JP4518408B2 - Image processing apparatus, method, and program - Google Patents

Description

The present invention relates to a color recording apparatus that performs color reproduction using a plurality of color materials including dark and light inks, and in particular, a color image processing apparatus, method, and method for determining an output signal value for each color material to be printed by removing undercolor a program, for example, a color MFP, color laser printer, a technique suitable to an ink jet printer.

  Conventionally, in the field of color printing and color hard copy, a black material is added to three color materials of yellow, magenta, and cyan to reproduce colors using four color materials. In general, since only the three colors of yellow, magenta, and cyan cannot reproduce the gray in the high density portion, it is possible to reproduce the high density achromatic color portion using the high density black color material. In addition, there is an effect that the amount of color material per pixel can be reduced by performing under color removal processing in which a black component of a color to be reproduced with three color materials of yellow, magenta, and cyan is replaced with a black color material.

  As a method for removing the under color, the simplest method is to obtain the minimum value MIN of the signal amounts Y, M, and C of yellow, magenta, and cyan, for example, as shown in FIG. Let rMIN (where r is a coefficient) be the black signal amount K0. A method is known in which correction data Y0, M0, and C0 are obtained by subtracting the K0 signal amount from the Y, M, and C signal amounts.

  Further, in the above-described under color removal method, the saturation is lowered for the chromatic color and the image quality is deteriorated. Therefore, in Patent Document 1, Y0 is obtained based on the black signal amount with respect to the data after the under color removal. , A method of performing undercolor printing (UCA) for increasing M0 and C0 has also been proposed.

  By the way, in recent years, in an ink jet printer, not only CMYK ink but also a printer using a light ink and a printer using a special color are developed. As a color separation method for multi-color ink, the device RGB signal is separated into CMYK signals using the conventional UCR / UCA method as described above, and the separated CMYK signals are further divided into a light / dark ink distribution table. A method of color separation is common (FIG. 13). However, in this method, since the distribution table for dark and light inks is fixed, it is not possible to flexibly control the dark and light ink separation pattern in consideration of the graininess and the total amount of ink that can be printed. Therefore, in order to maximize the effectiveness of the dark and light ink, a new color separation method different from the conventional UCR / UCA processing is required.

  For example, in Patent Document 2, the amount of dark and light ink used is determined in consideration of graininess and a color reproduction range by performing color separation using a three-dimensional memory map interpolation calculation. In this method, first, a color separation table is created for each outermost line in a color space (CMY space) defined by yellow, magenta, and cyan signal amounts Y, M, and C. At this time, the output level of the light ink in which the granularity of the dark ink is not noticeable is determined, and the start of the dark ink is determined. When the color separation tables for all the outermost lines are determined, the amount of color material ink in the color space is determined using interpolation, and the separation pattern of dark and light inks is flexibly controlled.

Japanese Examined Patent Publication No. 6-44801 JP 2002-33927 A

  However, the dark and light inks in Patent Document 2 are only primary colors of C, M, and Y. For example, the case where dark and light black ink is used is not considered. Also, in recent printers using secondary color inks (R, G, B) or tertiary color inks (black) having different densities, a color separation method is required that can flexibly control the separation pattern of light and dark inks. It is coming.

The present invention has been made in view of the above problems,
An object of the present invention is to provide a separation pattern of dark and light black ink (or secondary color ink) in a color separation apparatus that separates an input CMY signal into a plurality of color signals including black ink (or secondary color ink) having different densities. The present invention is to provide an image processing apparatus and method for performing under color processing with little color change, in which the under color of an input CMY signal can be accurately replaced with dark and light ink even when the color is controlled flexibly by color.

The present invention provides an image processing apparatus for color-separating an input color signal into an output color signal including black ink having different densities, color conversion means for obtaining an output value of the black ink having different densities from the input color signals, and the density different and mixed converting means for converting the color by mixing the different black ink density based on the output value of the black ink, using the color signal value after the mixing conversion, and under color removal processing means for performing under color removal of The most important feature is to have

In the image processing apparatus of the present invention, when the input color signal is color-separated into an output color signal including black ink (or secondary color ink) (hereinafter, dark and light ink) having different densities, it is based on the output value of the dark and light ink. Secondary color or tertiary color ink because it has a color mixture conversion means for converting dark and light ink into a mixed color and a color removal processing means for performing undercolor removal using the color signal value after the color mixture conversion. Even when the black generation process is performed in which the separation pattern of the dark and light inks is controlled for each hue, the color signal can be separated into color signals with little color change.

In the image processing apparatus of the present invention, the under color using the printing rate for the printing rate calculation means for obtaining the lower color printing rate from the output value of the dark and light ink and the color signal after the under color removal processing. Therefore, even when black generation processing is performed to control the separation pattern of dark and light inks for each hue, color separation with little reduction in saturation can be performed.

In the image processing apparatus of the present invention , the printing is performed so that the output saturation when the input color signal before color separation processing is output matches the output saturation when the output color signal after color separation processing is output. Since the rate is set, even when the setting of the dark and light ink separation pattern is changed, the input CMY characteristics do not change greatly and an image output with little color change can be performed.

In the image processing method of the present invention, when the input color signal is color-separated into the output color signal including the dark and light ink, the dark and light ink is converted into a color mixed based on the output value of the dark and light ink, and the color after the mixed color conversion Even when black generation processing is performed such that the separation pattern of dark and light inks is controlled for each hue because undercolor removal is performed using signal values, color separation can be performed into color signals with little color change.

In the program of the present invention, even when black generation processing is performed in which the separation pattern of dark and light inks is controlled for each hue, the program for color separation into color signals with little color change can be executed by a computer.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Example 1:
FIG. 2 shows the configuration of the image processing apparatus in the embodiment of the present invention. The color correction means 1 converts the input 8-bit RGB signals into C0M0Y0 signals (each 8 bits) as intermediate color signals, and the color separation means 2 converts the C0M0Y0 signal into cyan (C), magenta (M), Conversion into five-color signals (8 bits each) of yellow (Y), dark black (Bk), and light black (Lk). The converted CMYBkLk signals are sequentially processed by the gamma conversion unit 3, the total amount regulating unit 4, and the halftone processing unit 5, and finally output to a medium such as paper by the printer engine. Here, the gamma conversion means 3 adjusts gray balance, gradation, etc. by one-dimensional table conversion. As a result, when C = M = Y, achromatic gray is represented. The total amount regulating means 4 performs processing for keeping the total color material amount of CMYBkLk within the total amount (260% in this embodiment) that can be output by the printer engine. The halftone processing means 5 converts the CMYBkLk value having multi-level gradation into an area modulation format using a small number of gradations that can be reproduced by the printer engine.

  As the color correction unit 1, a method such as masking calculation or interpolation calculation using a three-dimensional lookup table (3D-LUT) 6 can be used. Here, the color correction parameter is set so that the input RGB signal and the C0M0Y0 signal reproduce the same color, but the color reproduced by C0M0Y0 changes depending on the conversion relationship of the color separation means at the subsequent stage. For this reason, the color correction parameter is created after the color separation means is determined.

  FIG. 1 shows the configuration of color separation processing means of the first embodiment. In the first exemplary embodiment, the black color generation density is determined using the black generation processing unit 11 that determines the output values of the dark black ink (Bk) and the light black ink (Lk) from the input CMY signal, and the generated Bk output value and Lk output value. It comprises a color mixture conversion unit 12 and a UCR processing unit 13 that performs undercolor removal processing.

  FIG. 3 is a diagram illustrating the black generation process. First, with the intermediate color signal C0M0Y0 as an input, tetrahedral interpolation calculation using a 3D-LUT similar to that of the color correction unit is performed. However, in the example of FIG. 3, the C0M0Y0 space is treated as one cubic lattice, and interpolation calculation is performed with eight lattice points. Therefore, point W (0, 0, 0), point Y (0, 0, 255), point M (0, 255, 0), point R (0, 255, 255), point C (255, 0, 0) ), Points G (255, 0, 255), points B (255, 255, 0), and outputs Bk and Lk corresponding to the points K (255, 255, 255) are provided as conversion tables. Of the signals calculated by the interpolation calculation, the Bk and Lk signals are the output signals Bk and Lk as they are.

Tetrahedral interpolation is a method in which a cubic lattice is decomposed into six tetrahedrons sharing a diagonal as shown in FIG. 4 and interpolation is performed using conversion table values of four vertices of the tetrahedron. As shown in FIG. 4, when the table values of each lattice point of the cubic lattice are expressed as P000, P001, P010, P011, P100, P101, P110, and P111, an arbitrary input signal exists in any tetrahedron of T1 to T6. This is determined by the determination formula in the table of FIG. 5, and the output value can be obtained using the following formula using the coefficients A, B, C, and D at that time.
(Output value) = (coefficient A) × C0 + (coefficient B) × M0 + (coefficient C) × Y0 + (coefficient D)
In the above description, the C0M0Y0 space is a single cubic lattice. However, the C0M0Y0 space may be divided into a plurality of cubic lattices and a correction amount corresponding to each lattice point may be held. Further, a hue division masking operation may be used instead of the interpolation operation.

  If the black generation process using the interpolation operation as described above is performed, it is possible to control the Bk and Lk output patterns flexibly for each hue by changing the output values of Bk and Lk assigned to the grid points. . FIG. 6 shows an example of Bk and Lk output values of hue A and hue B determined by the black generation process. The horizontal axis represents MIN (C0, M0, Y0), and the vertical axis represents the output values of Bk and Lk. FIG. 6A shows the relationship between Bk and Lk in, for example, a Y solid to black line. In the Y solid to black line, if the starting point of the Bk ink is early, the roughness is easily noticeable. Therefore, the start point of Bk ink is delayed as much as possible. FIG. 6B shows an example in which the roughness is not noticeable even when the Bk ink is applied from the beginning, such as a blue hue. In such a case, the start point of Bk is advanced in order to reduce the total amount of ink. Thus, the black generation process in the present embodiment can freely control the color separation of Bk and Lk for each hue.

  When the output values of Bk and Lk are obtained by the black generation process, the mixed color conversion unit 12 next obtains the mixed color density of the dark and light black ink, and the UCR processing unit 13 performs the under color removal process. Hereinafter, as an example, the maximum density of dark black ink will be described as Dd, and the maximum density of light black ink will be described as Dl.

  Incidentally, the density Ddl when the dark black ink and the light black ink are mixed is not simply obtained by the sum of the dark black density and the light black density. For example, when a mixed color output is performed with 100% solids, the amount of ink adhesion is large, so that the influence of reflection on the ink surface becomes large, and the density is often lower than Dd + Dl. Furthermore, there is a problem that the color mixing characteristics differ because the dot overlap ratio changes depending on the density ratio of the dark black ink and the light black ink.

Therefore, the color mixture conversion unit 12 obtains the color mixture density using a color mixture model. In order to construct a color mixture model, a plurality of color mixture patches with different output densities of Bk and Lk are output, and the density of the output patches is measured. Then, a conversion model is constructed using the colorimetric density and the Bk and Lk signal values. As a conversion model, for example, Ddl = a1 * Bk + a2 * Lk + a3 * Bk * Lk
The coefficients are optimized so that the colorimetric density prediction error is minimized. Also, a nonlinear model such as a neural network can be used instead of the polynomial. Next, under color removal processing is performed in the UCR processing unit 13 in accordance with the mixed color density obtained above. The under color removal process is a process for replacing the gray components of C0, M0, and Y0 with black ink, and can be typically performed by the following calculation.

C1 = C0-αK
M1 = M0−αK
Y1 = Y0-αK
In the present invention, since dark and light black ink is used, the mixed color density Ddl constructed above is used instead of the K signal. Here, the density Dcmy when C, M, and Y are mixed at 100%, respectively, and the density when the dark black ink and the light black ink are output with 100% solids are Dmax. In general, the color gamut that can be reproduced with CMYK is wider than the color gamut that can be reproduced with three colors of CMY, and Dcmy <Dmax in many cases. As a result, when the gray component having the same density as the CMY gray component is replaced with the black ink, a shadow float occurs. Therefore, when replacing with black ink, when C0 = M0 = Y0 = 255 is input, the undercolor removal amount is normalized so that Bk = Lk = 255. That is, when expressed by the formula, C1 = C0-255 (Ddl / Dmax)
M1 = M0-255 (Ddl / Dmax)
Y1 = Y0-255 (Ddl / Dmax)
It becomes. here,
It is desirable to adjust the Bk and Lk decomposition patterns in advance in the black generation processing unit so that the signal values of C0, M0, and Y0 are larger than 255 (Ddl / Dmax).

  By the above UCR processing, it is possible to perform UCR processing with a high degree of freedom with respect to the dark and light black ink separation pattern. In addition, the color gamut can be expanded by normalizing the UCR amount, thereby realizing clear shadow reproduction. Further, in the above description, the density when the dark black ink and the light black ink are each 100% output is Dmax. However, if the total amount regulation is 260%, in addition to Bk = Lk = 100%, It is possible to overprint with C = M = Y = 20%. In such a case, Dmax may be set to Bk = Lk = 100% and C = M = Y = 20%.

Example 2:
In the first embodiment, only the UCR process is used as the lower color process. However, with only the lower color process, the saturation is lowered and the color gamut is narrowed. Therefore, in the second embodiment, the color gamut is expanded by adding UCA processing to the lower color processing.

  FIG. 7 shows the configuration of the color separation processing means of the second embodiment. In the second embodiment, the black color generation density is determined using the black generation processing unit 21 that determines the output values of the dark black ink (Bk) and the light black ink (Lk) from the input CMY signal, and the generated Bk output value and Lk output value. The color mixing conversion unit 22, the UCR processing unit 23 that performs under color removal processing, the UCA rate calculation unit 24 that predicts the UCA rate using the generated Bk output value and Lk output value, and UCA (under color printing) It comprises a UCA processing unit 25 that performs processing.

In the configuration of FIG. 7, the black generation processing unit 21, the color mixture conversion unit 22, and the UCR processing unit 23 are the same as those in the first embodiment, and therefore, the UCA rate calculation unit 24 and the UCA processing unit 25 will be described here. Before describing the UCA process according to the present invention, a general UCA process will be described. A typical calculation method for UCR / UCA processing is C ′ = (C−αK) (1 + αK / 255).
M ′ = (M−αK) (1 + αK / 255)
Y ′ = (Y−αK) (1 + αK / 255)
It is. The above equation serves to increase the saturation by multiplying the chromatic component that has been reduced by the UCR processing by (1 + αK / 255). The reason for the decrease in saturation is that the black dots and chromatic color dots partially overlap when the CMY gray component is replaced with black ink.

  Here, the same phenomenon occurs in the case of UCA processing using dark and light black ink. As an example, the case of UCR processing with Bk ink of the same density and the case of UCR processing with Lk ink will be described with reference to FIG. Each circle in FIG. 8 schematically shows each halftone dot of each color image reproduced by the halftone dot area method. (A) represents the state of C, M, and Y dots before UCR processing. (B) shows the dot state after UCR processing with only Lk ink, and (c) shows the dot state after UCR processing with only Bk ink. Referring to FIG. 8, even when UCR processing is performed with black ink of the same density, the area ratio is higher in Lk. Therefore, even if the density is the same, the higher the ratio of the Lk ink, the higher the overlapping ratio between the C, M, and Y dots and the light and dark black dots. As described above, even when the mixed color densities of Bk and Lk are the same, it is preferable to control the UCA rate in accordance with the separation pattern of the light and light inks because the dot overlap is different. Therefore, the UCA rate calculation unit 24 has a function of obtaining an appropriate UCA rate according to the signal amounts of Bk and Lk. The processing may be performed at a constant UCA rate regardless of the dark and light ink signal amount, but the saturation can be further increased by controlling the UCA rate according to the dark and light ink signal amount.

  Next, a specific procedure for setting the UCA rate will be described with reference to the flowchart of FIG. FIG. 9 shows a calculation flow of the UCA rate for Ct, Mt, and Yt. First, in S101, the saturation S for Ct, Mt, and Yt is obtained. This is the saturation when the Ct, Mt, and Yt signal values are output by the printer. The saturation S can be easily calculated by constructing a color prediction formula that models the relationship between the output signal of the printer and the brightness, saturation, and hue obtained by measuring the output patch. Various color prediction formulas such as those based on neural networks and interpolation calculations have been proposed. Next, in S102, Ct, Mt, and Yt are regarded as input signals, and black generation and UCR processing are performed. The black generation and UCR processing uses the same processing as in the first embodiment. Next, a convergence calculation is performed in S103 to S106 to obtain the UCA rate γ. S103 corresponds to UCA processing, and increases the saturation by multiplying the chromatic colors C1, M1, and Y1 by γ. In S104, the saturation S 'for C2, M2, Y2, Bk, and Lk after color separation is calculated using the color prediction formula as in S102. If S and S ′ do not match, S103 to S106 are repeated while correcting γ until S ′ matches S.

  By calculating the above processing for various colors (Ct, Mt, Yt), a large number of data sets of output values of Bk and Lk and UCA rate γ can be obtained. Finally, a function F (Bk, Lk) for obtaining γ from Bk and Lk is created using these many data sets. Although there is no limitation in particular as a function, For example, a multi-degree polynomial etc. can be used. In addition, when the signal values of Bk and Lk are 0 by the black generation process, γ = 1, so the UCA function F (0, 0) is 1.

As described above, the UCA rate is calculated by the UCA rate calculation unit 24 using the obtained UCA function, and the UCA processing unit 25 performs the UCA process. In the case of the present embodiment, the UCA process is simply multiplication as shown in the following equation.
C2 = F (Bk, Lk) C1
M2 = F (Bk, Lk) M1
Y2 = F (Bk, Lk) Y1
As described above, in this embodiment, UCA processing is performed based on the Bk and Lk signal values, and high-saturation color reproduction is possible regardless of the black separation pattern. In addition, since the UCA rate is determined based on the saturation during CMY three-color reproduction, stable color reproduction can be performed regardless of the black generation pattern.

Example 3:
In the above-described embodiment, the undercolor process is executed by a calculation formula. The present invention is not limited to this, and can be realized by a 3D-LUT. That is, a 3D-LUT having the output values of C, M, Y, Bk, and Lk after color separation obtained by the above method as grid point output values is created, and the CMYBkLk for the input CMY signal is generated using the 3D-LUT. The output value may be obtained using an interpolation calculation circuit.

Example 4:
The present invention can be applied not only to the undercolor removal process using the dark and light black ink but also to the undercolor removal process of the secondary color. FIG. 10 shows the configuration of Example 3 in the case where undercolor removal is performed with dark red ink. However, it is assumed that the hue of the mixed color of 100% magenta ink and 100% yellow ink and the hue of red ink match.

  The red generation unit 31 generates dark red Dr and light red Lr according to the values of the inputs M0 and Y0 as in the second embodiment. Then, the UCR processing unit 33 removes the red component from the M and Y components. Further, by performing the UCA process 35 and correcting the saturation, color separation can be performed with a light and dark ink separation pattern with a high degree of freedom even when dark and light secondary color inks are used.

Example 5:
The above-described embodiment is an embodiment of hardware, but the present invention can also be executed by software. FIG. 11 shows an apparatus for realizing the color separation processing method. As shown in FIG. 11, the image processing apparatus according to the present embodiment includes an apparatus main body 100 including a microcomputer, a color measurement apparatus 150 for reading color values of each patch of a color chart, data and commands. The image forming apparatus 140 includes a mouse 110 and a keyboard 120 for inputting, a display 130 for displaying image data, and an image forming apparatus 140 such as a color printer. The color measuring device 150 is a device for outputting the data as a numerical expression of the color, irradiating the object (original) with light, and using the photoelectric principle of the intensity of reflected light or transmitted light from the object. It is for measuring. Basically, it consists of a light source and a photometer. The apparatus main body 100 includes a CPU 210, a ROM 220, a RAM 230, a hard disk 240 in which a control program for the CPU 210 is stored, a NIC 250 for exchanging data between the main body and another apparatus, and data and commands are input / output to / from these. It consists of buses connected as possible. In this system, the CPU 210 can be provided with the function as the color separation processing method of the present invention. Note that the function as such a color separation processing method in the CPU 210 can be provided in the form of an information recording medium such as a software package-specifically, a CD-ROM, for example. Therefore, in the example of FIG. A medium driving device 310 is provided for driving the information recording medium when it is set. In other words, the color separation processing method of the present invention allows a general-purpose computer system equipped with an image scanner, a display, etc., to read a program recorded on an information recording medium such as a CD-ROM, and The present invention can also be implemented in an apparatus configuration that causes a processor to execute a color signal processing method and a color conversion profile generation method. In this case, a program for executing the color separation processing method of the present invention, that is, a program used in the hardware system is provided in a state recorded on a medium. The information recording medium on which the program is recorded is not limited to the CD-ROM, and a ROM, RAM, flexible disk, memory card, or the like may be used. The program recorded on the medium is installed in a storage device incorporated in the hardware system, for example, the hard disk 240, so that the program can be executed to realize the color correction function and the color separation processing function. . Further, the program for realizing the color separation processing method of the present invention is not only provided in the form of a medium, but may be provided by a server, for example, by communication.

1 shows a configuration of color separation processing means according to a first embodiment. 1 shows an overall configuration of an image processing apparatus. It is a figure explaining a black generation process. It is a figure explaining tetrahedral interpolation. Indicates the coefficient of tetrahedral interpolation. An example of generation of dark and light black ink is shown. The structure of the color separation process means of Example 2 is shown. It shows the overlap of dark and light ink dots. The processing flowchart of a UCA rate is shown. The structure of the color separation process means of Example 3 is shown. The structural example of Example 4 is shown. It is a figure explaining the conventional CMYK color separation process. It is a figure explaining the color separation method to the conventional dark and light ink.

Explanation of symbols

11 Black Generation Processing Unit 12 Color Mixing Conversion Unit 13 UCR Processing Unit

Claims (8)

In an image processing apparatus for color-separating an input color signal into an output color signal including black ink having a different density, color conversion means for obtaining an output value of the black ink having a different density from the input color signal, and the black ink having a different density A color mixing conversion unit that converts black ink having different densities into a mixed color based on the output value, and a lower color removal processing unit that performs under color removal using the color signal value after the color conversion. An image processing apparatus. In an image processing apparatus for color-separating an input color signal into an output color signal containing secondary color inks having different densities, color conversion means for obtaining an output value of the secondary color ink having different densities from the input color signals, and the density Color conversion means for converting secondary color inks having different densities into mixed colors based on the output values of the secondary color inks having different colors, and a lower color for performing undercolor removal using the color signal values after the color mixture conversion An image processing apparatus comprising: a removal processing unit. A printing rate calculation means for obtaining a lower color printing rate from the output values of the black inks having different densities, and a lower color printing process using the printing rate for the color signal after the lower color removal processing is performed. 2. The image processing apparatus according to claim 1, further comprising undercolor printing means. A printing rate calculation means for obtaining a lower color printing rate from the output values of the secondary color inks having different densities, and a lower color printing process using the printing rate for the color signal after the lower color removal processing The image processing apparatus according to claim 2 , further comprising: a lower color printing unit that performs the operation. The retouching rate calculating means is configured so that the output saturation when the input color signal before color separation processing is output matches the output saturation when the output color signal after color separation processing is output. The image processing apparatus according to claim 3, wherein: is set. In an image processing method for color-separating an input color signal into an output color signal including black ink having a different density, a color conversion step for obtaining an output value of the black ink having a different density from the input color signal, and the black ink having a different density A color mixing conversion step of converting black inks having different densities into mixed colors based on the output value of the color, and a lower color removal processing step of removing the lower color using the color signal value after the color mixture conversion. An image processing method. In an image processing method for color-separating an input color signal into an output color signal including secondary color inks having different densities, a color conversion step for obtaining an output value of the secondary color ink having different densities from the input color signals, and the density a mixed conversion step of converting the color by mixing the two different color ink density based on the output value of the different secondary color inks, by using the color signal value after the mixing conversion, undercolor performing under color removal An image processing method comprising a removal processing step. Program for realizing the image processing method according to claim 6 or 7, wherein the computer.

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