JPH09254411A - Method and apparatus for forming image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an operation load to obtain binary data and to prevent the deterioration in image quality due to the reduction of the operation load in image forming using two kinds of light and dark inks. SOLUTION: The respective inequalities of a dither pattern Du for light ink and a dither pattern Dk for dark ink are made 1 to reduce an operation load for binary processing. Simultaneously, for image data in which gradation levels (x) are distributed uniformly, a dither pattern having a threshold value and its pattern is made so that light ink dots are always formed in the adjacent picture element when dark ink dots are formed, and the diameter of each formed dot is enlarged to decrease a ratio indicating the ground of a medium to be printed to reduce granular feeling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus and an image forming method, and more particularly to generation of binary image data when an image is formed using a plurality of inks having different densities.

[0002]

2. Description of the Related Art Conventionally, various methods such as a dither method and a density pattern method have been known as methods for generating binary image data from multi-valued image data. Among these methods, an error diffusion method, an average density preservation method, etc. are known as a binarization method for expressing a natural image in high quality.

The output result printed by an image forming apparatus such as an ink jet printer based on the data binarized by these binarization methods can reflect the gradation of an image to be output relatively faithfully. When compared with the output result output by a sublimation type thermal transfer printer or the like based on the value image data, the image quality is often inferior in halftone expression. That is, the halftone expression based on the binary data is eventually expressed by the ratio of the density of dots to be printed, whichever binarization method is used. In this case, as the density of the dot itself is higher than the surface of the paper on which the dot is printed, and as the diameter of the dot is larger, the graininess becomes more conspicuous and the image quality is inferior. Therefore, in order to improve the image quality of halftone expression, the dot diameter is made smaller by reducing the amount of ink droplets forming one dot.

The density of ink is reduced to reduce the density of one dot.

There are two possible methods.

First, in the method, in order to reduce the ink amount of one dot, it is necessary to make the ejected ink into smaller droplets. For that purpose, fine processing of the ink ejection port of the ink jet head or the like is required. There is a problem that it takes a relatively long time to improve the technology.

Next, in the method, the maximum density in the image cannot be realized by simply reducing the density of the ink, and the image to be printed is not conspicuous.

On the other hand, there is known a method of ensuring image quality using two kinds of inks having different densities. That is, when expressing a low-density halftone, printing is performed using low-density ink to realize an image in which graininess is less noticeable, and when expressing a high-density halftone,
It is possible to print by using ink of high density and ensure an image without a decrease in density.

FIG. 1 shows an example in which low-density ink (hereinafter referred to as light ink) and high-density ink (hereinafter referred to as dark ink) are used to express 33 gradations in a 4 × 4 dither pattern. Is shown. In the figure, shaded circles represent dots formed with light ink, and circles filled with black represent dots formed with dark ink. As shown in the figure, the minimum level is 0 when only a white background of the paper on which dots are not formed is set, and the maximum level is 32 when 16 dots are formed by dark ink in the entire 4 × 4 area,
33 gradations can be expressed by the levels in between.

FIG. 2 is a graph showing the number of dots printed with light ink and dark ink for each of the gradation expressions shown in FIG. In the figure, the horizontal axis represents the gradation level, and the vertical axis represents the number of printed dots. In addition, a curve indicated by A in the drawing shows the number of dots of light ink hit, and a curve indicated by B shows the number of dots of dark ink hit.

As is apparent from FIG. 2, when the gradation level is 0, the number of dots printed for both light ink and dark ink is zero. Thereafter, the number of dots of the light ink increases by 1 every time the gradation level rises by one rank, and the number of dots of the light ink shot at the gradation level 16 becomes 16. After that, each time the gradation level rises by one rank, the gradation level is increased by replacing the dots of the light ink with the dark ink, and at the gradation level 24, the number of dots of the light ink and the dark ink is equalized. At the adjustment level 32, the light ink is completely replaced with the dark ink, the number of dots of light ink is zero, and the number of dots of dark ink is 16
become.

FIGS. 3 (a) and 3 (b) are diagrams for explaining the operation contents of the dither pattern shown in FIG. 1, that is, the threshold value. The figure (a) is an arithmetic expression pattern Du for light ink, and the figure (b) is an arithmetic expression pattern D for dark ink.
indicates k.

The image data is represented as gradation level data x for each pixel, and this data x is standardized as 0≤x≤32. Binary data for each pixel is performed by sequentially corresponding the dither patterns Du and Dk of the above-described arithmetic expression to 4 pixels × 4 pixels of the image data represented in this way and performing the operation based on each threshold value. To generate.

FIGS. 4A and 4B are diagrams schematically showing binary data generation using a dither pattern. FIG.
(A) shows the binarization process of the image data 1 having uniform density, and (b) shows the binarization process of the image data 2 having nonuniform density distribution.

As shown in FIGS. 3 (a) and 3 (b), respectively, the results obtained by associating the image data 1 or 2 with the patterns Du and Dk of the arithmetic expressions shown in FIGS. 3 (a) and 3 (b). As for the image data 1, binary image data 3 for forming light ink dots and binary image data 4 for forming dark ink dots are obtained. In the binary image data 3 and 4, binary data of "1" is generated for a pixel in which dots are to be formed, and "0" is generated for a pixel in which dots are not to be formed. Similarly, for image data 2, binary image data 5 for forming light ink dots and binary image data 6 for forming dark ink dots are obtained. In the binary image data 5 and 6, "1" and "0" have the same meanings as described above.

[0016]

In the binary image data generation example using the dark and light inks shown above, in the case of the image data 1 having a uniform density, it is clear from the data 3 and 4 in FIG. 4 (a). As such, the data does not exist only for white background and dark ink dot data. In other words, in general, when forming an image of uniform density, the paper surface is covered with any combination of white background and light ink or light ink and dark ink. Therefore, it is possible to obtain high image quality with suppressed graininess.

By the way, as shown in FIG. 3A, each arithmetic expression of the light ink dither pattern Du performs two inequalities on the input multi-valued data x. This is because when the binary data for light ink is generated, the number of dots printed with light ink is decreased as the density of the image increases as shown in FIG. Therefore, the arithmetic expression of the dither pattern is also required to be represented by two inequalities. However, performing the calculation of two inequalities using two threshold values in each pixel as described above has a problem of increasing the calculation load.

That is, in general, the binary image is generated on the host computer side, and the binary image data is transferred to the printer for printing, and the binary image data is transferred to the printer as it is. In some cases, the binary image data generation circuit on the printer side generates and prints the binary image data. In this case, if the load of the binary image generation operation is relatively large as described above, the time for image data generation becomes longer in the former case, and the circuit scale becomes larger in the latter case. There is a problem that it becomes large.

The present invention has been made in view of the above-mentioned problems, and an image forming apparatus and an image capable of reducing the calculation load for binarization and preventing deterioration of image quality due to the reduction of the load. It is to provide a forming method.

[0020]

According to the present invention, there is provided:
In an image forming apparatus that prints by forming dark and light ink dots on a print medium, binarization processing is performed on a value indicated by image data by using a dither pattern to print dark and light ink dots. 2 to form
Binarizing means for generating value data, wherein the threshold values of the dither pattern corresponding to dark and light inks are
Based on the binarizing means which is set to a value such that the number of dots formed in each of the dark and light inks does not decrease at least as the value of the image data increases, and the binary data generated by the binarizing means. Printing means for forming dots of dark and light ink on a print medium, respectively.

Further, in an image forming apparatus for forming dots of dark and light ink on a print medium for printing, binarization processing is performed on the value indicated by the image data by using a dither pattern to obtain dark and light. It is a binarization unit that generates binary data for forming dots of each ink, and the threshold value of the dither pattern corresponding to each of the dark and light inks is the number of dots of the light ink formed. The value increases up to a predetermined value of the image data, and the number is constant above the predetermined value, and the number of formed dark ink dots is 0 up to the predetermined value, and above the predetermined value. A binarizing unit that is determined to increase and prevents binary data of each of the dark and light ink dots from being complementarily generated in the dither pattern, and the binarizing unit. Based on the binary data that, concentrated respectively, characterized in that comprises a printing means, the forming the light ink dots onto the print medium.

Preferably, the printing means sets the diameter of each of the dots of the dark and light inks to be formed to be larger than the pixel pitch.

Further, in an image forming method for forming dots of dark and light inks on a print medium for printing, the dither pattern data corresponding to the dark and light inks, and the respective threshold values, Prepare dither pattern data set to a value such that the number of dots formed in each of dark and light ink does not decrease at least as the value of image data increases, and use the dither pattern data to determine the value indicated by the image data. Binarization processing is performed on the above to generate binary data for forming respective dots of dark and light inks, and based on the generated binary data, dots of dark and light inks are respectively printed on the print medium. And forming each step.

In the image forming method of forming dots of dark and light inks on a print medium for printing, the dither pattern data corresponding to each of the dark and light inks, and the threshold value of the dither pattern data. However, the number of light ink dots formed increases up to a predetermined value indicated by the image data, and when the number of light ink dots is greater than or equal to the predetermined value, the number is constant, and the number of dark ink dots formed is the predetermined number. Up to the value is 0, and dither pattern data is prepared so as to increase above the predetermined value, and binary data of each of the dark and light ink dots is not complementary generated in the dither pattern data. To use the dither pattern data to perform binarization processing on the value indicated by the image data to form dots of dark and light ink, respectively. It generates binary data, on the basis of the binary data the generated dark respectively, to form a light ink dots onto the print medium, characterized by having the steps.

According to the above construction, the threshold value of the dither pattern used for the binarization processing should be such that the number of dots formed in each of the dark and light inks does not decrease at least as the image data value increases. Since the above threshold value only needs to set the lower limit of the image data value, it is possible to always have one inequality using the threshold value.

In the area of the image data value where the light ink dots and the dark ink dots are mixed, the light ink dots are always formed when the dark ink dots are formed, and these are formed to overlap each other. Instead, light ink dots are formed around the dark ink dots. Further, by making these dots larger than the pixel pitch, it is possible to reduce the portion where the printed medium appears.

[0027]

DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

FIGS. 5 (a) and 5 (b) are diagrams showing the dither pattern for generating binary data according to the embodiment of the present invention in terms of the calculation contents, and FIG. 5 (a) is the dither pattern for light ink. The pattern Du and FIG. 9B show the dither pattern Dk for dark ink. In the example shown in the figure, the dot is not formed regardless of the value of the multi-valued data x of the input image.

As is apparent from FIGS. 5A and 5B, in the binarization processing of this embodiment, the inequality operation may be performed once on the data x of each pixel.
This can reduce the calculation load.

Further, in the case of image data in which gradation levels are evenly distributed, when a dark ink dot is formed, a light ink dot is always formed in all the adjacent pixels, so that the dark ink dot and the white background are formed. It is possible to prevent the graininess from becoming conspicuous due to the mixture of and.

In this embodiment, in addition to using the dither pattern as described above, printing is performed in which the diameter of formed dots is large.

FIGS. 6A and 6B are print examples according to the present embodiment, in which the data of each pixel is x = 16 and 1
The case of printing with 100% duty, that is, solid printing is shown.

In the example shown in the figure, the diameter of the ink dot is 1.8 times the pixel pitch. In addition, 1.
One way to form dots of 8 times the diameter is to increase the amount of ink that is ejected to the pixels.
A known structure can be adopted as a structure for that purpose. For example, a method is known in which the structure of an inkjet head is set to a large discharge amount, or the energy applied for discharging is increased.

FIG. 6A shows a print result when solid printing is performed with light ink. In the figure, A is a portion where dots are not formed and remains as a white background, and B is light ink dots. The area covered by. In the case of this embodiment in which the dot diameter is 1.8 times the pixel pitch, the white background portion A is an area of about 2% of the entire area. On the other hand, FIG.
(B) shows a printing result when solid printing is performed with dark ink, in which C is a portion where dots are not formed and remains as a white background, and D is an area covered with dark ink dots. is there. When performing solid printing, since the dots of the light ink and the dark ink shown in FIGS. 6A and 6B are actually overlapped, the white background portion is completely filled with the light ink or the dark ink. Does not exist.

On the other hand, as shown in FIGS. 7A and 7B, even in solid printing, when the value of the image data x is x = 11 in each pixel, FIG. The five portions of A, B, C, D, and E that are not covered with the light ink dots are left uncovered by the dark ink dots shown in FIG. However, by setting the dot diameter to 1.8 times the pixel pitch as described above, the ratio of the area occupied by the white background to the entire image is relatively small, about 1.3%.

Similarly, when the area ratio of the white background portion is obtained for the solid print result in which the input level is 8 ≦ x ≦ 14 in which the light ink and the dark ink are mixed, the image data area ratio x = 9 1.8% x = 10 1.5% x = 11 1.3% x = 121% x = 13 0.8% x = 14 0.5% x = 15 0.3%. Originally, in the case of performing mixed printing of light ink and dark ink, it is desirable that the white background portion be 0%, but in the subjective evaluation of the image by the inventor's experiment, if the area occupancy ratio is relatively small as described above, , It has been found that it has little effect on image quality.

As described above, by using the dither pattern of this embodiment, the calculation load is reduced, and when dark ink dots are formed, light ink dots are always formed around them. By increasing the dot diameter, it is possible to reduce the ratio of the white background and further reduce the graininess due to the contrast between the white background and the dark ink dots.

Next, the amount of ink used when an image is formed using the dither pattern of the embodiment will be described. When the amount of ink required to cover the entire surface of the paper without overlapping coating per unit area is 100%, the amount of ink used is as shown below.

Image data Ink usage x = 0 0% x = 1 13% x = 2 25% x = 3 38% x = 4 50% x = 5 63% x = 6 75% x = 7 88% x = 8 101% x = 9 113% x = 10 126% x = 11 138% x = 12 151% x = 13 163% x = 14 176% x = 15 188% x = 16 202% On the other hand, how much ink can be ejected depends on the ink receiving ability of paper or the like, and the higher the receiving ability, the higher the technical difficulty for paper production. In this case, the paper is required to have a receptive capacity of about 200% at the maximum density, that is, an ability to absorb twice as much ink as necessary to cover the entire surface of the paper. The absorption capacity is low in technical difficulty, and is a level that does not require a special drying device such as a heater to increase the absorption capacity. Further, in the case of full-color printing, even if the receiving capacity of 400%, which is twice as much as the secondary color, is required, it is within a range that can be dealt with only by applying a low-cost paper surface coating agent.

FIG. 8 is a schematic perspective view showing an example of an ink jet recording apparatus according to the gradation recording method.

In this figure, the carriage 706 includes:
Eight ink tanks 7 that store dark and light inks for black, cyan, magenta, and yellow inks
01 and eight multi-heads 702 for respectively ejecting the dark and light inks of the above four colors are mounted.

Further, reference numeral 703 denotes a paper feed roller, which rotates together with the auxiliary roller 704 in the direction of the arrow in the drawing while applying a constant frictional force to the recording paper 707, and conveys the recording paper 707 at any time in the y direction. Also, reference numeral 705
Denotes a paper feed roller for feeding the recording paper 707, and also has a function of imparting a certain strength to the recording paper 707, like the rollers 703 and 704. Carriage 706
Is in a position (home position h) indicated by a broken line in the drawing when recording is not performed or when the ejection recovery process of the multi-head 702 is performed, and a predetermined process is performed by a recovery mechanism (not shown) such as a cap.

Before the start of recording, the carriage 706 at the home position h starts moving in the x direction along the carriage guide shaft 708 when a recording start command is issued. Along with this movement, based on the position signals of the carriage read by the linear encoder 709, four dark and light inks of four colors are ejected from the n ejection ports of the multi-head 702 in accordance with the recording signals, so that on the recording paper 707. Then, printing is performed only for the ejection port array width D of the printing head. By this recording scan, ink is landed on the recording paper in the order of dark black ink, light black ink, dark cyan ink, light cyan ink, dark magenta ink, light magenta ink, dark yellow ink, light yellow ink, and dots are formed. It is formed. When the recording up to the edge of the recording paper is completed, the carriage 706
Moves to the home position h by moving in the opposite direction, and performs recording in the x direction again in the same manner as above. Further, the paper feeding roller 703 rotates in the arrow direction from the end of the previous recording to the start of the next recording, so that the paper is fed by the recording width in the y direction. In this way, the recording on the recording paper is completed by repeating the recording of the width of the multi-head and the paper feeding for each scanning of the carriage 706.

(Others) It should be noted that the present invention includes a means (for example, an electrothermal converter or a laser beam) for generating thermal energy as energy used for discharging ink, particularly in an ink jet recording system. An excellent effect is obtained in a recording head and a recording apparatus of a type in which the state of ink is changed by the thermal energy. This is because according to such a method, it is possible to achieve higher density and higher definition of recording.

The typical configuration and principle are described in, for example, US Pat. Nos. 4,723,129 and 4,740.
It is preferable to use the basic principle disclosed in the specification of Japanese Patent No. 796. This method is a so-called on-demand type,
Although it can be applied to any type of continuous type, in particular, in the case of the on-demand type, it can be applied to a sheet holding liquid (ink) or an electrothermal converter arranged corresponding to the liquid path. By applying at least one drive signal corresponding to the recorded information and giving a rapid temperature rise exceeding the nucleate boiling, heat energy is generated in the electrothermal transducer, and film boiling occurs on the heat acting surface of the recording head. Liquid (ink) corresponding to this drive signal on a one-to-one basis.
This is effective because air bubbles inside can be formed. The liquid (ink) is ejected through the ejection opening by the growth and contraction of the bubble to form at least one droplet. When the drive signal is formed into a pulse shape, the growth and shrinkage of the bubble are performed immediately and appropriately, so that the ejection of a liquid (ink) having particularly excellent responsiveness can be achieved, which is more preferable. As the pulse-shaped drive signal, those described in US Pat. Nos. 4,463,359 and 4,345,262 are suitable. If the conditions described in US Pat. No. 4,313,124 of the invention relating to the rate of temperature rise on the heat acting surface are adopted, more excellent recording can be performed.

As the configuration of the recording head, in addition to the combination of the discharge port, the liquid path, and the electrothermal converter (linear liquid flow path or right-angled liquid flow path) as disclosed in the above-mentioned respective specifications, U.S. Pat. No. 4,558,333 and U.S. Pat. No. 44,558 which disclose a configuration in which a heat acting portion is arranged in a bending region.
A configuration using the specification of Japanese Patent No. 59600 is also included in the present invention. In addition, Japanese Unexamined Patent Publication No. 59-123670 discloses a configuration in which a common slit is used as a discharge portion of the electrothermal converter for a plurality of electrothermal converters, and an opening for absorbing a pressure wave of thermal energy is provided. The effect of the present invention is effective even if the configuration corresponding to the ejection portion is disclosed in JP-A-59-138461. That is, according to the present invention, recording can be surely and efficiently performed regardless of the form of the recording head.

Further, the present invention can be effectively applied to a full line type recording head having a length corresponding to the maximum width of a recording medium which can be recorded by the recording apparatus. Such a recording head may have a configuration that satisfies the length by a combination of a plurality of recording heads or a configuration as one recording head integrally formed.

In addition, even in the case of the serial type as described above, a recording head fixed to the apparatus main body or an electric connection with the apparatus main body or ink from the apparatus main body by being attached to the apparatus main body. The present invention is also effective when a replaceable chip-type recording head that can be supplied or a cartridge-type recording head in which an ink tank is provided integrally with the recording head itself is used.

Further, as the constitution of the recording apparatus of the present invention, it is preferable to add the ejection recovery means of the recording head, the preliminary auxiliary means and the like because the effects of the present invention can be further stabilized. If these are specifically mentioned, the recording head is heated using capping means, cleaning means, pressurizing or suction means, an electrothermal transducer, another heating element or a combination thereof. Pre-heating means for performing the pre-heating and pre-discharging means for performing the discharging other than the recording can be used.

The type and number of recording heads to be mounted are, for example, different from those provided with only one corresponding to a single color ink, and those corresponding to a plurality of inks having different recording colors and densities. A plurality may be provided. That is, for example, the recording mode of the recording apparatus is not limited to the recording mode of only the mainstream color such as black, but it may be either the recording head is integrally formed or a plurality of combinations may be used. The present invention is also extremely effective for an apparatus provided with at least one of full-color recording modes by color mixing.

In addition, in the above-described embodiments of the present invention, the ink is described as a liquid, but an ink that solidifies at room temperature or lower and that softens or liquefies at room temperature may be used. Or, in the inkjet system, it is common to control the temperature of the ink itself within the range of 30 ° C. or higher and 70 ° C. or lower to control the temperature so that the viscosity of the ink is within the stable ejection range. Sometimes, a liquid ink may be used. In addition, the temperature rise due to thermal energy is positively prevented by using it as the energy of the state change from the solid state of the ink to the liquid state, or in order to prevent the evaporation of the ink, it is solidified and heated in the standing state. You may use the ink liquefied by. In any case, by applying heat energy such as ink that is liquefied by applying heat energy according to the recording signal and liquid ink is ejected or that begins to solidify when it reaches the recording medium. The present invention can be applied to the case where an ink having a property of being liquefied for the first time is used. In this case, the ink is
JP-A-54-56847 or JP-A-60-7
As described in Japanese Patent Publication No. 1260, it is also possible to adopt a form in which the sheet is opposed to the electrothermal converter in a state where it is held as a liquid or solid substance in the concave portion or through hole of the porous sheet. In the present invention, the most effective one for each of the above-mentioned inks is to execute the above-mentioned film boiling method.

In addition, the ink jet recording apparatus of the present invention may be used as an image output terminal of an information processing apparatus such as a computer, a copying apparatus combined with a reader or the like, and a facsimile apparatus having a transmitting / receiving function. It may be a form or the like.

[0053]

As described above, according to the present invention,
The threshold value of the dither pattern used for the binarization processing is determined so that the number of dots formed in each of the dark and light inks does not decrease at least as the image data value increases. Only the lower limit of the image data value needs to be defined, so that there can always be one inequality using the threshold value.

In the area of the image data value where the light ink dots and the dark ink dots are mixed, the light ink dots are always formed when the dark ink dots are formed, and these are formed to overlap each other. Instead, light ink dots are formed around the dark ink dots. Further, by making these dots larger than the pixel pitch, it is possible to reduce the portion where the printed medium appears.

As a result, since a binary image is generated with a simple threshold pattern, the generation time can be shortened when the image is generated by the host computer, and the image is generated by the printer. In addition, the circuit scale can be reduced.

Further, even if the binarization process is simple as described above, it is possible to generate two kinds of images of light and shade in which the deterioration of the image quality is suppressed without impairing the graininess of the image.

Furthermore, it is not necessary for the print medium to have a high ink receiving ability.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing dot data of dark and light inks by a binarization process using a conventional dither pattern.

FIG. 2 is a diagram showing the relationship between the input data gradation level and the number of dots printed in each of dark and light inks in the conventional example.

3A and 3B are diagrams showing arithmetic expressions in dither patterns for the dark and light inks in the conventional example.

4A and 4B are diagrams for explaining a binarization process of image data of uniform density and image data of non-uniform density distribution, respectively, in the conventional example.

5A and 5B are diagrams schematically showing respective arithmetic expressions of a dither pattern for light ink and respective arithmetic expressions of a dither pattern for dark ink according to an embodiment of the present invention.

6A and 6B are schematic diagrams showing dot patterns formed on the basis of data obtained by binarization processing using the dither pattern of the above embodiment in the case of the maximum gradation level. Is.

FIG. 7 is a diagram similar to the case of a level lower than the maximum gradation level.

FIG. 8 is a schematic view showing an example of an inkjet printing apparatus that can be used in the embodiment of the present invention.

[Explanation of symbols]

 701 Ink tank 702 Head 706 Carriage

Claims (10)

[Claims] 1. An image forming apparatus for forming dots of dark and light inks on a print medium for printing, and binarizing a value indicated by image data using a dither pattern to obtain dark and light inks. Binarizing means for generating binary data for forming dots of each ink,
The threshold value of the dither pattern corresponding to each of the dark and light inks is set to a value such that the number of dots formed in each of the dark and light inks does not decrease at least as the value of the image data increases. An image forming apparatus comprising: a printing unit and a printing unit that forms dark and light ink dots on a print medium based on the binary data generated by the binarizing unit. 2. The threshold value of the dither pattern increases until the number of light ink dots formed reaches a predetermined value indicated by image data, and the number is constant above the predetermined value, and The image forming apparatus according to claim 1, wherein the number of formed dark ink dots is 0 up to the predetermined value, and is set to increase at the predetermined value or more. 3. An image forming apparatus that prints by forming dots of dark and light ink on a print medium, respectively, and performs binarization processing on a value indicated by image data using a dither pattern to obtain dark and light. Binarizing means for generating binary data for forming dots of each ink,
The threshold value of the dither pattern corresponding to each of the dark and light inks increases until the number of formed dots of the light ink reaches a predetermined value indicated by the image data, and when the number is equal to or more than the predetermined value, the number is constant. Yes, and the number of formed dark ink dots is 0 up to the predetermined value, and is determined to increase above the predetermined value, and the binary data of each of the dark and light ink dots is set in the dither pattern. And a printing means for forming dark and light ink dots on the print medium based on the binary data generated by the binarization means. An image forming apparatus characterized by the above. 4. The image forming apparatus according to claim 3, wherein the printing unit sets the diameter of each of the dots of dark and light ink to be formed to be larger than the pixel pitch. 5. The image forming apparatus according to claim 4, wherein the diameter of the dots is at least 1.8 times the pixel pitch. 6. An image forming method for forming dots of dark and light inks on a print medium for printing, wherein the dither pattern data corresponding to each of the dark and light inks has respective threshold values, Prepare dither pattern data that has a value such that the number of dots formed in each of dark and light ink does not decrease at least as the value of image data increases, and use the dither pattern data to determine the value indicated by the image data. Binarization processing is performed on the above to generate binary data for forming dots of dark and light inks, and based on the generated binary data, dots of dark and light inks are respectively printed on the print medium. An image forming method comprising forming each step. 7. The threshold value of the dither pattern increases until the number of light ink dots formed reaches a predetermined value indicated by image data, and when the number of light ink dots is equal to or larger than the predetermined value, the number is constant, and 7. The image forming method according to claim 6, wherein the number of formed dark ink dots is 0 up to the predetermined value, and increases when the number is greater than or equal to the predetermined value. 8. An image forming method for forming dots of dark ink and light ink on a print medium for printing, wherein the dither pattern data corresponds to each of the dark ink and the light ink, and the threshold of the dither pattern data. The value increases until the number of light ink dots formed reaches a predetermined value indicated by the image data, and when the number is equal to or more than the predetermined value, the number is constant, and the number of dark ink dots formed is the above. Prepared is dither pattern data that is 0 up to a predetermined value, is determined to increase above the predetermined value, and prevents binary data of dark and light ink dots from being complementary generated in the dither pattern data. Then, using the dither pattern data, binarization processing is performed on the value indicated by the image data to form dots of dark and light ink, respectively. It generates binary data, on the basis of the binary data the generated, each dark image forming method characterized by light ink dots of forming onto the print medium, having the steps. 9. The image forming method according to claim 8, wherein the dot diameter is at least 1.8 times the pixel pitch. 10. An image forming method for printing by forming dark and light ink dots on a print medium, wherein the areas where the dark and light ink dots are formed do not overlap. Method.