JPS61126868A - Color half tone image processing method - Google Patents

Abstract

PURPOSE:To obtain a regenerated image of high quality by denoting an optional integral number of threshold unit whose threshold is distributed high and low in two dimensions as N, n combining two rectangular matrics of NXN and nXn and disposing the threshold unit densely on the two dimensional plane. CONSTITUTION:When the regular matrix NXN and auxiliary matrix nXn are rectangular matrixes respectively in case that N, n are optional integral numbers, screen slope n/N or screen slope N/n is given. If an allotment of threshold value is performed in such a way that apparent difference of density appears and at least two sorts of regular matrix and auxiliary matrix whose characters are respectively different are prepared to be placed alternately toward a direction of screen slope indicated by alternate long and short dashes line, a different screen slope can be obtained. If newly obtained screen slope is represented by N, n, it can be generalized as (N+n)/(N-n) or (N-n)/(N+n). The screen slope of rational number can approximate the desired angle of screen properly in relation to number of tone wedges of density.

Description

[Detailed Description of the Invention] ■Field of the Invention The present invention relates to color image information processing of halftone images such as photographs and paintings, and in particular uses dots in a binary state of "recorded" and "non-recorded". The present invention relates to a method of reproducing gradations according to signals representing the density of each color by changing the ratio of "recorded" dots within a unit area.

■Conventional technology Methods for processing halftone images, that is, methods for reproducing the density of the original image, can be roughly divided into two methods: expressing the gradation by changing the size of the recorded dots and the density of the transferred ink dots. Density gradation method; ``recording'' and ``non-recording''
Use a dot in the binary state of .

There is an area gradation method that expresses gradation by changing the proportion of "recorded" dots within a unit area.

Of these, the size of the device itself can be reduced.

The area gradation method is often used because it is maintenance-free and the device can be provided at a low price.

Further, typical examples of this area gradation method include a density pattern method and a dither method.

In the density pattern method, the minimum reading unit (hereinafter referred to as a pixel) of the original image is associated with a region of a certain area (hereinafter referred to as the corresponding area) that has multiple dots in the reproduced image. Various patterns (density patterns) for recording the dots in the pixel are stored in advance, and a suitable density pattern is selected based on the density of the pixel from the group of patterns to represent the gradation in a pseudo manner. For example, if one pixel of the original image is N
When mapping to a square area (N is an arbitrary integer), various patterns in which N2 dots in the corresponding square area are "recorded n, II non-recorded" are stored in advance in the fixed memory in the device. By storing the data, it is possible to select, for each gradation, the dot pattern that best and naturally represents that gradation.

However, it is difficult to reduce the size of the dots below a certain level due to various constraints, and by increasing the number of expression gradations, the corresponding area becomes larger and the resolution deteriorates (the outline of the reproduced image becomes blurred).

On the other hand, if the corresponding area is made smaller, the resolution becomes higher, but the number of gradations per expression decreases, and smooth gradation expression cannot be expected. Further, since various density patterns must be stored in accordance with the required number of gradations, a large fixed memory is required.

For example, all density patterns based on the square corresponding area of N×N [dat2] are 2 N×Njl.

On the other hand, in the dither method, a threshold value that changes for each arriving pixel is compared with the density of the pixel, and the pixel is binarized. In the basic dithering method, one pixel of the original image is compared to an l threshold, where one threshold corresponds to one dot. The transitional threshold in the dither method can also be obtained by randomly generating a threshold corresponding to the number of read gradations (independent decision method), but in this case, the incidence of noise and the incidence of image information in the reproduced image are Therefore, a clear reproduced image cannot be obtained. Therefore,
A matrix (dither matrix) of threshold values that transition in a certain pattern (threshold value array) is stored, and by repeatedly using this (systematic dither method), binarization is performed with a certain regularity. In the systematic dither method (hereinafter simply referred to as the dither method), it is usually sufficient to store one dither matrix, so a small fixed memory capacity is required.

An example of a frequently used dither matrix threshold array is shown in FIGS. 3a to 3c. FIG. 3a shows what is called a Bayer type, and the arrangement is such that the difference between adjacent threshold values is as large as possible. In FIG. 3b, the elements are arranged in a meandering manner so as to form a spiral outward from the center as the threshold value increases (or vice versa), which is called a spiral type.

In Figure 3c, multiple thresholds with similar values are concentrated and arranged, and when viewed as a whole, the "record" dots intersect the matrix in a net shape at an angle of 45 degrees, so it is called a dot type. It is.

However, when one pixel and one dot are associated in this way, especially when the size of the read pixel of the original image is relatively large, that part may appear white or appear as a black dot in the reproduced image, so-called isolation. This produces a pixel effect. Therefore, in such cases, a method may be used in which a plurality of dots are assigned to one pixel of the original image in order to achieve a compromise with the density pattern method. Figures 2a to 2c show various embodiments of the dithering method (in the following description).

When the density of a pixel is greater than the threshold value, it is considered a "record" dot). Figure 2a shows the case where one threshold value corresponds to one pixel of the original image, and in a digital printer of equal size, the size of the read pixel and the dot are equal.6 Figure 2b shows the case where one pixel of the original image is - This is a case where one pixel of the original image corresponds to a part of the dither matrix, and FIG. 2C shows a case where one pixel of the original image corresponds to the entire dither matrix. The embodiment of the dither matrix shown in FIG. 2b or 2c is also used for enlarging an original image in a digital link or the like, or for receiving pixel density data from information sources having different reading levels. 2a to 2c are dither methods that repeatedly use a dither matrix, and will not be particularly distinguished in the following description.

Such a halftone image processing method is also used in digital color printers and the like that reproduce color halftone images. In these, yellow (Y).

3 colors of magenta (M) and cyan (C) (3 colors of paint)
The gradation of a color halftone image is expressed by subtractive color mixing in which gradation is given and superimposed based on the amount of each color component of the original image based on the above-mentioned method.

It is said that it is possible to express all colors (full color) by subtracting the three primary colors of this paint, but it is difficult to obtain vivid colors when they are combined in one layer. For example, black produced by superimposing three colors becomes a dull color, and black (B) is sometimes added to these in order to improve the quality of one image. In addition, due to factors such as the transparency of the ink, lower colors often do not come out well and do not follow subtractive color mixing.For this reason, in the field of conventional color printing, it is necessary to prevent each color component from overlapping. The color development through additive color mixing is stunning.

In this method, printing is performed by giving different forming angles (screen angles) to each printing plate formed in accordance with the above-mentioned area gradation method for each color. Figure 4a shows this screen angle in four-color printing with black added. A1
indicates the vertical and horizontal directions of the printing paper, and these are assumed to be the screen angle of 0 degrees. Bl + ci p Di has A1 at an angle of 4°.

It is rotated by 41+42 (to the left).

By the way, when printing is performed by giving different screen angles to each color, the occurrence of moiré becomes a problem. Moire is when one group of curved (straight) lines intersects another group of curved (straight) lines at an angle of 45 degrees or less, a new portion of the intersection points is created. This is a phenomenon in which the relative change in high frequencies between groups of curved (straight) lines is divided by a curved line and appears as a low-frequency striped pattern.This frequency division ratio increases as the angle becomes more acute.The human eye is sensitive to spatial frequency. Since it exhibits a low-pass characteristic in , if the angle at which each image (straight) line intersects is increased, the frequency of moiré generation shifts to the high frequency side, so that the striped pattern becomes invisible.

The above screen angle is set based on this theory. However, in this case, since orthogonal grids are targeted, the screen angle is set within a range of 90 degrees. If the degree of stimulation of each color arranged in each grid given a screen angle is equal, this angle divides 90 degrees equally (hereinafter,
(called equal distribution of screen angles).

In general, in the case of three-color printing using the three primary colors of yellow, magenta, and cyan, the screen angles are equally distributed assuming that the stimulation levels of each color are approximately equal, and the angle between each grid is 30
degree. These grids are 1 for a screen angle of 0 degrees.
A rotation of 5 degrees is given, and 4o is set to 15 degrees, 41 is set to 45 degrees, and 42 is set to 75 degrees.

Based on the above theory, if the screen angles are evenly distributed for the four colors (three primary colors of paint plus black), the angle between each grid will approach an acute angle, and the frequency of moiré occurrence will be lowered, making it more noticeable. . However, as mentioned above, yellow
Although it is assumed that the irritating dust of each of the three primary colors of magenta and cyan paint is approximately equal, they are actually quite different. Among coloring media such as ink, yellow is the most monochromatic and has the lowest irritating dust. Therefore, in the case of four-color printing, the screen angles are usually not equally distributed. In other words, rather than creating noticeable moiré between cyan or magenta, which has a relatively high amount of irritating dust, and black, which has zero saturation and has the highest amount of irritating dust, it is easier to visually create moiré between yellow. A method of absorption will be adopted.

In FIG. 4a, the grid A1 has a screen angle of 0 degrees.
Since the angle 43 or 44 between the grid A1 and the grid A1 is 15 degrees, the moiré frequency is the lowest, but by coloring this grid A1 with yellow, the irritant dust of yellow can be reduced as described above. It's so low that it's hardly noticeable. As for the other colors, for example, the grid B1 is colored magenta, the grid D1 is colored cyan, and the grid C1 is colored black.

Following such conventional printing technology, digital color printers that use the above-mentioned halftone image processing are also experimenting with color development that includes elements of additive color mixing by minimizing the overlap of each color component. . However, in digital color printers, etc., reading the original image,
Since all prints and the like are digitally processed, it is not possible to give angles θ to the density patterns or dither matrices M L , M 2 , etc. used for each color component, as shown in FIG.

Therefore, for example, there is a color halftone image processing method (Japanese Patent Application Laid-Open No. 146361/1983) using a dither method that uses a different dither matrix for each color. According to this, since the dither matrix is different for each color, inks of a plurality of different colors are not unnecessarily concentrated on the same pixel, and the image quality is significantly improved, especially in bright areas. However, in this method, the screen angles of the different dither matrices given to each color are equally 0 degrees, and the texture peculiar to the dither method becomes noticeable in the reproduced image due to the dither pattern that repeats at a low frequency.

On the other hand, a method of obtaining a screen angle by a density pattern method is disclosed in Japanese Patent Laid-Open No. 182372/1983. In this case, the density pattern is set so that the arrangement of "recorded" dots in the reproduced image forms a screen angle. For example, in FIGS. 6a to 6c, a 4×4 density pattern representing 16 gradations has a screen angle of 75
Indicates the condition that forms the degree. Figure 6a shows gradation 1, Figure 6b
The figure shows an example of gradation 2, and FIG. 6c shows an example of gradation 3. In the case of 16 gradations, the number of density patterns is 16 for gradation 1, 9 for gradation 2, 120 for gradation 2, and 560 for gradation 3.
It is expressed by combinations such as ``as''. Therefore, if this method is used to select the optimal density pattern that provides the desired screen angle, a huge amount of density patterns will have to be stored in a fixed memory. If the patterns are limited to 16 patterns for each gradation as shown in FIG.

■Purpose of the Invention The object of the present invention is to provide a color halftone image processing method that allows the screen angle to be easily set without requiring a large-capacity fixed memory, and that allows high-quality reproduced images to be obtained. .

■Structure of the Invention In order to achieve the above object, it is appropriate to use a dither method.

For digital color printers, etc., the above-mentioned 6th
As shown in the example of figure a, the line segment connecting the recording dots ``'' in the same gradation spread becomes the screen angle. That is, in a dither method that repeatedly uses one unit of dither matrix, the screen angle can be defined by a line segment connecting the centers of the dither matrices. Therefore, if a dither matrix is repeatedly used so that its center lies on a line segment indicating the desired screen angle, the resulting collection of dither matrices will form the set screen angle.

FIG. 7 shows a state in which identical rectangular matrices Ma are arranged so as to form a screen angle of 75 degrees.

This is 75 degrees left (
Since the shape is twisted by 15 degrees to the left, region Mb is generated as a lattice defect. Although this area Mb becomes a meaningless area in the reproduced image, it can be solved by providing an appropriate threshold value as another rectangular matrix (hereinafter referred to as a complementary matrix). However, as mentioned above, digital color
In a printer or the like, processing will be inconvenient unless the above-mentioned twist is applied using the minimum unit of one division (corresponding to one dot of the printer; hereinafter abbreviated as Div) defined by one threshold value in the dither matrix. Further, LDiv of a new complementary matrix due to a defect is made equal to LDiv of the original dither matrix (hereinafter referred to as a positive matrix).

That is, the above is equivalent to approximating the tangent of the screen angle θ to a rational number. For example, screen angle θ=
75a, jan75°=3.7320... Therefore, if we approximate this as 15/4, arc
tanl 5/4=75.0685...0.

Screen angle aret, an 15/4 [degrees]
(hereinafter referred to as screen gradient position 5/4), the positive matrix may be a 15x15 rectangular matrix, and the complementary matrix may be a 4x4 rectangular matrix.

In addition, by reversing the arrangement in Figure 7, an arrangement can be obtained in which the screen gradient is the reciprocal of the original screen gradient.6 The above means that if N and n are arbitrary integers, the positive matrix is N. ×N, and when the complementary matrices are n×n rectangular matrices, it can be generalized as giving a screen gradient position /N or a screen gradient N/n.

By the way, in FIG. 7, at least two types of positive matrices and complementary matrices with mutually different characteristics are prepared by allocating threshold values so that a clear difference in density appears, and the screen shown by the two-dot chain line is created. By alternately arranging them in the gradient direction, different screen gradients can be obtained.

This newly obtained screen gradient is defined as N above.

If expressed using n, (N + n) / (N -
n) or can be generalized as (N-n)/(N+n).

In this way, the screen slope of the rational number can appropriately approximate the desired screen angle in relation to the number of density gradations.

From the above viewpoint, the inventor has developed an N×N positive matrix and n
From the complementary matrix of ×n (N and ri are each integers),
As shown in Figure 8a, one side overlaps.

Create a new non-rectangular dither unit (hereinafter referred to as an N:n dither unit) with two vertical angles connected together; use this dither unit repeatedly to The screen slope position /N is obtained by densely arranging the dither units on a two-dimensional plane as shown in Fig. 8b; and by repeatedly using at least two types of dither units whose characteristics differ from each other depending on the threshold values they include, By arranging the screen densely on a two-dimensional plane in a roughly zigzag pattern, the screen gradient (N+n)/(N n)
He invented a method to obtain this and filed an application first. In this case, dithering is performed as shown in Fig. 9a, which is the inverse of Fig. 8a.
By configuring the unit (hereinafter referred to as a transposed dither unit for the Nun dither unit), the 9th
As shown in Figures b and 9c, screen slopes N/n and (N-n)/(Nten) can be obtained which provide supplementary angles to the screen angles in Figures 8a and 8c, respectively.

In the examples shown in Figures 8a-8c and 9a-9c, N:n = 4:1, and the respective screen slopes are bl = 1/4. ct=5in3゜b2=4.
C2=315.

The screen angle obtained by densely arranging such dither units or transposed dither units on a two-dimensional plane is not limited to that shown in FIG. 4a.

According to the theory of moire generation mentioned above, yellow, magenta,
In three-color printing using the three primary colors of cyan paint, it is sufficient that the grid spacing between each color is 30 degrees.

That is, as shown in Fig. 4b, if 45 is set to 30 degrees and 47 is set to 60 degrees, then grating A2, grating B2, and grating D2 are set at an angle of 30 degrees to each other, and the frequency of moiré generation is on the high frequency side. . In this case, if black is to be added, a grid C2 with a screen angle 46 of 45 degrees is provided. At this time, the distance between grid C2 and grid B2 or grid D2 is 15 degrees, but in the same way as above, grid C2 is colored yellow, which is less irritating, so that moiré is visually absorbed. Good.

In the present invention, +N: screen gradient position obtained by densely arranging n dither units or these transposed dither units on a two-dimensional plane /N
or (N+n)/ (N-n), or N/n
or (N n)/(N+n), approximately constitute a dense lattice that provides a screen angle of 30 degrees and a dense lattice that provides a screen angle of 60 degrees; these dense lattices and the known screen angle of 0 degrees Yellow (Y
), magenta (M), or cyan (C) to perform color halftone image processing.

In addition, as described above, approximately, a dense lattice that can obtain a screen angle of 30 degrees, a dense lattice that can obtain a screen angle of 60 degrees, a dense lattice that uses a dither matrix with a threshold array that can obtain a known screen angle of 0 degrees, and a dense lattice that can obtain a screen angle of 0 degrees, and Color halftone image processing is performed by associating yellow (Y), magenta (M), cyan (C), or black (B) with each dense lattice formed by a dither matrix with a threshold array to obtain a screen angle of 45 degrees.

However, when the arctangent of the screen slope is within the range of 30±10 degrees, it is approximated to 30 degrees, and when the arctangent of the screen slope is within the range of 60±IO degrees, it is approximated to 60 degrees.

On the other hand, if we densely arrange dither units with an N×N positive matrix and an n×n complementary matrix, then F= (N
” +n '2) Xm (However, 1m is the greatest common divisor of N and n, and N=N
Periodicity appears in F expressed as 'Xm, n=n'Xm).

Therefore, in a preferred embodiment of the present invention, an arbitrary rectangular matrix (hereinafter referred to as
The F matrix is stored in a fixed memory and used repeatedly to perform color halftone image processing. This allows the screen angle to be set extremely easily. In addition, by swapping the vertical and horizontal addresses for threshold value readout of this F matrix, a supplementary angle to the set screen angle can be obtained, which is equivalent to the dense lattice created by the transposed dither unit described above. Memory capacity can be further reduced.

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

FIG. 1a is a block diagram schematically illustrating a digital color copier embodying one aspect of the present invention.

This device consists of a scanner 1. A/D converter 2. Line buffer section 3. System control unit 4. Comparison section 5゜Printer 6
and a dither ROM, etc., and each component circuit is controlled by a signal from a system control section. The system control section is mainly composed of a CPU consisting of a microprocessor, etc., and includes a clock generator CLK; a timing control device TC that generates each transfer lock, synchronization signal, etc. based on the clock of CLK; It consists of an address control circuit, a keyboard, etc., which creates a threshold designation address for the dither ROM and each latch signal using a line synchronization signal and a pixel synchronization signal.

The binarization process of this color copying machine is as shown in the timing diagram shown in FIG. 1b. After reading and storing one line of an image, each pixel is sequentially binarized.
Printing is performed by synchronizing with a delay and transferring data in parallel. The general operation will be explained below.

When a document is set on the scanner 1 and a start command is input from the keyboard, line reading is started by the COD. The CCD consists of three rows of 2048 photoelectric conversion elements that read the image on the document surface at B[d a t /+w++1, and each element row is blue.

It is masked by a green or red filter plate, and its complementary color is yellow (Y).

Each color component of magenta (CM) or cyan (C) is read. The document is fed out by the step motor MoO every time one line of reading is completed. In this case, the reading by the CCD is called main scanning, and the feeding of the document by the step motor MoO is called sub-scanning.

When the line synchronization signal arrives from the CPU and the timing control device TC generates the scanner reading clock SCL 1, the COD converts the original image into yellow (Y), pixel by pixel,
It is separated into magenta (M) and cyan (C) components, and their densities are read by photoelectric conversion. The color components (densities) Y, M, and C of these pixels are analog signals, and the A/D of the first stage
The color data D is digitally converted in the converter 2.
Y.

Dm, DC and Db.

The A/D converter 2 includes A/D converters ADy', A[)m
.

It consists of ADc, equalizer EQ, etc.

The A/D converter ADy, ADm, or ADc converts the color component (density) Y, M, or C of the pixel into a digital value, and also divides and corrects the required number of gradations between the saturation density level and white level of each color. It has a γ conversion circuit to obtain each color data D 3' + D m or Dc. The equalizer EQ digitally converts the product component (density; synonymous with minimum value) of the color components Y, M, and C of a pixel, and corrects it using a coefficient determined through experiments (UCR correction) to adjust the required range between the white level and black level. Black color data Db&- is created by dividing into the number of gradations and correcting (γ conversion).

The color data Dy, Dm, Dc and Db are each transferred to the line buffer section 3. The line buffer section 3 includes a shift register Sry.

S rm , S re and Srb, and each shift register has an OR gate circuit Gl, G2 . A shift clock SCK is applied through G3 or G4, and when SCK occurs, each shift register takes in the corresponding color data.

By repeating the above steps, when all color data for one main scanning line is stored in each shift register, binarization processing is performed for each pixel.

The binarization process is performed by each digital comparator CPy, CPm, CPc or CPb of the threshold value comparison unit 5, using each color data DYyDm, Dc or Db and the dither R.
Threshold value shy read from OM.

This is done by comparing S hm , She or shb.

The dither ROM is a dither matrix (F
The threshold value is specified by the address A d dr of the address control circuit AC output. This threshold reading is yellow.

Magenta, cyan, and black are transferred in this order, and the threshold values for each digital comparator are transferred by transfer locks LCy, LCm, LCc, and LCb.
Retiming controlled. These transfer locks are also applied to each shift register at the same time, so each color data is transferred to a corresponding digital comparator, and the two are compared and binarized.

In this way, since each color is sequentially binarized, the outputs of the digital comparators do not coincide in time. Therefore,
The latches By, Bm, Be, and Bb are synchronized by the synchronization clock LC.

That is, latches BYyBmyBe and Bb are synchronous delay buffers.

Each binary data Sy, Sm, Sc or sb synchronized by the latch By, Bm, Be or Bb is connected to an AND gate circuit Gy+Gm.

Gc or NAND gate circuit Gb.

Here, "black replacement" replaces the product component of color components (densities) Y, M, and C with the equivalent color component B;
Alternatively, the binarized data is corrected depending on whether it is "black addition" in which an equivalent color component B is further added to the product component of the color components Y, M, and C.

This switching signal SB is input from the keyboard.

When it is "black replacement", the switching signal SB becomes '1';
The NAND gate circuit Gb output becomes re O# when sb is 1 (record)''; this is applied to the AND gate circuits Gy, Gm and Gc, and Gy, Gm and G
The output of c is the input (binarized data Sy, Sm or Sc
), it becomes PA0 (not written B)'' and is replaced with black.On the contrary, when it is "black addition", the switching signal SB becomes II O77, so the Gb output is always '1'. '', and all binarized data is output as is.

Binarized data Syo, Sag that has undergone such correction
o.

Sea and Sbo are transferred to the printer 6. Bu is a delay buffer for synchronizing Sbo.

The printer 6 has ink jet nozzles N y for each color.
, Nm, Nc and Nb are fixed, nozzle drivers NDy, NDm, NDc and NDb corresponding to each nozzle, and recording paper,
It is composed of a recording paper wrapping drum Dr and the like. Each nozzle
The driver inputs binary data D yo , D m
o, Dco, or Dbo to drive each ink jet nozzle on the carriage. Each ink jet nozzle produces a color print on the recording paper with an ink drop Iy, Im, Ic or Ib.

The above process is repeated while moving the carriage Car in the main scanning direction by the step motor Mol.
When printing for 1 minute is completed, the step motor Mo2 is driven in synchronization with the sub-scanning, the recording paper is fed out, and the same process is repeated.

These step motors are driven by the outputs of the timing controller TC, 5CL2. Controlled by PL or P2.

In the manner described above, the color image of the original document is reproduced on recording paper.

In the color copying machine shown in FIG. 1a, when performing three-color copying without using black CB), equalizer EQ,
The black processing systems such as the digital comparator CPb and the nozzle driver NDb are de-energized and the switching signal SB becomes It_Ol#.

This is a three-color copy of yellow (Y), magenta (M), and cyan (C).

Next, a detailed description will be given of a screen angle setting method using a dither matrix (F matrix) stored in the dither ROM.

As shown in FIGS. 8a and 9a, the dither unit of Nun divides one side of the n×n complementary matrix into a part of one side of the N×N positive matrix so that one vertex angle is adjacent to the other. This transposed dither unit is created by exchanging the positional relationship.

Such a dither unit may be used alone or as a set of a plurality of units having the same characteristics, and either of these units may be used repeatedly, and two dither units may be used as shown in FIGS. 8b and 9b.
Densely arranged in a dimensional plane (see below).

This is called distributed arrangement because there is no bias in the threshold array)
Screen slope position / N or N
A dense lattice with /n is obtained.

The following Table 1 shows the screen angle θ1 [degrees] which gives the screen slope position /N, and the screen angle 01'' [degrees] which gives the screen slope N/n, obtained for the value of the ratio Nun in one distributed arrangement. It shows the relationship between

That is, θ1 = arctan n / N to 1″
= arct, an N/n, and these θ1 and θl' are mutually complementary angles.

On the other hand, two types of dithers with different characteristics,
At least one of each is used repeatedly as a set, and the 8th c.
As shown in FIG.
As mentioned above, by densely arranging the grids on a dimensional plane (hereinafter referred to as concentrated arrangement because the threshold values are biased), a dense lattice having a screen gradient different from that in Table 1 can be obtained. The screen gradient in this case is (N + n) / (
N-n) or (N-n)/(N+N). The following table 2 shows the screen slope (N+n)/(N-n) obtained for the value of the ratio Nun in the concentrated arrangement.
It shows the relationship between the screen angle θ2 [degrees] which gives the screen gradient (N-n)/(N+n), and the screen angle 02'' [degrees] which gives the screen slope (N-n)/(N+n).

That is, if N, = N+n, N-=N-n, θ2
= arctan N + / N - tθ2'
= arct, anN −/N ten, and these θ
1 and 01'' are mutually complementary angles.

Table 2 In the present invention, color halftone image processing is performed using a dense lattice that can approximately obtain a screen angle of 30 degrees and a III lattice that can approximately obtain a screen angle of 60 degrees. These are 30 degree screen angle and 60 degree screen angle.
Degrees are complementary angles to each other, and this is the same as 01 and 01' above.
, or matches the relationship between θ2 and 02''.

In addition, in Tables 1 and 2 above, when the arctangent of the screen slope is within the range of 30±10 degrees, it is considered to be an approximate value of the screen angle of 30 degrees, and the arctangent of the screen slope is within the range of 60±10 degrees. Screen angle 60 when in range
If considered as approximate values of degrees, 26.6 degrees, 33.7 degrees, 31 degrees
．． 0 degree, 29.7 degree and 29.0 degree are screen angle 3
The approximate values are 0 degrees, 63.4 degrees, 56.3 degrees, 59.
0 degrees.

60.3 degrees and 61.0 degrees are approximate values for the screen angle of 60 degrees. These approximate values and the ratio Nun values in Table 1 or Table 2 are summarized as shown in Table 3 below.

Table 3 Figures 10a, 10b to 13a, and 13b are as follows:
This is a plan view of a part of Table 3.

In this, FIGS. 10a and 10b or 1
1a and 11b show the case where Nun dither units are arranged in a concentrated manner (hatched dither units and white dither units have different characteristics depending on the threshold values included); Figure 12b or Figures 13a and 13b show Nun's dither
This shows the case where units are distributed.

For example, if a 3:l dither unit is placed centrally,
A screen angle of 63.4 degrees is obtained as shown in FIG. 10a, and a supplementary angle of 26.6 degrees is obtained by the centralized arrangement of the transposed dither unit as shown in FIG. 10b. Figures 12a and 12b show that this same screen angle can be obtained with a 2=1 dither unit and a distributed arrangement of this transposed dither unit. Similarly, the screen angle of 59.0 degrees can be achieved by a concentrated arrangement of 4=1 dither units as shown in Figure 11a or a distributed arrangement of transposed 5=3 dither units as shown in Figure 13b. The obtained screen angle is 31.0 degrees.
A concentrated arrangement of 4:1 dither units with transposition as shown in Figure 11b or a 5=3 dither unit as shown in Figure 13a.
It is shown that this can be obtained by distributing dither units.

On the other hand, Fig. 14 shows the dithering that gives a screen angle of 0 degrees.
A dense lattice of the matrix is shown. This corresponds to section 12a above.
This is obtained by distributing rectangular dither matrices in the same manner as in the examples shown in FIGS. 13b to 13b. FIG. 15 also shows a dense grid of dither matrices providing a screen angle of 45 degrees. In this case, the characteristics of the hatched dither matrix and the white dither matrix are different, and the concentrated arrangement of rectangular dither matrices is performed in the same manner as in the examples shown in FIGS. 10a to 11b above. .

Next, N:n dither units are distributed or
A specific threshold array in the case of centralized arrangement will be described.

As described above, in the case of distributed arrangement, at least one dither unit can be used as one unit, but in the case of centralized arrangement, at least two dither units can be used as one unit. In color halftone image processing, it is preferable that the number of gradations for each color be as equal as possible, so in this embodiment, four dither units are combined into one composite unit and threshold arraying is performed.

FIG. 16a shows an example of a threshold array in a 40-gradation composite unit that obtains a screen gradient of 2 using a 3:1 dither unit. In this case, each dither unit is arranged in a substantially zigzag pattern so that the threshold values included are biased, so when the original image darkens, the dither units sequentially
In the figure, "record" dots appear in areas of the reproduced image (hereinafter referred to as reproduction areas) corresponding to the dither units at the upper right and lower left. After all of these playback areas become “recorded,” the playback areas of the upper left and lower right dither units are
Recording dots appear.Also, when melting, the opposite is true.The upper right and lower left dither units have different characteristics from the upper left and lower right dither units, so by densely arranging them, A screen slope of 2 will be obtained.

Figure 16b shows an example of a threshold array in a composite unit that obtains a screen slope position of /2 by means of a transposed dither unit. This is equivalent to exchanging the vertical address and the horizontal address in the threshold reading of the composite unit shown in FIG. 16a.

FIG. 17a shows an example of a threshold array to obtain a screen gradient of 5/3 by arranging threshold values in the same way as FIG. 16a in a 68-gradation composite unit using a 4:1 dither unit; The figure shows a threshold array in which the threshold read addresses in FIG. 17a are exchanged.

FIG. 18a shows an example of a threshold array in a 20-gradation composite unit that obtains a screen slope position of /2 with a 2:1 dither unit. In this case, there is no bias in the threshold values arranged in each dither unit, and when both y and K images are darkened, the reproduction area of each dither unit has an average value of
The record ``dot'' is cursed (the opposite is true when it melts). Therefore, each dither unit in a composite unit can be considered to have the same characteristics, and when composite units are densely arranged on a two-dimensional plane, The screen slope position /2 is obtained.If the threshold readout address in FIG. 18a is exchanged,
The threshold value array shown in FIG. 18b is obtained.

FIG. 19a shows an example of a threshold array for obtaining a screen gradient 315 in a 68-gradation composite unit using a 5:3 dither unit. In this a combination unit, two dither units having the same characteristic are densely arranged. When the original image darkens, ``record'' dots appear on average in all reproduction areas.Also,
In the case of melting, the opposite is true. Therefore, a screen slope 315 will be obtained with this composite unit.

If the threshold reading addresses in FIG. 19a are exchanged, the 19th
The threshold value array shown in figure b is obtained.

Figures 20a-20c show examples of threshold arrays in a rectangular dither matrix to obtain a screen angle of 0 degrees.

Figure 20a shows the 16-level dither matrix and the 20th level dither matrix.
FIG. b shows a dither matrix with 36 gradations, and FIG. 20c shows a dither matrix with 64 gradations, which are modifications of the spiral threshold array shown in FIG. 3b.

FIGS. 2A to 21C show examples of threshold arrays in a rectangular dither matrix that obtains a screen angle of 45 degrees. In this method, each dither matrix is equally divided into four sub-matrices, and a concentrated arrangement is performed in which the threshold values included in each sub-matrix are biased in a checkerboard pattern. FIG. 21a shows a dither matrix with 16 gradations, FIG. 21b shows a dither matrix with 36 gradations, and FIG. 21c shows a 641! This figure shows a ta-tone dither matrix, which is a modification of the halftone threshold array shown in FIG.

As mentioned above, in color halftone image processing, it is preferable that the number of gradations for each color be as equal as possible, so in this embodiment, a dither matrix with 16 gradations is combined with a composite unit with 20 gradations.

A dither matrix with 36 tones is a composite unit with 40 tones, and a dither matrix with 2 tones and 64 tones is 6 tones.
Used in combination with No. 8 composite unit.

By the way, N: n dither units (N, n are integers)
A dense arrangement of is obtained as a repeating rectangular pattern. Figure 22a shows 4: which forms a dense lattice on a plane.
1 shows a part of a collection of dither units. 22nd a
This dithering is shown with hatching in the figure.
The units are arranged in a rectangular shape with a certain periodicity. Therefore, this aggregate can be regarded as a repetition of the rectangular pattern shown by the thick solid line. If this period is F, then in general, F = (N'' + n'2) Xm (where m is the greatest common divisor of N and n.

As mentioned above, N==N'Xm, n=n'Xm).

Similar periodicity can be determined based on the value of F even in a dense arrangement of composite units on a plane. Therefore, in a preferred embodiment of the present invention, a rectangular matrix (F matrix) is extracted from a dense lattice made up of a collection of composite units based on this F, and this is repeatedly used.

For example, forming a dense lattice on a plane in Figure 22b 3:
A part of a composite unit assembly including a set of four l dither units is shown. As shown by hatching in FIG. 22b, it can be seen that this composite unit is similarly arranged in a rectangular manner with a certain periodicity. That is, it can be regarded as a repetition of the rectangular pattern shown by the thick solid line. This repetition becomes a cycle of 2F in the case of a composite unit created by combining two dither units in the substantially horizontal direction and two dither units in the substantially vertical direction as described above.

In this case, the value of N and n is, for example, N==3. If n==1 is given, the above F=10, so it is sufficient to extract an arbitrary F matrix of 20×20 from the plane of the dense lattice and use it repeatedly.

Each threshold value in the F matrix is specified by a vertical address and a horizontal address, and can be used repeatedly in the same way as a normal dither matrix.

In addition, such an F matrix can be used to obtain the supplementary angle of the screen angle given by the original F matrix by exchanging the successive addresses in the horizontal direction and the successive addresses in the vertical direction when reading the threshold value (hereinafter referred to as address exchange). Obtainable. In other words, it is equivalent to the F matrix extracted from the dense lattice of the complex unit by the transposed dither unit, and 1
By storing two F matrices, it is possible to obtain two types of screen angles. This allows the required fixed memory capacity to be further reduced.

Fig. 23a combines the F matrix shown in Fig. 22b extracted from a dense grid with a screen slope of 5/3 by the composite unit shown in Fig. 17a, and the dither matrix with a screen angle of 0 degrees shown in Fig. 20c. The manner in which "recording" dots of gradation 4 appear in the reproduced image in this case will be described. However, in this case, the F matrix is used as an equivalent F matrix of the screen gradient 315 by performing address exchange. In Figure 23a, the cross mark indicates the "record" dot with a screen slope of 5/3 (59,0 degrees).
The X mark represents a ``record'' dot with a screen slope of 315 (310 degrees), and the * mark represents a ``record'' dot with a screen angle of 0 degrees.For example, the * mark represents a yellow dot, and the x mark represents a magenta dot. The dots are arranged in different colors, and the cyan dots are arranged in the O mark.

Three-color printing is performed.

FIG. 23b shows how the "record" dots on the floor WR4 appear in the reproduced image when a screen angle of 45 degrees is further combined with the case of FIG. 23a. The 0 mark indicates a dot on the record with a screen angle of 45 degrees.In this case, for example, a yellow dot is placed on the O mark,
Mark with a magenta dot.

Four-color printing is performed with a cyan dot for the sign and a black dot for the X.

In FIG. 23a or 23b, one dot recorded for each color is set at a screen angle of 31.0 degrees.

59.0 degrees or 0 degrees; or screen angle 0
Since the angle of 31.0 degrees, 59.0 degrees, or 45 degrees is accurately maintained, the frequency at which moiré occurs is high, and unnatural striped patterns are no longer noticeable. At the same time, by using different screen angles for each color, the occurrence of texture is suppressed.

■Effects of the Invention As stated above, according to the present invention, it can be used repeatedly in the same way as a normal dither matrix, so the screen angle can be easily set without the need for a large-capacity fixed memory. It is possible to obtain high-quality, full-color reproduced images.

[Brief explanation of drawings]

FIG. 1a is a block diagram showing a schematic configuration of a digital color copying machine embodying one aspect of the present invention, and FIG.
3 is a timing diagram showing an outline of the processing of the device shown in the figure. Figures 2a, 2b and 2c are plan views showing the theory of dithering. FIGS. 3a, 3b, and 3c are plan views showing an example of a threshold array of a dither matrix. FIG. 4a is a plan view showing the screen angle in the conventional printing technique, and FIG. 4b is a plan view showing the screen angle used in the present invention. Figure 5 is a plan view S showing the grid that constitutes one screen angle.
be. FIGS. 6a, 6b, and 6c are plan views showing conventional examples. FIG. 7 is a plan view showing the basic principle of the present invention. 8a and 9a are plan views showing an example of a dither unit (threshold unit); FIGS. 8b and 9b are plan views showing a dense grid with a dense arrangement of dither units; FIGS. Figure 9c shows dithering with different characteristics.
FIG. 3 is a plan view showing a state in which units are arranged in a substantially zigzag pattern. Figures LOa, 10b, 11a and 11b show the screen angles obtained by the concentrated arrangement of dither units;
Figure 3b is a plan view showing the screen angle obtained by the distributed arrangement of dither units. FIG. 14 is a plan view showing a dither matrix with a screen angle of 0 degrees, and FIG. 15 is a plan view showing a dither matrix with a screen angle of 45 degrees. 16a@, 16b, 17a, 17b, 18a, 18b, 19a, 19b, 2
Figure 0a, Figure 20b, Figure 20c. FIGS. 21a, 21b, and 2Lc are plan views showing examples of threshold arrays. FIGS. 22a and 22b are plan views showing the principle of extracting the F matrix. Figures 23a and 23b are plan views showing the effects of the present invention. l: Scanner 2: A/D conversion section 3 Second line buffer section 4 System control section 5: Comparison section
6: Printer Kabuto 38A Part 3b ■
Kei 3c ■ No. 5 ■ Skull 48 A No. 4b ■ Fire 11a Corpse 11b ■ Kudzu+2a Kirikazu 13a ■ 'A 16a ■ Shu 17a ■ No. 16b ■ Kabuto 17b ■ Fan 20b Zukan 20c What No. 21b ■ Skull 21c Figure ￥ 123a■ Chapter 23b■

Claims (4)

[Claims] (1) Threshold units have thresholds that are distributed regularly or randomly, continuously or discontinuously, in height in two dimensions in association with the reading pixel divisions of the image. Identifies it in association with a two-dimensional address, compares the density of the primary color component of the pixel with the identified threshold value, and obtains a binarized image signal based on the height relationship between the two.
In color halftone image processing: A threshold unit in which the threshold values are distributed in height in two dimensions is formed by combining an N×N rectangular matrix and an n×n rectangular matrix, where N and n are arbitrary integers. ;n/N, or N/n, or (N+n) of a grid of line segments connecting thresholds of the same value with respect to a dense grid of thresholds obtained by densely arranging the threshold units on a two-dimensional plane; /(N-n) or (N-n)/(N+n) slope is 30 degrees, or
A color halftone image processing method that performs halftone image processing using a dense grid of a threshold value that can be regarded as an approximate value of a tangent of 60 degrees. (2) The gradient of the dense lattice of the threshold value is considered to be an approximate value of the tangent of 30 degrees when the value of the arc tangent of the gradient is in the range of more than 20 degrees and less than 40 degrees, and the gradient When the value of the arctangent of is in the range of 50 degrees or more and less than 70 degrees, 6
Claim No. 1 (1) which is regarded as an approximate value of the tangent of 0 degrees
) The color halftone image processing method described in section 2. (3) A dense lattice of the threshold value whose slope is an approximation of a tangent of 30 degrees, a dense lattice of the threshold value whose slope is an approximation of a tangent of 60 degrees, and a dither whose slope matches the dense lattice. - The color halftone image processing method according to claim (2), which uses a matrix to develop colors depending on the presence or absence of yellow, magenta, and cyan components. (4) A dense lattice of the threshold value whose slope is an approximation of a tangent of 30 degrees, a dense lattice of the threshold value whose slope is an approximation of a tangent of 60 degrees, and a dither grid whose slope matches the dense lattice. Claim (2): Using a matrix and a dither matrix whose gradient has a tangent value of 45 degrees, color development is performed with or without each of yellow, magenta, cyan, and black components.
The color halftone image processing method described in .