JPH1141477A - Image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily adjust only color areas of a stored color, gray, etc., to a free color tone, to maintain natural gradations, and to eliminate the occurrence of a false outline, etc. SOLUTION: A virtual YMC converting circuit 221 converts L*a*b* data from a 1st color converting means 210 into data in the YMC color space of a virtual image forming device having linear characteristics with conversion characteristics reverse to those of a post-stage three-dimensional interpolation color converting circuit 222. The three-dimensional interpolation color converting circuit 222 represents a color reproduction range of an image forming device 300 on the L*a*b* color space as a dodecahedron having its apexes at the maximum density points of red, green, blue, yellow, magenta, cyan, and gray, and the coordinates of white dots on a form used by the image forming device 300, divides the dodecahdron into six tetrahedrons including the maximum density points of gray and the white dots on the form, and performs interpolating operation in the respective tetrahedrons to reconvert the YMC data from the virtual YMC converting circuit 221 into, the L*a*b* data. A parameter setting means 240 changes the coordinates of the apexes of the tetrahedrons.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for converting color image signals such as image data obtained by reading an original and image data generated by a computer system.
The present invention relates to an image processing apparatus for converting an image recording signal to be transmitted to a color image forming apparatus such as a color printer.

[0002]

2. Description of the Related Art Many color image forming systems, such as digital color copiers, computer printers, and network printers, which are currently manufactured, use an input device or a color space of an input image signal as an image forming device (image output device). By mounting an image processing apparatus having a color conversion function for converting into a color space of (1), it is possible to obtain color reproduction in which output colors match input documents and input image signals.

However, in recent years, with the development of color conversion technology,
Although more precise color conversion is possible, it is difficult to achieve perfect color matching such that the color difference between the input document and the input image signal is completely zero. For this reason, it is necessary to finally perform color adjustment so that color reproduction of a portion that the user focuses on, such as a flesh color portion of a person image, can be performed well.

The reason that the color difference between an input document and an input image signal cannot be completely zero is as follows. One is the problem of the conversion accuracy inherent in the color conversion technology. In a matrix type color conversion apparatus widely used in digital color copiers, it is known that the conversion accuracy is improved by adding a nonlinear term, but the conversion performance is still CIEL ^* a ^* b ^*. The average color difference in the color space is about 3 to 7, and it is impossible to make the color difference zero.

In recent years, a color conversion apparatus of the DLUT (direct look-up table) type, which has been widely used due to its high conversion performance, should be able to reduce the color difference to zero in principle. But in practice, D
Only the same accuracy as that of a printer model that models the relationship between an input image signal and an output color of the image forming apparatus used for obtaining the LUT coefficient can be obtained. Various printer models have been proposed, but even if a printer model using a neural network which is said to have the highest accuracy is used, the color prediction accuracy of the model itself is about 3 to 5 in average color difference. Therefore, it is impossible to completely reduce the color difference to zero.

As described above, since the conversion accuracy of the currently used color conversion technology is about 3 to 10 as the average color difference,
In order to match the color of the part of the user's attention, such as the memory color, with the input document or input image signal, it is necessary to perform color adjustment.

Also, unlike the case of a CRT monitor in which a stable color is always emitted by the fluorescent material, in the color reproduction of the hard copy, the reproduced color is delicately changed by various environmental conditions, and an ideal image recording signal is reproduced. Combinations do not always reproduce ideal colors. Therefore, it is necessary to perform color adjustment in order to correct color reproduction characteristics that fluctuate depending on the environment.

On the other hand, it is necessary not only to accurately match the color reproduction of the output printed matter with the input document or input image signal, but also to perform color adjustment according to the user's preference. For example, with respect to memory colors such as skin colors, ideal colors have large individual differences. For example, it has been reported that Japanese people prefer pink skin color to real skin color, while Japanese people image pink color rather than real skin color.

Further, regarding color television,
In NTSC, the reference white of the system is C
The light source is selected, but since the user prefers white having a high color temperature, the D93 light source is selected for the reference white color monitor. With regard to hard copies, gray balance with a slight blue color is preferred, and there is a demand for making the gray balance slightly blue compared to the standard state.

In recent years, with the spread of color management systems, it has been widely practiced to match colors in various input / output devices. However, in a color management system, a color space such as RGB (red, green, blue) depending on an input device, or a YMCK (yellow,
Instead of using a color space such as magenta, cyan, and black), a device-independent color space such as an XYZ space or a CIEL ^* a ^* b ^* space is used. Since the image is converted to a space and then transferred to the image output device, the input image signal to the image output device is a signal in a device-independent color space. Therefore, it is desired that the image processing device on the image output device side has a function of performing color adjustment when performing color conversion from a device-independent color space to a color space such as YMCK.

Conventionally, as a color adjustment method, for example, for each of YMCK signals, which are image recording signals of four colors, a method of adjusting the gradation characteristics using a one-dimensional LUT (look-up table) is widely used. I have.

Japanese Patent Application Laid-Open No. 5-292303 discloses that an input image signal is converted into a color space represented by lightness, saturation, and hue, and the lightness, saturation, and hue are adjusted independently by an LUT. How to do is shown.

Japanese Patent Application Laid-Open No. 6-121159 discloses that a memory color portion is extracted from an input image, and the extracted portion is converted into a preferable color obtained by a sensory evaluation test in advance. The method of printing out is shown.

Further, Japanese Patent No. 2537997 discloses that a memory color reproduction target area is designated by hue and saturation, and only colors near the target area are moved to other colors in the lightness, saturation and hue space. In this method, the color is adjusted to thereby convert the color into a color that is preferable for the user while maintaining the natural gradation on the reproduced image.

[0015]

A method of adjusting the gradation characteristics of an input image signal or an image recording signal using a one-dimensional LUT is suitable for a demand for directly adjusting the amount of each color, for example, for making a skin color reddish. It is certainly effective.

However, since this method operates on the entire color space, it is not possible to adjust only the color of a specific area such as a memory color portion in an image. For example, in response to a request to make the flesh color in the image reddish, if the color corresponding to the flesh color part is adjusted to be larger, the color of the flesh color part is certainly adjusted, but the gray part in the image is If it exists, the gray part becomes red, and there is a problem that the impression of the gray part deteriorates. Conversely, when trying to reproduce gray as bluish, the skin color becomes bluish and a desirable color cannot be obtained.

The method disclosed in Japanese Patent Application Laid-Open No. 5-292303 allows adjustment in a color space of lightness, saturation, and hue close to human perception, and is easily sensible to a target color. It is certainly effective.

However, Japanese Patent Application Laid-Open No. 5-29230 discloses
The method of No. 3 has a problem that the brightness and the saturation of different hues cannot be adjusted independently. Moreover, it is theoretically impossible to adjust the color of the gray portion.

In the method disclosed in Japanese Patent Application Laid-Open No. 6-121159, a memory color portion is extracted from an image and adjusted, so that the memory color can be visually changed to a visually preferable color without affecting other colors. It is certainly effective in that it can be reproduced.

However, Japanese Patent Application Laid-Open No.
In the method of No. 9, since the extracted memory color area is subjected to a different color correction process from that of the other areas, a discontinuity occurs between the memory color and the other colors as shown in FIG. However, there is a drawback that good color reproduction cannot always be obtained, such as generation of color. In addition, only one kind of ideal image recording signal combination for reproducing the memory color is set for each color area, and since one ideal color is reproduced in this area, a subtle change in the tint of the original image is obtained. And the gradation is destroyed.

Furthermore, the method disclosed in Japanese Patent No. 2537997 is certainly capable of adjusting the color of only the target area such as the memory color without affecting the other colors while maintaining the color continuity. It is effective.

However, this patent No. 2537997
As in the method of JP-A-5-292303, it is basically impossible to adjust the color of the gray portion. In addition, although the continuity of colors is maintained, pseudo contours are likely to occur because the color change near the memory color in the color space is sharp. Furthermore, while specifying the median of the target area is simple,
It is difficult to specify the adjustment range.

In consideration of the above points, the present invention can easily and easily adjust only a color region close to that color to a free color tone when performing color adjustment of a portion of the image, such as a memory color or gray, which is noticed by a human. It is an object of the present invention to realize an image processing apparatus which can be adjusted and which can obtain a printed matter which does not cause an image defect such as a pseudo contour while maintaining a natural gradation on a reproduced image.

Further, by providing a color adjustment function when converting a device-independent color signal into an image recording signal to be sent to an image forming apparatus, an image capable of performing color adjustment corresponding to a color management system is provided. It is intended to realize a processing device.

[0025]

According to the first aspect of the present invention, first color conversion means for converting an input color image signal into a three-variable color signal independent of a device, and a color independent of the three-variable color signal. Color adjusting means for adjusting in the space, and second color converting means for converting the color-adjusted three-variable color signal into an image recording signal to be sent to the image forming apparatus, wherein the color adjusting means comprises: First color adjustment means for converting the three-variable color signal from the color conversion means into an image recording signal of a virtual image forming apparatus having linear characteristics, and converting the image recording signal of the virtual image forming apparatus to the original And a second color adjusting means for converting the color space into three variable color signals.

According to the second aspect of the present invention, the first color conversion means for converting the input color image signal into a device-independent three-variable color signal, and the three-variable color signal into an image recording signal to be sent to the image forming apparatus. Second color conversion means for performing
Color adjustment means for calculating and determining a color conversion parameter of the color conversion means, wherein the color adjustment means converts the color conversion parameter represented by the color space of the three-variable color signal from the first color conversion means into First color adjustment means for converting color conversion parameters expressed in a color space of a virtual image forming apparatus having linear characteristics into color conversion parameters, and color conversion parameters expressed in a color space of the virtual image forming apparatus; And a second color adjusting means for converting the color conversion parameters into the color conversion parameters expressed in the color space.

In the invention of claim 1 or 2,
The second color adjustment unit sets a color reproduction range of an image forming apparatus to which an image recording signal from the second color conversion unit is transmitted, in a uniform color space independent of a device, in red, green, blue, yellow, Magenta, cyan, and gray maximum density points and the white point of the image forming medium used in the image forming apparatus are represented by a dodecahedron having vertices, and the dodecahedron is represented by the gray maximum density point and the gray point, respectively. The image forming medium can be configured by a color conversion unit that divides the image into four tetrahedrons including a white point and performs color conversion by performing an interpolation operation inside each tetrahedron.

Further, the first color adjusting means adjusts the color reproduction range of the image forming apparatus to which the image recording signal from the second color converting means is sent out, in a uniform color space independent of devices, to red, green, and red. , The maximum density point of blue, yellow, magenta, cyan and gray and the white point of the image forming medium used in the image forming apparatus are represented by a dodecahedron having vertexes, and the dodecahedron is represented by the gray 6 including the maximum density point and the white point of the image forming medium
It can be configured by a color conversion unit having a conversion characteristic opposite to that of the color conversion unit that divides into four tetrahedrons and performs color conversion by performing an interpolation operation inside each tetrahedron.

[0029]

In the image processing apparatus according to the first aspect of the present invention, the input color image signal is converted into a device-independent three-variable color signal by the first color conversion means, for example, one of the uniform color spaces. Is converted into a signal in a CIEL ^* a ^* b ^* color space, and the three-variable color signal is adjusted in a device-independent color space by color adjustment means, and the color-adjusted three-variable color signal is An image recording signal to be sent to the image forming apparatus, for example, YM
It is converted to a CK signal.

In the color adjusting means, a three-variable color signal from the first color converting means, for example, L
^{The *} a ^* b ^* signal is converted to an image recording signal of a virtual image forming apparatus having a linear characteristic, for example, a YMC signal, and further, the image recording signal of the virtual image forming apparatus is converted by a second color adjusting unit at a subsequent stage. Is converted to a three-variable color signal in the original color space.

The second color adjusting means includes, for example, a second color adjusting means.
The color reproduction range of the image forming apparatus to which the image recording signal is sent from the color conversion means is set to a red, green, blue, and yellow color on a device independent color space, for example, on an L ^* a ^* b ^* color space. , Magenta, cyan, and gray maximum densities and the white point of the image forming medium used in the image forming apparatus are represented by a dodecahedron having vertices, and the dodecahedron is represented by a gray maximum density point and an image, respectively. Divided into six tetrahedrons containing the white point of the forming medium,
A color conversion unit that converts an image recording signal of the virtual image forming apparatus from the first color adjustment unit into a three-variable color signal of the original color space by performing an interpolation operation inside each tetrahedron. The first color adjusting means can be constituted by, for example, a color converting means having a conversion characteristic opposite to that of the color converting means constituting the second color adjusting means.

Therefore, the second color adjustment means changes the coordinates of one or more vertices of one or more tetrahedrons of the six tetrahedrons modeling the color reproduction range of the image forming apparatus. Accordingly, the three-variable color signal from the first color conversion means can be color-adjusted in a uniform color space, and can be compatible with a color management system, and can be adjusted for each hue of an adjustment area such as each memory color. The color adjustment can be performed independently without affecting other colors.

Also, by changing the coordinates of the maximum density point of the dodecahedron gray and the white point of the image forming medium so as to change the chromaticity, the other colors are not affected.
Adjusting the chromaticity of gray and performing linear interpolation inside the tetrahedron can maintain the linearity of gradations in the color space, thus causing image defects such as false contours. There is no. Further, since the number of parameters to be adjusted is small, it is possible to easily adjust to a desired color.

In the image processing apparatus according to the second aspect of the present invention, the three-variable color signal itself from the first color conversion means is color-adjusted by the color adjustment means as in the image processing apparatus according to the first invention. Instead, the color adjustment means calculates and sets the color conversion parameter of the second color conversion means, so that the three-variable color signal from the first color conversion means is converted into an image by the second color conversion means. In the process of being converted into an image recording signal to be sent to the forming apparatus, the color is adjusted. Therefore, as a result, the same effect as that of the image processing apparatus according to the first aspect is obtained.

[0035]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. 1 shows an embodiment of a color image output system using a first example of an image processing apparatus according to the present invention. Processing device 200, image forming device 3
00 and an external input device 400.

The image input device 100 is used to print or
5mm color negative film or positive film, or A
A silver halide photographic film typified by PS film etc.
In this example, the image data is read by a CD sensor, and in this example, a color image signal composed of RGB data of a total of 24 bits of 8 bits and 256 gradations for each color of RGB is output.

The image processing apparatus 200 has a first
The RGB data input from the image input device 100 is composed of a color conversion unit 210, a color adjustment unit 220, a second color conversion unit 230, and a parameter setting unit 240. It is converted into data of a CIEL ^* a ^* b ^* color space, which is one of the color spaces.

The converted L ^* a ^* b ^* data is subjected to a desired color adjustment process in the L ^* a ^* b ^* color space by the color adjustment means 220 as described later. The color adjustment parameters of the color adjustment means 220 are
Set by 0. Further, the data of the L ^* a ^* b ^* color space from the external input device 400 is supplied to the color adjusting means 220, and the color is similarly adjusted.

The external input device 400 includes the image processing device 20
0 or a computer system connected directly or via a network, for example, KOD
A CD-ROM drive for reading image data from a CD-ROM in the AK / PhotoCD format, a device for capturing image data from a digital camera, and a color edited by a user using a computer and stored on a recording medium represented by MO or Zip. A recording media drive for reading image data from the recording medium,
Or a printer transfer device for transferring an image displayed on a CRT monitor in a computer system.

The L ^* a ^* b ^* data after the color adjustment by the color adjustment unit 220 is converted by the second color conversion unit 230 into an image recording signal of the color space of the image forming apparatus 300, in this example, four colors of YMCK. The data is converted into data and transferred to the image forming apparatus 300. Then, the image forming apparatus 300
, An image is formed on a sheet by the YMCK data.

The input color signal from the image input device 100 is
Most generally, it is RGB data, and the following example shows the case of RGB data, but data in another color space such as a YMC color space or a YCC color space used in PhotoCD may be used.

As the first color conversion means 210, a means for converting an input color signal into an L ^* a ^* b ^* color space can be typically used. In the following example, the L ^* a ^* b ^* color is also used. Although a case of converting into a space is shown, any color space that does not depend on a device may be converted into another color space such as an XYZ color space or an L ^* u ^* v ^* color space. However, it is desirable that the image data be converted into a uniform color space.

In this example, a known matrix type color conversion circuit is used as the first color conversion means 210, and the color difference between the input image and the converted color is minimized so that the output color matches the input image. A matrix coefficient is obtained and used as a conversion coefficient of the first color conversion means 210.

Also, as the external input device 400, a device that converts various input color signals having different color reproduction ranges into an L ^* a ^* b ^* color space can be typically used ^. Although the case of converting to the a ^* b ^* color space is shown, if the device-independent color space is an XYZ color space or L ^* u ^* v ^*
The image data may be converted to another color space such as a color space. However, it is desirable that the image data be converted into a uniform color space.

Further, the color space of the image forming apparatus 300 is
The following example shows the case of the YMCK color space, but other color spaces such as the YMC color space and the RGB color space may be used. The image forming medium used in the image forming apparatus 300 is not limited to paper, but the following example shows the case of paper.

FIG. 2 shows an example of the image forming apparatus 300. In this example, a single-engine type electrophotographic color printer is used. The four colors of YMCK data from the image processing apparatus 200 are converted into binary data whose pulse width is modulated by a screen generator 390 in accordance with the data value. The signal is converted into a screen signal.

The laser signal 381 of the laser light scanner 380 is driven by the screen signal, and the laser light L is obtained from the laser diode 381, that is, the laser light scanner 380. Is irradiated.

The photosensitive drum 310 is charged by a charger 320 for forming an electrostatic latent image, and a laser beam scanner 380 is provided.
Is irradiated with the laser light L, an electrostatic latent image is formed on the photosensitive drum 310.

The photosensitive drum 3 on which the electrostatic latent image is formed
The developing devices 331, 332, 333, and 334 of the four colors KYMC of the rotary developing device 330 are brought into contact with the developing device 10, whereby the electrostatic latent images of the respective colors formed on the photosensitive drum 310 are developed into toner images. .

Then, the paper on the paper tray 301 is fed onto the transfer drum 340 by the paper feed unit 302 and is wound therearound, and corona discharge is given from the back of the paper by the transfer charger 341, so that The developed toner image on the photosensitive drum 310 is transferred onto a sheet. When a multi-color image is to be obtained, the paper is repeatedly brought into contact with the photosensitive drum 310 two to four times, whereby multiple images of the four colors of KYMC are transferred in a multiplex manner.

The sheet after the transfer is sent to the fixing device 370,
The toner image is fixed on the paper by being heated and melted. After the toner image is transferred onto the sheet, the photoconductor drum 310 is cleaned by the cleaner 350 and is prepared for reuse by the pre-exposure device 360.

The example of FIG. 2 is a case of a single engine system. However, the image forming apparatus 300 may be of another type such as a tandem engine system or an image-on-image system which forms a color image on a photosensitive drum and collectively transfers the images. May be used.

As will be apparent from the embodiments described below, the present invention can be applied to an image forming apparatus other than the electrophotographic system, such as a silver halide photographic system, a thermal transfer system, or an ink jet system. As a result, the same result as when applied to an electrophotographic image forming apparatus can be obtained.

FIG. 3 shows the image processing apparatus 200 shown in FIG.
An example is shown below. In this example, the color adjustment means 220 is configured by a virtual YMC conversion circuit 221 to be described later, a special three-dimensional interpolation color conversion circuit 222, and an LCH color adjustment circuit 223. DLUT
This is a case where it is configured by an interpolation operation circuit 231 and a gradation correction circuit 232.

The virtual YMC conversion circuit 221 converts the L ^* a ^* b ^* data input from the first color conversion means 210 or the external input device 400 into a linear characteristic in an L ^* a ^* b ^* color space described later. The data is converted into data in the YMC color space of the virtual image forming apparatus having the image data and sent to the three-dimensional interpolation color conversion circuit 222.

The three-dimensional interpolation color conversion circuit 222 converts the data of the virtual YMC color space into parameter setting means 240
Is converted into data in the L ^* a ^* b ^* color space in accordance with the parameters set in the above step, whereby the virtual YMC conversion circuit 2
The L ^* a ^* b ^* data input to 21 is color-adjusted in the L ^* a ^* b ^* color space and input to the LCH color adjustment circuit 223.

The LCH color adjustment circuit 223 converts the data of the L ^* a ^* b ^* color space from the three-dimensional interpolation color conversion circuit 222 into the L ^* C ^* H ° color space according to the parameters set by the parameter setting means 240. , And for each of lightness, saturation, and hue, the color is adjusted by a LUT (look-up table). Then, the data is converted to data in the L ^* a ^* b ^* color space. That is, the L ^* a ^* b ^* data after the color adjustment is output from the color adjustment unit 220.

Further, the L ^* a ^* b ^* data after the color adjustment is provided to the second color conversion means 230 by a DLUT (direct look-up table) and an interpolation operation circuit. DLUT interpolation arithmetic circuit 231
, The YMCK data corresponding to the L ^* a ^* b ^* data is converted, and the converted YMCK data is subjected to gradation correction by the gradation correction circuit 232 and
, That is, YMCK data after gradation correction as an image recording signal is obtained from the second color conversion means 230.

Then, an image recording signal composed of the YMCK data after the gradation correction is transferred to the image forming apparatus 300, and an image is formed on a sheet, for example, as shown in FIG.

In this example, the first color conversion means 210, the color adjustment means 220 and the second color conversion means 230 are configured as hardware by a dedicated image processing circuit in order to perform the image processing operation at high speed. However, all or some of these may be processed on a microprocessor by software operation.

The DLUT interpolation operation circuit 231 is composed of the DLUT and the interpolation operation circuit as described above,
The upper 4 bits of each of the 8-bit L ^* a ^* b ^* data starting from 0 generates an address of a grid point near the point determined by the L ^* a ^* b ^* data. Data of neighboring grid points is read from the DLUT, and the read grid point data is L ^* a ^* b
^* Interpolated by the lower 4 bits of the data to obtain output YMCK data.

For example, “Display and Imaging, SCI, Volume2, Number1 (199)
3) In P17 to P25, refer to the method of performing cubic interpolation with reference to the lattice points of eight neighboring points, the method of performing prism interpolation with reference to the lattice points of six neighboring points, and referencing the lattice points of four neighboring points. The method of performing tetrahedral interpolation is shown, and in this example,
Those methods, for example, a method of performing prism interpolation can be used. However, the neighboring grid point addresses are
There is no need to limit to bits.

The gradation correction circuit 232 is provided in the image forming apparatus 30.
0 to correct non-linear gradation characteristics.
Each of the K data can be constituted by an 8-bit LUT. The contents of the LUT of the gradation correction circuit 232 may be periodically rewritten so as to correct the temporal change of the image forming apparatus 300.

In this example, L ^* a ^* b ^* data is converted to YM
In the case where a DLUT interpolation arithmetic circuit 231 is used as the second color conversion means 230 in order to convert the CK data with high accuracy, a matrix type color generally widely used as the color conversion means is used as the second color conversion means 230. Similar results can be obtained by using another color conversion circuit such as a conversion circuit or a neural network type color conversion circuit.

The color adjusting means 2 which is one of the features of this example
The 20 virtual YMC conversion circuits 221 and the three-dimensional interpolation color conversion circuit 222 are constituted by color conversion circuits having conversion characteristics opposite to each other. To make it easier to understand,
Regarding the color conversion method by the three-dimensional interpolation color conversion circuit 222,
This will be described below with reference to FIGS.

In the color conversion method by the three-dimensional interpolation color conversion circuit 222, as shown in FIGS. 4A and 4B, L ^* a ^* b ^*
The color reproduction range of the image forming apparatus 300 in the color space is
Red, green, blue, yellow, magenta, cyan, and gray maximum density points R, G, B, Y, M, C, S,
And white point W of paper used in image forming apparatus 300
Are represented by a dodecahedron having a total of eight L ^* a ^* b ^* coordinates as vertices.

The parameter setting means 240 calculates the L ^* of the eight vertices R, G, B, Y, M, C, S and W of the dodecahedron ^.
The configuration is such that the a ^* b ^* coordinates can be set freely. However, if the user sets the coordinates may be configured to be able to set L ^* a ^* b ^* coordinates L ^* C ^* H ° coordinate system by closer to the human sense.

This dodecahedron is divided into six tetrahedrons each including a gray maximum density point S and a paper white point W as shown in regions 1 to 6 in FIG. It is determined according to the inequality expression shown in FIG. 4A which of the tetrahedrons the data is.
For example, input Y of Y = 100, M = 128, C = 255
Since the MC data satisfies C ≧ M and M ≧ Y, it is determined that the MC data is data in the tetrahedron of the area 4.

Next, the input YMC data in a certain tetrahedron is converted into the corresponding L ^* by an interpolation operation as shown below ^.
It is converted to a ^* b ^* data.

That is, as shown in FIG. 5A, the vertices (xi, yi, zi), (xj, yj, z
j), (xp, yp, zp), L ^* a ^* b ^* data corresponding to (xq, yq, zq) are Di, Dj, D
When given as p and Dq, the interpolation value D corresponding to the point (x, y, z) in the tetrahedron is D = DiΦi (x, y), as shown in FIG. , Z) + DjΦj (x, y, z) + DpΦp (x, y, z) + DqΦq (x, y, z) (1)

Where Φi (x, y, z) and Φj (x,
y, z), Φp (x, y, z), Φq (x, y, z)
Are the equations (2), (3), and (4) shown in FIG.
Given by (5).

As described above, the color reproduction range of the image forming apparatus 300 is described by a dodecahedron, the dodecahedron is divided into six tetrahedrons, and a linear interpolation operation is performed inside each tetrahedron. Input YMC data can be converted to L ^* a ^* b ^* data.

The color conversion method according to this method has the following advantages as compared with a color conversion method such as a matrix type, a neural network type or a DLUT type widely used as a conventional color conversion method.

First, the color reproduction range of the image forming apparatus 300 can be used to the maximum, and as described later, the linearity in the lightness direction, the saturation direction, and the hue direction can be guaranteed inside the apparatus. It is possible.

Second, if the color conversion parameter is 8
Since the coordinates of the vertices R, G, B, Y, M, C, S, and W are small and the dodecahedron is divided into tetrahedrons for each hue, color adjustment for each hue can be easily performed as described later. The setting of the gray axis can be performed without affecting other colors.

Third, since the gray axis is expressed as one side of a tetrahedron, continuity of the gray axis can be ensured.

Further, according to this method, the hardware of the conventional DLUT color conversion means of the tetrahedral interpolation method can be diverted, and in this case, the address values stored in the DLUT need only be the coordinates of eight vertices. Therefore, very low-cost implementation is possible.

On the other hand, the virtual YMC conversion circuit 221 is constituted by a color conversion circuit having conversion characteristics opposite to those of the three-dimensional interpolation color conversion circuit 222. Therefore, in the three-dimensional interpolation color conversion circuit 222, the eight vertices R, G, B,
Y, M, C, S and W are represented by red of the image forming apparatus 300,
Green, blue, yellow, magenta, cyan, and when set to L ^* a ^* b ^* coordinates of the white point of the paper used in the maximum density point and the image forming apparatus 300 gray, from YMC color space L ^* a ^* b ^* Considering the conversion characteristic to the color space, the virtual YMC conversion circuit 221 sets the conversion characteristic to the reverse.

Although the conversion from the actual L ^* a ^* b ^* color space to the YMC color space is very strong in nonlinearity, the conversion in the virtual YMC conversion circuit 221 is based on the linearity of tetrahedral interpolation. As is apparent, this is equivalent to converting the L ^* a ^* b ^* data into data of the YMC color space of the virtual image forming apparatus having linear characteristics.

In this example, the virtual YMC conversion circuit 221
Is composed of a DLUT-type color conversion circuit similar to the DLUT interpolation calculation circuit 231. However, this is L ^* a ^* b
^* To convert data into YMC data with high precision, as a virtual YMC conversion circuit 221, other color conversion circuits such as a matrix type color conversion circuit and a neural network type color conversion circuit generally widely used as color conversion means. Can obtain the same result.

As described above, in the color adjusting means 220,
The virtual YMC conversion circuit 221 allows the first color conversion unit 2
The L ^* a ^* b ^* data from 10 is converted from the L ^* a ^* b ^* color space to the virtual YMC color space for the following reasons.

As shown in FIG. 4, the color reproduction range of the image forming apparatus 300 is modeled by a dodecahedron in the L ^* a ^* b ^* color space, and the coordinates of the vertexes are changed.
Considering that color adjustment is realized, the first color conversion means 21
0 L ^* a ^* b ^* data directly from, L ^* a ^* b ^* to interpolation calculation in the color space, is outside the color reproduction range of the image forming apparatus 300, i.e., the data outside of the dodecahedral It means to convert and is impossible.

On the other hand, since the YMC color space has only color information within the color reproduction range of the image forming apparatus 300,
Conversion from the YMC space to data in the L ^* a ^* b ^* color space using the dodecahedral model shown in FIG. 4 can be executed without any problem.

Therefore, the conversion of the input L ^* a ^* b ^* data from the L ^* a ^* b ^* color space to the virtual YMC color space by the virtual YMC conversion circuit 221 is equivalent to the image formation from the input L ^* a ^* b ^* data. This is equivalent to extracting only data within the color reproduction range of the device 300.

Therefore, the data of the virtual YMC color space converted from the L ^* a ^* b ^* color space by the virtual YMC conversion circuit 221 is converted by the three-dimensional interpolation color conversion circuit 222 into L
^The desired color adjustment can be realized by re-conversion to the ^* a ^* b ^* color space.

As described above, L ^* when the virtual YMC conversion circuit 221 and the three-dimensional interpolation color conversion circuit 222 are combined ^.
The color adjustment in the a ^* b ^* color space is specifically performed as follows.

The 12 in the three-dimensional interpolation color conversion circuit 222
When the coordinate value of each vertex of the face is set to a value indicating the color reproduction range of the image forming apparatus 300 used for obtaining the conversion coefficient in the virtual YMC conversion circuit 221, the L ^* a ^* b ^* color space Also, the color adjustment is not working.

From this state, consider, for example, adjusting the skin color, which is one of the memory colors. Considering that the hue of the flesh color is increased by 5 ° in a preferable direction, the saturation is increased by +5, and the lightness is preserved, the flesh color region is included in the regions 1 and 2 in FIG. . Therefore, the hue of the L ^* a ^* b ^* coordinate value of the vertex R is increased by 5 °, and the L of the vertex R is increased by +5 so that the saturation corresponding to the saturation of the skin color is increased by +5.
^* a ^* b ^* Set the saturation of the coordinate value large.

More specifically, if the saturation of the skin color is 20 and the vertex R
Is 80, the saturation of the skin color portion is 25 (=
20 × 1.25), the saturation of the vertex R is also set to 100 (= 80 × 1.25) in proportion, and when the hue of the vertex R is 45 °, the hue is set to 50. ° (= 4
(5 ° + 5 °).

By adjusting the colors in this manner, the areas other than the areas 1 and 2 are not affected at all.
The hue and saturation of the skin color can be adjusted independently.
Brightness can be similarly adjusted.

Further, by performing linear interpolation inside the area, linear characteristics in a uniform color space are not destroyed, so that there is no image defect such as a false contour in an image after color adjustment.

Furthermore, since the parameters to be adjusted are few, for example, only the L ^* a ^* b ^* coordinates of the vertex R in the case of skin color adjustment, the adjustment can be performed easily and can be performed in a human sense. Adjustments can be made for near lightness, saturation and hue.

Further, by setting the coordinate values of the vertices W and S to arbitrary values, the color balance of the gray portion can be adjusted without affecting the color balance of the entire image.

Note that the image forming apparatus 300 outputs a reference patch on the photosensitive drum 310 or paper shown in FIG. When the reference patch is measured, the operation amount related to image formation is adjusted so that the measured value matches the target value, and the image quality is controlled, the image forming apparatus at that time is controlled each time the adjustment control is performed. The conversion characteristics of the virtual YMC conversion circuit 221 are reset using the L ^* a ^* b ^* coordinates of the eight vertices R, G, B, Y, M, C, S, and W indicating a color reproduction range of 300. It is desirable.

FIG. 6 shows an example of the LCH color adjustment circuit 223 of the color adjustment means 220. In this example, a ^* b ^* data in L ^* a ^* b ^* data from 3D interpolation color converting circuit 222 as described above is supplied to the chroma hue conversion circuit 224, the following equation (6) ( 7), C ^* = {(a ^* ) ² + (b ^* ) ² } ^1/2 (6) H ° = tan ⁻¹ (b ^* / a ^* ) (7) It is converted into C ^* and hue data H °.

The L ^* data from the three-dimensional interpolation color conversion circuit 222 is input to the L ^* LUT 225. The saturation data C ^* from the saturation hue conversion circuit 224 is stored in the C ^* LUT 22
6 and the hue data H ° is input to the H ° LUT 227.

The L ^* LUT 225, C ^* LUT 226 and H ° LUT 227 are one-dimensional LUTs, respectively ^.
Adjustment of the lightness direction by the LUT 225 is performed by the C ^* LUT 22
6 to adjust the saturation direction, and the H ° LUT 227 to adjust the hue direction.

FIGS. 7A, 7B and 7C show L ^* L
UT 225, C ^* LUT 226 and H ° LUT 227
Of the L ^* LUT 225 and the C ^* LUT 22
In 6, the minimum value of the input / output data is 0, and the maximum value is 2
Standardize to 55. In this example, L ^* LUT 225 is
The point with the highest brightness (L ^* = 95) is normalized to 0, the point with the lowest brightness (L ^* = 5) is normalized to 255, and the C ^* LUT 226 is set.
Standardizes the point with the lowest saturation (C ^* = 0) to 0 and the point with the highest saturation (C ^* = 128) to 255.

C ^* LUT 226 and H ° LUT 227
The saturation data C ^* and the hue data H ° after the adjustment are input to the a ^* b ^* conversion circuit 228, and the following equations (8) and (9): a ^* = C ^* cos (H °) ^{... (8) b * = C} * according to sin (H °) ... (9 ) is converted to a ^* b ^* data, after the conversion a ^* b ^*
The data is supplied to the DLUT interpolation calculation circuit 231 of the second color conversion means 230 together with the L ^* data adjusted by the L ^* LUT 225.

Note that the saturation / hue conversion circuit 224 and a ^*
The b ^* conversion circuit 228 may be configured with a two-dimensional LUT, instead of having the configuration that calculates the defining equation as described above.

The parameter setting means 240 uses the L ^* LUT
225, the table values of the C ^* LUT 226 and the H ° LUT 227 can be set freely. When the user sets table values, LUTs of various set values are used.
Are prepared, and LUs having desired characteristics are prepared from the
You may be comprised so that T can be selected.

As described above, L ^* a ^* b ^* data is converted to L ^* C ^* H
° By adjusting the LUT in the color space, the data linearly arranged in the L ^* a ^* b ^* color space by the three-dimensional interpolation color conversion circuit 222 can be arbitrarily set independently for each of lightness, saturation, and hue. It becomes possible to adjust to the characteristic of.

A three-dimensional interpolation color conversion circuit 222 having linear color adjustment characteristics and an LC having nonlinear color adjustment characteristics
By performing color adjustment by combining the H color adjustment circuit 223, it is possible to easily perform flexible color adjustment with a higher degree of freedom.

However, on the subsequent stage side of the three-dimensional interpolation color conversion circuit 222, the L ^* a ^* b ^* data is converted into another color space instead of the L ^* C ^* H ° color space and adjusted. The color adjustment may be performed by an adjustment method. If only linear color adjustment is sufficient, the color adjustment means 220
The configuration may be such that a color adjustment unit such as the LCH color adjustment circuit 223 is not provided at the subsequent stage of the three-dimensional interpolation color conversion circuit 222.

According to the above-described example, the image forming apparatus 300
Is represented by the dodecahedron shown in FIG. 4 and linear interpolation is performed inside the tetrahedral region divided for each hue, so that the linear characteristics in the uniform color space are not destroyed. No image defects such as false contours occur in the image after color adjustment. In addition, a desired color such as a flesh color can be independently adjusted without affecting the area other than the adjusted area at all.

Further, since the parameters to be adjusted are small only in the L ^* a ^* b ^* coordinates of the vertices of the dodecahedron, the adjustment can be performed easily, and the adjustment of the lightness, saturation and hue close to the human sense can be performed. Can be applied.

Further, by setting the coordinate values of the vertices W and S of the dodecahedron to arbitrary values, the color balance of the gray portion can be adjusted without affecting the color balance of the entire image.

[Embodiment 2] FIG. 8 shows a second example of the image processing apparatus 200 of the above-described color image output system.
In this example, without providing the color adjusting means 220 of the first embodiment,
The L ^* a ^* b ^* data from the first color conversion means 210 or the external input device 400 is directly supplied to the second color conversion means 230 to be converted into YMCK data, and the parameter setting means 24
According to the set value set at 0, the color adjustment parameter calculation unit 250 calculates the color conversion parameter of the second color conversion unit 230 and sets the color conversion parameter in the second color conversion unit 230 to obtain the same color as in the first embodiment. Make adjustment results available.

FIG. 9 shows a color adjustment parameter calculating means 250.
An example is shown below. In this example, the color adjustment parameter calculating means 250 includes an address setting means 251, an address changing means 252, a virtual YMC calculating means 253, a three-dimensional interpolation calculating means 254, and an LCH / LUT calculating means 255.

The address setting means 251 outputs L ^* a ^* b from the DLUT interpolation calculation circuit 231 of the second color conversion means 230.
^* Receiving grid point address data expressed in a color space and transferring it to the virtual YMC calculating means 253.

The virtual YMC calculation means 253 converts the grid point address data expressed in the L ^* a ^* b ^* color space into the virtual YMC color space by the same color conversion processing as that of the virtual YMC conversion circuit 221 of the first embodiment. Convert to data.

The three-dimensional interpolation calculating means 254 adjusts the color of the grid point address data expressed in the virtual YMC color space to desired characteristics by the same color conversion processing as the three-dimensional interpolation color conversion circuit 222 of the first embodiment. Then, the data is reconverted into data in the L ^* a ^* b ^* color space.

The LCH / LUT calculation means 255 converts the grid point address data expressed in the L ^* a ^* b ^* color space into L ^* C ^* by the same color conversion processing as the LCH color adjustment circuit 223 of the first embodiment ^. The color is adjusted to desired characteristics in the H ° color space.

The address changing means 252 is provided with the LCH / LU
Upon receiving the grid point address data for which the color adjustment calculation has been completed from the T calculation means 255, the second color conversion means 230
Is set as the grid point address data expressed in the L ^* a ^* b ^* color space in the DLUT interpolation calculation circuit 231.

In this example, in order to realize highly accurate color conversion and color adjustment, D second
This is a case where the LUT interpolation calculation circuit 231 is used.
As the color conversion means 230, another color conversion circuit such as a matrix type color conversion circuit or a neural network type color conversion circuit which is generally widely used as the color conversion means is used. A similar result can be obtained even if the color conversion parameters for performing the conversion and the color adjustment are calculated and set.

In this example, the first color conversion means 210, DL
The UT interpolation calculation circuit 231 and the gradation correction circuit 232
Although it is configured as hardware as in the first embodiment, the color adjustment parameter calculation unit 250 is configured to perform processing on a microprocessor by software calculation. This is because, in the color adjustment parameter calculation means 250, the data to be calculated is as small as only the lattice point address data, so that the calculation can be performed at high speed by software.

Thus, in this example, the hardware of the color adjustment means 220 required in the first embodiment is not required, and thus the cost of the image processing apparatus 200 can be reduced. However, the color adjustment parameter calculating means 250 may be configured as hardware to further speed up the calculation.

According to the second embodiment, the color adjustment parameter calculation means 250 determines the grid point address data of the DLUT interpolation calculation circuit 231 so that the color adjustment is performed so as to obtain desired characteristics. Device 20
0 cost reduction can be realized. Further, the image processing time when the image processing apparatus 200 is realized by software can be significantly reduced.

[0119]

As described above, according to the present invention, when color adjustment is performed on a portion of an image, such as a memory color or gray, which a human focuses on, only a color region close to that color is freely adjusted. In addition, it is possible to realize an image processing apparatus that can easily adjust the image quality and can obtain a printed material that maintains a natural gradation on a reproduced image and does not cause an image defect such as a false contour.

Further, by providing a color adjustment function when converting a device-independent color signal into an image recording signal to be sent to an image forming apparatus, an image capable of performing color adjustment corresponding to a color management system is provided. A processing device can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a color image output system including an example of an image processing apparatus according to the present invention.

FIG. 2 is a diagram illustrating an example of an image forming apparatus of the system of FIG. 1;

FIG. 3 is a diagram illustrating a specific example of the image processing apparatus of FIG. 1;

FIG. 4 is a diagram showing a dodecahedron and an interpolation area in the three-dimensional interpolation color conversion circuit of FIG. 3;

FIG. 5 is a diagram illustrating tetrahedral interpolation in the three-dimensional interpolation color conversion circuit of FIG. 3;

FIG. 6 is a diagram illustrating a specific example of the LCH color adjustment circuit of FIG. 3;

7 is a diagram illustrating a setting example of each LUT constituting the LCH color adjustment circuit of FIG. 6;

FIG. 8 is a diagram illustrating a color image output system including another example of the image processing apparatus of the present invention.

FIG. 9 is a diagram illustrating a specific example of the image processing apparatus in FIG. 8;

FIG. 10 is a diagram provided for explanation of a conventional image processing method.

[Explanation of symbols]

Reference Signs List 200 Image processing device 210 First color conversion means 220 Color adjustment means 221 Virtual YMC conversion circuit 222 Three-dimensional interpolation color conversion circuit 223 LCH color adjustment circuit 224 Saturation hue conversion circuit 225 L ^* LUT 226 C ^* LUT 227 H ° LUT 228 a ^* b ^* conversion circuit 230 second color conversion means 231 DLUT interpolation calculation circuit 232 gradation correction circuit 240 parameter setting means 250 color adjustment parameter calculation means 251 address setting means 252 address change means 253 virtual YMC calculation means 254 three-dimensional interpolation calculation Means 255 LCH / LUT calculation means

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masahiro Takamatsu 430 Nakai-cho, Nakai-cho, Ashigara-kami, Kanagawa Prefecture Inside Fuji Xerox Fuji Xerox Co., Ltd. Inside the corporation

Claims (7)

[Claims] A first color conversion unit for converting an input color image signal into a device-independent three-variable color signal; a color adjustment unit for adjusting the three-variable color signal in a device-independent color space; Second color conversion means for converting the color-adjusted three-variable color signal into an image recording signal to be sent to an image forming apparatus, wherein the color adjustment means converts the three-variable color signal from the first color conversion means into A first color adjusting unit that converts an image recording signal of the virtual image forming apparatus having linear characteristics into an image recording signal, and a second color adjusting unit that converts the image recording signal of the virtual image forming apparatus into a three-variable color signal of an original color space. An image processing apparatus comprising: a two-color adjusting unit. 2. A first color conversion means for converting an input color image signal into a device-independent three-variable color signal, and a second color conversion for converting the three-variable color signal into an image recording signal to be sent to an image forming apparatus. Means for calculating and setting a color conversion parameter of the second color conversion means. The color adjustment means is represented by a color space of a three-variable color signal from the first color conversion means. First color adjusting means for converting the color conversion parameters obtained in the color space of the virtual image forming apparatus having a linear characteristic into color conversion parameters, and a color expressed in the color space of the virtual image forming apparatus. Conversion parameters
An image processing apparatus comprising: a second color adjustment unit configured to convert color conversion parameters represented in an original color space into color conversion parameters. 3. The image processing apparatus according to claim 2, wherein a color conversion circuit of a direct look-up table type is used as the second color conversion means, and the color conversion parameter is a grid of the direct look-up table. An image processing apparatus characterized by point address data. 4. An image processing apparatus according to claim 1, wherein said second color adjustment means is a color reproduction range of an image forming apparatus to which an image recording signal from said second color conversion means is transmitted. With the maximum density point of red, green, blue, yellow, magenta, cyan, and gray in the uniform color space independent of the device and the white point of the image forming medium used in the image forming apparatus as a vertex. By expressing the dodecahedron in the form of a tetrahedron, the dodecahedron is divided into six tetrahedrons each containing the maximum density point of the gray and the white point of the image forming medium, and interpolation is performed inside each tetrahedron to obtain a color. An image processing apparatus comprising a color conversion unit for performing conversion. 5. The image processing apparatus according to claim 1, wherein said first color adjustment means is a color reproduction range of an image forming apparatus to which an image recording signal from said second color conversion means is transmitted. With the maximum density point of red, green, blue, yellow, magenta, cyan, and gray in the uniform color space independent of the device and the white point of the image forming medium used in the image forming apparatus as a vertex. By expressing the dodecahedron in the form of a tetrahedron, the dodecahedron is divided into six tetrahedrons each containing the maximum density point of the gray and the white point of the image forming medium, and interpolation is performed inside each tetrahedron to obtain a color. An image processing apparatus comprising a color conversion unit having a conversion characteristic opposite to a conversion characteristic of a color conversion unit for performing conversion. 6. The image processing apparatus according to claim 1, wherein the second color adjustment unit converts a color space of the virtual image forming apparatus into a color space of the three-variable color signal. An image processing apparatus comprising: a color conversion unit; and a color adjustment unit that performs color adjustment using a look-up table for each of lightness, saturation, and hue in a color space of the three-variable color signal. . 7. The image processing device according to claim 1, wherein the color space of the three-variable color signal is a CIEL ^* a ^* b ^* color space, which is one of uniform color spaces. An image processing apparatus characterized by the above-mentioned.