JP2003069850A - Method for preparing color conversion definition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare a color conversion definition with high accuracy which improves errors of a highlight part and a shadow part concerning a color conversion definition preparing method for preparing, for example, color conversion definition that reflects the ability of a skilled person of color adjustment in order to convert image data with colors represented with a certain color specification coordinate system into image data with colors represented with a different color specification coordinate system. SOLUTION: A virtual dot % instead of 0% and 100% is estimated from remaining data obtained by removing data of 0% and 100% halftone dot area ratio, and color conversion definition is prepared by using data including the estimated data of dot %.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to, for example, color adjustment for converting image data in which a color is expressed in one color coordinate system into image data in which a color is expressed in another color coordinate system. The present invention relates to a color conversion definition creating method for creating a color conversion definition that reflects the skill of an expert.

[0002]

2. Description of the Related Art Conventionally, in the field of printing, a device called a drum type scanner has been used as a device for reading a document image and obtaining image data suitable for printing by a printing machine. This drum-type scanner is configured to be able to adjust various parameters when reading a document image to obtain image data, and the document image is adjusted in a state where adjustment is performed by a color adjustment expert. Converted to image data. In this drum type scanner, C (cyan), M
Image data suitable for printing, which represents the dot area ratios of four colors (magenta), Y (yellow), and K (black), can be obtained.

Here, the drum type scanner is large in size and difficult to handle, and it is desired to adopt a small-sized and easy-to-use scanner (herein, this scanner is referred to as "target scanner") in place of this drum type scanner. Is big. The target scanner scans a document image and, for example, coordinates of C (cyan), M (magenta), and Y (yellow) color spaces (CMY color space), or R (red), G (green), and This is a scanner that obtains image data in which colors are expressed in the coordinates of the B (blue) color space (RGB color space). In this case, in order to obtain image data suitable for printing, the image data obtained by the target scanner is obtained. , CM
It is necessary to convert the image data in which the color is expressed in the coordinates of the Y color space or the RGB color space into the halftone dot area ratio data in which the color is expressed in the coordinates of the CMYK color space.

Here, by this color conversion, the CMY color space or RGB color space of the target scanner and the drum scanner can be obtained so that data of the same dot area ratio as that obtained by the drum scanner can be obtained. If a color conversion definition with the CMYK color space can be created, it is possible to perform color conversion that incorporates the skill of an expert in color adjustment of a drum scanner.

As a basic method for creating a color conversion definition that incorporates the skill of a color adjustment expert, the same color chart is applied to a drum type scanner and an object whose performance is to be copied. The data is read by both the scanner and the drum type scanner to obtain CMYK halftone dot area ratio data, and the target scanner obtains CMY or RGB data. In this way, the data in both color systems are obtained, and by associating them with each other, the CMY or RGB data obtained by the target scanner is converted into the same halftone dot area ratio as that of the CMYK obtained by the drum scanner. A color conversion definition for converting the point area ratio data is created.

[0006]

[Patent Document 1] Japanese Patent Laid-Open No. 10-
Japanese Patent Publication No. 70668 proposes a method of creating a color conversion definition for converting the first color system data having an irregular grid into the second color system data having an irregular grid. The CMYK halftone dot area ratio data and the CMY or RGB data obtained by reading the same color chart with the drum type scanner and the target scanner as described above are both data composed of an irregular lattice, and are proposed in this publication. By adopting the method described above, the data of the first color system (here, the CMY or RGB color system of the target scanner) including the irregular grid is converted into the second color system (also including the irregular grid) ( Here, it is possible to create a color conversion definition for converting the data of the dot area ratio of the CMYK color system of the drum type scanner.

However, when the method disclosed in the above publication is adopted, the error tends to increase in the highlight portion and the shadow portion, and this error may reach an unacceptable level.

In view of the above circumstances, it is an object of the present invention to provide a color conversion definition creating method capable of creating a highly accurate color conversion definition in which an error in a highlight part and a shadow part is improved. To do.

[0009]

A color conversion definition creating method of the present invention which achieves the above object, is a first input device for reading an image to generate image data color-represented in a first color coordinate system. Is used to obtain a first color data by reading a color chart composed of a plurality of color patches to which respective indexes are assigned, and the first color data is obtained, and the first color data is obtained between the index and the coordinates in the first color coordinate system. First of
And a second input device that reads an image to generate image data that is color-represented in a second color coordinate system. A second step of obtaining color data and obtaining a second correspondence relationship between the index and coordinates in the second color coordinate system; and an unsaturated second color of the second color data. Based on the third step of correcting the second correspondence relationship based on the data, the first correspondence relationship, and the second correspondence relationship corrected in the third step, the first color specification A fourth step of creating a color conversion definition for converting the coordinates in the coordinate system into the coordinates in the second color coordinate system.

For example, taking the above-mentioned drum type scanner as an example, when the data of the halftone dot area ratio output from the drum type scanner indicates 0% or 100%, the color of the color chart is that of the drum. When the color chart is read out from the color reproduction area of the type scanner and the color chart is read by the drum type scanner, 0% or 100 which is the limit of the color reproduction area of the drum type scanner.
It is highly possible that it has been saturated to%. If a color conversion definition is created based on data including such saturated data, there will be an error in the highlight portion near the halftone dot area ratio of 0% and the shadow portion near the halftone dot area ratio of 100%.

Therefore, in the present invention, the unsaturated second color data of the second color data obtained in the second step (in the above example, the halftone dot area ratio is 0% and the halftone dot area ratio is 0%). 100
Since the third step for correcting the second correspondence relationship (correspondence relationship between the index and the halftone dot area ratio in the above example) based on the halftone dot area ratio data excluding%,
The error caused by the inclusion of saturated data is removed, and a highly accurate color conversion definition is created for the highlight part and the shadow part.

Here, in the color conversion definition creating method according to the present invention, the third step is the non-saturated second color data of the second color data when the index is a variable.
It is preferable to correct the second correspondence by applying the least squares method to the color data.

By applying the least squares method, the second correspondence can be corrected with high accuracy.

In the color conversion definition creating method of the present invention, the second color coordinate system is C (cyan), M (magenta), Y (yellow), as illustrated above.
Alternatively, a color coordinate system may be used in which the dot area ratio for each of K and K (black) is the coordinate axis.

In the color conversion definition creating method of the present invention, typically, the first step is to first read the color chart with the first input device to obtain the first color data. The first correspondence in the forward direction for obtaining the coordinates on the first color coordinate system from the index is obtained, and then the first correspondence for the forward direction is calculated from the coordinates on the first color coordinate system. First in the opposite direction to find the index
In the second step and the third step, respectively, by reading the color chart with the second input device to obtain the second color data, and the second table from the index. Forward step based on the step of obtaining the second correspondence in the forward direction for obtaining the coordinates on the color coordinate system and the unsaturated second color data of the second color data obtained in the second step Is a step of correcting the second corresponding relationship in the first direction to obtain the corrected second corresponding relationship in the forward direction, wherein the fourth step is the first corresponding relationship in the reverse direction obtained in the first step. And a color conversion definition based on the corrected second correspondence in the forward direction obtained in the third step.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

FIG. 1 is a diagram showing an outline of a document image reading-printing system.

The original image 10 is read by the drum type scanner 100, and data of CMYK halftone dot area ratio (halftone%) is generated. Here, the halftone dot area ratio (halftone%) is C (cyan), M (magenta), Y suitable for the printing machine 300.
CMYK consisting of (yellow) and K (black)
CMYK color coordinate system C, which is suitable for this printing, for the sake of later distinction.
M _out , Y _out , and K _out respectively as C _out , M _out , Y _out , and K _out
It is written as.

The image data representing the halftone dot area ratio in which the color is expressed by the coordinates of the C _out M _out Y _out K _out color system is sent to the printing machine 300, and a film original plate is produced based on the image data. A printing plate is created based on the film original plate, printing is performed, and a print image 20 is created. Here, the drum-type scanner 100 has undergone color adjustment by a skilled person, and the printed image 20 is an image having extremely good color reproduction with respect to the original image 10.

Next, instead of the drum scanner 100,
A case where the original image 10 is read by using another scanner (target scanner) 200 will be described. The original image 10 is read by the scanner 200, and image data in which colors are expressed by the coordinates of the CMY three color system is generated. Here, C in the CMY color system, M, and Y, and _{C out M o ut Y out K} out color system of the image data representing halftone dot area ratio obtained in the drum-type scanner 100 (halftone%) In order to discriminate between them, they are expressed as C _in , M _in , and Y _in here.

Color conversion 210 is applied to the image data obtained by scanning the original image 10 with the scanner 200 and _in which the color is reproduced _{in the} C _in M _in Y _in color system. The color conversion here is C _in M _in Y _in obtained by the scanner 200.
The scanner 2 converts the image data in which the color is expressed in the color system.
C obtained when the drum-type scanner 100 reads the same original image as the original image 10 read by C
_out M _out Y _out K _out The image data is converted into image data equivalent to image data representing the dot area ratio (dot%) in which the color is expressed by the coordinates of the color system. A method of creating a color conversion definition used in this color conversion 210 will be described later.

When this color conversion 210 includes the skill of the color adjustment of the drum type scanner 100 by an expert as it is, C _out M which is the same as the output of the drum type scanner 100 by this color conversion 210. _out Y _out K _out Converted into image data representing the halftone dot area ratio expressed in the color system.

C generated by this color conversion 210
_The image data in which the halftone dot area ratio is expressed in the color system of _out M _out Y _out K _out is sent to the printing machine 300 and undergoes the same procedure as described above to obtain a printed image 20 having a correct color expression.
Is obtained.

Next, a method of creating a color conversion definition which reflects the skill of a skilled person and is adopted in the color conversion 210 will be described. In this case, instead of the original image 10, a color chart in which a large number of color patches are arranged is read by both the drum type scanner 100 and the target scanner 200, and the respective halftone dots of the C _out M _out Y _out K _out color system. The patch reference data representing the area ratio and the patch density data in which the colors are expressed _in the C _in , Min, Y _in color system are generated.

FIG. 2 is a flow chart showing a method of creating a color conversion definition adopted in the color conversion 210 of FIG. 1, and FIG. 3 is a diagram showing a color chart used when creating this color conversion definition.

The color chart 30 shown in FIG. 3 is obtained by writing on a color reversal film with a laser and developing, and C = 0, 1, ... In order from the upper left to the lower right of FIG. ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, & There are a total of nine blocks, each of which has an index of 8.
8. From top to bottom, Y = 0,1, ...,
Each index of 8 is attached. Where C = 0,
The density of C (cyan) increases in this order for 1, ..., 8 and M = 0,1, ..., 8, Y = 0, 1 ,. The density of magenta) and Y (yellow) are high. That is, with respect to any of C, M, and Y, it is guaranteed that the density of C, M, and Y will increase as the numerical value of the index increases. Here, each value of C, M, and Y represents an index, so that the indices C, M, and Y representing the colors of this color chart are C _index , M _index , and Y here.
_{Notated as index} .

In the target scanner 200 and the drum type scanner 100 shown in FIG. 1, the color chart 30 with indexes shown in FIG. 3 is read (steps (a) and (b) in FIG. 2), and the target scanner 200 is Patch density data representing coordinates _in the C _in M _in Y _in color system of each patch forming the color chart 30 is generated,
In the drum type scanner 100, the color chart 30
The patch reference data representing the halftone dot area ratio of the C _out M _out Y _out K _out color system of each patch constituting

At this stage, the conversion table from the chart index to the patch density data is created on the target scanner side, and the conversion table from the same chart index to the patch reference data is created on the drum scanner side.

FIG. 4 is a diagram showing the color chart shown in FIG. 3 in the index space.

C _index , M _index , Y shown in FIG.
_{On the index} space with _index as an axis, each patch of the color chart shown in FIG. 3 has C _index , M _index , and Y.
It is arranged on each grid point of 0, 1, 2, ..., 8 for each _index .

FIG. 5 is for the color chart shown in FIG.
C obtained by scanning with the scanner 200 _inM_inY_inIn color space
Diagram showing the coordinates of each patch in the color chart
Is.

In the color chart 30 shown in FIG. 3, each patch is arranged on each grid point in the index space as shown in FIG. 4, but the patch density obtained by reading the color chart with the target scanner 200 is shown. The coordinates represented by the data have a distorted shape as shown in the image in FIG. 5 _in the C _in M _in Y _in color space. However, for example, the maximum density of C (cyan) obtained by scanning with the target scanner 200 is 3.6, and the maximum density of C that can be expressed by the color reversal film having the color chart 30 shown in FIG. For example, the patch density data obtained by the target scanner 200 is non-saturated data in which the value of the patch density data changes as the density of C, M, and Y of each patch on the color chart 30 changes.

FIG. 6 shows C _out M _out Y _out K of patch reference data representing the dot area ratio obtained by reading the color chart shown in FIG. 3 with the drum scanner 100.
It is the figure which showed the coordinate in _out color space typically. However, for the sake of representation, except for the K _{ou t} here, C _out, M
It is represented by three axes, _out and Y _out .

Since the halftone dot area ratio is adopted here,
There is no data that the net% is less than 0% and exceeds 100%.
Among a large number of patches constituting the color chart 30 shown in FIG. 3, on C _out M _out Y _out K _out color space, as the halftone dot area ratio of greater than 0% less than or 100% if the virtual representation Some patches have colors to be expressed, and as a result of being read by the drum scanner 100, those patches are saturated to 0% or 100%.

The shaded area in FIG. 6 is 0% -100.
% Of a cube (FIG. 6 is represented by the three axes of C _out , M _out , and Y _out ) that is in contact with the surface and has a saturated value (0% or 100%). It is a representation.

When the color conversion definition is created by directly using the data saturated to 0% or 100% as described above,
In particular, a color conversion definition with a large error is created in the highlight part or the shadow part. Therefore, here, in step (d) of FIG. 2, the output value (patch reference data) is corrected.

Here, first, the step (step (c)) of inverse function generation shown in FIG. 2 will be described.

When the color chart 30 shown in FIG. 3 is read by the target scanner 200 to generate patch density data, a function (table) for converting the chart index into patch density data is generated. Therefore, in step (c), an inverse function (table) for converting the patch density data into a chart index is created by performing an operation such as an interpolation operation. This inverse function is a grid point (density grid point; C _in M _in Y _in color space shown _in FIG. 5).
It is for converting an integer) into a chart index (where this chart index contains values after the decimal point).

On the other hand, the function (table) for converting the chart index into the patch reference data obtained by reading the color chart shown in FIG. 3 with the drum scanner 100 is as follows in step (d). Will be corrected.

FIG. 7 is an explanatory diagram of a method of correcting the function. A one-dimensional function is shown here for ease of illustration and description.

Here, the patch of index 0 is virtually a patch in which halftone dot% should be expressed as a negative value, but since the minimum halftone dot% is 0%, it is saturated to 0%.
The indexes 1, 2, ..., 7 have large net% as the index value increases. The patch of index 8 is a patch that is supposed to be expressed by a halftone dot percentage that exceeds 100%, but the maximum halftone dot percentage is 100%, so it is saturated to 100%.

Therefore, here, 0% of index 0
, And the data of 100% of index 8 are not used, the least-squares approximation curve is obtained from the seven data of indexes 1, 2, ..., 7 and the virtual negative net% (corresponding to index 0 △), and the net percentage corresponding to index 8 that exceeds 100% (△)
Is required.

In this way, minus net%, 100
Corrected reference data including halftone% exceeding% is generated. At this stage, a function (table) for converting the chart index into the corrected reference data is created.

Next, in step (e) of FIG.
The concentration grid point data found in step (c)
Inverse function to convert to index and in step (d)
Calculated and corrected chart index reference
Concentration grid point data is created by combining with the function to convert to data
(C which is the output of the target scanner 200 shown in FIG._inM _in
Y_inReference data (data in color space)
The same C as the output of the drum type scanner 100 shown_outM_out
Y_outK_outData representing the dot area ratio in the color space)
A function (table) for conversion is created. This function
In creating the (table), at the final stage,
If the net% of the eggplant is required, the negative net%
When 0% is set to 0% and net% exceeding 100% is required,
The net percentage exceeding 100% is replaced by 100%
It

FIG. 7 conceptually shows an error generated when the correction of step (d) of FIG. 2 is not performed and a correct value obtained by performing the correction of step (d) of FIG. ing. That is, the index 0.9
The halftone dot% of the point is obtained as a% when the step (d) is not corrected, but is obtained as b% when the step (d) is corrected, and the step (d) is not corrected. In the case, the result includes an error of (a% -b%). The vicinity of the dot% of 0% is a highlight part, and a slight difference in the dot% appears as a large difference in color. Therefore, this error may be an unacceptably large error.

If the correction in step (d) is not carried out, the same error will be included on the shadow side where the dot% is close to 100%. The error on the shadow side may be an unacceptable error when a negative image (a black-and-white image is an image in which black and white are reversed) is generated. Since the correction in step (d) is performed here, this error is eliminated, a highly accurate color conversion definition is created, and the original image 10 is read by the target scanner 200 shown in FIG. 1 to obtain image data. By performing the color conversion 210 using the color conversion definition obtained as described above, the drum scanner in which the target scanner 200 is used without using the drum scanner 100 and the color is adjusted with high accuracy by an expert. It is possible to obtain halftone dot data in which colors are expressed with the same precision as when 100 is used.

[0047]

As described above, according to the present invention, a highly accurate color conversion definition can be created.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of a document image reading-printing system.

FIG. 2 is a flowchart showing a method for creating a color conversion definition adopted in the color conversion of FIG.

FIG. 3 is a diagram showing a color chart used when creating a color conversion definition.

FIG. 4 is a diagram showing the color chart shown in FIG. 3 on an index space.

5 is a diagram schematically showing the coordinates of each patch of the color chart _in the C _in M _in Y _in color space obtained by reading the color chart shown in FIG. 3 with a target scanner.

FIG. 6 schematically shows coordinates of patch reference data representing a dot area ratio obtained by reading the color chart shown in FIG. 3 with a drum scanner in a C _out M _out Y _out K _out color space. It is a figure.

FIG. 7 is an explanatory diagram of a method of correcting a function for converting a chart index into patch reference data.

[Explanation of symbols]

10 original image 20 printed images 100 drum scanner 200 scanner 210 color conversion 300 printing machines

Continued front page    F term (reference) 2C087 AA15 BA03 BA07 BB10 BD36                 5B057 AA01 BA02 CA01 CA08 CA12                       CA16 CB01 CB08 CB12 CB16                       CE18 CH07                 5C077 LL19 MP08 PP32 PP33 PP37                       PQ12 PQ23 TT08                 5C079 HB01 HB02 HB03 HB11 LA02                       LB02 MA05 MA10 NA03 NA29                       PA07

Claims (4)

[Claims] 1. A first input device that reads an image to generate image data that is color-represented in a first color coordinate system, and is composed of a plurality of color patches to which respective indexes are assigned. Obtaining a first color data by reading the color chart, and obtaining a first correspondence between the index and the coordinates in the first color coordinate system; a first step of reading the image; Second color data is obtained by reading the color chart using a second input device that generates image data that is color-expressed in the second color coordinate system, and the index and the second color A second step of obtaining a second correspondence relationship between the coordinates in the coordinate system, and a second step of correcting the second correspondence relationship based on the unsaturated second color data of the second color data. Three Based on the step, the first correspondence, and the second correspondence corrected in the third step, the coordinates in the first color coordinate system are set to the second color coordinate system. And a fourth step of creating a color conversion definition to be converted into coordinates. 2. The second step comprises applying a least squares method to the unsaturated second color data of the second color data when the index is a variable, 2. The color conversion definition creating method according to claim 1, wherein the correspondence relationship of is corrected. 3. The color coordinate system according to claim 1, wherein the second color coordinate system is a color coordinate system having a halftone dot area ratio for each of cyan, magenta, yellow, and black as a coordinate axis. How to create color conversion definition. 4. In the first step, first, by reading the color chart with the first input device to obtain the first color data, the coordinates on the first color coordinate system are obtained from the index. To obtain the first correspondence in the forward direction, and then to obtain the first correspondence in the forward direction from the coordinates in the first color coordinate system to the first correspondence in the reverse direction. The second step and the third step are each a step of converting into a relationship, and the second step and the third step read the color chart with the second input device to obtain the second color data. The step of obtaining the second correspondence in the forward direction for obtaining the coordinates on the color coordinate system of No. 2, and the unsaturated second color data of the second color data obtained at the second step. A step of correcting the second corresponding relationship in the forward direction on the basis of the data to obtain a corrected second corresponding relationship in the forward direction, wherein the fourth step is the reverse step obtained in the first step. A step of creating the color conversion definition based on a first correspondence relationship in the direction and the corrected second correspondence relationship in the forward direction obtained in the third step. Item 1. A color conversion definition creating method according to item 1.