JP2001016475A - Image forming device and image processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device by which an image with high definition is formed concerning a gray scale part in a color image. SOLUTION: The driving quantity of CMYK ink is controlled to permit the calorimetric value of the gray scale in the formed image to be within the negative quadrant side, of (b) on the ab plane of the CIEL ab space and also within the hue angle range of a 90 deg. and, besides, to permit the hue with black ink to be continuously connected. Thus, the fluctuation of the hue of the gray scale is suppressed to a min. level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image forming apparatus and an image processing method, for example, an image forming apparatus and an image processing method for forming a color image on a recording medium.

[0002]

2. Description of the Related Art A color ink jet printer is known as an example of an apparatus for forming a color image on a recording medium. In general, in a color inkjet printer, a predetermined amount of each of the color components cyan (C), magenta (M), yellow (Y), and black (K) is stably ejected from a recording head. By doing so, the color tone is maintained.

In color image data represented by red (R), green (G), and blue (B) color components, data whose value is R = G = B is so-called gray scale data. Show.

In recent color ink-jet printers, even if K color ink is provided, thin gray scale portions are mainly used for C, M, and Y colors in order to suppress the generation of graininess in a formed image. Many employ so-called composite black formed by ink. In this case, the number of C, M, and Y dots to be printed is controlled by an application of a printer driver or the like, and is controlled to be as achromatic as possible in colorimetry, ie, a = b = 0 in CIELab chromaticity coordinates.

[0005]

However, when realizing composite black in the above-mentioned conventional color ink jet printer, achromatic color of a = b = 0 is completely realized by controlling the number of dots of C, M, and Y inks. This is actually difficult in terms of measurement errors and control errors.

[0006] Further, regarding the ink ejection amount, the negative effect of the ink supply system due to manufacturing variations of the recording head, changes in the surrounding environment such as temperature and humidity, temperature changes due to head driving during the recording operation, and the remaining amount of ink in the ink tank. There are various variables, such as changes in pressure. Therefore, even if control means for compensating for these fluctuation factors is provided, it has been difficult to stably control the ink ejection amount.

Further, not only the ink discharge amount but also the recording medium may change due to its type, variation, humidity and the like.

[0008] These fluctuations in the ink ejection amount and the recording medium appear as grayscale color tone changes. Since this color tone change appears colorimetrically as a change in chromaticity across the point of a = b = 0 in the CIELab chromaticity coordinates,
If there is a gray gradation, a gray multi-step tone patch, or the like in the color image data, they appear to slightly change into various hues. As a result, the image quality in the gray scale is reduced, and further, the gray portion is affected, resulting in a reduction in the quality of the entire image centering on a portion close to an achromatic color.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and has as its object to provide an image forming apparatus capable of forming a high-quality image on a gray scale portion of a color image.

It is another object of the present invention to provide an image processing method capable of creating a table for realizing an appropriate color space conversion for a gray scale portion in a color image.

[0011]

As one means for achieving the above object, the image forming apparatus of the present invention has the following arrangement.

That is, input means for inputting an image signal of a first format, conversion means for converting the image signal of the first format into a second format comprising a plurality of color components, and input means for converting the image signal of the second format. Correcting means for correcting the value of each color component, and a second
Image forming means for forming a color image on a recording medium based on an image signal of a format, wherein the correction means indicates that the image signal indicating achromatic color in the first format indicates achromatic colorimetrically Each color component value of the second format is corrected so as to be formed as a hue within a predetermined range deviated from the hue.

[0013] As a technique for achieving the above object, the image processing method of the present invention includes the following steps.

That is, an obtaining step of obtaining the tint of an achromatic sample image output on a recording medium based on a predetermined color processing condition, and determining whether the tint is within a predetermined color range. A determining step of performing, based on the determination result, an adjusting step of adjusting the color processing conditions so that the color falls within a predetermined color range; and, as a result of the determination, the color falls within a predetermined color range. If so, a table creation step of creating a color conversion table based on the color processing conditions, wherein the predetermined color range is within a predetermined hue range.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to the present invention will be described below in detail with reference to the drawings. Hereinafter, a color image forming apparatus according to the present invention will be described with reference to the drawings.

FIG. 1 is an external view showing the configuration of an ink jet printer, which is a color image forming apparatus to which the present embodiment is applied.

In FIG. 1, an ink jet printer 1
3 has four head cartridges 1A, 1B, 1C, 1
D is exchangeably mounted on the carriage 2. Less than,
When referring to all or any one of the plurality of recording units 1A to 1D, it is simply referred to as a recording head 1.

Each of the recording heads 1 has an ink tank section at the top and a recording head section (ink ejection section) at the bottom, and has a structure in which the recording head and the ink tank are integrated.

Each of the recording heads 1 is mounted on the carriage 2 so as to be exchangeable and positioned, and each of the recording heads 1 has a connector (not shown) for receiving a signal for driving the recording head unit and the like. Is provided. On the other hand, the carriage 2 is provided with a connector holder (electric connection portion) for transmitting a drive signal and the like to each recording head 1 through the connector. Each recording head 1 on the carriage 2 is connected to a control circuit on the apparatus main body side by a flexible cable for supplying a signal pulse current and a temperature control current.

In the ink tank section above each recording head 1, black (K) pigment ink in the head cartridge 1A, and cyan (C), magenta (M), and yellow ink in the head cartridges 1B, 1C, and 1D, respectively. The dye ink of (Y) is stored. Discharge ports for discharging ink droplets are formed in a predetermined arrangement on a surface of each recording head 1 facing the recording medium 8 (downward discharge port surface in the illustrated example), and each of the discharge ports is accommodated. The ink is ejected.

The carriage 2 is guided and supported so as to reciprocate along a guide shaft 3 installed in the apparatus main body so as to extend in the main scanning direction. And the carriage 2
The motor pulley 5 and the driven pulley 6 are driven by the main scanning motor 4.
And a timing belt 7 to control the position and movement thereof.

A recording medium 8 such as a recording sheet or a plastic sheet is provided with two pairs of conveying rollers 9, 10 and 11, 12.
The recording roller is conveyed through a position (recording unit) facing the discharge port surface of the recording head 1 by the rotation of these transport rollers. The recording medium 8 has its back surface supported by a platen (not shown) so as to form a flat recording surface in the recording section. In this case, each of the recording heads 1 mounted on the carriage 2 is held such that their discharge port surfaces protrude downward from the carriage 2 and are parallel to the recording medium 8 between the two pairs of conveying rollers. I have.

The ink jet printer 13 having the above configuration is connected to a personal computer (hereinafter simply referred to as a personal computer) 14 via a cable 15 and is controlled by driver software installed in the personal computer 14.

The outline of the processing performed by the driver software 20 will be described with reference to FIG.

The processing performed by the driver software 20 is such that a program related to the driver software 20 stored in a hard disk (HD) 31 in the personal computer 14
It operates using M32 as a work memory. This is realized based on the control of the CPU 30.

The driver software 20 receives, for example, RGB image data representing an image created by an application, and converts the RGB image data into C ₃ M ₃ Y ₃ K ₃ data for the ink jet printer 13.

First, RGB image data is converted into C ₁ M ₁ Y ₁ K ₁ data by a look-up table (LUT) 21 created by a method described later. Converted C
_{The 1} M ₁ Y ₁ K ₁ data is gamma-corrected in a gamma correction unit 22, subjected to a quantization process such as an error diffusion method in a quantization processing unit 23, and converted into C ₂ M ₂ Y ₂ K ₂ data. .

Hereinafter, the gray scale control performed by the LUT 21 will be described in detail with reference to FIGS.

FIG. 3 shows the amount of CMYK ink to be applied when the ink jet printer 13 forms an image for an 8-bit grayscale output value (0 to 255) based on control from driver software in the personal computer 14. It is a graph shown. Note that the gray scale levels from 0 to 255 shown in the graph correspond to the actual RGB values (R = G =
Conversely, B) corresponds to 255 to 0.

FIG. 4 shows an ink jet printer 13.
Is a colorimetric value when an image is formed with respect to an 8-bit grayscale output value (0 to 255), that is, ab of the CIELab chromaticity coordinates of the grayscale level shown in FIG.
It is a figure showing a behavior in a plane. Here, since the measurement was performed using the base color of the recording paper as a white reference, the white point (point W in FIG. 4) is a = b = 0. In addition, since each point in FIG. 4 indicates an actual measured value, some variation occurs.

First, in FIG. 3, it can be seen that the thin portions of the gray scale are formed by the three CMY inks, that is, composite black. As for the ink ejection amount in this portion, the ratio increases in the order of C, M, and Y. As a result, as shown in FIG. 4, it can be seen that the hue obtained by measuring the formed image is a minus side of the b-axis in the ab plane, that is, a slightly bluish gray. In this way, by adding a little bluish color to the gray scale portion, it is possible to suppress the visual discomfort to gray as compared with a case where a little reddish color or yellowish color is added.

According to FIG. 3, K ink is ejected in a gray scale portion which is somewhat dark (in the example of FIG. 3, gray level 176 or more). Then, in the part of the gray level 255 (that is, black data of R = G = B = 0), an image is formed using only the K ink.

However, the K ink is often not completely achromatic due to its characteristics. Therefore, in this embodiment, as shown in FIG. 4, as the gray scale level becomes darker, the hue thereof is slightly changed toward the hue of black ink (point K in FIG. 4), It is controlled so that it changes continuously. As a result, the change in the visual tint from gray to black is minimized, that is, the change in the tone of the gray scale is seen very naturally.

By controlling the hue angle within the range of 90 ° in the ab plane for all gray scale gradations, it is also possible to suppress a change in the gray color visually.

That is, the conversion is performed such that the correspondence between the grayscale image data formed by the ink jet printer 13 and the respective density values of CMYK is as shown in FIG.
This is realized using T21. As a result, in the grayscale colorimetric values actually formed on the recording medium, FIG.
Can be obtained.

The above conversion (RGB → C) will now be described with reference to FIG.
₁ M ₁ Y ₁ K ₁₎ for LUT21 creation method for realizing be described.

First, a gray patch (R = G = B = 0-2)
55 patches are printed using the set gray color separation parameters (S50), and the printed gray patches are measured or visually checked (S51). Here, as a method of measuring a gray patch, for example, Lab using a colorimeter is used.
Value measurement, or measurement of optical density through an RGB filter.

Based on the result of the color measurement in step S51, it is determined whether or not the gray tint is within the allowable range for all gradations (S52).

Here, the following example is given as a determination method in step S52.

When the Lab value is measured, it is determined whether or not the gray colorimetric value is on the minus side of b on the ab plane and within a predetermined angle in all gradations from 0 to 255. . If this condition is satisfied, it is determined that the value falls within the allowable range.

When the optical density is measured by the RGB filter, the optical density after passing through the B filter is different from that of black ink, which is at the same level at the high K density of human visual sensitivity. It is determined whether the optical density is smaller by a predetermined amount than the optical density after passing through the two filters.
If this condition is satisfied, it is determined that the value falls within the allowable range.

When the color is visually determined, it is determined whether or not the printed gray patch is appropriately bluish. If it is appropriately bluish, it is determined that it is within the allowable range.

If it is determined in step S52 that the value is not within the allowable range, the color separation parameters of gray are adjusted to adjust the balance of CMYK data (S53).
Steps S50 to S52 are repeated.

For example, if the bluish is not enough, the gray color separation parameters are adjusted so that cyan (C) data is increased and magenta (M) and yellow (Y) data is reduced. Conversely, if the bluish tone is too strong, the gray color separation parameters are adjusted so that C is reduced and M and Y are increased.

If it is determined in step S52 that the LUT is within the allowable range, the LUT 21 is created based on the gray color separation parameters determined to be within the allowable range (S54).

Here, for the input RGB, C ₁ M ₁ Y ₁ K ₁
The following color separation parameters are used when creating the LUT 21 that outputs.

C = 255-RM = 255-G Y = 255-B uc = min (C, M, Y) C ₁ = C-uc + UCC [uc] M ₁ = M-uc + UCM [uc] Y ₁ = Y −uc + UCY [uc] K ₁ = UCK [uc] Here, UCC, UCM, UCY, and UCK represent CMYK lines (functions for undercolor removal control) of gray color separation parameters.

The LUT 21 may be updated by incorporating the above-described LUT creation function into a printer driver and executing it based on a user's manual instruction. By doing so, the ink jet printer 13
Even if the output color reproducibility changes due to a change over time or a change in the recording medium, good color reproduction can be easily realized.

As described above, according to the present embodiment,
By controlling the amount of CMYK ink applied so that the grayscale colorimetric value slightly deviates from the achromatic color to a predetermined hue, variations and fluctuations in the ink discharge amount and the recording medium due to some fluctuation factors may occur. However, the variation of the hue of the grayscale or graytone patch can be kept within a certain range of the hue angle. Therefore, it is possible to improve the quality of not only gray but also the entire image.

In particular, for all gray scale gradations, the hue actually output is defined as ab in CIELab space.
By setting the angle within a range of 90 ° on a plane, the effect of suppressing a change in the visual gray tint becomes more remarkable.

The hue in the gray scale is represented by a
By controlling so as to be in the quadrant on the minus side of b on the b plane, a bluish gray can be formed as a whole. Therefore, at the same time as suppressing the visual discomfort with the ideal neutral gray, even if there is a change in the gray level, the gray generally becomes a reddish or yellowish gray with a large discomfort. Can be avoided.

Further, by substantially matching the hue of the gray scale with the hue of the K ink, the connection of the hue from gray to black can be made continuous and the gray scale hue change can be made natural.

In the present embodiment, the description has been made assuming that the droplets ejected from the recording head 1 are ink, and that the liquid stored in the ink tank is ink. It is not limited. For example, an ink tank may contain a processing liquid discharged to a recording medium in order to improve the fixability and water resistance of the recorded image or to improve the image quality.

[0054]

[Other Embodiments] The above-described embodiment is provided with a means (for example, an electrothermal converter or a laser beam) for generating thermal energy as energy used for performing ink ejection, particularly in an ink jet recording system. By using a method in which a change in the state of the ink is caused by the thermal energy, it is possible to achieve higher density and higher definition of recording.

Further, the present invention is not limited to this embodiment, and various applications are possible within the scope of known techniques. First, as the ink jet printer, in the above-described embodiment, ink of four colors of CMYK is used.
Y3 colors, or 6 or 7 colors of ink with CM or CMY equipped with dark and light inks may be used. Further, the present invention is not limited to the ink jet type, and may be applied to an electrophotographic type or thermal transfer type printer such as a laser beam printer.
Furthermore, as a color output device, not limited to a printer,
It is also applicable to displays such as CRTs and liquid crystals.

Even if the present invention is applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), a device including one device (for example, a copier, a facsimile). Device).

Further, an object of the present invention is to supply a storage medium (or a recording medium) in which program codes of software for realizing the functions of the above-described embodiments are recorded to a system or an apparatus, and to provide a computer (or a computer) of the system or apparatus. It is needless to say that the present invention can also be achieved by a CPU or an MPU) reading and executing the program code stored in the storage medium. In this case, the program code itself read from the storage medium implements the functions of the above-described embodiment, and the storage medium storing the program code constitutes the present invention. Also,
When the computer executes the readout program code, not only the functions of the above-described embodiments are realized, but also the operating system (OS) running on the computer based on the instructions of the program code.
It goes without saying that a case where the functions of the above-described embodiments are implemented by performing some or all of the actual processing, and the processing performs the functions of the above-described embodiments.

Further, after the program code read from the storage medium is written into the memory provided in the function expansion card inserted into the computer or the function expansion unit connected to the computer, the program code is read based on the instruction of the program code. Needless to say, the CPU included in the function expansion card or the function expansion unit performs part or all of the actual processing, and the processing realizes the functions of the above-described embodiments.

[0059]

As described above, according to the present invention, a high-quality image can be formed in a gray scale portion of a color image.

Further, it is possible to easily create a table for realizing appropriate color space conversion for a gray scale portion in a color image.

[Brief description of the drawings]

FIG. 1 is an external view illustrating a configuration of an inkjet printer according to an embodiment of the invention.

FIG. 2 is a block diagram showing a functional configuration of driver software according to the embodiment;

FIG. 3 is a graph showing C with respect to a gray scale in the embodiment;
A graph showing the MYK dot implantation ratio,

FIG. 4 is a graph showing chromaticity coordinates in an ab plane of a gray scale according to the embodiment;

FIG. 5 is a flowchart illustrating an LUT creation process according to the embodiment.

[Explanation of symbols]

 Reference Signs List 1 recording head 2 carriage 3 guide shaft 4 main scanning motor 5 motor pulley 6 driven pulley 7 timing belt 8 recording medium 9, 10, 11, 12 transport roller pair 13 inkjet printer 14 personal computer 15 cable 21 input unit 22 logarithmic conversion unit 23 Black generation unit 24 γ correction unit 25 Output filter 26 Gray control unit 27 Recording unit 29 Main controller 30 CPU 31 ROM 32 RAM

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Claims (24)

[Claims] An input unit for inputting an image signal of a first format; a conversion unit for converting the image signal of the first format into a second format comprising a plurality of color components; A correction unit configured to correct the value of each color component; and an image forming unit configured to form a color image on a recording medium based on the corrected image signal in the second format. Wherein each color component value in the second format is corrected so that an image signal indicating an achromatic color is formed as a hue within a predetermined range shifted from a hue indicating an achromatic color in colorimetry. Forming equipment. 2. The image forming apparatus according to claim 1, wherein the first format is composed of each color component of RGB. 3. The image forming apparatus according to claim 1, wherein the second format comprises CMY color components. 4. The image forming apparatus according to claim 1, wherein the second format is composed of CMYK color components. 5. The method according to claim 1, wherein the hue within the predetermined range is CIELab.
2. The image forming apparatus according to claim 1, wherein the hue is in a range forming a predetermined angle on an ab plane of the space. 6. The image forming apparatus according to claim 5, wherein the predetermined angle is 90 degrees. 7. The hue within the predetermined range is CIELab
2. The image forming apparatus according to claim 1, wherein the hue is in a quadrant on the minus side of the b component on the ab plane of the space. 8. The image processing apparatus according to claim 2, wherein the correction unit is configured to form an image signal indicating an achromatic color in the first format as a hue that continuously changes in colorimetry according to its density level. 2. The image forming apparatus according to claim 1, wherein each color component value is corrected. 9. The image forming means is capable of forming an image with black ink, and the correcting means is configured to convert an image signal indicating an achromatic color having a highest density level in the first format into a colorimetric hue of the black ink. 9. The image forming apparatus according to claim 8, wherein each color component value of the second format is corrected so as to be formed as a hue that conforms to the following. 10. The apparatus according to claim 1, wherein the input unit inputs the first format image signal from an external device.
The image forming apparatus as described in the above. 11. The image forming apparatus according to claim 10, wherein said external device is a personal computer. 12. The image forming apparatus according to claim 1, wherein the image forming unit includes an ink jet recording head that performs recording by discharging ink. 13. The ink jet recording head according to claim 1, wherein the recording head ejects ink by using thermal energy, and includes a thermal energy converter for generating thermal energy to be applied to the ink. The image forming apparatus according to claim 12. 14. An acquiring step of acquiring a tint of an achromatic sample image output on a recording medium based on a predetermined color processing condition, and determining whether the tint is within a predetermined color range. Determining, based on the determination result, adjusting the color processing conditions so that the color is within a predetermined color range; and, as a result of the determination, the color is within a predetermined color range. if there is,
A table creation step of creating a color conversion table based on the color processing conditions, wherein the predetermined color range is within a predetermined hue range. 15. The image processing method according to claim 14, wherein in the acquiring step, Lab values are acquired by measuring the color of the sample image. 16. The method according to claim 16, wherein in the determining step, the Lab
16. The image processing method according to claim 15, wherein if the value is in a range forming a predetermined angle on an ab plane in the CIELab space, it is determined that the value is within the predetermined color range. 17. The image processing method according to claim 16, wherein the predetermined angle is 90 degrees. 18. The method according to claim 18, wherein in the determining step, the Lab
16. The image processing method according to claim 15, wherein the value is a negative range of the b component on the ab plane in the CIELab space. 19. The image processing method according to claim 14, wherein, in the obtaining step, the color is obtained by measuring an optical density value of the sample image through an RGB filter. 20. In the determining step, if the density value passed through the B filter is smaller than the density value passed through another filter by a predetermined amount, it is determined that the color range is the predetermined color range. The image processing method according to claim 19. 21. The image processing method according to claim 14, wherein in the obtaining step, the color is obtained by a visual judgment of a user. 22. The image processing method according to claim 21, wherein in the determining step, if the color is bluish, it is determined that the color falls within the predetermined color range. 23. The image processing method according to claim 14, wherein the predetermined hue is a blue hue. 24. A recording medium on which a program code for image processing is recorded, wherein the program code acquires at least the tint of an achromatic sample image output on the recording medium based on predetermined color processing conditions. A code of an acquiring step to be performed, a code of a determining step of determining whether or not the color is within a predetermined color range, and a code for determining that the color is within a predetermined color range based on the determination result. A code of an adjustment step for adjusting color processing conditions, and as a result of the determination, if the color is within a predetermined color range,
And a code for a table creation step of creating a color conversion table based on the color processing conditions, wherein the predetermined color range is within a predetermined hue range.