JP2004050791A - Imaging apparatus and imaging method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an imaging apparatus in which a high gradation number can be attained, consumption of image recording material, e.g. ink, can be minimized, and excellent gradation reproduction characteristics are attained. <P>SOLUTION: Using threshold value data T1 obtained by converting a reference threshold value matrix by a first threshold value matrix conversion table LUT1, continuous gradation image data G is converted into binary image data D1 and using threshold value data T2 obtained by converting a reference threshold value matrix by a second threshold value matrix conversion table LUT2, the continuous gradation image data G is converted into binary image data D2. A pixel is formed by driving a C head 28 based on the binary image data D1, and a pixel constituting the same gradation unit is formed by driving an LC head 32 based on the logical product data of the binary image data D1 inverted by an inversion circuit 33 and the binary image data D2. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention converts M gradation image data (M is an integer of 3 or more) into binary image data using a threshold matrix, and represents gradations by a plurality of pixels based on the binary image data. The present invention relates to an image forming apparatus and method for forming a unit and forming a gradation image as a set of gradation units.
[0002]
[Prior art]
In the field of printing, for example, after making plate plates of C, M, Y, and K colors with a plate making device such as an image setter, CTP (Computer To Plate), or CTC (Computer To Cylinder), each plate Is used to produce a color printed matter having a desired color and gradation.
[0003]
By the way, since a number of processing steps are required for producing a color printed matter, it is desirable that a proof image is output in advance and the color of the image and the halftone image structure can be confirmed. Conventionally, in order to create a proof image of a color printed matter, a proof image creation device having an output resolution comparable to that of a plate making device is used. However, such a proof image creating apparatus is considerably expensive, and a considerable burden is imposed on the site where the demand for printed matter is small.
[0004]
Therefore, in recent years, relatively inexpensive image forming apparatuses such as ink jet printers have appeared, and attempts have been made to create a proof image of a color print using such an image forming apparatus.
[0005]
In the ink jet printer, as shown in FIG. 8, the continuous tone image data G is compared with each threshold value data T constituting the threshold value matrix in the comparison unit 2, and when G ≧ T, 1 (pixel formation), G < In the case of T, binary image data D that is 0 (no pixel is formed) is generated, and the printer driver 4 is driven based on this binary image data D, whereby a halftone image can be formed. FIG. 9 shows the relationship between the continuous tone image data G and the gradation of the obtained halftone image.
[0006]
However, in the case of an ink jet printer, since it is difficult to form one pixel small, the substantial output resolution cannot be increased to the level of the output resolution of the plate making apparatus. Therefore, if the number of screen lines representing the number of halftone dots formed per inch is set to be the same as that of the plate making apparatus, a halftone image having the same gradation as that of a color printed matter cannot be obtained. End up. For example, in the case of the halftone image shown in FIG. 9, it is possible to express only up to 10 gradations including the case where pixels are not formed.
[0007]
Therefore, in an inkjet printer, in order to secure the number of gradations, it is common to convert continuous tone image data G to binary image data D using an error diffusion method to form an image. In this case, since the error diffusion method has a problem that the obtained image has a rough feeling, the ink constituting the color image is composed of cyan (C), magenta (M), yellow (Y), and black (K). In addition to normal four-color ink, light cyan (LC) with the same hue lower in density than cyan and light magenta (LM) with the same hue lower in density than magenta are used to create a color image with six inks. There is something to be formed.
[0008]
In this case, if a halftone image is formed using the above-described six colors of ink, it is expected that the halftone image particularly for cyan and magenta can be increased in gradation.
[0009]
[Problems to be solved by the invention]
However, in the conventional ink jet printer, the pixel forming process using the ink of each color is individually performed. For example, there is a possibility that a plurality of inks may be superimposed on the same position constituting the halftone image to form a pixel. . In this case, there arises a problem that the ink absorbing capacity of the paper as the image recording medium is exceeded and the paper is deformed or it takes a long time for the ink to dry. Also, the change in density due to the overlapping of inks of the same hue with different densities is generally small, so that not only the desired number of gradations cannot be obtained, but also a problem of wasteful consumption of ink occurs. become.
[0010]
The present invention has been made to solve the above-mentioned problems, and can obtain a high number of gradations, can minimize consumption of image recording materials such as ink, and is excellent. An object of the present invention is to provide an image forming apparatus and method capable of obtaining gradation reproduction characteristics.
[0011]
[Means for Solving the Problems]
In the present invention, M gradation image data is converted into first binary image data by using a first threshold value matrix for forming gradation units (halftone dots) representing gradation by a plurality of pixels. First pixels having a predetermined density are formed from the first binary image data. On the other hand, the M gradation image data is converted into the second binary image data by using the second threshold value matrix for forming the gradation unit in cooperation with the first threshold value matrix. Next, the first binary image data is inverted to generate inverted binary image data, logical product data of the inverted binary image data and the second binary image data is obtained, and the logical product data To form a second pixel having a predetermined density different from that of the first pixel.
[0012]
In this case, since the first pixel and the second pixel are not formed at the same position, ink or the like is not consumed wastefully, and the first pixel and the second pixel having different densities have a higher level. It is possible to form gradation units consisting of tones.
[0013]
Further, from the threshold value data T2 constituting the second threshold value matrix and the desired overlap adjustment parameter P, from the M gradation image data G,
When G <T2, D2 = 0
When 0 ≦ G−T2 <P, D2 = 1
When P ≦ G−T2, D2 = 0
After the second binary image data D2 to be obtained is obtained, a gradation unit can be formed from the first binary image data and the second binary image data D2.
[0014]
In this case, the overlap adjustment parameter P is adjusted so that the first pixel and the second pixel are not formed at the same position, or the first pixel and the second pixel overlap in a desired gradation range. Thus, a gradation unit that is not formed can be formed.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a configuration block diagram of an image forming apparatus 10 of the present embodiment that can be configured by, for example, an ink jet printer. In this image forming apparatus 10, in addition to the four colors of C, M, Y, and K, the LC ink having the same hue lighter than C and the LM ink having the same hue lighter than M are used. It is assumed that a color halftone image that is a toned image can be formed. In FIG. 1, only the configuration blocks for C and LC are shown, and descriptions of the other M, LM, Y, and K are omitted.
[0016]
The image forming apparatus 10 includes a reference threshold matrix storage unit 12 that stores a reference threshold matrix composed of 3 × 3 threshold data T shown in FIG. 2, and a first threshold matrix that converts the reference threshold matrix into a first threshold matrix composed of threshold data T1. 1 threshold matrix converter 14 and a second threshold matrix converter 16 that converts the reference threshold matrix into a second threshold matrix composed of threshold data T2.
[0017]
The first threshold value matrix is converted from the reference threshold value matrix by the first threshold value matrix conversion table LUT1 stored in the first threshold value matrix conversion table storage unit 18. The second threshold value matrix is converted from the reference threshold value matrix by the second threshold value matrix conversion table LUT2 stored in the second threshold value matrix conversion table storage unit 20. FIG. 3 shows an example of the first threshold matrix conversion table LUT1 and the second threshold matrix conversion table LUT2.
[0018]
In addition, the image forming apparatus 10 compares the threshold value data T1 supplied from the first threshold value matrix conversion unit 14 and the continuous tone image data G with a first comparison unit 22 (first binary image data conversion unit). ) And a second comparison unit 24 (second binary image data conversion unit) that compares the threshold data T2 supplied from the second threshold matrix conversion unit 16 with the continuous tone image data G.
[0019]
The binary image data D1 (first binary image data), which is a comparison result by the first comparison unit 22, is supplied to the cyan (C) driver 26, and the inversion circuit 33 (binary image data inversion unit) and The data is supplied to the light cyan (LC) driver 30 via the AND gate 35 (logical product data calculation unit). The binary image data D2 (second binary image data), which is a comparison result by the second comparison unit 24, is supplied to the LC driver 30 via the AND gate 35.
[0020]
A drive signal that is an output of the C driver 26 based on the binary image data D1 is supplied to the cyan (C) head 28 (first pixel forming unit), and is an output of the LC driver 30 based on the binary image data D1 and D2. A certain drive signal is supplied to a light cyan (LC) head 32 (second pixel formation unit). Note that the C head 28 and the LC head 32 form a halftone image relating to the hue of cyan by ejecting cyan ink and light cyan ink onto the recording medium 34.
[0021]
The image forming apparatus 10 according to the present embodiment is basically configured as described above. Next, the operation and effects thereof will be described.
[0022]
First, the first threshold value matrix conversion unit 14 reads out the 3 × 3 threshold data T constituting the reference threshold value matrix set as shown in FIG. 2 from the reference threshold value matrix storage unit 12, and the first threshold value matrix conversion. The first threshold value matrix conversion table LUT1 set in the relationship shown in FIG. 3 is read from the table storage unit 18, the threshold value data T is converted by the first threshold value matrix conversion table LUT1, and the threshold value data T1 is created.
[0023]
Similarly, in the second threshold value matrix conversion unit 16, the threshold value data T is read from the reference threshold value matrix storage unit 12, and the second threshold value matrix set in the relationship shown in FIG. 3 from the second threshold value matrix conversion table storage unit 20. The conversion table LUT2 is read, the threshold data T is converted by the second threshold matrix conversion table LUT2, and the threshold data T2 is created.
[0024]
In this case, threshold data T1 and T2 capable of expressing 21 gradations are created from threshold data T capable of expressing 10 gradations. As shown in FIG. 3, the threshold data T1 and T2 corresponding to the same threshold data T are set in a relationship of T1> T2. Further, the threshold data T1 and T2 corresponding to the highlight side of the continuous tone image data G are set in a relationship of T1 >> T2. The reason for this will be described later.
[0025]
Next, the first comparison unit 22 compares the magnitudes of the continuous tone image data G (M tone image data: M is an integer of 3 or more) and the threshold data T1 supplied from the first threshold matrix conversion unit 14. Then, binary image data D1 which is 1 (pixel formation) when G ≧ T1 and 0 (pixel non-formation) when G <T1 is supplied to the C driver 26, and the binary image data D1. Is inverted by the inversion circuit 33 and supplied to one input terminal of the AND gate 35 as inverted binary image data.
[0026]
On the other hand, the second comparison unit 24 compares the magnitudes of the continuous tone image data G and the threshold value data T2 supplied from the second threshold value matrix conversion unit 16, and if G ≧ T2, 1 and G <T2 are satisfied. In this case, binary image data D2 that is 0 (pixel not formed) is supplied to the other input terminal of the AND gate 35. Accordingly, the AND gate 35 always supplies 0 to the LC driver 30 when the binary image data D1 is 1, and supplies the logical product data according to the binary image data D2 to the LC driver 30 when the binary image data D1 is 0.
[0027]
The C driver 26 drives the C head 28 in accordance with a drive signal based on the binary image data D1, and forms a pixel (first pixel) with cyan ink on the recording medium 34. Further, the LC driver 30 drives the LC head 32 in accordance with a drive signal based on the binary image data D1 and D2, and forms a pixel (second pixel) of light cyan ink on the recording medium 34.
[0028]
FIG. 4 is an explanatory diagram of gradation units (halftone dots) constituting a halftone image formed by the cyan ink and light cyan ink formed as described above. A pixel indicated by hatching represents a pixel formed by light cyan ink having a low density, and a pixel indicated by cross hatching represents a pixel formed by cyan ink having a high density.
[0029]
In this case, the threshold data T1 and the threshold data T2 are set so that T1 >> T2 in the region where the threshold data T is small, as shown in FIG. Therefore, when the continuous tone image data G is in the range of 1 to 4, the binary image data D1 is all 0, and the C head 28 does not form cyan ink pixels. On the other hand, the binary image data D1 of 0 is inverted by the inversion circuit 33 and supplied to one input terminal of the AND gate 35 as inverted binary image data of 1. At this time, when continuous tone image data G in the range of 1 to 4 is supplied to the second comparison unit 24, the binary image data D2 that becomes 0 or 1 corresponding to the threshold data T2 = 1 to 4 is The logical product data corresponding to the binary image data D2 supplied to the other input terminal of the AND gate 35 is supplied to the LC driver 30. Therefore, the LC head 32 forms pixels with light cyan ink only by the LC head 32 at each position on the recording medium 34 corresponding to the threshold data T2 = 1 to 4 based on the drive signal from the LC driver 30.
[0030]
As a result, on the highlight side corresponding to the continuous tone image data G = 1 to 4, a fine gradation is expressed only by the light cyan ink whose density change is smaller than that of the cyan ink.
[0031]
When the continuous tone image data G is 5, the binary image data D1 corresponding to the threshold data T1 = 5 is 1, so that the C head 28 is positioned on the recording medium 34 corresponding to the threshold data T1 = 5. Thus, cyan ink pixels are formed. At this time, since one of the input terminals of the AND gate 35 is supplied with the inverted binary image data of 0 inverted by the inverter circuit 33, the output of the AND gate 35 corresponding to the threshold data T2 = 1 is 2 It is 0 regardless of the value image data D2. Therefore, light cyan ink pixels are not formed by the LC head 32 at positions on the recording medium 34 corresponding to the threshold data T1 = 5 and T2 = 1.
[0032]
As a result, as shown in FIG. 4, the gradation unit formed from the continuous gradation image data G = 5 has pixels of only light cyan ink at positions on the recording medium 34 corresponding to the threshold data T2 = 2 to 4. On the other hand, pixels of only cyan ink are formed at positions on the recording medium 34 corresponding to the threshold data T1 = 5.
[0033]
Similarly, for continuous tone image data G = 6 to 18, a gradation unit is formed at a position on the recording medium 34 where pixels are formed by the C head 28 without pixels formed by the LC head 32. Is done. In this case, since the cyan ink and the light cyan ink are not superimposed on the same position of the recording medium 34, the recording medium 34 is deformed by excessive ink, and it takes a long time to dry the ink. Does not occur. In addition, since only one of the inks is selected to form each pixel, ink consumption can be minimized, which is economical.
[0034]
In this way, a halftone image having approximately twice the gradation as in the case of the prior art shown in FIG. 9 can be formed by a combination of low density LC and high density C ink.
[0035]
In the image forming apparatus 10 described above, the binary image data D2 for driving the LC head 32 is controlled using the binary image data D1 for driving the C head 28, as shown in FIG. As in the image forming apparatus 40, the binary image data D2 can be controlled using a preset overlap adjustment parameter P. In the image forming apparatus 40, the same components as those of the image forming apparatus 10 are denoted by the same reference numerals, and description thereof is omitted.
[0036]
The first threshold value matrix storage unit 42 and the second threshold value matrix storage unit 44 store a first threshold value matrix and a second threshold value matrix configured by the threshold value data T1 and T2 shown in FIG. The first threshold value matrix and the second threshold value matrix are obtained by converting from the reference threshold value matrix using the first threshold value matrix conversion table and the second threshold value matrix conversion table, as in the case of the image forming apparatus 10. May be.
[0037]
The second comparison unit 46 (second binary image data conversion unit) uses the threshold data T2 supplied from the second threshold matrix storage unit 44 and the overlap adjustment parameter P supplied from the overlap adjustment parameter storage unit 48. The binary image data D2 is calculated from the continuous tone image data G as follows.
[0038]
That is, the second comparison unit 46
When G <T2, D2 = 0 (1)
When 0 ≦ G−T2 <P, D2 = 1 (2)
When P ≦ G−T2, D2 = 0 (3)
As described above, the binary image data D2 is calculated from the continuous tone image data G.
[0039]
Here, the overlap adjustment parameter P is set to
P = T1-T2 (T1> T2) (4)
The case where it is set as will be described.
[0040]
First, since the binary image data D1 and D2 are both 0 when the condition of the expression (1) is satisfied, no pixel is formed on the recording medium 34.
[0041]
Further, when the condition of the expression (2) is satisfied, the relationship of T2 ≦ G <T1 is obtained from the expression (4), so that D1 = 0 and D2 = 1, and the corresponding position on the recording medium 34 is obtained. In this case, pixels of only light cyan ink are formed.
[0042]
Further, when the condition of the expression (3) is satisfied, since the relationship of T1 ≦ G is obtained from the expression (4), D1 = 1 and D2 = 0, and the corresponding position on the recording medium 34 is Pixels using only cyan ink are formed.
[0043]
As described above, by obtaining the binary image data D2 according to the relationship of the expressions (1) to (4), the cyan ink and the light cyan ink are the same on the recording medium 34 as in the case of the image forming apparatus 10. It is possible to obtain the image forming apparatus 40 that does not overlap with the position to form pixels.
[0044]
Next, the overlap adjustment parameter P is set to
P> T1-T2 (T1> T2) (5)
The case where it is set as will be described.
[0045]
In this case, when the condition of the expression (1) is satisfied, the binary image data D1 and D2 are both 0 regardless of the relationship of the expression (5). Not formed.
[0046]
Further, when the conditions of the expressions (2) and (3) are satisfied, the relationship of T2 ≦ G <T1 + α (α> 0) is obtained from the expression (5), and therefore, the continuous floor where D1 = D2 = 1 is satisfied. A range of tonal image data G is generated. Therefore, the light cyan ink and the cyan ink are in a predetermined range during the transition from the state where the pixels are formed only with the light cyan ink on the highlight side to the state where the pixels are formed on the shadow side without overlapping the light cyan ink and the cyan ink. A partially overlapping pixel is formed.
[0047]
In the image forming apparatus 40, the binary image data D2 is controlled using the overlap adjustment parameter P set in advance according to the difference between the threshold data T1 and T2, but the image forming apparatus shown in FIG. 50, the overlap adjustment parameter P is selected from the overlap adjustment parameter storage unit 52 according to the continuous tone image data G, and the binary image data D2 is generated according to the equations (1) to (3). Also good. In this case, a gradation unit formed by superimposing light cyan ink and cyan ink can be set according to the continuous gradation image data G.
[0048]
Further, as in the image forming apparatus 60 shown in FIG. 7, the overlap adjustment parameter P is selected from the overlap adjustment parameter storage unit 62 according to the threshold data T2, and the binary image data D2 according to the expressions (1) to (3). May be generated. In this case, it is possible to arbitrarily set whether or not the light cyan ink and the cyan ink are formed so as to overlap each pixel constituting the gradation unit.
[0049]
In the above-described embodiment, the case of an inkjet printer that forms a halftone image using light cyan ink and cyan ink has been described. However, any image forming apparatus that can form pixels having the same hue and different densities. The present invention can also be applied to image forming apparatuses other than inkjet printers.
[0050]
In the above-described embodiment, a case has been described in which M tone image data (M is an integer of 3 or more) is converted to binary image data to form a halftone dot image. It is also possible to extend and apply when forming a halftone image by converting to image data (n is an integer of 2 or more).
[0051]
For example, in the image forming apparatus 10 shown in FIG. 1, the C head 28 and the LC head 32 are configured so that the size of pixels formed on the recording medium 34 can be controlled to three types: large, medium, and small. A quaternary image data is output from the first comparison unit 22 and the second comparison unit 24 and supplied to the C head 28 and the LC head 32 via the C driver 26 and the LC driver 30, whereby a network based on the quaternary image data is obtained. A point image can be formed. As a simple method for generating quaternary image data, a method of supplying three different sets of threshold data to each of the first comparison unit 22 and the second comparison unit 24 and comparing the magnitude with M gradation image data. Can be mentioned.
[0052]
【The invention's effect】
As described above, according to the present invention, it is possible to form a high gradation halftone dot image using a plurality of pixel forming portions capable of forming pixels having different densities. In this case, since it is possible to suitably avoid a situation where pixels having different densities are formed at the same position, for example, wasteful consumption of image recording material such as ink can be suppressed, and excessively Problems such as deformation of the image recording medium due to the image recording material being supplied to the image recording medium can be avoided.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a configuration of an image forming apparatus according to an exemplary embodiment.
FIG. 2 is an explanatory diagram of two threshold data generated from reference threshold data.
FIG. 3 is an explanatory diagram of a threshold matrix conversion table for generating two threshold data shown in FIG. 2;
FIG. 4 is an explanatory diagram of a halftone image generated using two threshold data shown in FIG.
FIG. 5 is a block diagram illustrating a configuration of an image forming apparatus according to another embodiment.
FIG. 6 is a block diagram illustrating a configuration of an image forming apparatus according to another embodiment.
FIG. 7 is a configuration block diagram of an image forming apparatus according to another embodiment.
FIG. 8 is an explanatory diagram of a halftone image generation method in the prior art.
FIG. 9 is an explanatory diagram of a halftone dot image generated by the prior art shown in FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10, 40, 50, 60 ... Image forming apparatus 12 ... Reference | standard threshold value matrix memory | storage part 14 ... 1st threshold value matrix conversion part 16 ... 2nd threshold value matrix conversion part 18 ... 1st threshold value matrix conversion table storage part 20 ... 2nd threshold value matrix Conversion table storage unit 22 ... first comparison unit 24, 46 ... second comparison unit 26 ... C driver 28 ... C head 30 ... LC driver 32 ... LC head 33 ... reversing circuit 34 ... recording medium 35 ... and gate 42 ... first Threshold matrix storage unit 44 ... second threshold matrix storage units 48, 52, 62 ... overlap adjustment parameter storage unit

Claims (8)

M gradation image data (M is an integer of 3 or more) is converted into binary image data using a threshold matrix, and gradation units representing gradation are formed by a plurality of pixels based on the binary image data. An image forming apparatus for forming a gradation image as a set of gradation units,
A first binary image data conversion unit that converts the M gradation image data into first binary image data using a first threshold value matrix for forming the gradation unit;
A second binary image for converting the M gradation image data into second binary image data using a second threshold matrix for forming the gradation unit in cooperation with the first threshold matrix. A data converter,
A binary image data reversing unit that inverts the first binary image data and generates inverted binary image data;
A logical product data calculation unit for obtaining logical product data of the inverted binary image data and the second binary image data;
A first pixel forming unit for forming first pixels having a predetermined density based on the first binary image data;
A second pixel forming unit for forming a second pixel having a predetermined density different from that of the first pixel based on the logical product data;
And the gradation unit is formed by the first pixel and the second pixel. The apparatus of claim 1.
The threshold value data constituting the first threshold value matrix is set to a larger value than the threshold value data constituting the second threshold value matrix, and the density of the first pixel formed by the first pixel forming unit is the first threshold value. An image forming apparatus, wherein the density is set to be higher than the density of the second pixel formed by the two-pixel forming unit. M gradation image data G (M is an integer of 3 or more) is converted into binary image data using a threshold matrix, and a gradation unit representing gradation is formed by a plurality of pixels based on the binary image data. An image forming apparatus for forming a gradation image as a set of gradation units,
A first binary image data conversion unit for converting the M gradation image data G into first binary image data using a first threshold matrix for forming the gradation unit; and the first threshold. Using the threshold data T2 constituting the second threshold matrix for forming the gradation unit in cooperation with the matrix and the desired overlap adjustment parameter P, the M gradation image data G is
When G <T2, D2 = 0
When 0 ≦ G−T2 <P, D2 = 1
When P ≦ G−T2, D2 = 0
A second binary image data conversion unit that converts the second binary image data D2 into
A first pixel forming unit for forming first pixels having a predetermined density based on the first binary image data;
A second pixel forming unit that forms second pixels having a predetermined density different from that of the first pixels, based on the second binary image data D2.
And the gradation unit is formed by the first pixel and the second pixel. The apparatus of claim 3.
2. The image forming apparatus according to claim 1, wherein the overlap adjustment parameter P is set according to the M gradation image data G. The apparatus of claim 3.
The image forming apparatus, wherein the overlap adjustment parameter P is set according to the threshold data T2. The apparatus of claim 3.
The overlap adjustment parameter P is threshold data constituting the first threshold value matrix T1,
P ≧ T1-T2 (however, T1> T2)
An image forming apparatus that is set as: M gradation image data (M is an integer of 3 or more) is converted into binary image data using a threshold matrix, and gradation units representing gradation are formed by a plurality of pixels based on the binary image data. An image forming method for forming a gradation image as a set of gradation units,
Converting the M gradation image data into first binary image data using a first threshold matrix for forming the gradation unit;
Converting the M grayscale image data into second binary image data using a second threshold matrix for forming the grayscale unit in cooperation with the first threshold matrix;
Obtaining logical product data of inverted binary image data obtained by inverting the first binary image data and the second binary image data;
Forming a first pixel having a predetermined density based on the first binary image data;
Forming a second pixel having a predetermined density different from that of the first pixel based on the logical product data;
And forming the gradation unit by the first pixel and the second pixel. M gradation image data G (M is an integer of 3 or more) is converted into binary image data using a threshold matrix, and a gradation unit representing gradation is formed by a plurality of pixels based on the binary image data. An image forming method for forming a gradation image as a set of gradation units,
Converting the M gradation image data G into first binary image data using a first threshold matrix for forming the gradation unit;
Using the threshold data T2 constituting the second threshold matrix for forming the gradation unit in cooperation with the first threshold matrix and the desired overlap adjustment parameter P, the M gradation image data G is ,
When G <T2, D2 = 0
When 0 ≦ G−T2 <P, D2 = 1
When P ≦ G−T2, D2 = 0
Converting to second binary image data D2
Forming a first pixel having a predetermined density based on the first binary image data;
Forming a second pixel having a predetermined density different from that of the first pixel based on the second binary image data D2.
And forming the gradation unit by the first pixel and the second pixel.

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