JP2000350026A - Method and device for recording dot image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a reproducibility in the density of a dot image by considering the influence to be given on adjacent pixels from each dot by comparing a threshold matrix with a multi-level image signal expressing an image to be printed. SOLUTION: A density influence degree table is prepared, which expresses the influence of a density, which each pixel receives by the dot generated at an adjacent pixel position (S1). Order for turning on the pixel is set (S2) for every dot cell in a repeating block. Order of turning-on the data of each dot cell is read into a memory area inside a threshold deciding part, and also a plurality of density measuring points are set in the repeating block (S3). A bit map memory for turning-on and that for a density value, which are used for deciding a threshold distribution are secured inside a main memory (S4). The threshold of each pixel is decided, based on prepared data (density influence degree table and access information).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for forming an image composed of halftone dots.

[0002]

2. Description of the Related Art Ordinary printed materials are often formed using halftone dots. As an image formed by halftone dots, a halftone film for producing a printing plate, a monochrome print,
There are color prints and the like. In this specification, an image formed by halftone dots is called a "halftone image" or a "halftone image". The smallest recording point forming a halftone dot is called a "small dot" or simply "dot". Note that a halftone dot is also called a “halftone dot” and is a collection of one or more small dots.

When a halftone image is generated, a threshold value read from a threshold matrix prepared in advance is compared with a multi-tone image signal to determine whether or not to generate a dot at each pixel position. Is generally generated.

[0004] As a method of creating a threshold matrix for forming halftone dots, many methods have been proposed so far.
For example, Japanese Patent Application Laid-Open No. 10-844 disclosed by the present applicant
In the method described in JP-A-77-77, a threshold distribution in a threshold matrix is devised so that unevenness in image density is reduced as much as possible.

[0005]

In the conventional method, when creating a threshold matrix, the size of one pixel area (referred to as "pixel size") and the size of a dot (referred to as "dot size") are determined. Were assumed to match. However, in practice, the pixel size often does not match the dot size. FIG. 1 is an explanatory diagram illustrating an example of a relationship between a pixel size and a dot size. FIG. 1 (A)
This shows a case where ideal dots are formed such that one pixel area of a rectangular shape can be painted without gaps. Actually, the case where the dot is smaller than one pixel area as shown in FIG. 1B or the case where the dot is larger than one pixel area as shown in FIG. Note that a dot portion that extends beyond one pixel region is also called a “fringe”.

In the method described in Japanese Patent Application Laid-Open No. Hei 10-84477, it is assumed that an ideal dot as shown in FIG. A threshold matrix was created. That is, in such a threshold matrix, the influence of a dot formed at one pixel position on another adjacent pixel position is not considered. Therefore, if the relationship between the pixel size and the dot size deviates significantly from this ideal state, the density of the image may not be correctly reproduced.

The problem of the reproducibility of the image density is a problem common to various dot image recording devices and dot image printing devices. For example, in an image setter or a plate setter that records a halftone image by exposing a photosensitive material using a light spot, and in a direct printing machine that directly prints a color print on a print medium from a color image signal, Problems arise.

[0008] In recent years, color printers for general-purpose computers such as personal computers and workstations have become widespread. Although such a color printer is relatively inexpensive, the image quality is quite good, and there is a demand for producing a proof print (proof print) using such a color printer. Since the proof print is for confirming the finish of the printed matter, it is preferable to print the proof print using the same halftone dots as the printed matter.

[0009] However, color printers for general-purpose computers are not compatible with dither matrices and error diffusion.
It is provided mainly for performing printing in accordance with an image density reproduction method other than the halftone method. When printing a halftone image using such a color printer, it is necessary to prepare a threshold matrix in advance, and at this time, the same problem as in the case described above occurs. That is, in the normal method, the influence of one dot on the adjacent pixels is not considered, and thus there is a problem that the density of the image cannot be reproduced well.

In recent years, a color printer (referred to as a "multi-valued color printer") capable of reproducing the density of one pixel with a plurality of gradations has also been used. Even when a halftone image is printed by such a multi-valued color printer, it is particularly difficult to print the halftone image in consideration of the influence of each dot formed in one pixel area on adjacent pixels. there were.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems in the prior art, and is intended to improve the reproducibility of the density of a halftone dot image in consideration of the influence of each dot on adjacent pixels. The purpose is to provide.

[0012]

In order to solve at least a part of the above-mentioned problems, a first configuration of the present invention uses an image recording device for visually recording an image. , And visually record the halftone image. At this time, in consideration of the difference between the size of the pixel in the image recording apparatus and the size of the dot formed at each pixel position, a threshold matrix is created that reflects the effective density reproduction characteristics of the image resulting from the difference. Further, a halftone signal representing a halftone image is generated by comparing the threshold matrix with a multi-tone image signal representing an image to be printed. And
By supplying a dot signal to the image recording apparatus, the image recording apparatus records a dot image composed of halftone dots. Since the threshold matrix is created in consideration of the influence of the density value given to the adjacent pixel position by the dot formed at each pixel position, the reproducibility of the density of the halftone image can be improved.

In a second configuration of the present invention, a color printer for printing a color image on a print medium using a plurality of printing primary colors having different hues is prepared, and a halftone image is formed using the color printer. Print. At this time, a threshold matrix reflecting the density reproduction characteristics of the color printer is prepared for each of the plurality of printing primary colors. Further, using these threshold matrices, a halftone signal is generated from a color image signal representing a color image to be printed. And
By supplying this dot signal to a color printer, a color image composed of dots is printed by the color printer. This threshold matrix reflects the density reproduction characteristics of the color printer and takes into account the effect of each dot on adjacent pixels, so that the density reproducibility of a halftone dot image can be improved.

As the color printer, a printer provided mainly for printing a color image on a print medium in accordance with an image density reproduction method other than the dot method can be used. Since the threshold value matrix reflects the density reproduction characteristics of such a printer, it is possible to print a halftone image using such a printer.

As the above-mentioned color printer, a multi-value printer capable of reproducing each pixel with a plurality of density gradations for at least one of the plurality of printing primary colors can be used. At this time, the threshold matrix for the printing primary colors that can be reproduced with a plurality of density gradations has a plurality of thresholds per pixel. In addition, when generating a halftone signal, a plurality of threshold values for each pixel are compared with a color image signal for a printing primary color that can be reproduced at a plurality of density gradations, thereby obtaining a density gradation of each pixel. Is generated. This makes it possible to print a color halftone image using a multi-valued printer.

With respect to the printing primary colors reproducible with a plurality of density gradations, at least some of the plurality of halftone dots constituting the printed color image are arranged from the center of the halftone dots to the periphery. The threshold value matrix may be configured such that the density gradation of the pixel decreases toward the portion. In this case, the density of the image can be reproduced more accurately.

With respect to the printing primary colors reproducible at a plurality of density gradations, at least some of the plurality of halftone dots constituting a printed color image have a plurality of halftone dots. The threshold matrix may be configured such that a region of a relatively low density gradation which is not the highest gradation of can be a region having a width of a plurality of pixels at the periphery of the halftone dot. Even with such a threshold matrix, it is possible to accurately reproduce the density of an image.

A plurality of density gradations may be realized by a difference in the size of a dot recorded in one pixel area.

Alternatively, a plurality of density gradations may be realized by a difference in the density of ink forming dots recorded in one pixel area.

As the color printer, a binary printer that reproduces each pixel in binary with respect to a plurality of printing primary colors having different hues can be used. At this time,
Each threshold matrix has one threshold per pixel. The maximum value of the threshold value in each threshold value matrix is smaller than the number of pixels included in each threshold value matrix. In this way, using a binary color printer,
It is possible to print a halftone image.

When a binary color printer is used, the thresholds in each threshold matrix can be set so that there is a jump in the values when the thresholds are arranged in order of magnitude.
In this case, there is a possibility that the density of the image can be reproduced more accurately.

The threshold value in each threshold value matrix is at least two such that the threshold value of the adjacent pixel position becomes the same value.
It can be set so that there are two pixel positions.
Even in this case, there is a possibility that the density of the image can be reproduced more accurately.

The present invention relates to a dot image recording method and apparatus, a dot image printing method and apparatus, a dot signal generating method and apparatus, a computer program for realizing the functions of those methods or apparatuses, and a computer for the same. The present invention can be realized in various forms, such as a recording medium on which a program is recorded, a data signal including the computer program and embodied in a carrier wave.

[0024]

DETAILED DESCRIPTION OF THE INVENTION Repeating unit area for forming halftone dots: Before describing the embodiments of the present invention, a repeating unit area (also referred to as a "repeated block") used for forming halftone dots will be described first. Here, the “repetition unit area” means an area that is repeatedly applied in a tile shape on the image plane, and the threshold matrix is obtained by assigning a threshold to each pixel position in the repetition unit area. It is. When forming a halftone dot, by comparing the threshold value in the repeating unit area with the multi-tone image signal,
A dot signal is formed.

As a method of defining a repeating unit area,
There is a method in which one dot area where one dot is formed is used as one repeating unit area, and a method in which a larger area including a plurality of dot areas is used as one repeating unit area.

FIG. 2 is an explanatory diagram showing a system in which one halftone dot area (also called "halftone dot cell") is used as one repeating unit area. In this scheme, the four corners of each halftone dot region coincide with the corners of the pixel grid. Therefore, the entire image plane can be covered by repeatedly applying the halftone dot area in a tile shape. However, this method has a problem that the number of halftone lines (also called “screen ruling”) and halftone angle (also called “screen angle”) that can be realized are considerably limited. This is because the four corners of each halftone dot region must match the corners of the pixel grid.

FIG. 3 is an explanatory diagram showing a method in which a wide area including a plurality of halftone dot areas is used as one repetition unit area. In this example, an area including 4 × 4 halftone areas is 1
One repetition unit area (also referred to as a “repetition block”). Such a repeating unit area including a plurality of halftone dot areas is generally called a “supercell”. Although the four vertices of the supercell match the pixel grid points, the four vertices of each halftone dot region do not necessarily match the pixel grid points. In the supercell method, 1
Since there is flexibility in the number of halftone dot regions constituting one supercell, the number of halftone lines and halftone angle can be more freely realized by the so-called rational tangent method. Here, the rational tangent method is a method of forming a halftone dot such that the tangent (tan) of the halftone angle is a rational number.

In the following embodiment, a threshold matrix is created using a repetitive unit area of the supercell system. However, it is also possible to use a repetition unit area of the one dot system.

B. First Embodiment: In a first embodiment of the present invention, first, a threshold matrix used for recording a halftone image using a binary image recording apparatus is created, and then a threshold matrix is used using this threshold matrix. Perform visual recording of point images. A binary image recording apparatus is an image recording apparatus of a type that forms dots of a predetermined size and a predetermined density in one pixel area. Specifically, an image setter for forming a halftone film, a binary There are color printers and the like. The “binary color printer” means a printer that generates a binary print signal indicating whether or not to create a dot at each pixel position for a plurality of inks and prints a color image according to the binary print image. The following mainly describes a case in which a proof print of a halftone image is created using a binary color printer connected to a general-purpose computer.

FIG. 4 is a block diagram showing a configuration of a threshold matrix creating apparatus used in the embodiment of the present invention. This apparatus includes a density influence degree table memory 102, a threshold value matrix memory 104, a threshold value determination unit 106, a density calculation unit 108, a lighting bitmap memory 110, and a density value bitmap memory 112. I have. This threshold matrix creation device can be realized by a general-purpose computer such as a personal computer. That is,
The four memories 102, 104, 110, and 112 are secured in the RAM of a general-purpose computer, and the functions of the threshold value determination unit 106 and the density calculation unit 108 are realized by the computer executing a predetermined computer program. . The functions of these units will be described later.

FIG. 5 is a flowchart showing the processing procedure of the first embodiment. In step S1, a density influence degree table showing the influence of the density on each pixel due to dots formed at adjacent pixel positions is prepared.

FIG. 6 is an explanatory diagram showing the contents of the density influence degree table used in the first embodiment. FIG.
Different types of dot formation patterns are shown. Each dot formation pattern is composed of 3 × 3 pixels, and each dot formation pattern has a pattern classification number P from 0 to 15.
IDs are respectively assigned. A white square indicates a pixel on which no dot is formed, and a solid square indicates a pixel on which a dot is formed. In any of the patterns, dots are not always formed at the center pixel of interest. The numerical value described in the frame of the target pixel indicates the substantial density of the target pixel due to the influence of dots formed in the periphery. The density value is represented by 8 bits, and can take a value in the range of 0 to 255. The density value of a pixel painted with ink is 255. The actual dot has a shape as shown in FIG. 1C, but in FIG. 6, for convenience of illustration,
The inside of the rectangular pixel area is drawn as being painted.

As shown in FIG. 1C, when there is a fringe (that is, when the actual dot size is larger than one pixel area), the dot formed at a certain pixel position is It also affects the density of neighboring pixels. The density influence degree table indicates the density that the target pixel substantially has as a result of the influence of the dots formed at the pixel positions in the vicinity of 4 of the target pixel. Specifically, in a state where no dot is formed at the position of the target pixel, the substantial density of the target pixel is determined according to the presence or absence of the dot at the four neighboring pixel positions.

As can be understood from FIG. 6, as more dots are formed near the target pixel, the substantial density value of the target pixel increases. FIG. 7 shows a specific configuration of the density influence degree table. That is, the density influence table includes the pattern classification number PID and the real density value due to the influence of the neighboring dots. The density influence table is created for each of a plurality of ink colors (primary colors for printing) used in the binary color printer.

It should be noted that, depending on the relationship between the pixel size and the dot size, the dots formed in four neighboring pixels (so-called eight neighboring pixels) obliquely adjacent to the target pixel may also affect the density of the target pixel. is there. In this embodiment,
Such effects of the dots at the eight neighboring pixel positions are ignored, and only the effects of the dots at the four neighboring pixel positions are considered. However, the real density value of the pixel of interest may be determined in consideration of the influence of the dots at the eight neighboring pixel positions.

The real density value of the target pixel in each dot formation pattern is determined by actually forming each dot formation pattern for each ink using a binary color printer, and calculating the real density value of the target pixel in each dot formation pattern. Determined by measuring. FIG. 8 is an explanatory diagram showing a method of measuring the real density value of the target pixel. FIG. 8A shows a state in which the pattern identification number PID is 03 (FIG. 6) actually printed, and FIG. 8B shows a state in which this dot formation pattern is read by a high-resolution scanner. 2 shows a read image obtained by the above method. In this specification, a pixel defined by a dot pitch to be recorded is called a “recorded pixel”, and a pixel in a read image is called a “read pixel”. In the example of FIG. 8, the resolution of the recording pixel is 720
dpi, and the resolution of the read pixel is 72, which is ten times the resolution of the read pixel.
00 dpi. However, in FIG. 8B, for convenience of illustration, the ratio between the size of the recording pixel and the size of the reading pixel is not drawn correctly. In general, the resolution of the read pixels only needs to be higher than the resolution of the recording pixels.

The read image data has multi-tone pixel values for each read pixel. The substantial density value of the target pixel can be obtained by averaging the pixel values of a plurality of read pixels included in the target pixel area. The real density values of the target pixel shown in FIGS. 6 and 7 are determined in this manner.

It can be seen from FIG. 6 that even if the number of dots formed at the neighboring pixel positions is the same, the actual density value of the target pixel may be different (for example, the pattern identification numbers PID of 01 and 02 are different). Two patterns). The reason is that the actual density value is determined in consideration of the actual dot formation state as shown in FIG.

Note that the substantial density value of the target pixel due to the influence of the neighboring dots can be determined by a method other than the method shown in FIG. For example, when the size of a dot and the minute density distribution in the dot are known quite accurately in advance, it is possible to calculate the real density value of the pixel of interest based on such information. Specifically, for a binary image recording apparatus in which dots are formed by exposing a photosensitive material with a light beam, if the γ characteristic of the photosensitive material and the light amount distribution of the exposure beam are known, then FIG. The actual density value of the target pixel can be calculated without performing such actual measurement. In the case of an ink-jet printer, if the ink density, the amount of ink droplets for one dot, and the size of one dot are known, the actual density value of the target pixel is calculated without performing actual measurement. be able to.
Further, in the case of a laser printer (electrophotographic printer), if the characteristics of the photoconductor (drum), the distribution of the exposure amount on the photoconductor, and the toner characteristics are known, the actual pixel of the pixel of interest is not measured. A density value can be calculated.

The density influence table prepared in this way is stored in the density influence table memory 102 (FIG. 4) of the threshold value matrix creating device.

In step S2 of FIG. 5, the lighting order of the pixels is set for each dot cell in the repetition block. Note that "lighting" means that dots are formed at pixel positions. For example, in an image recording apparatus of a type that exposes a photosensitive material with a light beam, turning on the light beam is called “lighting” and forms a dot by ejecting ink droplets onto a print medium. Then, discharging the ink droplet is called "lighting". “Lighting order” means the order in which dots are formed. However, the lighting order set here is tentative, and a threshold value indicating the final lighting order is determined by processing described later.

FIG. 9 is an explanatory diagram showing a repetition block SC used in the first embodiment. The repetition block SC includes 5 × 5 halftone cells HC. Here, the position of each dot cell in the repetition block is specified by a two-digit number. This two-digit number is called a “dot cell number”. That is, the halftone cell HC00 having the halftone cell number “00” is located at the upper left corner of the repetition block SC, and the halftone cell HC44 having the halftone cell number “44” is
The repetition block SC is located at the lower right position. Step S
In 2, in the pixels in each of the halftone cells HC in such a repetition block SC, an independent lighting order is set for each halftone cell HC. FIG. 10 is an explanatory diagram showing an example of an arrangement of the lighting order in one dot cell HC. The lighting order is set for each pixel in the halftone cell HC so that the value gradually increases from the center to the periphery.

In step S3 of FIG. 5, the lighting order data of each dot cell in the repetition block SC is
6 is read into the internal memory area, and a plurality of density measurement points are set in the repetition block SC. FIG. 11 is an explanatory diagram showing the positions and types of measurement points. FIG. 11 (A)
Represents the repetition block SC and the halftone cells HC00 to HC44.
FIG. 11B shows the positions of the measurement points. In FIG. 11B, for reference, halftone dots formed when the halftone dot area ratio is 50% are shaded. As the measurement points, mountain-side measurement points P00 to P44 located at the center of each dot cell and valley-side measurement points B00 to B44 located at four corners of each dot cell are set. The code of the mountain side measurement point (for example, “P00”, “P4
The last two numbers of “4”) indicate the halftone cell number (FIG. 11A) including the mountain side measurement point. The last two numbers of the signs of the valley-side measurement points (for example, “B00” and “B44”) are set to the same numbers as the hill-side measurement points at the upper left of the valley-side measurement points. In addition, since the repeated block SC is repeatedly applied in a tile shape on the image plane, an equivalent measurement point (for example, B44, B40, etc.) appears a plurality of times at the measurement points on the sides of the repeated block SC.

In step S3, an adjacent relation table indicating the adjacent relation between the halftone cells is further created. FIG.
It is explanatory drawing which shows the adjacent relation table which shows the adjacent relation between halftone cells. For example, in FIG. 12 (A), the numbers of the halftone cells in the neighborhood of 8 with the 22nd halftone cell HC22 are 32, 31, 21, 11, 12, 13, 23, 33.
It is. The adjacent relation table shown in FIG. 12B is a table in which the numbers of the halftone cells having an adjacent relation of eight neighbors are registered for each halftone cell.

In step S4 of FIG. 5, the lighting bitmap memory 110 and the density value bitmap memory 1 used for determining the threshold distribution in the repetition block SC are used.
12 are secured in the main memory. In the lighting bitmap memory 110, 1 is set for a pixel to be lit,
A non-illuminated pixel is a 1-bit deep memory area in which 0 is set. In addition, the bitmap memory 112 for density value
Is an 8-bit deep memory area for storing data indicating a substantial density value determined for each pixel in consideration of the influence of the density value due to neighboring dots.

In step S4, the lighting bitmap data and the density value bitmap data are all 0.
Initially set to FIG. 13 is an explanatory diagram showing the size of the image area included in the lighting bitmap memory 110 and the density value bitmap memory 112. Although the two bitmap memories 110 and 112 have different depths, the image areas included therein (referred to as “bitmap areas”)
Have the same size as shown in FIG. This bitmap area is a wide area provided with an extended width for density measurement around a rectangular area (shown by a broken line) circumscribing the repetition block SC. That is, the repetition block S
When measuring the density of an image at a density measurement point (see FIG. 11B) located at the vertex of C, it is necessary to know the lighting state of a certain range around the image. Therefore, a wide area having a predetermined extension width around the repetition block SC is used as the bitmap area. Preferably, the expansion width is set to a value corresponding to a few halftone dot pitch.

The bit map area includes a repetition block SC
Since there is a wider area, there is a position where a plurality of equivalent pixels exist in the bitmap area. Such an equivalent plurality of pixels are associated with each other so as to be lit simultaneously. In this specification, the association of a plurality of equivalent pixels is called "nesting", and the information is called "nesting information". FIG. 14 is an explanatory diagram showing the contents of the nesting information. The coordinates (10) shown in FIG.
The pixel of (0, 120) is the pixel of lighting order 1 of the halftone cell HC00, as shown in FIG. FIG.
As shown by a black circle in FIG. 3A, there are three other pixels equivalent to this pixel in the lighting bitmap. The nesting information correlates the coordinates of these four pixels. On the other hand, in the access information of the lighting bitmap (information indicating the lighting order of each pixel and the presence / absence of lighting), the lighting order corresponding to the pixels of one repetition block SC is stored. Therefore, when setting the lighting state (1 or 0) of each pixel in the lighting bitmap memory 110, the same lighting state is always set for a plurality of pixels associated by the nesting information.

In step S5 of FIG.
Based on the data prepared in S4 (that is, the density influence table in FIG. 7 and the access information in FIG. 14), the threshold value of each pixel in the repetition block SC is determined. FIG. 15 is a flowchart showing a detailed procedure of step S5. In step S11, an initial lighting position in the repetition block SC is set, and a lighting bit (= 1) is set in the lighting bitmap memory 110. The initial lighting position can be set to a pixel having a lighting order of 1 in an arbitrary halftone cell. For example, a pixel of lighting order 1 of the halftone cell HC22 at the center of the repeating block SC and a pixel of lighting order 1 of the halftone cell HC00 at the upper left corner of the repeating block SC can be selected as the initial lighting position.

In step S12, the density calculator 108
Calculates the effective local density value of the pixel affected by the lighting. Here, the “local density value” means the density value of each pixel, and at each measurement point (FIG. 11B), a density value measured for a wider area (“wide area density value”). Call). The substantial local density value of each pixel is determined using a density influence table (FIGS. 6 and 7).

FIG. 16 is an explanatory diagram showing a method for determining a substantial local density value of each pixel. FIG. 16 (A-1)
FIG. 16B-1 shows a lighting bitmap before the central pixel is turned on for a region of × 5 pixels.
The bit map for the density value at that time is shown. FIG.
The local density value of each pixel in (B-1) conforms to the content of the density influence table shown in FIG. FIG. 16 (A
As shown in FIG. 16B-2, when the central pixel is turned on, the local pixel value of the central pixel and the local pixel values of the pixels in the vicinity thereof are changed according to the density influence degree table, as shown in FIG. Is determined. That is, the local pixel value of the lit pixel is set to 255, and the local pixel value of the neighboring pixels (the four neighboring pixels in this embodiment) is changed according to the density influence degree table. In the example of FIG. 16B-2, the number of the changed local density value is circled.

In step S13 in FIG. 15, the density calculator 108 measures the image density (wide area density value) at each of the measurement points shown in FIG. 11B, and the wide area density of the measurement point affected by the lighting is measured. Update the measured value. FIG. 17 is an explanatory diagram showing a method of measuring a wide area density value. The wide area density value D at the measurement point is integrated by multiplying the weighting function W (x, y) centered on the measurement point by the density bitmap data B (x, y), and this is integrated with the integration value of the weighting function W. It is a value standardized by. In other words, the wide area density value D is obtained by dividing the density value bitmap data B (x, y) in a certain range including a plurality of halftone cells (hereinafter referred to as a “measurement mask”) by a weight function W (x , Y). Weight function W (x,
As the distribution of y), it is preferable to employ a weight distribution that approximates a Gaussian distribution. Further, the weight function W (x,
The diameter of the range (measurement mask) in y) is the dot pitch of 4.
It is preferable to set it to about 25 times. The wide area density value D obtained in this manner is a value substantially equal to the density value of an image obtained using a densitometer having an aperture (opening) of the size of the measurement mask.

In step S13 in FIG. 15, first, in FIG.
At each measurement point shown in FIG. 1 (B), a wide area density value is calculated according to the method shown in FIG. Then, for the measurement point at which the wide area density measurement value has been changed, the wide area density measurement value is updated.

FIG. 18 is an explanatory diagram showing an example of the result of the wide area density measurement value. The horizontal axis in FIG. 18A indicates the position of the measurement point, and the vertical axis indicates the density value. Although the peak-side measurement points and the valley-side measurement points are alternately arranged on the horizontal axis, these axes are separately shown for convenience of illustration. In the example of FIG. 18, the mountain side measurement point P of the halftone cell HC22
22 indicates the lowest density value.

In step S14 of FIG. 15, a plurality of lighting candidates to be turned on next are selected from the vicinity of the measurement point indicating the lowest density value. FIG. 19 is an explanatory diagram illustrating an outline of a lighting candidate selection method. In FIG. 19, a dot cell HC
It is assumed that the peak measurement point Pmn or the valley measurement point Bmn of mn indicates the lowest density value among the measurement points.

When the peak-side measurement point Pmn indicates the lowest density value, a predetermined number (for example, three) of non-lighted pixels in the halftone dot cell HCmn are selected in accordance with the lighting order, and lighting candidates are selected. And FIG. 20 is an explanatory diagram illustrating a specific method of selecting lighting candidates. FIG. 20 shows a dot cell HC
22 shows a case in which three lighting candidates are selected. Among the non-lighted pixels in the halftone cell HC22, the three pixels with the fastest lighting order have the lighting order of 102, 10
3,105 pixels. Therefore, these three pixels are selected as lighting candidates.

When the halftone dot area ratio is close to 100% and a predetermined number of unlit pixels are not present in the halftone cell HCmn, the remaining lighting candidates are selected from the adjacent halftone cells. I do. At this time, the adjacency relation table shown in FIG. 12B is used. That is, first, the halftone cells (left, right, upper,
Lighting candidates are selected from the lower halftone cells (one of the lower halftone cells). If there is a shortage, lighting candidates are selected from the other eight neighboring halftone cells.

On the other hand, when the valley-side measurement point Bmn indicates the lowest density value, it is desired to select a lighting candidate from the vicinity of the valley-side measurement point Bmn. Therefore, in this case, as shown in FIG. 19 (C), a predetermined number of pixels are selected from the unlit pixels in the four halftone cells around the valley-side measurement point Bmn according to the lighting order. , Lighting candidates.

In step S15 of FIG.
The pixels of the predetermined number of lighting candidates selected in step 14 are tentatively lit one by one, and in a state where each lighting candidate is lit, a wide area density value is measured at each measurement point. FIG. 21 shows a calculation result of the wide area density value obtained in a state where each of the three lighting candidates is turned on. As illustrated in FIGS. 21B to 21D, density deviations are obtained in the distribution of the wide-area density measurement values obtained when each of the lighting candidates is turned on. Here, the “density deviation” is the difference between the highest density value and the lowest density value. The fact that the density deviation is small means that the unevenness of the image is small. Therefore, in step S16 of FIG. 15, the lighting candidate with the smallest density deviation among the plurality of lighting candidates is determined as the next lighting position. To the pixel determined as the next lighting position, the lighting order among all the pixels in the repetition block SC is assigned as a provisional threshold value of the pixel. That is, when the lighting position is the pixel that lights up i-th in the repetition block SC, (i−1) is assigned as the provisional threshold.

In step S16, the effective halftone dot area ratio Reff in the state where the i-th pixel is turned on is calculated using the following equation (1). Reff = (Σd / N × 255) × 100 (1) Here, Σd is the total value of the local density values of each pixel in the repetition block SC, and N is the total number of pixels in the repetition block SC.

The effective halftone dot area ratio is an effective halftone dot area ratio (that is, an effective image density) in consideration of the influence of fringes of dots. FIG. 22 is an explanatory diagram showing the relationship between the provisional threshold value thus obtained and the effective halftone dot area ratio. As will be described later, the final threshold distribution is determined based on the relationship between the provisional threshold and the effective halftone dot area ratio.

In step S17 of FIG. 15, it is determined whether or not the provisional threshold has been assigned to all the pixels in the repetition block SC. If the processing has not been completed, the process returns to step S12, and the processing of steps S12 to S16 described above is repeated. After the assignment of the provisional thresholds for all the pixels is completed, in step S18, the provisional thresholds for all the pixels included in the repeated block are stored in a storage device such as a hard disk.

Thereafter, in step S19, the provisional threshold is normalized, and the final threshold is determined. FIG. 23 is an explanatory diagram of the content of the process of threshold normalization.
Here, when the number of gradations in the threshold matrix is M,
Consider a process of determining a normalized threshold value j (j is an integer satisfying 0 ≦ j ≦ M−1). The threshold value j after the normalization is assigned to a pixel satisfying the condition of the following expression (2). Rid (j-1) <(Effective halftone dot area ratio Reff at the time of lighting) ≦ Rid (j) (2)

Here, Rid (j) is a halftone dot area ratio calculated from the threshold value j, and is an ideal halftone dot area ratio when there is no fringe of dots. In other words, the final threshold value j is assigned to a pixel which achieves an effective halftone dot area ratio close to the ideal halftone dot ratio Rid (j) when there is no fringing of dots.

In an ideal state without dot fringe,
When the pixels to which the 0th to jth thresholds are assigned are turned on, (j + 1) pixels out of M are turned on. Therefore, the ideal halftone dot area ratio Rid (j) at that time is given by the following equation (3). Rid (j) = {(j + 1) / M} × 100 (3)

In FIG. 23, three thresholds (j-1), j,
The ideal dot area ratio for (j + 1) is R
id (j-1) = 51.10, Rid (j) = 51.25,
It is assumed that Rid (j + 1) = 51.40. At this time, the threshold j is assigned to a pixel having a provisional threshold that achieves the effective halftone dot area ratio Reff satisfying the above equation (2). Specifically, the threshold j is an effective halftone dot area ratio R
eff is less than or equal to the ideal halftone dot area ratio Rid (j) = 51.25, and the immediately preceding ideal halftone dot area ratio Rid (j)
-1) = Temporary threshold (n−
1) is assigned to the pixel having Also, the threshold (j +
1) is that the effective halftone dot area ratio Reff is equal to the ideal halftone dot area ratio Reff.
id (j + 1) = 51.40 or less and 1
Pixels having provisional threshold values n and (n + 1) that are larger than the previous ideal halftone dot area ratio Rid (j) = 51.25 are assigned.

As can be seen from the above example, the same final threshold may be assigned to a plurality of pixels in some cases. Although not shown in FIG. 23, a threshold value that is not assigned to any pixel (that is, a threshold number is omitted)
May occur.

FIG. 24 is an explanatory diagram showing a threshold value matrix generated in the first embodiment. This threshold matrix is
This is a matrix having a threshold value for a halftone dot area of 8 × 8 pixels surrounded by a thick line, and is composed of threshold values in the range of 0 to 63. However, FIG. 24 also shows thresholds that appear at pixel positions adjacent to the periphery of the threshold matrix when the 8 × 8 threshold matrix is repeatedly applied.

The threshold matrix of FIG. 24 has the following two features. (1) Missing numbers for thresholds (1, 4, 6, 11, 18, 33, 3
6, 38). (2) The same threshold value may be assigned to adjacent pixels.

The reason why a missing number occurs in the threshold value (that is, the value jumps when the threshold values are arranged in order of magnitude) is that when a certain pixel is turned on, the effective halftone dot area ratio greatly changes ( That is, the dot area ratio may jump). That is, for a pixel position at which the effective halftone dot area ratio changes significantly at the time of lighting, the threshold value and the effective halftone dot area ratio are set by setting the threshold value to a value apart from the immediately preceding threshold value. Can be correlated correctly. The reason why the same threshold is assigned to adjacent pixel positions is that even if those pixel positions are turned on at the same time,
This is because the effective halftone dot area ratio does not change much. If there are at least two pixel positions where the threshold values of adjacent pixel positions have the same value, an effective halftone dot area ratio can often be accurately reproduced. The threshold matrix generated according to the first embodiment has an advantage that the halftone dot area ratio of an image can be reproduced more accurately due to such features.

As described above, in the first embodiment, the local density value of each pixel is determined in consideration of the influence of the fringe of the dot, and the effective halftone dot area ratio when each pixel is turned on is determined. Since the threshold matrix is determined, it is possible to create a threshold matrix that more accurately reproduces the image density by reflecting the effective density reproduction characteristics of the image under the influence of the fringe of the dots. Further, in a state where a plurality of lighting candidates are selected and these are tentatively lit, the lighting candidate with the smallest density deviation is adopted as the next lighting position, so that the density deviation in the image can be suppressed to a small value. In addition, unevenness of an image can be reduced. Note that the method of the first embodiment can be similarly applied to a case where the dot size is smaller than the pixel area.

FIG. 25 is a block diagram showing the configuration of the binary halftone dot image recording apparatus in the first embodiment. The binary halftone dot image recording apparatus includes a CPU 30, a main memory (ROM and RAM) 32, a floppy disk drive 34, an SPM memory 36, a sub-scanning address counter 38, a main scanning address counter 40, a comparator 42. The SPM memory 36 is a memory that stores a threshold matrix.

The binary halftone dot image recording apparatus also includes an exposure device (not shown) for recording a halftone dot image on a recording medium by a light beam. It is also possible to use a binary color printer instead of the exposure device. When a binary color printer is used, the comparator 42 and the SPM memory 36 are separately provided for each of a plurality of printing ink colors used in the binary color printer.

This binary image recording apparatus can also be realized by a general-purpose computer such as a personal computer. In this case, the address counters 38 and 40 and the comparator 42 are realized by a computer program.
The SPM memory 36 is secured in the main memory 32 of the general-purpose computer.

A binary color printer connected to a general-purpose computer is usually provided mainly for printing a color image on a print medium in accordance with an image density reproduction method other than the halftone dot method. When a normal color image is printed using a binary color printer, a print signal is created using a dedicated printer driver attached to the binary color printer, and the print signal is converted to a binary color image. Supply it to the printer and execute printing. However, since such a binary color printer is not intended to print a halftone image, it is impossible to print a color halftone image using a printer driver dedicated to the binary color printer as it is. is there. Therefore, when printing a color halftone image using a binary color printer, a printer driver for printing a halftone image having the functions of the SPM memory 36 and the comparator 42 is created and installed in a general-purpose computer. In this case, it is possible to generate a dot signal for printing a color dot image using the printer driver for printing a dot image, and supply it to the binary color printer.

The sub-scanning address counter 38 receives the sub-scanning start signal Rx and the sub-scanning clock signal Cx. The sub-scanning start signal Rx is a signal that generates one pulse when the sub-scanning coordinates of the light beam are reset to the initial position. The sub-scanning clock signal is a signal that generates one pulse each time the sub-scanning coordinates of the light beam are updated. The sub-scanning address counter 38 outputs these signals R
x and Cx, the sub-scanning coordinates of the light beam in the repetition unit block are generated and stored in the SPM memory 36.
As a sub-scanning address. Similarly, the main scanning address counter 40 generates main scanning coordinates of the light beam in the repeating unit block according to the main scanning start signal Ry and the main scanning clock signal Cy, and stores the generated main scanning coordinates in the SPM memory 36 as a main scanning address. Supply. According to the address given from these two address counters 38 and 40, 1 is calculated from the threshold value matrix in the SPM memory 36.
The two thresholds Ss are read and supplied to the comparator 42.

The comparator 42 compares the threshold value Ss with the input image signal Im and generates a binary halftone signal (binary output) according to the comparison result. The level of the binary dot signal is
It is as follows. When Ss <Im: H level (lighting) When Im ≦ Ss: L level (non-lighting) When the input image signal Im is in the range of 0 to 64, the range of the threshold value Ss is 0 to 63. Become. When the input image signal Im is in the range of 0 to 255, the threshold value Ss
Ranges from 0 to 254.

An exposure device (not shown) exposes a photosensitive recording medium (for example, a photosensitive film) with a light beam in accordance with the binary halftone signal, thereby forming a halftone image on the recording medium. In this way, a halftone image of each color plane of YMCK is created, and these halftone images are overprinted with inks of the respective colors, whereby a multicolor print can be obtained.

When a binary color printer is used in place of the exposure device, the input image signal I
By comparing m with the threshold value Ss, a binary dot signal for each ink is created and supplied to a binary color printer to execute printing of a color image.

FIGS. 26 to 28 are graphs showing the density deviation of a halftone dot image formed by using the threshold value matrix prepared according to the first embodiment in comparison with the conventional example and the ideal state. 26 to 28 show that the output resolution is 4000d.
pi, the screen ruling (screen ruling) is 400 lpi, and the screen angles (screen angle) are 15 ° and 45 °, respectively.
This relates to a halftone image in the case of ° and 75 °. The graph annotated as “considering fringe” shows the change in the actual density deviation of the halftone image printed using the threshold matrix created in the first embodiment. Further, “does not consider fringe” indicates a change in the actual density deviation of a halftone dot image printed using the threshold matrix created by the method described in JP-A-10-84477 described above. “Ideal” indicates an ideal density deviation of a halftone image in an ideal state in which the halftone dot area ratio is equal to the ratio of the number of lightings to all pixels.

Under most of the conditions shown in FIGS. 26 to 28, the density deviation of the first embodiment considering the fringe is
It is understood that the density deviation is smaller than that of the conventional example in which the fringe is not considered.

Regarding the first embodiment described above, the following various modifications are possible.

(1) In the first embodiment, one repetition block SC is targeted for the threshold value allocation processing. However, a wider area including a plurality of repetition blocks SC is adopted as the processing target area. Is also possible.

(2) As the measurement points, points other than those shown in FIG. 11B can be adopted. For example, all the pixels in the repetition block can be used as measurement points. Alternatively, it is also possible to arrange measurement points at fixed intervals at positions different from the peak-side measurement points and the valley-side measurement points in FIG. Alternatively, measurement points may be randomly arranged in the repetition block SC. By arranging the measurement points at regular intervals, there is an advantage that the correlation between the density deviation and the unevenness of the image can be increased.

(3) Weight function W for performing density measurement
As (x, y), various types other than those shown in FIG. 17 can be used. For example, a weight function described in FIG. 5 of Japanese Patent Publication No. 61-27683 disclosed by the present applicant may be used.

(4) In step S15 of FIG.
As a criterion for determining the next lighting position, the criterion that “the density deviation is minimized” has been adopted, but other criterion can be adopted. For example, FIG.
It is also possible to adopt a lighting candidate that maximizes the density value at the lowest density measurement point obtained in step S13 as the next lighting position. At this time, a limitation may be added that the next lighting position is adopted only when the density value at the lowest density measurement point does not become the highest density value. Alternatively, other evaluation criteria (for example, the smaller the peripheral length of a halftone dot (lighted portion) is better) may be considered together with the density deviation of a plurality of lighting candidates.

(5) As a method of selecting a plurality of lighting candidates in step S14 of FIG. 15, a method other than the method shown in FIGS. 19 and 20 can be adopted. FIG.
It is explanatory drawing which shows the other method of selecting a lighting candidate. In this method, the area is not divided for each halftone cell as shown in FIG. 29A, and the mountain area (H is used) as shown in FIGS. 29B and 29C.
1 to H9) and valley regions (V1 to V4, etc.). The mountain region is a region including pixels that are turned on when the dot area ratio is about 50% or less. The valley region is a region formed of pixels that are not turned on when the halftone dot area ratio is about 50% or less.

As shown in FIG. 29B, when the lowest density measurement point exists in the valley region V1, lighting candidates are selected from the hill regions H1, H2, H4, and H5 adjacent to the valley region V1. select. The reason why a lighting candidate is selected from adjacent mountain regions is that, in general, the shape of a halftone dot becomes clearer when pixels in the mountain region are turned on first than in a valley region.
When the halftone dot area ratio approaches 50%, these peak regions H
In some cases, a sufficient number of lighting candidates cannot be selected from among 1, H2, H4, and H5. In this case, a lighting candidate is selected from the valley regions near the mountain region. For example, when there are not enough lighting candidates in the mountain region H5,
Lighting candidates are selected from the valley regions V1, V2, V3, and V4 in the vicinity.

On the other hand, as shown in FIG. 29C, when the lowest density measurement point exists in the mountain region H5, a plurality of lighting candidates are selected from the mountain region H5. When a sufficient number of lighting candidates cannot be selected in the mountain region H5, the surrounding mountain regions H2, H4, H6, H8, H1,
A lighting candidate is selected from H3, H7, and H9. When the halftone dot area ratio approaches 50%, a sufficient number of lighting candidates may not be selected from these mountain regions. In this case, a lighting candidate is selected from a valley region in the vicinity. For example, when there are not enough lighting candidates in the mountain region H6, the lighting candidates are selected from the valley regions V2 and V4 in the vicinity.

C. Second Embodiment In a second embodiment of the present invention, first, a threshold matrix used when a halftone image is recorded using a multi-valued image recording apparatus is created, and then a network is formed using this threshold matrix. Perform visual recording of point images. A multi-level image recording apparatus is an image recording apparatus of a type capable of forming a plurality of types of dots having different local density values (density per pixel) in one pixel area.
There are a multi-value color printer for a general-purpose computer and a multi-value direct printing machine.

The “multi-value color printer” has at least one
For each type of print color, generate a multi-valued print signal indicating which of a plurality of types of dots are to be created at each pixel position,
It means a printer that prints a color image in accordance with the multi-level print signal. Here, the “print color” means a color of an ink having at least substantially the same hue, and usually four print colors of cyan, magenta, yellow, and black are used.

In recent years, the densities of some printing colors (for example, magenta and cyan) are different from each other.
Some printers use a plurality of types of inks having substantially the same hue. In such a printer, dots formed with relatively high-density ink and dots formed with relatively low-density ink have different local density values. In addition, some printers use the same ink and change the dot size by changing the ejection amount of ink droplets to change the local density value. As can be seen from these examples, in a multi-value printer, as a method of realizing a plurality of local density values for one printing basic color, a method of changing the density of the ink itself, a method of changing the ink amount, And a method combining the two. The present invention is applicable to multi-valued printers utilizing these various methods. In addition,
The following mainly describes a case where a multi-valued color printer that realizes a plurality of local density values by changing the density of the ink itself is used.

FIG. 30 is a flowchart showing the processing procedure of the second embodiment. Steps S1a to S5a in FIG.
Among them, step S3 is the same as the procedure of the first embodiment shown in FIG. 5, and other steps are slightly changed from the first embodiment.

In step S1a, a density influence table is prepared which indicates the influence of the density on each pixel due to dots formed at adjacent pixel positions.

FIG. 31 is an explanatory diagram showing the contents of the density influence degree table used in the second embodiment. In the first embodiment shown in FIG. 6, a numerical value indicating the influence of the density value due to the neighboring dots is shown at the position of the center pixel, but the numerical value is omitted in FIG. In the second embodiment, each pixel can be reproduced with three density gradations using two types of dots having different local density values. That is, in each dot formation pattern in FIG. 31, a white square indicates a pixel where no dot is formed, and a square filled with a light color is a dot having a relatively low density formed with light ink. The formed pixels and the squares filled with dark color indicate the pixels formed with dots of relatively high density formed of dark ink. Light ink and dark ink mean inks having substantially the same hue but different densities. Light ink is also called “photo ink”.
Note that, instead of using inks having different densities, a plurality of density gradations of each pixel may be realized by changing the dot size (ink amount) using the same ink.

In the second embodiment, it is assumed that the density of dark ink dots is about 100% and that of light ink dots is about 50%. Therefore, hereinafter, the local density level of the pixel on which the light ink dot is formed is called “１ ／ density level”, and the local density level of the pixel on which the dark ink dot is formed is “2/2 density level”. Call. The actual local density of each pixel is measured in the same manner as in the first embodiment, and a threshold matrix is determined by reflecting the measured value. Therefore, even when the actual densities of the dark ink dots and the light ink dots are different from the above values,
The same processing as described below can be applied.

In the above-described first embodiment (FIG. 6), it has been assumed that no dot is always formed at the central target pixel in any of the dot formation patterns. On the other hand, in the second embodiment, the pattern classification number PI
As shown in the pattern of D = 05 to 08, the influence of the density value on the central pixel due to neighboring dots is also considered for a dot forming pattern in which light ink dots are formed at the central pixel. The reason is that the density of the light ink is low, so even if a light ink dot is formed at a certain pixel position, when a dot is formed at a pixel position in the vicinity thereof,
This is because the substantial local density value at the pixel position changes.

In step S1a of FIG. 30, FIG.
For many dot forming patterns including the pattern shown in FIG. 1, the influence of the density of the neighboring dots on the center pixel is evaluated, and a density influence degree table is created. Since the method of evaluating the degree of influence is the same as the method in the first embodiment described with reference to FIG. 8, detailed description will be omitted.

In step S2a of FIG. 30, the lighting order of the pixels is set for each dot cell in the repetition block.
FIG. 32 is an explanatory diagram illustrating a method of creating a lighting order in the second embodiment. FIG. 32A shows an example of one halftone dot area. In step S2a, first, FIG.
A reference lighting order as shown in FIG. In the reference lighting order, one pixel is divided into four small sections, and a value that gradually increases from the center to the periphery is assigned to each small section as the reference lighting order.

Next, the parameter i for determining the lighting order
Is sequentially increased one by one from 0, and this parameter i
And the value of the reference lighting order, the lighting order of the 1/2 density level for the light ink dot (FIG. 32 (C
-1)) and the lighting order of the 2/2 density level for dark ink dots (FIG. 32 (C-2)). 1
The lighting order of the / 2 density level is consecutively arranged in order from the two small sections having the reference lighting order having a value larger than the parameter i among the four small sections in one pixel. It is a numerical value assigned. On the other hand, the lighting order of the 2/2 density level is, in order from four small sections having a reference lighting order having a value larger than the parameter i among four small sections in one pixel, in order. It assigns consecutive integer values. Note that these two lighting orders do not include the same value twice or more, and one value is uniquely assigned to one pixel.

As described above, if the reference lighting order is set in advance and the lighting order for the two density levels is created by comparing the parameter i with the reference lighting order, the operator can manually turn on the lighting order. There is an advantage that it is not necessary to determine the order, and the lighting order can be automatically determined. That is, if the operator inputs the size of one dot area, the setting of the reference lighting order and the creation of the lighting order for each density level using the reference lighting order are automatically executed by a predetermined computer program. It is possible. Such an advantage is particularly remarkable when the size of one pixel region or a repetition block increases.

In step S3 of FIG. 30, the lighting order data of each halftone cell in the repetition block SC is determined by the threshold determination unit 1.
06 (FIG. 4), and a plurality of density measurement points are set in the repetition block SC. This processing is almost the same as the processing in the first embodiment, and the density measurement points shown in FIG. 11 and the adjacent relation table shown in FIG. 12 are used as they are.

In step S4a of FIG. 30, the lighting bitmap memory 110 and the density value bitmap memory 112 (FIG. 4) used for determining the threshold distribution in the repetition block SC are secured in the main memory. You. The density value bit map memory 112 is the same as that used in the first embodiment, and is a memory area having a depth of 8 bits. On the other hand, the lighting bitmap memory 110 used in the second embodiment is a memory area having a depth of 2 bits. In the second embodiment, the value “0” is assigned to a non-illuminated pixel.
“0” (binary number) is set in the lighting bitmap, and “01” is set for a pixel lighting at a half density level.
“10” is set for each pixel that is lit at the 2/2 density level.

The nesting information (FIG. 14) of the lighting bit map of the second embodiment is different from that of the first embodiment.
FIG. 33 is an explanatory diagram showing the contents of the nesting information in the second embodiment. As can be seen from a comparison with FIG. 14, in the second embodiment, two values can be set as the lighting depth z. The lighting position where the lighting depth z is 1 is １ ／
Lighting at the density level means that the lighting depth z
The lighting position where is 2 means that the lighting is performed at the 2/2 density level. As a result, two lighting orders are set for the same lighting position. For example, the coordinate value (10) in the cell whose dot cell number is 00
(0, 120), the lighting order “1” where the lighting depth z is 1 and the lighting order “3” where the lighting depth z is 2
Is set.

At step S5a in FIG.
Based on the data prepared in 1a to S4a (that is, the density influence degree table in FIG. 31 and the access information in FIG. 33),
The threshold value of each pixel in the repetition block SC is determined. FIG. 34 is a flowchart showing the detailed procedure of step S5a. Among the steps shown in FIG. 34, step S1
Steps S1, S13 and S15 to S19 are the same as those in the first embodiment shown in FIG. 15, and the other steps S12 and S14 are slightly changed from the first embodiment.

In step S11, the repetition block SC
Is set, and the lighting bit (= 1) is set in the lighting bitmap memory 110. This process
This is the same as the first embodiment.

In step S12a, the density calculator 108
Calculates the effective local density value of the pixel affected by the lighting. FIG. 35 is an explanatory diagram showing a method for determining the substantial local density value of each pixel in the second embodiment. FIG.
(A-1) and (A-2) show lighting bitmaps before and after lighting of the central pixel of the 3 × 3 pixel area, respectively. FIGS. 35 (B-1) and (B-2) show the density influence degree tables for the central pixel before and after lighting, respectively. As can be seen from FIG. 35 (A-1), before lighting, 1/2 density level dots are formed at the left and lower pixel positions among the four pixel positions near the central pixel. As shown in FIG. 35 (B-1), the substantial local density value of the central pixel at this time is 1
26. On the other hand, as can be seen from FIG.
In the state after lighting, a dot of a half density level is formed in the center pixel itself. As a result, as shown in FIG. 35 (B-1), the substantial local density value of the center pixel becomes 189. Thus, the effective local density value for the center pixel is
It can be seen that it increases by 63.

As described above, in the present specification, it is assumed that the local density value of each pixel is affected by dots formed at positions near four of each pixel. Therefore, when the half density level is formed in the central pixel in FIG. 35B-2, not only the local density value of the central pixel is changed, but also the local density value of each pixel located in the four neighboring positions. Is also affected. FIG. 35C shows the difference (increment) between the local density values of all the pixels affected by the dot formed in the center pixel.

FIG. 35 (D-1) shows a density value bit map of a 5 × 5 pixel area before lighting.
The 3 × 3 pixel area at the center of the 5 × 5 pixel area corresponds to the 3 × 3 pixel area shown in FIG. FIG.
The density value bitmap after lighting shown in FIG. 5 (D-2) is the same as the density value bitmap before lighting shown in FIG.
It is obtained by adding the difference of the density influence degree shown in FIG.

Note that FIGS. 35 (B-1), (B-2),
As can be seen by comparing (D-1) and (D-2), FIG.
The density value bit map after lighting from the density influence degree table without calculating the difference as in FIG. 5 (C) (FIG. 35 (D-2))
Can also be obtained directly. In this case, five types of dot formation, one type of dot formation pattern with the lit pixel as the center pixel and four types of dot formation patterns with the four pixels near the four pixels as the center pixel, respectively. By referring to the density influence tables for the patterns, the substantial local density values at these five pixel positions may be determined.

In step S13 in FIG. 34, the density calculator 108 measures the image density (wide area density value) at each measurement point shown in FIG. 11B, and the wide area density of the measurement point affected by the lighting is measured. Update the measured value. This processing is the same as in the first embodiment.

In step S14a, a plurality of lighting candidates to be turned on next are selected from the vicinity of the measurement point indicating the lowest density value. FIG. 36 is an explanatory diagram showing a specific method of selecting a lighting candidate in the second embodiment. FIG. 36 shows that 3 of the halftone cells HC22 having the halftone cell number of 22.
This shows a case where lighting candidates are selected. Among the non-lighted pixels in the halftone cell HC22, the three pixels with the fastest lighting order are in the three lighting states with the lighting order of 102, 103, and 105. Note that the lighting order 102 is 1 from no lighting.
/ 2 density level.
The lighting order 103 means that the lighting at the １ ／ density level changes from the lighting at the 濃度 density level to the lighting at the ／ density level. Similarly, the lighting order 105 also changes from the lighting at the 1/2 density level to 2 /
This means that the lighting changes to two density levels. As described above, in the second embodiment, since there are a plurality of lighting states having different density levels, even when a lighting candidate is selected,
Lighting states having different density levels are selected and distinguished from each other.

The processing contents of steps S15 to S19 in FIG. 34 are almost the same as those in the first embodiment. That is, in step S15, the predetermined number of lighting candidate pixels selected in step S14a are temporarily lit one by one, and in a state where each lighting candidate is lit, measurement of a wide area density value is performed at each measurement point. In step S16, among the plurality of lighting candidates, the lighting candidate with the smallest density deviation is determined as the next lighting position. In step S17, it is determined whether or not the provisional threshold has been assigned to all the pixels in the repetition block SC. If the processing has not been completed, the process returns to step S12a, and the processing in steps S12a to S16 described above is repeated. After the assignment of the provisional thresholds for all the pixels is completed, in step S18, the provisional thresholds for all the pixels included in the repeated block are stored in a storage device such as a hard disk. Thereafter, in step S19, the provisional threshold is normalized to determine the final threshold.

As shown in FIG. 33 (B), in the second embodiment, two lighting orders for 1/2 density level and 2/2 density level are assigned to the same pixel position. Is set. Therefore, also in the threshold value matrix, two different threshold values for 1/2 density level and 2/2 density level are assigned to the same pixel position.

FIG. 37 is a diagram showing a half of the data generated in the second embodiment.
FIG. 4 is an explanatory diagram showing a first threshold value matrix for a density level and a second threshold value matrix for a 2/2 density level.
These threshold matrices are shown in FIGS. 32 (C-1) and (C-
As can be understood from comparison with the lighting order shown in 2), in the final threshold matrix, a value different from the lighting order is assigned as a threshold. The reason for this is that, in the second embodiment, as in the first embodiment, the threshold matrix is devised so that the gradation can be reproduced more accurately in consideration of the influence of the fringe of the dots.

FIGS. 38A and 38B are explanatory diagrams showing another example of the threshold value matrix generated according to the second embodiment. FIG. 38 (C) shows one halftone dot of a ternary halftone image formed using these threshold matrices.
As can be seen from this figure, when the threshold matrix generated in the second embodiment is used, halftone dots are formed such that the density of pixels decreases from the center to the periphery of the halftone dots. The halftone dot has a substantially concentric multilayer structure (multi-region structure), and relatively low-density pixels existing at the periphery of the halftone dot constitute an area having a width of a plurality of pixels. . By using such halftone dots, the density of the image can be reproduced more smoothly.

In the case of a ternary halftone dot image, the number of region layers constituting one halftone dot having different densities is two.
In the value halftone dot image, the number of layers in the region is three. In general, N
In a value halftone image (that is, a halftone image using (N-1) types of dots having different densities), one halftone dot is represented by (N-
1) It is composed of one area layer. Halftone dots having such a multilayer structure typically have an image density of about 10% to about 40%.
% Is formed. In a region where the density of an image is lower than this, there is a case where a halftone dot is formed only by one kind of relatively low-density pixel without having a multilayer structure. In a region where the density of an image is higher than this, high-density pixels of adjacent halftone dots are continuous with each other.
In some cases, the multilayer structure as shown in FIG.
Therefore, usually, a halftone dot having a multilayer structure is a part of a plurality of halftone dots included in a halftone image.

FIG. 39 is a block diagram showing the configuration of a multi-valued halftone dot image recording apparatus according to the second embodiment. This multi-valued halftone dot image recording apparatus includes matrix memories 202 and 204 storing two threshold matrices for 1/2 density level and 2/2 density level, and two comparators 206 and 208.
, A bit adjuster 210, and a multi-valued printer 212.

Note that this multi-valued halftone dot image recording apparatus also has the configuration shown in FIG.
As in the recording apparatus according to the first embodiment shown in FIG.
A U30, a main memory 32, a sub-scanning address counter 38, and a main scanning address counter 40 are provided, but are not shown in FIG.

The components of the multi-valued halftone dot image recording apparatus other than the multi-value printer 212 can be realized by a general-purpose computer such as a personal computer. In this case, the comparators 206 and 208 and the bit adjuster 210 are realized by a computer program, and the threshold matrix memories 202 and 204 are secured in a main memory of a general-purpose computer. These components 202, 20
4, 206, 208, and 210 are individually provided for each of a plurality of printing colors (normally, cyan, magenta, yellow, and black) used in the multi-value printer. Only the color components are shown.

Since a multi-valued color printer connected to a general-purpose computer is not designed to print a halftone image, it is necessary to print a color halftone image using a printer driver dedicated to the multi-valued color printer as it is. Is impossible. Therefore, when printing a color halftone image using a multi-valued color printer, the threshold matrix memory 20
2, 204, comparators 206, 208 and bit adjuster 21
A printer driver for printing a halftone image having a function of “0” is created and installed on a general-purpose computer. In this way, it is possible to generate a dot signal for printing a color dot image using the printer driver for printing a dot image, and supply the generated dot signal to a multi-valued color printer.

The input image signal Im is supplied to two comparators 20
6, 208, two threshold matrices 202, 2
The threshold values are compared with the threshold values Ss0 and Ss1 read from the address S04. Specifically, the first comparator 206 determines the threshold S
s0 is compared with the input image signal Im, and a binary output signal B0 corresponding to the comparison result is generated. The level of the binary output signal B0 is as follows. When Ss0 <Im: H level (lit) When Im ≦ Ss0: L level (non-lit)

The second comparator 208 is also the first comparator 20
The same processing as in step 6 is executed. A first output signal B0 (hereinafter, referred to as “lower bit”) output from the first comparator 206 and a second output signal B1 (hereinafter, “higher bit”) output from the second comparator 208 ) Is input to the bit adjuster 210. The bit adjuster 210 performs the following 2-bit ternary halftone dot signal Smulti for the four combinations of the upper bit B1 and the lower bit B0.
Is generated and output. When (B1, B0) = (0, 0): Smulti = 00 When (B1, B0) = (0, 1): Smulti = 01 When (B1, B0) = (1, 0): Smulti = 10 When (B1, B0) = (1, 1): Smulti = 10

In other words, when the upper bit B1 and the lower bit B0 are both 1, the bit adjuster 210 outputs
By changing the lower bit B0 to 0, a ternary halftone signal Smuulti is generated. In other cases, the ternary halftone signal Smulti is generated using the upper bit B1 and the lower bit B0 as they are. As a result, a dot signal for recording a ternary dot is generated from one multi-tone input image signal Im. Note that the bit adjuster 210 is omitted,
The upper bit B1 and the lower bit B0 can be used as they are as the 2-bit ternary halftone dot signal Smulti.

When the ternary halftone signal Smulti is supplied to the multilevel printer 212, the multilevel printer 212
A dot image composed of dot values is recorded.

As described above, in the second embodiment, in order to print a halftone image by a multivalued image recording apparatus using a plurality of dots having different densities, a threshold matrix reflecting the fringe of dots is created, and the threshold matrix is created. A dot image composed of multi-valued halftone dots is printed using a matrix. In other words, since the threshold matrix that reflects the effective density reproduction characteristics of the image caused by the fringe of the dots (that is, the density reproduction characteristics of the printer) is used, the density of the image can be reproduced more accurately. It is possible.

By the way, with respect to a multi-value color printer which is mainly sold and used for being connected to a general-purpose computer, there is usually no threshold matrix for recording a dot image. In such a case, if the second embodiment is applied, a threshold matrix for a multi-valued color printer can be successfully created. As a result, for example, when creating a proof print of a printed matter using a multi-valued color printer, using halftone dots having the same reproducibility as halftone dots used in final printing,
It is possible to make proofs.

The present invention is not limited to the above examples and embodiments, but can be implemented in various modes without departing from the gist of the invention.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of a relationship between a pixel size and a dot size.

FIG. 2 is an explanatory diagram showing an example of a cell of the one dot system.

FIG. 3 is an explanatory diagram showing an example of a supercell.

FIG. 4 is a block diagram showing a configuration of a threshold matrix creation device used in an embodiment of the present invention.

FIG. 5 is a flowchart illustrating a processing procedure according to the first embodiment;

FIG. 6 is an explanatory diagram showing the contents of a density influence degree table in the first embodiment.

FIG. 7 is an explanatory diagram showing a specific configuration of a density influence degree table.

FIG. 8 is an explanatory diagram showing a method of measuring a real density value of a target pixel.

FIG. 9 shows a repetition block S used in the first embodiment.
Explanatory drawing which shows C.

FIG. 10 is an explanatory diagram showing an example of an arrangement of lighting order in one dot cell HC.

FIG. 11 is an explanatory diagram showing positions and types of measurement points.

FIG. 12 is an explanatory diagram showing an adjacency relation table showing an adjacency relation between halftone cells;

FIG. 13 is an explanatory diagram showing the size of a bitmap memory.

FIG. 14 is an explanatory diagram showing the contents of nesting information.

FIG. 15 is a flowchart showing a detailed procedure of step S4.

FIG. 16 is an explanatory diagram showing a method for determining a substantial local density value of each pixel.

FIG. 17 is an explanatory diagram showing a method of measuring a wide area density value.

FIG. 18 is an explanatory diagram showing an example of a result of a wide area density measurement value.

FIG. 19 is an explanatory diagram showing a lighting candidate selection method.

FIG. 20 is an explanatory diagram showing a specific method of selecting lighting candidates.

FIG. 21 is an explanatory diagram showing a calculation result of a density value obtained in a state where three lighting candidates are turned on.

FIG. 22 is an explanatory diagram showing a relationship between a provisional threshold and a dot area ratio.

FIG. 23 is an explanatory diagram showing processing contents of threshold normalization.

FIG. 24 is an explanatory diagram showing an example of a threshold matrix generated in the first embodiment.

FIG. 25 is a block diagram showing a configuration of a binary halftone dot image recording apparatus.

FIG. 26 is a graph showing a comparison between the density deviation of the embodiment and the density deviation of the conventional example.

FIG. 27 is a graph showing a comparison between the density deviation of the example and the density deviation of the conventional example.

FIG. 28 is a graph showing a comparison between the density deviation of the embodiment and the density deviation of the conventional example.

FIG. 29 is an explanatory diagram showing another method of selecting a lighting candidate.

FIG. 30 is a flowchart illustrating a processing procedure according to the second embodiment;

FIG. 31 is an explanatory diagram showing the contents of a density influence degree table in the second embodiment.

FIG. 32 is an explanatory diagram showing a method of creating a lighting order in the second embodiment.

FIG. 33 is an explanatory diagram showing the contents of nesting information in the second embodiment.

FIG. 34 is a flowchart showing a detailed procedure of step S5a.

FIG. 35 is an explanatory diagram showing a method for determining a substantial local density value of each pixel in the second embodiment.

FIG. 36 is an explanatory diagram showing a specific method of selecting lighting candidates in the second embodiment.

FIG. 37 is an explanatory diagram showing an example of a threshold matrix generated in the second embodiment.

FIG. 38 is an explanatory diagram showing another example of the threshold value matrix generated in the second embodiment.

FIG. 39 is a block diagram showing a configuration of a multi-valued halftone dot image recording device according to a second embodiment.

[Explanation of symbols]

 Reference Signs List 30 CPU 32 main memory 34 floppy disk device 36 SPM memory 38 sub-scanning address counter 40 main scanning address counter 42 comparator 102 density influence table memory 104 threshold matrix memory 106 threshold determining section 108 ... Density calculation unit 110... Lighting bitmap memory 112... Density value bitmap memory 206, 208.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 1/46 H04N 1/46 Z (72) Inventor Hiroshi Sano 4 (72) Inventor Hiroshi Asai 4-chome Tenjin Kitamachi 1-1-1 Dainippon Screen Manufacturing Co., Ltd. F-term (Ref.)

Claims (13)

[Claims] 1. A method of recording a halftone image represented by using a halftone dot composed of one or more dots, comprising:
(A) a step of preparing an image recording device for visually recording an image; and (b) an image resulting from a difference between the size of a pixel and the size of a dot formed at each pixel position in the image recording device. And (c) comparing the threshold matrix with a multi-tone image signal representing an image to be printed to represent a halftone dot image. Generating a halftone signal; and (d) causing the image recording apparatus to record a halftone image composed of halftone dots by supplying the halftone signal to the image recording apparatus. A dot image recording method characterized in that: 2. A method for printing a color image composed of halftone dots, comprising: (a) preparing a color printer for printing a color image on a print medium using a plurality of printing primary colors having different hues. (B) preparing, for each of the plurality of printing primary colors, a threshold matrix reflecting the density reproduction characteristics of the color printer; and (c).
Generating a halftone signal from a color image signal representing a color image to be printed using the threshold matrix; and (d) supplying the halftone signal to the color printer, thereby forming a halftone dot. Printing the color image on the color printer. 3. The method according to claim 2, wherein the color printer is provided mainly for printing a color image on a print medium in accordance with an image density reproduction method other than the halftone method. ,Method. 4. The method according to claim 2, wherein the color printer can reproduce each pixel with a plurality of density gradations for at least one of the plurality of printing primaries. A multi-valued printer, wherein the threshold matrix for the printing primary colors reproducible with the plurality of density gradations has a plurality of thresholds per pixel; Generating a halftone signal representing a density gradation of each pixel by comparing a plurality of threshold values for each pixel with the color image signal for a printing primary color that can be reproduced in gradation. . 5. The method according to claim 4, wherein the printing primary colors reproducible at the plurality of density gradations are at least a part of a plurality of halftone dots constituting the printed color image. In the method, the threshold value matrix is configured such that, in the halftone dot, the density gradation of the pixel decreases from the center to the peripheral portion of the halftone dot. 6. The method according to claim 5, wherein the printing primary colors reproducible at the plurality of density gradations are at least a part of a plurality of halftone dots constituting the printed color image. In the halftone dot, the threshold value matrix is set such that a region of a relatively low density gradation that is not the highest gradation among the plurality of density gradations can be a region having a width of a plurality of pixels at the periphery of the halftone dot. Is composed, the method. 7. The method according to claim 4, wherein the plurality of density gradations are realized by a difference in the size of a dot recorded in one pixel area. 8. The method according to claim 4, wherein the plurality of density gradations is realized by a difference in density of ink forming dots recorded in one pixel area. ,
Method. 9. The method according to claim 2, wherein the color printer is a binary printer that reproduces each pixel in a binary manner with respect to a plurality of printing primary colors having different hues, and each threshold matrix includes: A method, wherein there is one threshold per pixel, and the maximum value of the threshold in each threshold matrix is less than the number of pixels included in each threshold matrix. 10. The method according to claim 9, wherein the thresholds in each threshold matrix are set such that there is a jump in value when the thresholds are arranged in order of magnitude. 11. The method of claim 9, wherein the thresholds in each threshold matrix are set such that there are at least two pixel locations such that adjacent pixel locations have the same threshold. ,Method. 12. An apparatus for recording a halftone image represented using halftone dots composed of one or more dots, comprising: an image recording unit for visually recording an image; A memory for storing a threshold matrix reflecting an effective density reproduction characteristic of an image caused by a difference between a pixel size in a portion and a dot size formed at each pixel position; By generating a halftone signal representing a halftone image by comparing the multitone image signal representing the image, and supplying the halftone signal to the image recording unit, A dot image recording apparatus, comprising: a dot signal generation unit that causes an image to be recorded in the image recording unit. 13. An apparatus for printing a color image composed of halftone dots, comprising: a color printer for printing a color image on a print medium using a plurality of printing primary colors having different hues; With respect to the printing primary colors, a memory that stores a threshold matrix that reflects the density reproduction characteristics of the color printer, and using the threshold matrix, generates a dot signal from a color image signal representing a color image to be printed, By supplying the halftone signal to the color printer,
A dot signal generating unit for causing the color printer to print a color image composed of halftone dots;