JP2012129912A - Printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a printer using a profile optimized by using a more appropriate granular index when the profile is optimized by considering granularity of an ink amount.SOLUTION: The printer manages a color by using the profile making a coordinate value of a color space of input data correspond to the ink amount. In the profile, the ink amount is selected so that a value of a target function including the granular index showing granularity of the ink amount becomes low, and the granular index is set by adding index values showing use difficulty of respective inks, which correspond to a value of previously decided lightness of the ink amount, for every ink constituting the ink amount to the respective inks constituting the ink amount. The printer includes ink amount selecting means selecting the ink amount corresponding to input data based on the profile.

Description

  The present invention relates to color management that associates an input value from an input device with an output value from an output device.

  2. Description of the Related Art Conventionally, a color management system (CMS) is known as a technique for associating an output value of an input device with an output value of an output device. In a color management system in a printing apparatus, input data in a device-dependent color space supplied from an input device is converted into output data (also called ink amount) in a color material color space (for example, CMYK). At this time, the value is defined by the color management system so that the optimum output data is selected in consideration of the color reproducibility between devices and the characteristics and capabilities of the devices (see, for example, Patent Document 1).

  In the color management system described above, the graininess index is known as an evaluation value for evaluating the graininess of the ink amount. The graininess indicates the visibility of dots of the printing ink, and if this graininess is increased, graininess noise is generated in the image, which is not preferable. Here, the graininess noise is noise that is visually recognized at a certain level of ink dots forming an image and feels rough in a smooth region of the image. Therefore, conventionally, the degree of graininess is quantified as the graininess index described above to reduce graininess noise (see, for example, Patent Document 2).

JP 2008-302699 A JP 2007-281724 A

  The graininess index described above has a non-linear relationship with the ink amount, and sometimes falls into a local minimum in the process of searching for the optimum ink amount. In addition, since the graininess index and the ink amount are independent values, it may be difficult to adjust the graininess index in the ink amount search step.

  The present invention has been made in view of the above problems, and in optimizing a profile in consideration of the graininess of the ink amount, a printing apparatus using a profile optimized by using a more appropriate graininess index. For the purpose of provision.

  In order to solve the above-described problems, the present invention provides a printing apparatus that performs color management using a profile in which color space coordinate values of input data and an ink amount are associated with each other, and the profile includes the granularity of the ink amount. The ink amount is selected so that the value of the objective function including the granularity index indicating low is reduced, and the granularity index is predetermined for each ink constituting the ink amount. Ink for selecting the ink amount corresponding to the input data based on the profile is set by summing up index values indicating the difficulty of using each ink according to the value of lightness for each ink constituting the same ink amount. The configuration has a quantity selection means.

The invention configured as described above relates to a printing apparatus that performs color management using a profile in which coordinate values of a color space of input data are associated with ink amounts. In this profile, each ink amount is selected so that the objective function including the granularity index indicating the granularity of the ink amount is low. For example, in creating a profile, an ink amount associated with an input value is selected based on a well-known optimization method so that the granularity is low. The granularity index included in the objective function is set by summing up index values indicating the difficulty of using each ink with respect to the lightness value predetermined for each ink amount for each ink constituting the ink amount. Is done.
For this reason, an ink that is not desirable from the viewpoint of graininess with respect to a predetermined lightness at a certain ink amount has a high sum of index values indicating the difficulty of use, and thus the value of the objective function increases and the ink amount The possibility of being excluded from the candidate increases.
Furthermore, since the index value is a value dependent on the determined lightness, the graininess index calculated based on the index value has a linear relationship with the lightness, and the local minimum is searched for the ink amount. Can be prevented.

1 is a block diagram illustrating a configuration of a printing system. It is a flowchart which shows the whole process sequence at the time of LUT creation without color correction. It is explanatory drawing of FM converter and an inverse model initial stage LUT. FIG. 4A to FIG. 4C are diagrams illustrating a correspondence relationship between input grid points and coordinate values of the L * a * b * color space in the smoothing process. It is a flowchart which shows an example of the process sequence of a smoothing process (step S800 of FIG. 2). 6A to 6D are explanatory diagrams showing the processing contents of steps S820 to S850 in FIG. It is a figure for _{demonstrating the} lightness cost L * _Cost . It is a figure explaining the weight for every ink color. It is explanatory drawing which shows the production method of the color correction LUT using LUT without color correction. It is a figure which shows the position of the certain ink amount Ij in an a * b * plane. FIG. 6 is a flowchart for calculating an adjustment amount for an ink amount Ij belonging to a certain color gamut. It is a figure explaining standard adjustment amount table T1 as an example. It is a figure which shows the object ink amount Ij located in a WCB surface.

Hereinafter, embodiments of the present invention will be described in the following order.
1. First embodiment:
1.1. About the configuration of the printing system:
1.2. About creating a profile:
1.3. About setting the adjustment amount:
2. Other embodiments:

1. First embodiment:
1.1. About the configuration of the printing system:
FIG. 1 is a block diagram illustrating a configuration of a printing system. The printing system includes a computer 10, a printer 20, a display 30, and an operation unit 40. The computer 10 also includes a storage unit 11, a CPU 12, a RAM 13, a USB interface (I / F) 21, a video I / F 31, an input I / F 41, and the like. Then, the CPU 12 included in the computer 10 implements each function such as the LUT creation module by reading the program stored in the storage unit 11 and executing operations in accordance with the program while expanding the program in the RAM 13. Each function of the LUT creation module will be described later.

  The storage unit 11 is, for example, a hard disk drive (HDD), and stores a program, a color conversion profile (for example, an inverse model initial LUT 410, an LUT 510 without color correction, a color correction LUT 610 described later), a reference adjustment amount table T1, and the like. “LUT” is an abbreviation for lookup table. The LUT 510 without color correction is a color conversion for converting a gradation value of a predetermined input color space (for example, RGB color space) into a combination of a plurality of types of ink used in the printer 20 (hereinafter also referred to as ink amount Ij). It is a table. The RGB color space, which is the input color space of the LUT 510 without color correction, is not a so-called device-dependent color space, but a virtual color space (or abstract color space) set independently of a specific device. The LUT 510 without color correction is used, for example, when creating the color correction LUT 610. The color correction LUT 610 is a lookup table for converting a standard device-dependent color space (for example, an sRGB color space or a JAPAN COLOR 2001 color space) into an ink amount Ij of a specific printer. The inverse model initial LUT 410 will be described later.

The computer 10 also functions as a print control apparatus by executing calculations according to a program and controlling the printer 20 via the USB I / F 21 or the like. Specifically, the computer 10 obtains image data to be printed, color-converts the image data on a pixel-by-pixel basis using a color conversion profile (such as LUT510 without color correction), and performs halftone processing on the color-converted image data. Print data is generated by performing microwave processing. Then, the print data is output to the printer 20 via the USB I / F 21 or the like. As a result, the printer 20 executes printing based on the print data. The computer 10 and the printer 20 may be connected by other interfaces such as infrared rays and wireless LAN.
The computer 10 is connected to the display 30 via a video I / F 31 and is connected to an operation unit 40 such as a keyboard and a mouse via an input I / F 41. As described above, the computer 10 has the function of the ink amount selection unit.

  The LUT creation module of the computer 10 includes an initial value setting unit 14, a smoothing processing unit 15, a table creation unit 16, a gamut prediction unit 17, a forward model converter 18, and an adjustment amount setting unit 19. ing. The functions of these parts will be described later.

1.2. About creating a profile:
FIG. 2 is a flowchart showing an overall processing procedure when creating an LUT without color correction.
In step S100, ink data is set. Here, the ink data is the color and type of ink used in the printer 20 and the duty limit value. In this embodiment, a color printer (printer 20) that can use four types of ink, cyan (C), magenta (M), yellow (Y), and black (K) is assumed. Of course, the type of ink used by the printer 20 is not limited to four, and light cyan (Lc), light magenta (Lm), and light black (LK) may be added thereto.

  Also, the duty limit value indicates the ink amount Ij that can be driven into the unit area of the medium (for example, paper). This duty limit value depends on the type of medium and the resolution of the image to be formed. For this reason, the duty limit value is determined for each type of medium (for example, paper) and for each print resolution. In this embodiment, it is assumed that printing is performed on a photographic paper with a resolution of 1440 × 1440 dpi. The duty limit value includes a duty limit value for a single ink color and a duty limit value (total duty limit value) for a total of a plurality of colors. For example, the duty limit value at a certain resolution of a certain paper is determined to be 80% for a single color and 120% for each color in total.

  In step S200, a color patch is printed. Here, a color patch is printed by a simple combination of ink amounts Ij without using a duty limit value or a color conversion table. In the present specification, the term “color patch” is used in a broad sense including not only a chromatic color patch but also an achromatic color patch.

  In step S300, for example, color measurement of the color patch is performed using a colorimeter or a scanner (not shown). This color measurement result is taken into the computer 10, and a virtual sample is formed in which the ink amount Ij and the color measurement value (the coordinate value of the L * a * b * color space) are associated with each other.

  In step S400, a forward model (hereinafter also referred to as FM) is created based on the color patch colorimetry result (virtual sample). The “forward model” means a conversion model that converts the ink amount Ij into a color value (colorimetric value) in the device independent color space. Conversely, the “inverse model” means a conversion model that converts the color value of the device independent color space into the ink amount Ij. In the present embodiment, the CIE-L * a * b * color space is used as the device independent color space. Hereinafter, the color value of the CIE-L * a * b * color space is also simply referred to as “L * a * b * value” or “Lab value”.

  FIG. 3 is an explanatory diagram of the FM converter and the inverse model initial LUT. As shown in FIG. 3, the spectral printing converter RC constituting the preceding stage of the FM converter 18 converts the ink amounts Ij of a plurality of types of ink into spectral reflectances R (λ of color patches printed according to the ink amounts Ij. ). In this embodiment, it is assumed that the printer 20 uses the above-described four color inks, and the spectral printing model converter RC also receives the ink amounts Ij of these four types of inks. However, an arbitrary ink amount Ij can be used as the plurality of types of ink used in the printer 20. The color calculation unit CC calculates the color value of the L * a * b * color space from the spectral reflectance R (λ). For the calculation of the color value, a light source (for example, standard light D50) selected in advance is used as a color patch observation condition. As a method for creating the spectral printing model converter RC, for example, a method described in JP-T-2007-511175 can be employed.

  The inverse model initial LUT 410 is a look-up table having an L * a * b * value as an input and an ink amount Ij as an output. The initial LUT 410 is obtained by, for example, dividing the L * a * b * color space into a plurality of small cells and selecting and registering the optimum ink amount Ij for each small cell. This selection is performed in consideration of the image quality of the color patch printed with the ink amount Ij, for example. In general, there are many combinations of ink amounts Ij that reproduce a certain L * a * b * value. Therefore, in the initial LUT 410, a combination of a plurality of ink amounts Ij that reproduce substantially the same L * a * b * values is selected by selecting the optimum ink amount Ij from a desired viewpoint such as image quality. . The L * a * b * value that is the input value of the initial LUT 410 is a representative value of each small cell. On the other hand, the ink amount Ij that is an output value reproduces any L * a * b * value in the cell. Accordingly, in this initial LUT 410, the input value L * a * b * and the output value ink amount Ij do not correspond exactly, and the output value ink amount Ij is converted to L by the FM converter 18. When converted into * a * b * values, a value slightly different from the input value of the initial LUT 410 is obtained. However, as the initial LUT 410, an input value and an output value that completely correspond to each other may be used. It is also possible to create the LUT 510 without color correction without using the initial LUT 410. As a method for creating the initial LUT 410 by selecting the optimal ink amount Ij for each small cell, for example, the method described in the above-mentioned Japanese translations of PCT publication No. 2007-511175 can be employed.

  In step S500 in FIG. 2, gamut prediction is performed using the forward model (FM) and the ink duty limit value. In step S600, evaluation of the gamut predicted in step S500 (hereinafter also referred to as prediction gamut) is performed. This determination is made by the user, for example. If the evaluation of the predicted gamut is NG (NO in S600), the process returns to step S100.

  On the other hand, if the evaluation of the predicted gamut is OK (S600: YES), the initial input value for creating the LUT 510 without color correction is set by the user in step S700. As the input values of the LUT 510 without color correction, values at substantially equal intervals determined in advance as RGB values are set. Since a set of RGB values is considered to represent a point in the RGB color space, the set of RGB values is also referred to as an “input grid point”.

  In step S700, the initial value of the ink amount Ij for some small number of input grid points selected in advance from the plurality of input grid points is input by the user. It is preferable to select at least an input grid point corresponding to a vertex of a three-dimensional color solid in the RGB color space as the input grid point to which the initial input value is set. At the apex of the three-dimensional color solid, each RGB value takes the minimum value or the maximum value of the definition range. Specifically, when each RGB value is expressed by 8 bits, (R, G, B) = (0,0,0), (0,0,255), (0,255,0), (255, The initial input value of the ink amount Ij is set for eight input grid points that are (0,0), (0,255,255), (255,0,255), (255,255,0), and (255,255,255). Note that the ink amounts Ij for the input grid points of (R, G, B) = (255, 255, 255) are all set to zero. The initial input value of the ink amount Ij for the other input grid points is arbitrary, and is set to 0, for example.

In step S800 of FIG. 2, the smoothing processing unit 15 (FIG. 1) executes a smoothing process (smoothing process) based on the initial input value set in step S700.
FIG. 4A to FIG. 4C are diagrams illustrating a correspondence relationship between input grid points and coordinate values of the L * a * b * color space in the smoothing process. In FIG. 4B, the distribution of a plurality of color points in a state before the smoothing process is indicated by double circles and white circles. These coordinate values constitute a three-dimensional color solid CS in the L * a * b * color space. The L * a * b * value of each coordinate value is a value obtained by converting the ink amounts Ij at a plurality of input lattice points of the LUT 510 without color correction into L * a * b * values using the FM converter 18. . The initial value of the ink amount Ij for the other input grid points is set by the initial value setting unit 14 (see FIG. 1) from the initial input value.

  FIG. 4C shows the distribution of color points after the smoothing process. The smoothing process is a process of moving a plurality of coordinate values in the L * a * b * color space and smoothing the distribution of the coordinate values close to equal intervals. In the smoothing process, an optimum ink amount Ij is also determined to reproduce the L * a * b * values of the coordinate values after movement. At this time, the optimum ink amount Ij assigned to the input grid point is calculated based on the objective function E.

  Each of the vertices of the three-dimensional color solid CS (FIGS. 4B and 4C) in the L * a * b * color space is a three-dimensional color solid (FIG. 4A) of the input color space (RGB color space) of the LUT 510 without color correction. There is a one-to-one correspondence with the vertex. Also, the sides (ridge lines) connecting the vertices can be considered to correspond to each other in both color solids. Each coordinate value of the L * a * b * color space before the smoothing process (FIG. 4B) is associated with an input grid point of the LUT 510 without color correction, and therefore L * after the smoothing process (FIG. 4C). Each color point in the a * b * color space is also associated with an input grid point of the LUT 510 without color correction. Note that the input grid points of the LUT 510 without color correction are not changed by the smoothing process.

  The reason for performing the smoothing process in the L * a * b * space when creating the LUT 510 without color correction is as follows. In the LUT 510 without color correction, there is a demand for setting the ink amount Ij in the output color space so that a color gamut as large as possible can be reproduced. The color gamut that can be reproduced with a specific ink set is determined in consideration of a predetermined limit condition such as an ink duty limit value (a limit value of the ink amount Ij that can be ejected to a certain area). On the other hand, the forward model FM described above does not take these restriction conditions into consideration and is created regardless of the reproducible color gamut. Therefore, if the range that can be taken by the coordinate values in the L * a * b * space is determined in consideration of limiting conditions such as ink duty limitation during the smoothing process, the color gamut that can be reproduced with a specific ink set is determined. It becomes possible to do. As an algorithm for moving the coordinate value, for example, an algorithm using a dynamic model is used (see, for example, Japanese Patent Laid-Open No. 2006-197080 for the dynamic model).

  FIG. 5 is a flowchart showing an example of the processing procedure of the smoothing process (step S800 in FIG. 2). In step S810, the initial value setting unit 14 (FIG. 1) initially sets a plurality of coordinate values to be smoothed. Specifically, first, a temporary ink amount Ij for each coordinate value to be smoothed is determined from the initial input value of the ink amount Ij. Next, the L * a * b * value corresponding to the temporary ink amount Ij is obtained using the FM converter 18. Then, the obtained L * a * b * value is converted again into the ink amount Ij using the inverse model initial LUT 410 (FIG. 3).

As a result of the process in step S810 described above, the following initial values are determined for the coordinate values to be subjected to the smoothing process.
(I) Value of input grid point of LUT without color correction: (R, G, B)
(Ii) Initial coordinate values of L * a * b * color space coordinate values corresponding to each input grid point: L (R, G, B)
(Iii) Initial ink amount Ij corresponding to each input grid point: I (R, G, B)
As can be understood from the above description, the initial value setting unit 14 has a function of setting initial values related to other input grid points from input initial values related to typical input grid points. The initial value setting unit 14 may be included in the smoothing processing unit 15.

In step S820, the smoothing processing unit 15 moves the coordinate values in the L * a * b * space.
6A to 6D are explanatory diagrams showing the processing contents of steps S820 to S850 in FIG. As shown in FIG. 6A, before the smoothing process, the distribution of coordinate values is considerably biased. FIG. 6B shows the position of each color point after the minute time has elapsed. The L * a * b * value after this movement is called “target value LABt”. This is because the value “LABt” is used as a target value in the search process of the optimum value of the ink amount Ij described below.

ここで、Ijは目的関数Ｅを最小とするインク量Ij、LABFM(Ij)はインク量IjをＦＭコンバーター１８で変換して得られるＬ*ａ*ｂ*値、GI(Ij)はインク量Ijで印刷されるカラーパッチの粒状性指数（Graininess Index）、αは定数である。（１）式の右辺第１項は、２つのベクトルLABt, LABFM(Ij)で表される２つの座標値の距離の２乗を求める演算を意味している。 In step S830, the smoothing processing unit 15 searches for an optimal value of the ink amount Ij with respect to the target value LABt using the objective function E expressed by the following equation (1) (see FIG. 6C).

Here, Ij is an ink amount Ij that minimizes the objective function E, LABFM (Ij) is an L * a * b * value obtained by converting the ink amount Ij by the FM converter 18, and GI (Ij) is an ink amount Ij. The graininess index (α) of a color patch printed with is a constant. The first term on the right side of the equation (1) means an operation for obtaining the square of the distance between two coordinate values represented by two vectors LABt and LABFM (Ij).

  The above equation (1) shows that the graininess index GI is more in the ink amount Ij that reproduces the L * a * b * value close to the coordinate value LABt of the color point after moving by a minute amount in the process of step S820. This means that the small ink amount Ij is determined as the optimum ink amount Ij. The search for the optimum ink amount Ij is executed using an optimization method such as a quasi-Newton method. Further, the search for the ink amount Ij is started from the initial ink value of each input grid point set in step S810. Accordingly, the ink amount Ij obtained by the search is a value obtained by correcting the initial ink amount Ij.

In general, the search for the ink amount Ij by the optimization process is executed under the following conditions.
(Condition i) The objective function E is minimized (Condition ii) Only ink that has been designated in advance as usable is used.

  As an expression that gives the objective function E of the condition i, various expressions other than the above expression (1) can be used. In addition, as usable ink in the condition ii, there are a case where only some of the plurality of types of ink that can be used by the printer 20 are designated and a case where all types of ink are designated. The type of ink that can be used is preferably set in advance by the user for each color point to be smoothed. Specific examples of these optimization conditions will be described later.

ここで、Dutyはインク量Ijのデューティ制限値を示す。また、Ｌ* _Costは明度コストを示す。この明度コストは、あるターゲット明度LABt（予め決められた明度の値）における所定インクの使い難さを示す指標値であり、インク量Ijを構成するインク(i)（本実施形態では、ｉ：Ｃ，Ｍ,Ｙ,Ｋ）毎に設定される。そして、Weightは、インク色毎重みであり、インク(i)のＬ*値に応じて設定される値である。 In the present embodiment, the graininess index GI (Ij) is given by, for example, the following formula (2).

Here, Duty indicates the duty limit value of the ink amount Ij. L * _Cost represents the lightness cost. This lightness cost is an index value indicating the difficulty of using a predetermined ink at a certain target lightness LABt (predetermined lightness value), and ink (i) constituting the ink amount Ij (in this embodiment, i: C, M, Y, K). Weight is a weight for each ink color, and is a value set according to the L * value of ink (i).

ここで、ｅは自然対数の底である。また、ｘはインク量Ijのターゲット明度LABtである。そして、βは定数である。さらに、ｋは、インク色定義であり、デフォルトでは、インク(i)のベタ印字明度である。 FIG. 7 is a diagram for explaining the lightness cost L * _Cost . The brightness cost function ζβ (x) shown in FIG. 7 is a function having the target brightness LABt as an independent variable (horizontal axis) and the brightness cost as a dependent variable (vertical axis). This lightness cost calculation function ζβ (x) is a part of the sigmoid curve, and is represented by the following equation (3).

Here, e is the base of the natural logarithm. X is the target brightness LABt of the ink amount Ij. Β is a constant. Furthermore, k is an ink color definition, and by default is the solid print brightness of ink (i).

The lightness cost calculation function ζβ (i, x) indicates the difficulty in using the ink (i) with respect to the target lightness x of the ink amount Ij when reproducing the target lightness taking into account the granularity. Is calculated as follows. In general, the graininess increases as the contrast difference between the inks increases. Therefore, in order to lower the graininess, it is desirable that ink having a higher density in the high brightness region is not used. For example, in Figure 7A, shows the lightness cost L * _Cost of ink K (black), the target brightness x is lower region lower lightness cost L * _Cost, target brightness x is lightness cost L * _Cost in a high region Set to be higher. On the other hand, FIG. 7B shows the lightness cost L * _Cost in the ink Y (yellow), and is set so that the slope of the lightness cost L * _Cost is large in the region where the target lightness x is equal to or higher than the predetermined lightness.

Further, since the shape of the lightness cost calculation function ζβ (x, i) changes according to the value of the ink color definition k, the lightness cost in the ink (i) is controlled by changing the ink color definition k. be able to. For example, in the ink (i) whose ink color definition (lightness) is k1, when the ink color definition k1 is lowered from the actual lightness to k2, the curve of the lightness cost calculation function ζβ (x, i) changes (in the figure). ), The lightness cost L * _Cost increases as a whole even with the same target lightness. On the other hand, when the ink color definition k of the ink (i) is increased from the actual lightness, the lightness cost in the ink (i) is reduced, and the generation amount of the ink (i) is not suppressed. As described above, the ink color definition k based on the lightness of the ink (i) can be used as a parameter for adjusting the generation amount of the ink (i).

ここで、Ｗmaxは重みの最大値であり定数である。また、γは定数である。そして、インク色明度Ｌ*inkは、インクｉにおけるインク色定義ｋを０から１の範囲で規格化した値であり、例えば、Ｌ*inkは、以下に示す式（５−a）（５−b）から算出される。

FIG. 8 is a diagram illustrating the weight for each ink color. The weight for each ink color is set by the following equation (4).

Here, Wmax is a maximum value of the weight and is a constant. Γ is a constant. The ink color lightness L * ink is a value obtained by standardizing the ink color definition k in the ink i in the range of 0 to 1. For example, L * ink is expressed by the following equations (5-a) (5- Calculated from b).

  The weight for each ink color shown in Equation (4) changes the weight of ink generation according to the lightness. In other words, graininess is more likely to occur with dark ink (i), so by reducing the weight for bright ink (i) and increasing the weight for dark ink (i), a dark ink can be obtained. It becomes difficult to select and graininess can be suppressed. Further, as shown in FIG. 8, since the weight for each ink color is a function that monotonously decreases in accordance with the change in the ink color lightness L * ink, the ink color definition k that is the basis of the ink color lightness L * ink is actually set. The generation of ink (i) can be controlled by changing the brightness from (for example, the brightness 40 to the brightness 35).

  From the above description, the value of the ink (i) constituting the ink amount Ij is obtained by multiplying the lightness cost obtained based on the function ζβ (x), the duty limit value for the ink amount Ij, and the weight Weight for each ink color. ) To calculate the graininess index GI. Further, the graininess index GI constitutes the second term of the objective function E, so that it is possible to generate the ink amount Ij taking the graininess into consideration in the search for the optimum ink amount Ij. At this time, the values of the lightness cost and the weight for each ink color are determined by the lightness of the ink (i), so that the generation of the ink (i) can be controlled by the lightness. A method for setting the brightness adjustment amount used at this time will be described later.

  In step S840 in FIG. 5, the L * a * b * value corresponding to the ink amount Ij searched in step S830 is recalculated by the FM converter 18 (see FIG. 6D). Here, the reason for recalculating the L * a * b * value is that the searched ink amount Ij is the ink amount Ij that minimizes the objective function E, and therefore L * a * b * reproduced with the ink amount Ij. This is because the value is slightly deviated from the target value LABt of the optimization process. The L * a * b * values recalculated in this way are used as coordinate values after the movement of each color point.

  In step S850, it is determined whether or not the average value (ΔLab) ave of the movement amount of the coordinate value of each color point is equal to or less than a preset threshold value ε. When the average value (ΔLab) ave of the movement amount is larger than the threshold value ε, the process returns to step S820 and the smoothing process of steps S820 to S850 is continued. On the other hand, when the average value (ΔLab) ave of the movement amount is equal to or smaller than the threshold value ε, the distribution of the color points is sufficiently smoothed, and the smoothing process ends. Note that an appropriate value for the threshold ε is experimentally determined in advance.

  As described above, in the smoothing process (smoothing process) of the present embodiment, the optimal ink amount Ij corresponding to the color point after the movement is searched by the optimization method while moving each color point at every minute time interval. Then, these processes are continued until the movement amount of the color point becomes sufficiently small. As a result, a smooth color point distribution can be obtained as shown in FIG. 4C.

  In step S900 of FIG. 2, the table creation unit 16 creates the LUT 510 without color correction using the result of the smoothing process. That is, the table creation unit 16 registers the optimum ink amount Ij for reproducing the color point of the L * a * b * color space associated with each input grid point as the output value of the LUT 510 without color correction. In the smoothing process, in order to reduce the calculation load, it is possible to select only the color points corresponding to only a part of the input grid points of the LUT 510 without color correction as the processing target. For example, when the RGB value interval at the input grid point of the LUT 510 without color correction is 16, if the RGB value interval at the input grid point to be smoothed is set to 32, the load of the smoothing process is reduced by half. be able to. In this case, the table creating unit 16 determines and registers the ink amount Ij for all the input grid points of the LUT 510 without color correction by interpolating the smoothing processing result.

In step S1000 of FIG. 2, the LUT 510 without color correction is evaluated. If the evaluation is NG (NO in S1000), the process returns to step S100 again. On the other hand, if the evaluation is OK, the LUT 510 without color correction is registered in the storage unit 11.
The LUT creation module also creates the color correction LUT 610 using the LUT 510 without color correction.

  FIG. 9 is an explanatory diagram showing a method for creating a color correction LUT using an LUT without color correction. As shown in FIG. 9, the LUT 510 without color correction converts the RGB value into an ink amount Ij. This ink amount Ij represents the ink amounts Ij of the eight types of ink of the printer 20. At this time, the subscript j of the ink amount Ij is 1 to 8. The converted ink amount Ij is converted into an L * a * b * value by the FM converter 18. On the other hand, the sRGB values are converted into L * a * b * values according to a known conversion formula. The converted L * a * b * values are gamut-mapped so that the color gamut matches the color gamut of the L * a * b * values converted by the FM converter 18. On the other hand, a reverse conversion LUT 511 is created using the L * a * b * values converted from the RGB values through the LUT 510 without color correction and the FM converter 18 as a reverse lookup table. The gamut-mapped L * a * b * values are converted into RGB values by the inverse conversion LUT 511. This RGB value is further converted again into the ink amount Ij by the LUT 510 without color correction. The color correction LUT 610 can be created by registering the correspondence between the last ink amount Ij and the first sRGB value in the lookup table. The color correction LUT 610 is a color conversion table for converting the sRGB color space into the ink color space.

  As described above, the color correction LUT 610 is created and recorded in the HDD 11. Thereafter, the computer 10 uses the color correction LUT 610 to select the ink amount Ij corresponding to the input data and outputs it to the printer 20.

1.3. About setting the adjustment amount:
Hereinafter, a method for adjusting the generation amount of each ink (i) constituting the ink amount Ij when searching for the optimum ink amount Ij for reproducing the target lightness will be described. As described above, the search for the ink amount Ij is performed using the objective function E shown in the above equation (2), and the occurrence of each ink (i) is determined by the lightness cost constituting the granularity index GI and the weight for each ink color. Be controlled. Furthermore, the lightness cost and the weight for each ink color are values set according to the ink color definition (the lightness of ink (i)) as a parameter, so the lightness of a certain ink (i) is changed from the actual lightness. As a result, the value of the objective function E corresponding to the ink amount Ij also changes. For example, as shown in FIGS. 7 and 8, if the ink color definition is brighter than the actual ink brightness, the amount of ink (i) generated increases.

On the other hand, depending on the color gamut of the ink amount Ij, it is desirable that the adjustment amount of the ink color definition in each ink (i) is set as appropriate. Here, the adjustment amount of the ink color definition indicates an increase / decrease amount of change when the brightness (ink color definition) of a certain ink (i) is changed.
FIG. 10 is a diagram showing the position of a certain ink amount Ij on the a * b * plane. In the search for the optimum ink amount Ij using the granularity index GI according to the present embodiment, when the generation amount of the complementary color of the color gamut to which a certain ink (i) belongs is suppressed, the a * b * plane with respect to this color gamut There may be a case where the color gamut (gamut) opposite to each other decreases. For example, in FIG. 10A, when the generation amount of C ink that is a complementary color in the R direction is suppressed, the color gamuts in the G direction and the B direction may decrease. Therefore, it is not preferable to set the same adjustment amount for the ink (i), and it is desirable to set the adjustment amount according to the color gamut.

In the present embodiment, the adjustment amount is set by the following (Method i) (Method ii).
(Method i) An adjustment amount is calculated by interpolation calculation based on the saturation.
(Method ii) An adjustment amount is calculated by interpolation calculation based on the hue angle Hue.
In (Method ii) described above, (ii-1) A reference point is determined based on the position of the target lattice point in accordance with the position of the L * a * b * color space of the ink amount Ij. (Ii-2) Two methods are used in which the same reference point is used in the color gamut where the target lattice point is located.

FIG. 11 is a flowchart for calculating the adjustment amount for the ink amount Ij belonging to a certain color gamut.
In step S1, a reference adjustment amount serving as a reference for the adjustment amount is set. In the present embodiment, each vertex of R (red), G (green), B (blue), C (cyan), M (magenta), and Y (yellow) in the input color space of the LUT 510 without color correction, and W ( Considering each hue ridge line connecting white) or K (black) and the ink (i) belonging to the gray axis as a reference grid point, a reference adjustment amount is set to this reference grid point. As shown in FIG. 4, the vertices of the Lab color system three-dimensional color solid CS correspond one-to-one with the vertices in the input color space of the LUT 510 without color correction, and the sides connecting the vertices are both colors. It can be considered that they correspond to each other in three dimensions. Therefore, the process for calculating the adjustment amount shown below is executed in the input color space in the LUT 510 without color correction. Then, the correspondence between the reference grid point and the reference adjustment amount is recorded as a reference adjustment amount table T1. Although the method for setting the reference grid point is arbitrary, in this embodiment, the reference grid point is set as follows. (1) A point existing in the input color space of the LUT 510 without color correction and corresponding to each hue edge of the target product gamut and the ink (i) existing on the gray axis. (2) The intervals between the reference grid points are equal intervals.

  FIG. 12 is a diagram illustrating the reference adjustment amount table T1 as an example. In the reference adjustment amount table T1 shown in FIG. 12, at the reference lattice point of the C hue ridge line (the line segment connecting the vertex C and the vertex W), the reference lattice point ID and the ink amount value associated with this lattice point ( C, M, Y, and K ink amounts are shown) and adjustment amounts are associated with each other. The reference grid point ID is for identifying each reference grid point. The reference adjustment amount table T1 is similarly set for each hue ridgeline and gray axis for which reference grid points are set. Here, the adjustment amount set in the reference adjustment amount table T1 is appropriately set according to each color gamut.

  In step S2, the adjustment amount setting unit 19 determines whether or not the target ink amount Ij belongs to the gray axis. Specifically, the adjustment amount setting unit 19 converts the target ink amount Ij into an L * a * b * value using the FM converter 18, and the saturation set by the converted a * and b * values. If S is less than or equal to the threshold value Sdef, it is determined that this ink amount Ij belongs to the gray axis. If the target ink (i) belongs to the gray axis, the process proceeds to step S4, and the adjustment amount is calculated.

In step S4, the adjustment amount setting unit 19 _applies the adjustment amount Offset _Grayi specified for the target ink amount Ij with reference to the reference adjustment amount table T1 corresponding to the gray axis as the adjustment amount of each ink amount Ij. (Formula (6) below). Here, OffsetPi is an adjustment amount set for the ink (i) included in the target ink amount Ij.

  In step S3, the adjustment amount setting unit 19 determines the color gamut to which the target ink amount Ij belongs. For example, the adjustment amount setting unit 19 determines the color gamut to which the target ink amount Ij belongs based on the L * a * b * value acquired in step S2. Here, the target ink amount Ij belonging to other than the gray axis belongs to the outer or inner side of the reproduction color gamut. Further, as described above, the vertex of the Lab color system three-dimensional color solid CS has a one-to-one correspondence with the vertex in the input color space of the LUT 510 without color correction, and the side connecting each vertex also has both colors. It can be considered that they correspond to each other in three dimensions. Therefore, in the present embodiment, it is determined that the ink (i) existing in the outline of the gamut exists in the outline in the input color space of the LUT 510 without color correction.

  In step S5, the adjustment amount setting unit 19 determines whether or not the target ink amount Ij belongs to the outline of the gamut. As described above, when the target ink amount Ij exists in the outline of the gamut, it can be determined that it exists in the outline in the input color space of the LUT 510 without color correction. For example, when the ink amount Ij belongs to the WCBM surface, the value of the black ink K needs to be zero. In addition, when the ink amount Ij is composed only of the cyan ink C and the magenta ink M (Y = K = 0), it can be determined that the ink amount Ij is on the CBM surface. In this state, if the cyan ink C is 0, it can be determined that the target ink amount Ij is a point on the BM ridgeline, and if the magenta ink M is 0, the target ink amount Ij is a point on the CM ridgeline. It can be judged that there is. Further, blue B occurs when the ink amounts of cyan ink C and magenta ink M are equal. Therefore, in addition to the above conditions, if the ink amounts of cyan ink C and magenta M are equal, the target ink amount It can be determined that Ij is on the B ridge line. Further, when the yellow ink Y and the black ink K are 0, and neither the cyan ink C nor the magenta ink M is 0, if the cyan ink C is larger than the magenta ink M, the target ink amount Ij belongs to the WCB surface, If the cyan ink C is less than the magenta ink M, it can be determined that the target ink amount Ij belongs to the WMB surface. As described above, based on the value of the ink amount Ij, it can be determined whether or not it belongs to the outline and the area to which it belongs.

  When the target ink amount Ij belongs to the outline (step S5: YES), the process proceeds to step S6, and the adjustment amount setting unit 19 sets the adjustment amount of each ink (i) with respect to the target ink amount Ij. On the other hand, when the target ink amount Ij belongs to the inside of the gamut (step S5: NO), the adjustment amount setting unit 19 proceeds to step 7 and calculates the adjustment amount.

  FIG. 13 is a diagram illustrating the target ink amount Ij located on the WCB surface. When the ink amount Ij belongs to the outline of the gamut, in step S6, the adjustment amount setting unit 19 obtains reference grid points on the two ridge lines surrounding the color gamut to which the target ink amount Ij belongs. For example, if the ink amount Ij is composed of ink (Ci, Mi, Yi, Ki) when belonging to the WCB plane, the reference grid point on the C ridge line is color-corrected with reference to the reference adjustment amount table T1. The reference grid point on the B ridge line in the input color space of the none LUT 510 is set to (Ci, Mi, 0, 0) in the input color space of the LUT 510 without color correction (FIG. 13A). . Of course, if there is no reference grid point that satisfies the above conditions in the reference adjustment amount table T1, among the reference grid points recorded in the reference adjustment amount table T1, inks Ci, inks Mi and L * constituting the ink amount Ij are used. A reference grid point having the closest distance in the a * b * space may be selected.

  When the ink amount Ij does not belong to the outline of the gamut (that is, when it belongs to the inside of the gamut), in step S7, the adjustment amount setting unit 19 points on the two ridge lines surrounding the color gamut to which the target ink amount Ij belongs. Among them, the vertex of each hue is set as a reference lattice point (FIG. 13B). Here, in the value (C, M, Y, K) of the input color space of the LUT 510 without color correction, the value of each vertex is C = (Gn, 0,0,0), M = (0, Gn, 0 , 0), Y = (0,0, Gn, 0), R = (0, Gn, Gn, 0), G = (Gn, 0, Gn, 0), B = (Gn, Gn, 0,0) ). Gn is the value of each ink.

ここで、OffsetPi’は対象インク量Ijを構成するインク（ｉ）における仮の調整量、OffsetAi、OffsetBiは基準格子点（Ａ又はＢ）の調整量、色相角HuePiは、対象インク量Ijの色相角、色相角HueA、HueBは基準格子点の色相角を示す。また、色相角Hueは、色相（ａ*ｂ*平面）におけるある座標値が示す角度であり、基準格子点におけるインク量Ijに対応するＬ*ａ*ｂ*値から以下に示す式（８）により算出される。

In step S8, the adjustment amount setting unit 19 sets the reference adjustment amount associated with the reference lattice point set in step S6 or step S7, the hue angle Hue on the a * b * plane of the target ink amount Ij, and Based on the hue angles HueA and HueB of the reference grid points, a temporary adjustment amount is calculated using an adjustment amount calculation formula shown in the following formula (7).

Here, OffsetPi ′ is a temporary adjustment amount in ink (i) constituting the target ink amount Ij, OffsetAi and OffsetBi are adjustment amounts of the reference grid point (A or B), and the hue angle HuePi is the hue of the target ink amount Ij. Angles and hue angles HueA and HueB indicate the hue angles of the reference grid points. The hue angle Hue is an angle indicated by a certain coordinate value in the hue (a * b * plane), and the following equation (8) is derived from the L * a * b * value corresponding to the ink amount Ij at the reference grid point. Is calculated by

  In the above equation (7), the distance between each reference grid point on the a * b * plane and the target ink amount Ij is determined using the hue angle Hue, and a temporary adjustment amount for the target ink amount Ij is determined for each reference grid. It is obtained by interpolation from the adjustment amount of the points. In other words, the temporary adjustment amount of the target ink amount Ij is calculated by applying the adjustment amount set to each reference grid point according to the value of each hue angle Hue according to the equation (7).

ここで、OffsetHueiは、色相各Hueをもとに算出した対象インク量Ijに含まれるインク（ｉ）の調整量であり、具体的には、上記式（７）により算出される値である。また、SPは、対象インク量Ijの彩度である。 In step S9, if the saturation S of the target ink amount Ij is larger than the threshold Sth (step S9: YES), the adjustment amount setting unit 19 sets the calculated temporary adjustment amount as the adjustment amount. On the other hand, when the saturation of the target ink amount Ij is equal to or less than the threshold value Sth (step S9: NO), the adjustment amount setting unit 19 performs the adjustment amount of the ink amount Ij based on the following equation (9) in step S10. Is calculated.

Here, OffsetHuei is an adjustment amount of the ink (i) included in the target ink amount Ij calculated based on each hue Hue, and is specifically a value calculated by the above equation (7). SP is the saturation of the target ink amount Ij.

  As shown in FIG. 10B, since the ink amount Ij whose saturation S is the threshold value Sdef <S <threshold value Sth is closer to the gray axis, it is desirable to consider the degree of influence on the gray axis. Therefore, the influence from the gray axis is taken into account by performing the interpolation calculation according to the saturation S by the above equation (9).

  The processes in steps S1 to S10 are repeated until adjustment amounts are set for all ink amounts Ij corresponding to the grid points defined in the LUT 510 without color correction (step S11: YES). When adjustment amounts are set for all ink amounts Ij, in step S12, the adjustment amount setting unit 19 records the adjustment amounts set for the respective ink amounts Ij in the HDD 11. Therefore, when the operator adjusts the generation amount of ink (i) in the search for the optimal ink amount Ij, the ink amount definition is changed according to the adjustment amount recorded in step S12. The adjustment amount setting method has been described above.

2. Other embodiments:
There are various embodiments of the present invention.
For example, using the L * a * b * color space as the uniform color space is an example, and the L * u * v * color space may be used.

As described above, in the profile (LUT 510 without color correction) used by the printing apparatus, each ink amount Ij is selected such that the objective function including the graininess index indicating the graininess of the ink amount Ij is low. The granularity index included in the objective function is set by summing up index values indicating the difficulty of using each of the inks in the target lightness of the ink amount Ij for each ink constituting the ink amount Ij.
Further, since the index value is a value dependent on the lightness (target lightness), the granularity index calculated based on this index value is a value dependent on the lightness. As a result, it is possible to suppress falling into a local minimum in the search for the ink amount Ij.

The index value indicating the difficulty of use is set so that the darker the ink, the harder it is to use in the high brightness range of the target brightness. More specifically, the sigmoid function shown in the above equation (3) is set. Calculated based on For this reason, it is possible to obtain an index value indicating difficulty in use in the present invention by a function calculated from the target brightness.
Furthermore, the graininess index is calculated by multiplying an index value indicating difficulty in use by a weight set according to the brightness of each ink, thereby enabling more effective control of the ink generation amount. Become.

  Needless to say, the present invention is not limited to the above embodiments. That is, the mutually replaceable members and configurations disclosed in the above embodiments are applied by changing their combinations as appropriate. The members and configurations disclosed in the examples can be replaced with the members and configurations interchangeable with each other as appropriate, and the combination thereof is changed and applied. It is one of the present invention that a person skilled in the art can appropriately substitute the members and configurations that can be assumed as substitutes for the members and configurations disclosed in the above-described embodiments based on the above, and change the combination to apply. It is disclosed as an example.

  DESCRIPTION OF SYMBOLS 10 ... Computer, 11 ... Memory | storage part, 12 ... CPU, 13 ... RAM, 14 ... Initial value setting part, 15 ... Smoothing process part, 16 ... Table preparation part, 17 ... Gamut prediction part, 18 ... Forward model converter, 19 ... Adjustment amount setting unit, 20 ... printer, 21 ... USB I / F, 30 ... display, 31 ... video I / F, 40 ... operation unit, 41 ... input I / F

Claims (5)

A printing apparatus that performs color management using a profile that associates the coordinate value of the color space of input data with the ink amount,
In the profile, the ink amount is selected so that the value of the objective function including the granularity index indicating the granularity of the ink amount is low,
The granularity index forms an index value indicating the difficulty of using each ink according to a predetermined brightness value of the ink amount for each ink constituting the ink amount. The total is set for each ink,
A printing apparatus comprising: an ink amount selecting unit that selects an ink amount corresponding to input data based on the profile.   2. The printing according to claim 1, wherein the index value indicating the difficulty of use is set so that the darker the ink is, the harder it is to use in a high lightness range in the predetermined lightness value. apparatus. The index values indicating the difficulty of use are: x is a predetermined lightness value of the ink amount, i is an ink identifier, k is a lightness set for the ink, e is a natural logarithm, and β is a constant. The sigmoid function shown below

The printing apparatus according to claim 1, wherein the printing apparatus is calculated as a solution of:   4. The graininess index is calculated by multiplying an index value indicating the difficulty of use by a weight set according to the lightness of each ink. Printing device.   The printing apparatus according to any one of claims 1 to 4, wherein the selection of the ink amount is executed in a smoothing process in the profile.

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