JP2012231307A - Manufacturing method of printer, printer and printing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique capable of forming a color conversion LUT excellent in gradation.SOLUTION: There is provided a manufacturing method of a printer provided with a color conversion LUT for converting a color in a first color space to an ink amount set that can be used in the printer. In a case where there are at least two lattice points of which a correspondence between a lattice point to be processed and the ink amount set has been already determined such that the lattice point to be processed is interposed therebetween, a plane that passes a coordinate in a second color space corresponding to the lattice point to be processed while having a normal vector parallel to a straight line connecting two coordinates in the second color space corresponding to the at least two lattice points in the second color space is obtained. The Euclidean distance between the plane and a coordinate in the second color space corresponding to a candidate for the ink amount set is calculated as an evaluation value. An ink amount set associated with the lattice point to be processed is determined based on the evaluation value.

Description

  The present invention relates to a technique for performing color conversion processing for a printing apparatus.

  As an image output apparatus, a color ink jet printing apparatus is widely used. In a color ink jet printing apparatus, color conversion processing is performed to convert color values of image data expressed by a first color system into color values of a second color system corresponding to the type of ink. In this color conversion process, a lookup table in which the color values of the first color system and the color values of the second color system are associated (hereinafter also simply referred to as “color conversion LUT”) is referred to. .

  As a technique for creating a lookup table, for example, Patent Document 1 discloses that a color conversion LUT is created using various evaluation indexes that change according to the amount of ink of each color. As such evaluation indexes, for example, various evaluation indexes representing numerical values such as the granularity of the ink and the smoothness of the change in the ink amount have been used. However, a color conversion LUT that can express more excellent gradation is used. A technology that can be created was desired.

JP-T-2007-511161

  In view of the above problems, the problem to be solved by the present invention is to provide a technique capable of creating a color conversion lookup table with excellent gradation.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

Application Example 1 A storage unit in which a color conversion lookup table for converting a color in the first color space into an ink amount set that is a combination of a plurality of ink amounts usable in the printing apparatus is recorded A method for manufacturing a printing apparatus comprising: a selection step of selecting, as a processing target grid point, a grid point that associates the ink amount set from among grid points constituting the first color space; and the processing target grid point An evaluation value for determining an ink amount set to be associated with the processing target grid point is calculated based on a candidate determination step for determining an ink amount set candidate to be associated with the ink amount and the ink amount represented by the ink amount set candidate. Based on the calculation step and the calculated evaluation value, an ink amount set to be associated with the processing target grid point is determined from the ink amount set candidates, and the determined ink An ink amount set determination step of associating a set with the processing target grid point and recording the set in the color conversion lookup table; and the color conversion lookup in which the processing target grid point and the ink amount set are associated with each other A storage step of storing the table in the storage unit included in the printing apparatus, and in the calculation step, a lattice point for which a correspondence relationship between the processing target lattice point and the ink amount set has already been determined is the processing step. A normal vector parallel to a straight line connecting two coordinates in the second color space corresponding to the two grid points in the second color space when at least two target grid points exist. And a plane passing through the coordinates in the second color space corresponding to the processing target grid point, the plane and the second color corresponding to the ink amount set candidate The Euclidean distance between the coordinates in between, is calculated as the evaluation value, the manufacturing method of the printing apparatus.

  According to this method, when the ink amount set to be associated with the processing target grid point of the color conversion lookup table is determined, the grid point for which the correspondence between the processing target grid point and the ink amount set has already been determined is the processing target grid. When at least two points exist so as to sandwich a point, the second color space has a normal vector parallel to a straight line connecting two coordinates in the second color space corresponding to the two lattice points. A plane passing through the coordinates in the second color space corresponding to the processing target grid point is obtained, and the Euclidean distance between this plane and the coordinates in the second color space corresponding to the ink amount set candidate is evaluated. Calculate as If the ink amount set is determined based on such an evaluation value, the connection of the gradation change between the two grid points sandwiching the processing target grid point can be smoothed. A printing device having an uptable can be manufactured.

[Application Example 2] The printing apparatus manufacturing method according to Application Example 1, wherein in the calculation step, two grid points for which a correspondence relationship between the processing target grid point and the ink amount set has already been determined are processed. When there are at least two sets so as to sandwich the target grid point, in the second color space, for the two sets of grid points, each of the two planes is obtained and the intersection of the two planes is obtained, A method of manufacturing a printing apparatus, wherein a Euclidean distance between an intersection line and a coordinate in the second color space corresponding to the ink amount set candidate is calculated as the evaluation value.

  According to this method, when there are at least two sets of two grid points for which the correspondence relationship between the processing target grid point and the ink amount set has already been determined so as to sandwich the processing target grid point, from the two sets of grid points The Euclidean distance between the obtained intersection line of the two planes and the coordinates corresponding to the ink amount set candidate is calculated as an evaluation value. By determining the ink amount set based on such an evaluation value, it is possible to smooth the connection of each gradation change between two sets of grid points arranged around the processing target grid point. A printing apparatus having a look-up table with excellent gradation can be manufactured.

[Application Example 3] A method of manufacturing a printing apparatus according to Application Example 1 or Application Example 2, wherein two grids in which a correspondence relationship between the processing target grid point and the ink amount set has already been determined in the calculation step. When at least three sets of points exist so as to sandwich the processing target grid point, in the second color space, for the three sets of grid points, three planes are obtained and intersections of the three planes are obtained. A method for manufacturing a printing apparatus, wherein a Euclidean distance between the intersection and a coordinate in the second color space corresponding to the ink amount set candidate is calculated as the evaluation value.

  According to this method, when there are at least three sets of two grid points for which the correspondence relationship between the processing target grid points and the ink amount set has already been determined so as to sandwich the processing target grid points, The Euclidean distance between the intersection of the three planes obtained and the coordinates corresponding to the ink amount set candidate is calculated as an evaluation value. By determining the ink amount set based on such an evaluation value, it is possible to smooth the connection of the respective gradation changes between the three sets of grid points arranged around the processing target grid point. A printing apparatus having a look-up table with excellent gradation can be manufactured.

  The present invention can be configured as a printing method, a printing apparatus, a lookup table creation method, and a computer program in addition to the above-described configuration as a printing apparatus manufacturing method. Such a computer program may be recorded on a computer-readable recording medium. As the recording medium, various media such as a flexible disk, a CD-ROM, a DVD-ROM, a magneto-optical disk, a memory card, and a hard disk can be used.

1 is a schematic configuration diagram of a printing apparatus as an embodiment of the present invention. It is a flowchart of a printing process. It is a schematic block diagram of a color conversion LUT creation apparatus. It is the schematic of a forward model converter. It is a table | surface which shows the data structure of a color conversion LUT. It is explanatory drawing which shows the data structure of a color conversion LUT. It is a flowchart which shows a color conversion LUT creation routine. 3 is a flowchart illustrating a method for manufacturing a printing apparatus. It is explanatory drawing which shows the concept of the selection order of a process target grid point. It is a figure which shows the example of the other selection method of a process target grid point. It is a figure showing the calculation method of the 1st gradation property index in case two referenceable lattice points exist. It is a figure showing the calculation method of the 1st gradation property index | exponent in case four referenceable grid points exist. It is a figure showing the calculation method of the 1st gradation property index in case six referenceable grid points exist. It is explanatory drawing which shows the calculation method of 2nd gradation index | exponent SI. It is the figure which showed another method of calculating | requiring a 1st gradation index.

Hereinafter, in order to further clarify the operations and effects of the present invention described above, embodiments of the present invention will be described based on examples in the following order.
A. Printing device:
B. Color conversion LUT creation device:
C. How to select grid points for processing:
D. Evaluation value calculation method:
E. Variations:

A. Printing device:
FIG. 1 is a schematic configuration diagram of a printing apparatus as an embodiment of the present invention. The printing apparatus 10 is a color ink jet printing apparatus, and is mounted on the carriage 80, a mechanism for transporting the print medium P by the paper feed motor 74, a mechanism for reciprocating the carriage 80 in the axial direction of the platen 75 by the carriage motor 70, and the carriage 80. A mechanism for driving the print head 81 to eject ink and forming dots, a control unit 30 for controlling the paper feed motor 74, carriage motor 70, and print head 81, and a computer or recording medium (not shown). An image data supply unit 91 that acquires image data and supplies the image data to the control unit 30.

  On the carriage 80, for example, ink cartridges 82 to 85 for color ink that respectively contain cyan ink C, magenta ink M, yellow ink Y, and black ink K are mounted as color inks. In the print head 81 below the carriage 80, nozzle rows corresponding to the above-described color inks are formed. When these ink cartridges 82 to 85 are mounted on the carriage 80 from above, ink can be supplied from the ink cartridges 82 to 85 to the print head 81.

  The control unit 30 includes a CPU 40, a ROM 51, a RAM 52, and an EEPROM 60. The CPU 40 develops and executes a control program stored in advance in the ROM 51 in the RAM 52, thereby controlling the reciprocation and paper feed of the carriage 80 and driving the print head 81 to eject ink onto the print medium P. To control.

  A color conversion LUT 61 is stored in the EEPROM 60. The color conversion LUT 61 converts the color values of the RGB format image data ORG supplied from the image data supply unit 91 into the ink amounts of the inks (C, M, Y, K) stored in the ink cartridges 82 to 85. It is a table for converting into the ink amount set to represent. The color conversion LUT 61 is created in advance by a color conversion LUT creation device (see FIG. 3) described later.

  FIG. 2 is a flowchart of a printing process executed by the printing apparatus. When the user instructs print processing, the CPU 40 acquires RGB format image data ORG from the image data supply unit 91 (step S100). When the image data ORG is acquired, the CPU 40 refers to the color conversion LUT 61 stored in the EEPROM 60 and converts the color value of the image data ORG into an ink amount set (step S200). When the color conversion process from the RGB value to the ink amount set is completed, the CPU 40 performs a halftone process on the image data converted into the ink amount set (step S300). As a specific method of the halftone process, for example, a systematic dither method, an error diffusion method, a density pattern method, or the like can be employed. Next, the CPU 40 performs rasterization processing for rearranging the image data (step S400). When the rasterization process is performed, the CPU 40 drives the carriage motor 70, the paper feed motor 74, and the print head 81 based on the rasterized data, and discharges the color ink from the print head 81 to execute printing (step). S500).

B. Color conversion LUT creation device:
B1. Device configuration:
FIG. 3 is a schematic configuration diagram of the color conversion LUT creation apparatus 20 for creating the color conversion LUT 61 shown in FIG. The color conversion LUT creation apparatus 20 is configured as a computer including a CPU 41, a ROM 53, a RAM 54, an HDD 55, a monitor (not shown) and various input / output interfaces. The CPU 41 develops the program stored in the ROM 53 or the HDD 55 in the RAM 54 and executes it, thereby executing the processing target grid point selection unit 100, the ink amount set candidate calculation unit 200, the forward model converter 500, the evaluation value calculation unit 300, the optimum value. Each function of the ink amount set determination unit 400 and the like is realized. The HDD 55 stores a color conversion LUT 61 created by the color conversion LUT creation device 20.

FIG. 4 is a schematic diagram of a forward model converter 500. The forward model converter 500 has a function of converting the ink amount set into L ^* value, a ^* value, and b ^* value (hereinafter collectively referred to as “Lab value”) in the CIELAB color space. A converter 510 and a color converter 520 are provided.

  The spectral printing model converter 510 converts an ink amount set represented by each value of CMYK into a spectral reflectance R (λ). The spectral reflectance R (λ) represents the colorimetric value of the color patch formed by the ink amount set. The color converter 520 calculates a Lab value from the spectral reflectance R (λ) obtained by the spectral printing model converter 510. At this time, the color converter 520 calculates the Lab value based on a preset type of light source (for example, D65 which is a standard light source) and a type of printing paper (for example, glossy paper). That is, if the forward model converter 500 is used, the Lab value corresponding to the ink amount set can be uniquely acquired by inputting the ink amount set. The configurations and functions of the spectral printing model converter 510 and the color converter 520 are disclosed in, for example, JP-T-2007-511175 and JP-A-2008-263579.

  The ink amount set candidate calculation unit 200 (see FIG. 3) has a function of obtaining an optimum ink amount set candidate (hereinafter simply referred to as “ink amount set candidate”) to be stored for a processing target grid point in the color conversion LUT 61. Have. The processing target grid point and the optimum ink amount set will be described later.

  The evaluation value calculation unit 300 has a function of calculating an evaluation value for determining whether the ink amount set is an optimal ink amount set based on the ink amount set candidates.

  The optimum ink amount set determination unit 400 has a function of determining an optimum ink amount set (hereinafter referred to as “optimum ink amount set”) to be associated with the processing target grid point based on the evaluation value calculated by the evaluation value calculation unit 300. Have

  5 and 6 are a table and an explanatory diagram showing the data structure of the color conversion LUT 61 stored in the HDD. As shown in FIG. 5, the RGB value, the Lab value, and the optimum ink amount set are uniquely associated with the color conversion LUT 61. The Lab color space represented by the Lab value has a color reproduction range wider than that of the RGB color space represented by the RGB value, and has a property that it does not depend on the device. The R value, G value, and B value constituting the RGB value are each divided into 17 stages in this embodiment. That is, as shown in FIG. 6, the color conversion LUT 61 can be represented as a hexahedron having 17 × 17 × 17 lattice points in a three-dimensional color space having an R axis, a G axis, and a B axis. . Therefore, RGB values, Lab values, and ink amount sets are associated with each lattice point constituting the hexahedron. The RGB value associated with each grid point is (R, G, B) = (0, 0, 0), the Lab value is (L, a, b) = (0, 0, 0), and the ink amount is set. Can be represented by multi-dimensional coordinate values such as (C, M, Y, K) = (0, 0, 0, 0). The number of divisions is not limited to 17, and for example, the range that can be taken by the R value, the G value, and the B value can be divided into arbitrary numbers such as 11 and 32. Also, the interval between adjacent lattice points can be made non-uniform, such as narrowing in a certain range and widening in a certain range. Further, the interval between adjacent lattice points can be equal, or the range that can be taken by the R value, G value, and B value cannot be divided equally to the specified number of divisions (17 in this embodiment). It is also possible to make the intervals substantially equal. The RGB color space in this embodiment corresponds to the first color space of the present application, and the Lab color space corresponds to the second color space.

B2. Color Conversion LUT Creation Flow FIG. 7 is a flowchart showing a color conversion LUT creation routine executed by the color conversion LUT creation apparatus 20. First, the CPU 41 initializes the color conversion LUT 61 (step S10). Specifically, the Lab value and the initial value of the ink amount set (hereinafter referred to as the initial ink amount set) are associated with the RGB value of each grid point of the color conversion LUT 61. The initial ink amount set can be calculated based on, for example, the following expressions (1) and (2). The Lab value can be obtained by inputting the value of the initial ink amount set to the forward model converter 500.

Here, the subscript _j of the ink amount I _{j (R, G, B)} in the above equation (1) corresponds to the number of types of ink constituting the ink amount set. In this embodiment, since four types of inks (C, M, Y, K) are used, j = 1 to 4. As the ink amount I _{j (R, G, B) with} respect to the grid point whose RGB value is 0 or 255, a value input in advance by the user is used. According to the equations (1) and (2), the ink amount I _{j (R, G, B)} at an arbitrary RGB value can be obtained, and this ink amount I _{j (R, G, B)} is combined. The information is determined as an ink amount set at the arbitrary RGB value. Note that the value of the initial ink amount set may be set to a value determined by another method without depending on the calculation described above. The initial ink amount set can be corrected from a Lab value to a more preferable value by using, for example, an “inverse model initial LUT” described in JP-A-2009-33239.

  When the color conversion LUT 61 is initialized as described above, the processing target grid point selection unit 100 determines a grid point (hereinafter, referred to as an optimal ink amount set) from the color conversion LUT 61 according to a predetermined order. (Described as “processing target grid point”) is selected (step S20). The order of selecting the processing target grid points will be described later. When the processing target grid point is selected, the ink amount set candidate calculation unit 200 calculates and determines an optimum ink amount set candidate (ink amount set candidate) to be associated with the processing target grid point (step S30). The ink amount set candidate determined for the first time is the initial ink amount set recorded from the beginning in the color conversion LUT 61. When the ink amount set candidate is determined, the evaluation value calculation unit 300 calculates an evaluation value based on the determined ink amount set candidate (step S40). The evaluation value calculation method differs depending on which lattice point in the hexahedron shown in FIG. 6 corresponds to the processing target lattice point selected in step S20. Details of the evaluation value calculation method will be described later.

  When the evaluation value is calculated, the CPU 41 determines whether or not the evaluation value has become minimum (step S50). If the evaluation value is not the minimum, the CPU 41 returns the process to step S30. When the evaluation value is the minimum, the optimal ink amount set determination unit 400 determines the ink amount set candidate from which the evaluation value is calculated as the optimal ink amount set, and the current processing target of the color conversion LUT 61 Corresponding to grid points (step S60).

  In step S30, the ink amount set candidate calculation unit 200 obtains the ink amount set candidates as follows from the second time. First, a second ink amount set candidate is obtained by slightly changing the value of each ink amount constituting the ink amount set from the initial ink amount set. The direction of this change is the direction in which the evaluation value is optimized (that is, the direction in which the evaluation value decreases). Thereafter, the values of the respective ink amounts constituting the ink amount set candidates obtained in the previous time are successively made smaller. The next ink amount set candidate is obtained by changing. Step S40 and step S50 are executed for each ink amount set candidate. If the processing from step S30 to step S50 is repeated, the ink amount set candidate whose evaluation value is optimized (minimum) is finally obtained when an affirmative determination is made in step S50. In step S60, the obtained ink amount set candidate is determined as the optimum ink amount set.

  Specifically, the iterative processing from step S30 to step S50 is realized using a known steepest descent method, quasi-Newton method, simplex method, or the like. In step S50, the CPU 41 calculates the evaluation value based on the ink amount set candidate obtained n times (n is a natural number equal to or greater than 1) and the evaluation value based on the ink amount set candidate obtained (n + 1) times. When the difference value is equal to or less than the prescribed difference value, the ink amount set candidate obtained for the nth time or the (n + 1) th time may be determined as the optimum ink amount set. In addition, the number of times of processing of S50 is set in advance from step S30 to step (for example, 200 times), and the ink amount set candidate having the smallest evaluation value or difference value among them is determined as the optimum ink amount setting. May be determined as

  When the optimum ink amount set is associated with the processing target grid point, the CPU 41 determines whether there is another processing target grid point (step S70). If there are other processing target grid points, the process returns to step S20 and the above-described series of processing is repeatedly executed. If there is no processing target grid point, that is, if the optimum ink amount set is determined for all grid points, the color conversion LUT creation routine is terminated.

  FIG. 8 is a flowchart illustrating a method for manufacturing the printing apparatus. The color conversion LUT 61 created as described above is appropriately read out from the HDD 55 and written into the EEPROM 60 by an EEPROM writer or the like when the printing apparatus 10 is manufactured (step S80). Then, the EEPROM 60 is mounted on the control unit 30 in the printing apparatus 10 (step S90), whereby the printing apparatus 10 is manufactured. Note that other methods may be employed for writing the color conversion LUT 61 into the EEPROM 60. For example, the CPU 40 of the printing apparatus 10 can acquire the color conversion LUT 61 through the image data supply unit 91 and write it in the EEPROM 60. Further, when the color conversion LUT 61 is written in the EEPROM 60, the Lab value may be deleted from the color conversion LUT 61 in advance. By doing so, the color conversion LUT 61 represents only the correspondence between RGB values and ink amount sets.

C. How to select grid points for processing:
FIG. 9 is an explanatory diagram showing the concept of the selection order of processing target grid points in step S20 of FIG. As shown in FIG. 6, the color conversion LUT 61 can be regarded as a hexahedron including 17 × 17 × 17 lattice points in a three-dimensional space. In FIG. An example is shown in which one surface is constituted by 9 × 9 lattice points. The numbers shown in FIG. 9 indicate the selection order of the lattice points closest to the numbers, that is, the lattice points on the sides of the hexahedron. The smaller the number, the faster the lattice points are selected. .

  In the example illustrated in FIG. 9, the processing target lattice point selection unit 100 uses the lattice point (the lattice point assigned the selection order “1” in FIG. 9) located at the vertex of the hexahedron as the first processing target lattice point. Next, a grid point (in FIG. 9, the grid point assigned with the grid point processing order 2) located at the approximate center of the two first processing target grid points is selected as the second processing target grid point. . The reason why the position of the selected grid point is “substantially” in the center is that the interval between the grid points is not always equal, and an even number of grid points between the two grid points. Because there may be. In addition, the “lattice point positioned substantially at the center” means, in other words, the lattice point closest to the center of the two first processing target lattice points or the lattice point closest to the center of the two first processing target lattice points. It can be said that one of the points is a lattice point. It can also be said that the lattice point located at the approximate center is sandwiched between the two first processing target lattice points. After the first processing target grid point and the second processing target grid point are selected, a grid point (in FIG. 9, grid point processing) located at the approximate center between the first processing target grid point and the second processing target grid point. The grid point with the order 3) is selected as the third processing target grid point.

  As described above, in this embodiment, the lattice point located at the vertex of the hexahedron is first selected to determine the optimum ink amount set, and then the approximate ink amount set is positioned at the approximate center of the two lattice points. The optimal ink amount set for each grid point on the side of the hexahedron is determined by successively selecting grid points to be processed. Note that after determining the optimum ink amount set for the lattice points on the sides of the hexahedron, the optimum ink amount set is determined for each lattice point within the surface surrounded by the sides. At this time, as in the selection order of the grid points on the side, the grid point located at the approximate center of the two grid points on the opposite sides for which the optimal ink amount set has been determined is selected and the optimal ink amount set is selected. After that, by selecting one grid point at the approximate center of the two grid points for which the optimal ink amount set has been determined, the ink amount set for each grid point on the hexahedron plane is determined. To go. In addition, a lattice point located at approximately the center of four vertices or four lattice points is first selected, and then the lattice point and lattice points located at approximately the center of four vertices or four lattice points are respectively selected. It is also possible to sequentially select the processing target grid points. Thus, after all the optimal ink amount sets for the lattice points in the plane are determined, the optimal ink amount is determined for the lattice points inside the hexahedron surrounded by the six surfaces. At this time, as in the selection order of the lattice points on the sides of the hexahedron, it is possible to select a lattice point substantially in the middle of the two lattice points, and it is possible to select eight lattice points or six lattice points on the six surfaces. First, a lattice point located at approximately the center of the grid point is selected, and then the lattice point and each of the lattice points located at approximately the center of the six vertex points on the six vertices or the six faces are set as processing target lattice points. You can also select them in order. As described above, in this embodiment, the optimum ink amount set is determined in the order of the sides, the plane created by the sides, and the hexahedron created by the sides.

  FIG. 10 is a diagram illustrating an example of another selection method of processing target grid points. In the example shown in FIG. 10, an ordering index for selecting a processing target grid point is given in advance to each grid point, and the grid point is selected based on the index. In this method, first, it becomes an ordering index for selecting a processing target grid point with respect to the D (1) axis (for example, L axis) and the D (2) axis (for example, a axis) shown in FIG. Set the grid point label. When the lattice points shown in FIG. 10 are represented by lattice point labels, the lattice point labels of the third lattice point from the left on the D (1) axis and the fourth lattice point from the top on the D (2) axis are: (3, 4), the grid point label of the fifth grid point from the left on the D (1) axis and the fifth grid point from the top on the D (2) axis is (2, 2).

  Next, a grid point having a grid point label value of only 1 is selected from all grid points. When the set of lattice points shown in FIG. 10 is regarded as a quadrangle, the lattice point corresponding to the vertex of the quadrangle is the lattice point label (1, 1), and is selected as the first processing target lattice point. In the next stage, a grid point label having 1 or 2 as the numerical value of the grid point label is selected, and a grid point having a grid point label including 1 is preferentially selected. That is, lattice points whose lattice point labels are (2, 1) or (1, 2), (2, 2) are selected, and among them, lattice points whose lattice point labels are (1, 2), (2, 1). Points are selected with priority. The numerical values given to the lattice points shown in FIG. 10 are the selection order of the lattice points when the lattice points are selected by the method. In this way, if processing target grid points are selected according to the grid point labels given to the grid points, parallel processing can be executed, and the time required for creating the color conversion LUT 61 can be shortened. it can. Note that this method is not limited to two dimensions. When there are N input axes, an ordering index is assigned to each axis, and a grid point label is set for each grid point to be processed to obtain an optimal ink amount. A set can be determined. At this time, the optimum ink amount set can be determined after determining the order of processing the grid points, but can also be performed simultaneously with the determination of the grid point processing order.

D. Evaluation value calculation method:
Next, the evaluation value calculation method by the evaluation value calculation unit 300 in step S40 of FIG. 7 will be described. According to the forward model converter 500, a unique Lab value can be obtained by inputting an ink amount set. However, it is generally difficult to uniquely determine the ink amount set from one Lab value. This is because, for example, even if the value of the ink amount set made up of CMYK is (C, M, Y, K) = (0, 0, 0, 60), (C, M, Y, K) = (20, 20 , 20, 0) may show the same Lab value. Therefore, in this embodiment, an evaluation value Ei corresponding to the ink amount set candidate is obtained by an evaluation function represented by the following expression (3), and the ink amount set candidate having a smaller evaluation value Ei is determined as a processing target grid point. Is determined as the optimum ink amount set corresponding to

  The evaluation function shown in Formula (3) is represented by the sum of four different evaluation indexes. The first gradation index LabDist and the second gradation index SI are the smoothness of the gradation change of the Lab value, the graininess index GI is the graininess of the ink in the printed matter, and the non-color constancy index CII is under different light sources. It is an evaluation index for evaluating the constancy of color when a printed material is observed. As shown in Expression (3), each evaluation index is multiplied by a predetermined weight coefficient w. Each value of the weighting factor w can be set as appropriate according to the characteristics of the printing apparatus 10, user preferences, and the like. Further, by setting the weight coefficient w to zero, it is possible to select an evaluation index that is actually used from among the four evaluation indices. In this embodiment, the evaluation value is calculated based on at least the first gradation index LabDist among these four evaluation indices. Of course, in addition to these four evaluation indexes, it is also possible to use other evaluation indexes (for example, an index for evaluating the smoothness of the change in the ink amount).

FIG. 11 is a diagram showing a method of calculating the first tone index LabDist for the lattice points on the sides constituting the hexahedron shown in FIG. When there is a grid point (hereinafter also referred to as “referenceable grid point”) A and a referenceable grid point B for which the optimum ink amount set has already been determined, the selection of the processing target grid point shown in FIG. According to the order, a substantially central grid point that is a position between them is a processing target grid point. Therefore, first, the evaluation value calculation unit 300 calculates the coordinates A in the Lab space corresponding to the referenceable grid point A and the coordinates B in the Lab space corresponding to the referenceable grid point B as shown in FIG. Find the connecting line N _AB . Next, a plane Q _AB having a normal vector parallel to the straight line N _AB and passing through the coordinates C in the Lab space corresponding to the processing target grid point C is obtained. Then, a Lab value by the forward model converter 500 from the current ink amount set candidates, determine the length of a perpendicular T drawn from the Lab value X to the plane Q _AB. This length, that is, the Euclidean distance between the Lab value X and the plane Q _AB is the value of the first gradation index LabDist. As shown in Expression (3), the smaller the first gradation index LabDist is, the smaller the evaluation value Ei is. Therefore, an ink amount set candidate having a smaller gradation index LabDist is more likely to be determined as the optimum ink amount set.

FIG. 12 is a diagram illustrating a method of calculating the first gradation index LabDist when there are four (two sets) of referenceable grid points. This calculation method is a calculation method of the first gradation index LabDist when the lattice points in the plane constituting the hexahedron shown in FIG. 6 are the processing target lattice points. Processing target grid point F (corresponding to coordinate F in Lab space) located substantially at the center of four referenceable grid points A to D (corresponding to coordinates A to D in Lab space) constituting one surface 12, when the first tone index LabDist is obtained, the evaluation value calculation unit 300 calculates a normal vector parallel to a straight line N _AB connecting a pair of coordinates A and B on opposite sides as shown in FIG. And a plane Q _AB passing through the coordinate F and a plane Q _CD passing through the coordinate F having a normal vector parallel to a straight line N _CD connecting a pair of coordinates C and D on the opposite sides. Ask. Then, these two determined plane Q _AB, the line of intersection N _ABCD of Q _CD, and this intersection line N _ABCD, the Euclidean distance between the Lab value X determined by the forward model converter 500 from the current ink amount set candidate, the It is calculated as the value of the 1 gradation index LabDist. Thus, even when there are four coordinates in the Lab space corresponding to the referenceable lattice point, the coordinates A are the same as when there are two coordinates in the Lab space corresponding to the referenceable lattice point. Ink quantity set candidates that can maintain good gradation from the coordinates C to the coordinates B and the gradation from the coordinates C to the coordinates D are determined as the optimum ink quantity set. As shown in FIG. 12, the coordinates in the Lab space corresponding to the two sets of referenceable lattice points are present on different straight lines and are not present on the same straight line.

FIG. 13 is a diagram illustrating a method of calculating the first gradation index LabDist when there are six (three sets) reference lattice points. This calculation method is a calculation method of the first gradation index LabDist when the lattice point located inside the hexahedron shown in FIG. 6 is the processing target lattice point. As shown in FIG. 13, when there are coordinates A to F in Lab space corresponding to six referenceable lattice points A to F, one on each side of the hexahedron, Similarly, the evaluation value calculation unit 300 has a normal vector parallel to a straight line N _AB connecting a set of coordinates A and B, and passes the coordinates G in the Lab space corresponding to the processing target grid point. A plane Q _AB and a plane Q _CD having a normal vector parallel to the straight line N _CD connecting the set of coordinates C and D and passing through the coordinate G are obtained. Further, another set of coordinates E and F is obtained. A plane Q _EF having a normal vector parallel to the connecting straight line N _EF and passing through the coordinates G is obtained. Then, the intersection of these three planes Q _AB , Q _CD , and Q _EF is obtained, and the Euclidean distance between this intersection and the Lab value X calculated by the forward model converter 500 from the current ink amount set candidate is the first gradation. Calculated as the value of the sex index LabDist. In this way, even when there are six coordinates in the Lab space corresponding to the referenceable lattice points, the same as in the case where there are two and four coordinates in the Lab space corresponding to the referenceable lattice points. Ink amount set candidates that can maintain good gradation from coordinate A to coordinate B, gradation from coordinate C to coordinate D, and gradation from coordinate E to coordinate F are determined as the optimum ink amount set. Is done. Here, in the example shown in FIG. 13, the coordinate G and the intersection formed by the three planes exist at the same position. For this reason, the Euclidean distance between the coordinates G and the Lab value X may be calculated as the value of the first gradation index LabDist without obtaining three planes. As shown in FIG. 13, among the coordinates in the Lab space corresponding to the three sets of referable grid points, the two sets of coordinates exist on different straight lines, and are on the same straight line. Is a nonexistent coordinate. Among the coordinates in the Lab space corresponding to the three sets of referenceable lattice points, the remaining one set of coordinates exists on a plane different from the plane where the two sets of coordinates exist, and is the same. The coordinates do not exist on the plane.

  In the color conversion LUT creation routine described above, if the vertex of the hexahedron is a processing target grid point, there is no referenceable grid point, and therefore the color conversion LUT creation routine shown in FIG. The calculation of the one-gradation index LabDist may be omitted, and the value of the initial ink amount set may be directly associated with the optimum ink amount set in step S60.

  Hereinafter, other evaluation indexes included in Expression (3) will be described. FIG. 14 is an explanatory diagram illustrating a method of calculating the second tone index SI included in the equation (3). For the sake of simplicity, FIG. 14 shows an example for determining the optimum ink amount set at coordinates in the Lab space corresponding to the processing target grid points on the two dimensions represented by the input axes L and a. ing. In calculating the second gradation index SI, the evaluation value calculation unit 300 firstly calculates the coordinates A in the Lab space corresponding to the referenceable grid point A and the Lab space corresponding to the referenceable grid point B. An ink amount set candidate is determined for the coordinate C in the Lab space corresponding to the processing target grid point C located approximately at the center of the coordinate B. Then, the Lab value X is calculated from the ink amount set candidate using the forward model converter 500. Then, based on the following equation (4), the secondary differential value is calculated based on the Lab value of the coordinate A, the Lab value of the coordinate B, and the Lab value corresponding to the ink amount set candidate. This secondary differential value becomes the value of the second gradation index SI.

  As shown in FIG. 14, the closer the value of the second gradation index SI is to 0, the more the inclination from the coordinate A to the Lab value X represented by the ink amount set candidate, and the Lab value X to the coordinate B The gradient in the Lab space from the coordinate A to the coordinate B becomes smooth. That is, the closer the value of the second gradation index SI is to 0, the more the color conversion LUT 61 with excellent gradation can be created.

  Among the evaluation indices shown in Expression (3), the graininess index GI can be calculated using various graininess prediction models, and can be calculated based on, for example, the following Expression (5).

Here, a _L is a lightness correction coefficient, WS (u) is a winner spectrum of an image indicated by halftone data used for printing a color patch, VTF (u) is a visual spatial frequency characteristic, and u is a spatial frequency. . Halftone data is determined by halftone processing from the ink amount Ij of the color patch. Although the above equation (5) is expressed in one dimension, the spatial frequency of the two-dimensional image may be calculated as a function of the spatial frequencies u and v. Specific methods for calculating the graininess index GI include, for example, the method described in JP-T-2007-511161, Makoto Fujino, Image Quality Evaluation of Inkjet Prints, Japan Hardcopy '99, p.291-294. The described method can be used.

  Of the evaluation indices shown in Expression (3), the non-color constancy index CII can be calculated based on the following Expression (6), for example.

Here, ΔL is the brightness difference of the color patch under two different observation conditions (under different light sources), ΔC _ab is the chroma difference, and ΔH _ab is the hue difference. During the calculation of the non-color constancy index CII, Lab values under two different viewing conditions are converted to standard viewing conditions (eg, under standard light D65 viewing) using a chromatic adaptation transformation (CAT). For CII, Billmeyer and Saltzman's Principles of Color Technology, 3rd edition, John Wiley & Sons, Inc, 2000, p.129, pp. 213-215 can be referred to.

According to the color conversion LUT creation apparatus 20 of the present embodiment described above, the first tone characteristic index LabDist is used as the evaluation index in order to determine the optimum ink amount set associated with each grid point of the color conversion LUT 61. As shown in FIG. 11, the first gradation index LabDist is calculated from the Lab value X represented by the ink amount set candidate, the coordinate A in the Lab space corresponding to one set of referenceable grid points A, and the referenceable grid. Euclidean up to a plane Q _AB having a normal vector parallel to a straight line N _AB connecting the coordinates B in the Lab space corresponding to the point B and passing through the coordinates G in the Lab space corresponding to the processing target grid point G Represents distance. Therefore, if an ink amount set candidate having a small first gradation index LabDist is determined as the optimum ink amount set, the gradation change from the coordinate A to the Lab value X represented by the optimum ink amount set, and the Lab value X Therefore, it is possible to create a color conversion LUT 61 with excellent gradation characteristics. In the present embodiment, not only the processing target grid points existing between a set of referenceable grid points on the sides of the hexahedron but also the grid points on the hexahedron surface and inside the hexahedron are subjected to the optimum ink by the same method. Determine the quantity set. Therefore, it is possible to create a color conversion LUT 61 with high gradation over the entire color space, and to improve the color reproducibility of the printing apparatus 10.

E. Variations:
As mentioned above, although one Example of this invention was described, this invention is not limited to such an Example, A various structure can be taken in the range which does not deviate from the meaning. For example, a function realized by software may be realized by hardware. In addition, the following modifications are possible.

-Modification E1:
In the above embodiment, when there are four referenceable lattice points (see FIG. 12), the coordinate A in the Lab space corresponding to the referenceable lattice point A and the Lab space corresponding to the referenceable lattice point B are used. A plane Q _AB having a normal vector parallel to the straight line N _AB connecting the coordinates B of the image, a coordinate C in the Lab space corresponding to the reference lattice point C, and a Lab space corresponding to the reference lattice point D A plane Q _CD having a normal vector parallel to the straight line N _CD connecting the coordinates D is created, and obtained by the forward model converter 500 from the intersection line N _ABCD formed by these two planes and the current ink amount set candidate. Although the Euclidean distance from the Lab value X is the first gradation index LabDist, the Euclidean distance obtained by the method described below may be the LabDist.

FIG. 15 is a diagram showing another method for calculating the first gradation index LabDist when there are four (two sets) of referenceable lattice points. As shown in FIG. 15, the Lab value X corresponding to the ink amount set candidate and the distances between the plane Q _AB and the plane Q _CD are expressed by the first gradation index LabDist (1) and the first gradation index LabDist ( 2), and the sum thereof is calculated as a first tone index LabDist (total) shown in the following formula (7). By doing so, the evaluation value calculation unit 300 can calculate the first gradation index LabDist without obtaining the intersection line N _ABCD , so that the optimum ink amount set can be determined in a short time. .

  Further, even when there are six (three sets) reference lattice points (see FIG. 13), the distance to each plane is expressed by the first gradation index LabDist (1) as in the method described above. Alternatively, the first gradation index LabDist (2) and the first gradation index LabDist (3) may be calculated, and the sum thereof may be calculated as the first gradation index LabDist (total).

-Modification E2:
In the above-described embodiment, the Lab value corresponding to the ink amount set is acquired using the forward model converter 500. However, instead of the method using the forward model converter 500, the ink amount set candidate and the Lab value are previously determined. Corresponding correspondence tables may be created, and ink amount set candidates may be selected from them.

-Modification E3:
In the above-described embodiment, the RGB value, the Lab value, and the optimum ink amount set are uniquely associated with the created color conversion LUT 61 (see FIGS. 5 and 6). The value associated with the optimum ink amount set is not limited to the Lab value. For example, it is possible to employ a color system value capable of evaluating gradation such as L * C * h * value and L * u * v * value.

-Modification E4:
In the above-described embodiment, the carriage 80 included in the printing apparatus 10 is for color ink that contains, for example, cyan ink C, magenta ink M, yellow ink Y, and black ink K as color inks. Although the ink cartridges 82 to 85 are mounted, the mounted ink is not limited to these inks. For example, in addition to these inks, ink such as light cyan Lc and light magenta Lm may be mounted on the carriage 80. In this case, the color conversion LUT creation device 20 determines an optimal ink amount set including ink amounts such as light cyan Lc and light magenta Lm, and creates the color conversion LUT 61.

-Modification E5:
In the embodiment described above, the printing apparatus 10 is an ink jet printing apparatus, but instead of this, a laser printer or an offset printing apparatus that performs printing by attaching color toner onto a printing medium is employed. Can do.

-Modification E6:
In the above-described embodiment, the optimum ink amount set is determined for the lattice point located approximately at the center of the two lattice points for which the optimum ink amount set has already been determined. However, the optimal ink amount set may be determined from any grid point as long as the grid point is sandwiched between two grid points. Similarly, in the case of four or six referenceable grid points, the optimum ink amount set may be determined from the grid points at any position as long as the grid points are sandwiched between the grid points.

-Modification E7:
In the embodiment described above, the printing apparatus 10 and the color conversion LUT creation apparatus 20 are separated from each other. However, the function of the color conversion LUT creation apparatus 20 may be incorporated in the printing apparatus 10. That is, the printing apparatus 10 may create the color conversion LUT 61. In addition, a printing system including a computer and a printing apparatus may be regarded as a printing apparatus in a broad sense, and the computer may create a color conversion LUT 61, and the computer may perform color conversion processing using the color conversion LUT 61. In this case, the computer can further perform halftone processing and rasterization processing and control the printing apparatus to perform printing.

-Modification E8:
In the above-described embodiment, the color conversion LUT 61 is stored in the EEPROM 60. However, the color conversion LUT 61 may be stored in another storage device such as a ROM, a RAM, or an HDD.

DESCRIPTION OF SYMBOLS 10 ... Printing apparatus 20 ... Color conversion LUT creation apparatus 30 ... Control unit 40 ... CPU
41 ... CPU
51 ... ROM
52 ... RAM
53 ... ROM
54 ... RAM
55 ... HDD
60 ... EEPROM
61 ... Color conversion LUT
DESCRIPTION OF SYMBOLS 70 ... Carriage motor 74 ... Paper feed motor 75 ... Platen 80 ... Carriage 81 ... Print head 82-85 ... Ink cartridge 91 ... Image data supply part 100 ... Processing object grid point selection part 200 ... Ink amount set candidate calculation part 300 ... Evaluation Value calculating unit 400 ... Optimal ink amount set determining unit 500 ... Forward model converter 510 ... Spectral printing model converter 520 ... Color converter P ... Printing medium

Claims (5)

Manufacturing of a printing apparatus including a storage unit in which a color conversion lookup table for converting a color in the first color space into an ink amount set that is a combination of a plurality of ink amounts usable in the printing apparatus is recorded A method,
A selection step of selecting a grid point associating the ink amount set as a processing target grid point from grid points constituting the first color space;
A candidate determination step of determining a candidate of an ink amount set to be associated with the processing target grid point;
A calculation step of calculating an evaluation value for determining an ink amount set to be associated with the processing target grid point based on the ink amount represented by the ink amount set candidate;
Based on the calculated evaluation value, an ink amount set to be associated with the processing target grid point is determined from the ink amount set candidates, and the determined ink amount set is associated with the processing target lattice point. A step of determining an ink amount set to be recorded in the color conversion lookup table;
A storage step of storing the color conversion lookup table in which the processing target grid point and the ink amount set are associated with each other in the storage unit included in the printing apparatus;
In the calculation step, when there are at least two grid points for which the correspondence between the processing target grid point and the ink amount set has already been determined so as to sandwich the processing target grid point, the second color space , Having a normal vector parallel to a straight line connecting two coordinates in the second color space corresponding to the two grid points, and coordinates in the second color space corresponding to the processing target grid point. A plane passing therethrough is calculated, and a Euclidean distance between the plane and coordinates in the second color space corresponding to the ink amount set candidate is calculated as the evaluation value.
A method for manufacturing a printing apparatus. A method of manufacturing a printing apparatus according to claim 1,
In the calculating step, when there are at least two sets of two grid points for which the correspondence relationship between the processing target grid point and the ink amount set has already been determined so as to sandwich the processing target grid point, the second In the color space, for each of the two sets of grid points, the two planes are obtained and the intersection line of the two planes is obtained, and the intersection line and the second color space corresponding to the ink amount set candidate are obtained. Euclidean distance between the coordinates in the middle is calculated as the evaluation value,
A method for manufacturing a printing apparatus. A method of manufacturing a printing apparatus according to claim 1 or 2,
In the calculating step, when there are at least three sets of two grid points for which the correspondence relationship between the processing target grid point and the ink amount set has already been determined so as to sandwich the processing target grid point, the second color In the space, for each of the three sets of grid points, three planes are obtained and intersections of the three planes are obtained, and the intersection points and coordinates in the second color space corresponding to the ink amount set candidates are obtained. Euclidean distance between and is calculated as the evaluation value,
A method for manufacturing a printing apparatus.   The printing apparatus manufactured by the manufacturing method of the printing apparatus as described in any one of Claim 1- Claim 3. A printing method for performing printing using a color conversion look-up table for converting a color in a first color space into an ink amount set that is a combination of a plurality of ink amounts usable in a printing apparatus,
A selection step of selecting a grid point associating the ink amount set as a processing target grid point from grid points constituting the first color space;
A candidate determination step of determining a candidate of an ink amount set to be associated with the processing target grid point;
A calculation step of calculating an evaluation value for determining an ink amount set to be associated with the processing target grid point based on the ink amount represented by the ink amount set candidate;
Based on the calculated evaluation value, an ink amount set to be associated with the processing target grid point is determined from the ink amount set candidates, and the determined ink amount set is associated with the processing target lattice point. A step of determining an ink amount set to be recorded in the color conversion lookup table;
A storage step of storing in the storage unit the color conversion lookup table in which the processing target grid point and the ink amount set are associated with each other;
Obtaining image data represented by the first color space;
Converting the image data into the ink quantity set with reference to the color conversion lookup table;
Performing halftone processing on the converted ink amount set to generate print data;
A step of performing a printing process based on the print data,
In the calculation step, when there are at least two grid points for which the correspondence between the processing target grid point and the ink amount set has already been determined so as to sandwich the processing target grid point, the second color space , Having a normal vector parallel to a straight line connecting two coordinates in the second color space corresponding to the two grid points, and coordinates in the second color space corresponding to the processing target grid point. A plane passing therethrough is calculated, and a Euclidean distance between the plane and coordinates in the second color space corresponding to the ink amount set candidate is calculated as the evaluation value.
Printing method.

Images