are compensated for, and excellent image reproducibility is obtained. However, on the other hand, the number of operations is large and the influence on the processing speed becomes large.

  In this case, if the image processing apparatus is configured on the host PC and the image forming apparatus is simply used as a printer by improving the computer processing capability, it cannot be said that the error diffusion process itself is a very heavy process. However, in an image forming apparatus as a copier that is assumed to be used stand-alone, it is necessary to perform processing by a controller mounted on the apparatus body, and the weight of error diffusion processing is a problem that cannot be ignored.

Therefore, in the composite halftone processing according to the present invention, first, the calculation load is reduced by switching between dither processing and error diffusion processing for each gradation section.
Here, the gradation section is represented by a highlight portion represented by only the minimum output value among the multi-valued number, a shadow portion represented by only the maximum output value among the number of values, and the minimum output value. The output value is divided into middle parts that are expressed by output values that are one or more steps larger and that are one or more steps smaller than the maximum output value.

  Then, a dither process that performs quantization by paired comparison of the input value and the threshold matrix is applied to the highlight part and the shadow part, and a quantization error around the pixel of interest is added to the middle part to add a quantum error. An error diffusion process is applied.

  First, FIG. 9 shows an example of an output dot pattern of a shadow gradation portion by binary error diffusion processing. However, since dots are generated densely to express a dark gradation, the paper surface is filled up. Interference (moire) with an original pattern (such as a halftone original) is also less noticeable.

  Also, in an image forming apparatus capable of multi-value output that switches between output dot size and light and dark ink, gradations are expressed by replacing dots of different sizes and densities, making moire less noticeable.

  Next, with reference to FIG. 10, a description will be given of a method for expressing multi-values using three types of large, medium, and small dot sizes and dither processing. Tone expression is performed by arranging dots according to the threshold order of the dither matrix. In the following, a multi-value method for switching dot sizes will be described, but the same applies to a multi-value method for switching colorants having different densities.

  In this case, when the gradation section assigned to the small dots is “0 <small dot section ≦ gradation level n”, the paper surface is filled with small dots at the gradation level n. At gradation level n + 1, any of the filled small dots is replaced with medium dots, and the ratio of medium dots increases as the gradation rises (the same is true for switching between medium dots and large dots). is there.).

  If halftone processing with multiple values as described above is used, dots of any size will always be assigned to the shadow side gradation level where large dots are applied, except for the original white background. Moire is buried in the density of dots.

  Further, in the highlight portion where only the minimum dots are used, even if there is regularity of the original image, it is difficult to completely reproduce the original image quality by either error diffusion processing or dither processing. This is because the error diffusion process cannot sufficiently accumulate errors for data having a small input data and a narrow range (for example, data such as small dots shown in FIG. 12A) (FIG. 12). (B)) In the dither processing, a pattern that is not synchronized with the dither threshold matrix is forcibly turned off (FIG. 12C), and therefore, a narrower image is likely to be lost during output.

  In other words, since the influence of moiré on image quality is reduced at gradations where the dispersibility of individual dots is not noticeable, it can be used even with dither processing that is weak against moiré. Either dither processing or error diffusion processing can be used. It is possible to use dither processing even in a highlight portion where there is no great difference in image quality even when using.

  Therefore, in the present invention, multi-level dither processing is applied to the highlight portion and the shadow portion, and multi-level error diffusion processing is applied to the gradation between the highlight portion and the shadow portion. We are trying to make it.

  For example, the example shown in FIG. 12A is a four-value case, and the example shown in FIG. 12B is a five-value case. In the case of five values, it is divided into a middle section 1 and a middle section 2.

  At this time, the calculation of the error value is omitted (error value = 0) in the case where the input value is 0 and the gradation interval to which the dither processing is applied. When the input is 0, it is a portion that originally has no image, and therefore, unnecessary calculation processing is omitted, and it is possible to prevent the image quality from being deteriorated due to generation of a dot that is not in the original image.

  In addition, the input value of the target pixel itself is sufficiently large in the shadow portion, and the influence is small even if the quantization error is propagated to the peripheral pixels, and the accumulated error itself is small in the highlight portion. Since the influence is difficult to occur even if it is propagated, the calculation of the error value is omitted even in the highlight portion and the shadow portion to which the dither processing is applied. This can further increase the speed.

  Further, when different halftone processing is used in continuous gradations, the continuity of the dot pattern may be interrupted at the processing switching portion, and the tone jump may be noticed.

  As a method for solving such a problem, the threshold values used in the error diffusion process are made into a matrix, the same size as the dither matrix in the dither process in which switching is performed in the middle part and the shadow part, and the regularity of the threshold value allocation is the same To make tone jump inconspicuous.

  This is because the regularity of the threshold matrix affects the dispersibility of the error diffusion processing, and the manifestation state of the dither regularity can be controlled according to the amplitude of the threshold. By aligning the regularity of the dither matrix and threshold matrix of the dither processing to be switched, it is possible to form the same dot arrangement near the switching gradation as when dither processing is performed, and the dither processing pattern Will be smoothly connected.

  However, when the pattern tendency of the threshold value matrix becomes strong in this way, the interference with the original halftone dot, which is originally improved in the error diffusion process, becomes a problem again. By using a highly random dispersion matrix, it is possible to prevent moiré that generates a specific interference pattern, but since dots are randomly arranged, dots may be lost where there should originally be halftone dots. Yes, the graininess deteriorates and the image becomes blurred. For example, when an error diffusion process using a dispersion matrix threshold is performed on the halftone image shown in FIG. 13A, the image is as shown in FIG.

  This requires a certain amount of matrix size (preferably about 256 × 256 pixels) in order to ensure randomness, and when dots are dispersed using the entire matrix size, halftone dots are formed. This is because the probability of the dot being turned on at the power position is reduced. Of course, in the case of a continuous tone original that is not a halftone dot, a smoother image can be reproduced if the regularity is not visible. Therefore, the larger the threshold mask size, the better.

  Therefore, in the present invention, dispersion threshold matrices having different sizes are subjected to error diffusion processing depending on whether the original image (input) is a halftone image or not, here, whether it is a halftone image or a continuous tone image. The graininess is improved by applying the threshold value and the threshold value of the dither processing.

  FIG. 14 is a partially enlarged explanatory view of an original halftone image at 600 dpi (however, K plate, screen angle 45 °). A halftone original (halftone image) expresses gradation by gradually increasing the dots arranged at a specific pitch. When the pitch to be arranged is calculated with an output resolution of 600 dpi, it is as shown in FIG. In general printing, 175 lines are often used. In this case, the area covered by one halftone dot is approximately 5 × 5 pixels. In order to replace the halftone dots with output dots without excess or deficiency, a threshold matrix covering 5 × 5 pixels is sufficient. If the size is further increased, there is a higher possibility that the halftone dot portion is missing due to the property of dispersing the dots (FIG. 16A). Further, when the value becomes smaller, the dispersibility becomes lower, and a geometric texture is likely to be generated in the error diffusion process (FIG. 16B).

Therefore, when the original image (input image) is a halftone image, the following distributed threshold matrix of the first size is used as a threshold value for error diffusion processing and dither processing.
1st size = an integer closest to (halftone pitch / output resolution pitch)

For example, when outputting a 175-line halftone original (image) with an image forming apparatus having an output resolution of 600 dpi,
Threshold matrix size = (25.4 / 175 × √2) / (25.4 / 600)
= 4.88473
Thus, a matrix having a matrix size of 5 pixels, which is an integer closest thereto, is used.

  On the other hand, in a continuous tone image in which interference with a halftone dot need not be considered, the dispersibility of dots is also lowered in the threshold value matrix defined by the first size.

Therefore, when the input image is a continuous tone image, the following distributed threshold matrix of the second size is used as a threshold value for error diffusion processing and dither processing.
2nd size = integer closest to (1mm / output resolution pitch)

For example, when outputting with an image forming apparatus having a continuous tone document (image) output resolution of 600 dpi,
Threshold matrix size = 1 / (25.4 / 600)
= 23.622
A matrix having a matrix size of 24 pixels, which is an integer closest thereto, is used.

  For a halftone image, by setting the matrix size close to the halftone dot pitch, even if dispersion processing is performed depending on the characteristics of the matrix, it will be distributed within the range where the halftone dot grows, and the halftone dot of the original Reproducibility is improved, and moire and blurring due to missing halftone dots are improved.

  For continuous tone images, it is basically unnecessary to consider moiré, and a larger threshold matrix is better for improving graininess. That is, the more the dots are dispersed uniformly and more uniformly, the better the graininess. Therefore, the larger the size, the greater the degree of freedom of dot arrangement.

  However, as long as it is actually mounted in an apparatus or system, there is a size limitation, and it is difficult to mount a large threshold matrix due to the calculation speed and memory capacity. In this case, depending on the quality of the threshold matrix, a texture may be generated in the threshold matrix size cycle.

  Therefore, when a continuous tone manuscript is set, by setting the threshold mask size to a second size of 1 mm or less where human visual sensitivity falls, periodic textures are not noticeable even if the quality of the threshold matrix is somewhat poor. Can be done.

  Switching between the first size threshold matrix and the second size threshold matrix is performed according to whether or not the input image is a halftone image (halftone image or continuous tone image) as described above.

  Here, whether the input image is a halftone image or a continuous tone image can be determined (determined) based on the frequency characteristics of the input image. This is because, in the case of a halftone dot image, the periodic component of the halftone dot pitch protrudes, and on the contrary, if it is a continuous tone image, a clear periodicity hardly occurs. Note that when a periodic image is originally captured, even a continuous tone image has the same characteristics as a halftone image, and may be processed as a halftone image. In addition, since the number of halftone lines can be analyzed from the periodicity of the frequency characteristics, the input image is subjected to Fourier transform, etc., and the frequency characteristics are automatically analyzed to perform halftone image processing (or line number designation). The continuous tone image processing may be switched.

  Also, if the number of lines is lower than 175 lines, it is possible to easily determine whether or not the image is a halftone image even with the naked eye. Therefore, halftone image processing is performed by an input from the operation panel 514 or the like by the operator's visual determination. It is also possible to switch the continuous tone image processing.

  In this case, a measuring instrument such as a line number meter is required for measuring the number of halftone dots. However, since the number of halftone lines generally used is limited, 175 lines are printed finely, You may make it select and classify | categorize roughly like 100 lines.

  Furthermore, even if the image data is not input from the scanner but is image data created on application software, for example, the image may be thinned out in the form of halftone dots by functions such as transparency processing and hatching processing.

  In such a case, the image has regularity like a halftone dot image, so special processing such as transparency and hatching is applied to each application and each image object such as a figure or photograph. It is also possible to switch between halftone image processing and continuous tone image processing by determining whether or not it has been performed (for example, determination based on frequency analysis or information from application software).

Next, the overall flow of the above-described halftone processing will be described with reference to the flowchart of FIG.
First, when document data is set, the halftone analysis unit automatically analyzes the document image (input image) as described above to determine whether the document is a halftone document or a continuous tone document (or captures external input). To determine.)

  If the original is a halftone dot document, a threshold value matrix set of the first size is selected from the number of halftone dots and the output resolution. If the original is not a halftone original (if it is a continuous tone original), the second size is determined from the output resolution. Select a threshold matrix set.

  Then, it is determined whether or not the gradation is a dither processing section. If the gradation is a dither processing section, dither processing is performed using a dither processing threshold in the selected matrix set, and if it is not a dither processing section (error diffusion). If processing interval), error diffusion processing is performed using the error diffusion processing threshold in the selected matrix set.

  Then, when the halftone process of all the document data is completed, this process is exited.

  In this way, a pair of input value and threshold matrix is used for the highlight portion expressed only by the minimum output value among the multi-valued numbers and the shadow portion expressed only by the maximum output value among the multi-valued numbers. Applying a dither process that performs quantization by comparison, for a middle part that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value Applying an error diffusion process that performs quantization by adding a quantization error around the pixel of interest, depending on the halftone dot pitch of the input image or the presence or absence of a halftone dot, the variance threshold matrix used for the error diffusion process and dither process By adopting a configuration in which the size is switched, it is possible to reduce deterioration in image quality in the composite halftone process while reducing a calculation load caused by switching the halftone process.

  The above-described image processing is performed when image processing in an image forming apparatus such as an inkjet recording method or an electrophotographic method, or an image forming system in which the image forming apparatus and the image processing apparatus are combined, and image processing is performed by the image processing apparatus. It can be similarly applied to.

  In addition, the image processing (halftone processing) according to the present invention is not only installed as hardware, but can be incorporated into a printer driver as a program, for example, as halftone processing for a printer mode, as described above. This can reduce the computation load, and particularly when applied to an image forming apparatus that prints out image data from the host side as well as the processing in the image forming apparatus having the copy mode as in the above embodiment. The host computer can be released from the printing task more quickly. Furthermore, the present invention can also be applied to an information processing apparatus that sends print data to an image forming apparatus (the information processing apparatus in this case is also an image processing apparatus according to the present invention).

  Note that the image processing according to the present invention can also be applied to a system that performs software processing on a computer while relying on the high processing capability of the host computer. It is also possible to switch between another halftone process and the error diffusion process in the present invention. Although it is inferior in terms of computation load, which is a problem in the present invention, there are some halftone processes that can output higher quality for continuous tone originals. These halftone processes are classified by mode, for example, the image processing including the high-speed error diffusion processing of the present invention is performed in the copy / FAX mode in which halftone originals are frequently used, and the other halftone processes are performed in the printer mode mainly using continuous tone originals. By using tone processing, high-quality image reproduction is possible. These processes can be switched automatically according to the mode selection, or can be switched by an external input such as a user instruction.

  Further, there is an image forming apparatus as a multi-function machine that has a function called “Scan to E-mail” and distributes image data captured by a scanner through a network. As described above, even if an image is captured by an external scanner, if the original is a halftone document, moire or blur may occur at the time of output. Is.

  In the present invention, the processing load is small. For example, when a dedicated processing unit for image processing can be retrofitted, or when connected to a PC, part or all of the image processing can be processed by software. If possible, more sophisticated arithmetic processing is possible. When such an external arithmetic processing device can be utilized, it is possible to execute high-speed input correction processing and halftone processing including image area separation and frequency analysis processing at high speed. By switching from 1 to more advanced processing, it becomes possible to create a high-quality copy image. In this regard, it may be switched automatically in accordance with the detection of the external arithmetic processing unit, or may be switched by an external input such as a user instruction.

1 device body 2 image reading device (scanner)
32 Carriage 34 Recording Head 425 Halftone Processing Unit 501 Main Control Unit 516 Scanner Control Unit

Claims (9)

In a halftone processing method for converting a multi-tone image into multi-valued output data of three or more values smaller than input data using a threshold value,
For the highlight portion expressed by only the minimum output value of the multi-value number and the shadow portion expressed only by the maximum output value of the multi-value number, a pair comparison of the input value and the threshold matrix is performed. Apply dithering to quantize,
A quantization error around the pixel of interest is added to the middle portion that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value. Applying an error diffusion process that performs quantization,
An image processing method, wherein the size of a dispersion threshold matrix used for error diffusion processing and dither processing is switched according to a halftone dot pitch of an input image or the presence or absence of a halftone dot. Perform an image determination process to determine whether the input image is a halftone image for the input image,
If the input image is a halftone dot image as a result of the determination process, the next first-size distributed threshold matrix is displayed. If the input image is not a halftone image, the second second size distributed threshold matrix is displayed. Using the matrix as the error diffusion and dither threshold respectively
The first size = an integer closest to (halftone pitch / output resolution pitch),
The second size = an integer closest to (1 mm / output resolution pitch),
The image processing method according to claim 1, wherein:   The image processing method according to claim 2, wherein the image determination process is determined based on an external input or a frequency characteristic of the input image.   The threshold matrix used for the quantization determination in the error diffusion process has a size of g × h (both g and h are integers of 2 or more), the threshold matrix size and the threshold arrangement order, and the threshold matrix size in the dither process 4. The image processing method according to claim 1, wherein the threshold arrangement order is shared.   5. The quantization error value of a pixel of interest is set to 0 regardless of the result of dithering when the input value is 0 or outside the gradation range where error diffusion processing is performed. An image processing method according to claim 1. In an image processing apparatus that performs halftone processing for converting a multi-tone image into multi-valued output data of three or more values smaller than input data using a threshold value,
For the highlight portion expressed by only the minimum output value of the multi-value number and the shadow portion expressed only by the maximum output value of the multi-value number, a pair comparison of the input value and the threshold matrix is performed. Apply dithering to quantize,
A quantization error around the pixel of interest is added to the middle portion that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value. Applying an error diffusion process that performs quantization,
An image processing apparatus, comprising: means for switching a size of a dispersion threshold matrix used for error diffusion processing and dither processing according to a halftone dot pitch of an input image or the presence or absence of a halftone dot. In a program for causing a computer to perform halftone processing for converting a multi-tone image into multi-valued output data of three or more values smaller than input data using a threshold value,
For the highlight portion expressed by only the minimum output value of the multi-value number and the shadow portion expressed only by the maximum output value of the multi-value number, a pair comparison of the input value and the threshold matrix is performed. Apply dithering to quantize,
A quantization error around the pixel of interest is added to the middle portion that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value. Applying an error diffusion process that performs quantization,
A program for causing the computer to perform processing for switching the size of a dispersion threshold matrix used for error diffusion processing and dithering processing in accordance with the halftone dot pitch of an input image or the presence or absence of halftone dots.   A recording medium on which the program according to claim 7 is recorded. In an image forming apparatus that forms an image by performing halftone processing that converts a multi-tone image into multi-valued output data that is three or more values smaller than input data using a threshold value,
For the highlight portion expressed by only the minimum output value of the multi-value number and the shadow portion expressed only by the maximum output value of the multi-value number, a pair comparison of the input value and the threshold matrix is performed. Apply dithering to quantize,
A quantization error around the pixel of interest is added to the middle portion that is expressed by an output value that is one or more steps larger than the minimum output value and one or more steps smaller than the maximum output value. Applying an error diffusion process that performs quantization,
An image forming apparatus comprising: means for switching a size of a dispersion threshold matrix used for error diffusion processing and dither processing according to a halftone pitch of an input image or presence / absence of a halftone dot.

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