JPH06296231A - Color value transformation - Google Patents

Abstract

PURPOSE:To improve the transformation precision by providing an affine transformation means for color value in the preceding stage of a transformation means using a lock up table which transforms a color value to a color expression value peculiar to an output device. CONSTITUTION:An inputted L*a*b* value 1 is transformed to an FGH value 3 by an affine transformation means 2 and is inputted to transformation means 5, 9, and 13 using the look up table method with interpolation. The means 5 retrieves a look up table LUT 4 to transform the FGH value to a C value and calculates an interpolation value C and outputs a C value 7. In the same manner, means 9 and 13 retrieve LUTs 8 and 12 to transform the FGH value to an M value and a Y value respectively and calculate interpolation values M and Y and output an M value 11 and a Y value 15. In this constitution, transformation is performd with a high precision by adding relatively simple matrix calculation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for outputting or displaying colors in a color printer, a color printer, a color copying machine, a color facsimile, a color display, etc.
The present invention relates to a device for converting a color value into a color expression value specific to an output device.

[0002]

2. Description of the Related Art In an image editing apparatus that has been used in the field of conventional printing, halftone dots of cyan (C), magenta (M), yellow (Y) and black (K) of printing ink are used as quantities representing colors. Values proportional to% are often used and these values are obtained from a color scanner for printing.

Recently, various color printers, color copying machines, etc. have been connected to an image editing apparatus used in the field of printing and have been used as a substitute for a proofing machine. Photo, sublimation type thermal transfer,
There are various types such as inkjet, and various pigments and dyes are used as coloring materials. In addition, even with color displays, the color development performance differs depending on the model.

These output devices use CMY as data.
Values, CMYK values, RGB values, etc. are input, but the CMY values or CMYK of the image editing apparatus are input.
It was difficult to obtain the same color on the printed matter or on the screen simply by transferring the values as they were to an output device such as a color printer or a color display due to the difference in the image reproduction mechanism and the color materials.

Therefore, in the data representation used by the image editing apparatus,
CIEL instead of CMYK values^*a ^*b^*System-like output
The value that does not depend on the characteristics of the force device is used for various output devices.
CMY which is a color expression value peculiar to the output device when outputting
Values, CMYK values, RGB values, etc.
There is one.

As a method of converting the CIE L ^* a ^* b ^* values into CMY values, CMYK values, RGB values, etc. which are color expression values peculiar to various output devices, a method using a lookup table method with interpolation, a neural network The method of using is well known.

[0007]

[Problems to be Solved by the Invention]

Generally, the data space of the color expression value peculiar to the output device is filled with the circumscribed cube on the orthogonal space as shown in FIG. 1, while it depends on the output device such as the CIEL ^* a ^* b ^* values. The data space of color values that do not exist is not filled in the circumscribed cube parallel to the axis in the orthogonal space as shown in FIG.

Therefore, when the look-up table method with interpolation is used, a wasteful data space that is not searched is generated in the look-up table, so that the conversion accuracy is improved relative to the number of divisions of the look-up table. There was a problem not to do.

[0010]

To this end, the present invention provides a conversion device including conversion means for converting a color value into a color expression value specific to an output device by using a lookup table method with interpolation. It is characterized in that an affine transforming means for color values is provided before the transforming means using a lookup table.

[0011]

Although there are various color expression methods, CIEL ^* a ^* b ^* is a color expression method that does not depend on the output device ^.
There are space, CIEL ^* u ^* v ^* space, XYZ space, Munsell space, etc., and the color value defined in such a data space that does not depend on the input / output device is called a color value here, and all of them are three-dimensional. It is a space.

[0012] A printing machine also has a color expression method unique to the output device.
There are various values such as the% value of halftone dots, the value related to the adhesion amount of dyes and pigments, such as in color printers and color copiers, and the value related to the amount of electricity for causing the cathode ray tube to emit light, such as in color displays. It is a three-dimensional or four-dimensional data space.

According to the present invention, the above-mentioned color value is converted into a corresponding color expression value specific to the output device by using a lookup table method with interpolation.

Hereinafter, as an example, the color value is CIEL ^*.
Assuming a ^* b ^* space, the color reproduction values specific to the output device are cyan ink, magenta ink, which are printing inks,
The description will be made assuming the CMY value which is the dot% with respect to the yellow ink.

The color reproduction range of an output device such as a color printer or color display is shown in FIG. 2 in the CIEL ^* a ^* b ^* space. In FIG. 2, w, r, g, b, c, m, y, and k at each vertex indicate points corresponding to white, red, green, blue, cyan, magenta, yellow, and black, respectively. .

Reference numeral 102 indicates L *, which surrounds the color reproduction range.
A circumscribed cube parallel to the axis, the a * axis, and the b * axis is shown.

Conventionally, from CIE L ^* a ^* b ^* space to CMY
CIEL ^* a as shown in Fig. 2 when converting data to space
Prepare three three-dimensional lookup tables in which the circumscribed cube 102 is divided into equal intervals on each axis in the ^* b ^* space, and convert each L ^* a ^* b ^* value to a C value, L ^* a ^*. b ^*
It was used for converting values to M values and L ^* a ^* b ^* values to Y values.

Now, assume a three-dimensional FGH orthogonal space.

Although any vertex may be selected, for example, the vertex w is mapped to the origin of the FGH orthogonal space, and wy, w
The line connecting -m and w-c is the F axis, G axis of the FGH orthogonal space,
Consider a transformation that maps to the H axis.

This conversion can be performed using the matrix of 3 rows and 4 columns shown in FIG. 3, and is simply w-c, w-m, w-.
It is mathematically known that if the line connecting y is only mapped so as to be parallel to the F axis, G axis, and H axis, the matrix of 3 rows and 3 columns shown in FIG. 4 can be used. Such a transformation is called an affine transformation.

As a result of this affine transformation, the output reproduction range is as shown in FIG. 5 in the FGH orthogonal space.

Reference numeral 103 is a color reproduction range after affine transformation, and reference numeral 104 is a color reproduction range 103 after affine transformation.
It is a circumscribed cube that is parallel to the F-axis, G-axis, and H-axis surrounding the.

Three three-dimensional look-up tables are prepared on the FGH space, and are used for converting FGH values into C values, converting FGH values into M values, and converting FGH values into Y values.

That is, the L ^* a ^* b ^* value is once converted into an FGH value, which is then converted into a C value, an M value, and a Y value by using three look-up tables prepared on the FGH space.

As compared with FIG. 2, FIG. 5 has an improved filling rate in the circumscribed cube, and the method of the present invention reduces the waste of the look-up table and improves the conversion accuracy as compared with the conventional method.

Also in the method of obtaining the interpolated value using the look-up table, "primary interpolation using 8 points", "primary interpolation using 4 points", "primary interpolation using 6 points", " Various methods such as “quadratic interpolation using 27 points” are known, but they are effective in any case.

"Primary interpolation using 8 points" and "Primary interpolation using 4 points" are described in Electrophotographic Society of Japan, Vol. 29, No. 3, (1990), pages 299 to 305.

[0028]

EXAMPLES Examples will be described below with reference to the drawings.

Based on the present invention, a mechanism as shown in FIG. 6 has been realized.

As the interpolation method, "primary conversion using 8 points" is adopted.

In FIG. 6, reference numeral 1 is the input L ^* a.
^* b ^* value, reference numeral 2 is an affine transformation means for converting the L ^* a ^* b ^* value into an FGH value, and reference numeral 3 is the converted FGH value.

The L ^* a ^* b ^* value is converted to an FGH value by multiplication with a matrix of 3 rows and 3 columns, and the FGH value is a means for searching a lookup table for converting the FGH value into a C value. Interpolated value C obtained from the DFG values retrieved in 4 and 4
Means 5, a means 8 for searching a lookup table for converting an FGH value into an M value, means 11 for calculating an interpolated value M obtained from the FGH value searched by 8
It is input to the means 12 for searching the look-up table for converting the FGH value into the Y value, and the means 13 for calculating the interpolation value Y obtained from the FGH value searched by the 12.

The look-up table in reference numeral 4 has a three-dimensional structure. The C values of eight surrounding points surrounding the point to be obtained by the input FGH value are input to the means 5 through the reference numeral 6, and the means 5 is three-dimensional. The C value indicated by reference numeral 7 is obtained by the proportional calculation of

The M value indicated by reference numeral 11 and the Y value indicated by reference numeral 15 are also obtained by performing the same calculation as in the case of obtaining the C value.

For comparison, a mechanism based on the conventional method as shown in FIG. 7 was realized.

In FIG. 7, reference numeral 1 is the input L ^*.
a ^* b ^* has a value, L ^* a ^* b ^* means for the value to search a look-up table for converting the C value 16, 16
In means 20, 20 for in a retrieved L ^* a ^* b ^* means 17 for calculating an interpolated value C obtained from the values, L ^* a ^* b ^* value to search a look-up table for converting the M value means 21 for calculating the interpolation value M for determining from the retrieved L ^* a ^* b ^* values, search L ^* a ^* b ^* values by a means 24, 24 for searching a look-up table for converting the Y value It is input to the means 25 for calculating the interpolation value Y to be obtained from the L ^* a ^* b ^* value thus obtained.

The look-up table in 16 is constructed in three dimensions, and the C values of 8 points surrounding the point to be obtained by the input L ^* a ^* b ^* value are inputted to the means 17 through 18 and The means 17 obtains the C value indicated by reference numeral 19 by three-dimensional proportional calculation.

The M value indicated by the reference numeral 23 and the Y value indicated by the reference numeral 27 are also obtained by performing the same calculation as in the case of obtaining the C value.

An offset printing machine was used to change the halftone dot% of each ink of cyan, magenta, and yellow by 10% from 0 to 100 to print color chips of 1331 colors, and the color was measured to obtain L ^* a ^* b ^*. The values were obtained and the look-up tables within 4, 8, 12, 16, 20, 24 were created.

The lookup table was calculated with five divisions for each axis, and by comparing the calculation results of the conventional method and the present invention, the method of the present invention was about 1.3 times better in accuracy.

[0041]

As described above, according to the present invention, by adding a relatively simple matrix calculation, it is possible to obtain a conversion means for converting a color value into a color expression value specific to the output device with higher accuracy than in the prior art. The present invention can also be effectively used for computer color matching.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a data space of color expression values unique to an apparatus.

FIG. 2 is an explanatory diagram showing a range of reproducible color values.

FIG. 3 is an explanatory diagram showing an example of a matrix of 3 rows and 4 columns used for performing affine transformation.

FIG. 4 is an explanatory diagram showing an example of a matrix of 3 rows and 3 columns used for performing affine transformation.

FIG. 5 is an explanatory diagram showing a range of reproducible color values after affine transformation.

FIG. 6 is an explanatory diagram showing the relationship of each means of the present invention.

FIG. 7 is an explanatory diagram showing a relationship between respective means of a conventional method.

[Explanation of symbols]

1 chromatic value 2 affine transformation means for converting chromatic value into FGH coordinate system 3 FGH value 4 Look-up table for calculating C value from FGH value 5 Means for interpolating C value from FGH value. 6 C values of 8 points around the desired point. 7 Interpolated C value. 8 Look-up table for calculating M value from FGH value 9 Means for interpolating M value from FGH value. 10 M value of 8 points around the desired point. 11 Interpolated M value. 12 Look-up table for calculating Y value from FGH value 13 Means for interpolating Y value from FGH value. 14 Y values of 8 points around the desired point. 15 Interpolated Y value. 16 Look-up table for calculating C value from 16 color values 17 Means for interpolating C value from color value. 18 C value of 8 points around the desired point. 19 Interpolated C value. 20 Look-up table for calculating M value from color value 21 Means for interpolating M value from color value. 22 M value of 8 points around the point to be sought. 23 Interpolated M value. 24 Look-up table for calculating Y value from color value 25 Means for interpolating Y value from color value. 26 Y values of 8 points around the desired point. 27 Interpolated Y value. 100 Data space of color expression values specific to the output device. 101 Color value data space. 102 A circumscribed cube parallel to an axis that encloses a data space of color values. 103 A data space of the color expression value unique to the output device mapped to the FGH space. 104 A circumscribed cube parallel to the axis that encloses the data space of the color expression value specific to the output device mapped in the FGH space.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI Technical display location G09G 5/06 8121-5G H04N 1/46 9068-5C

Claims (1)

[Claims] 1. A conversion device including a conversion means for converting a color value into a color expression value specific to an output device by using a lookup table method with interpolation, and having an affine conversion means for the color value in a stage before the conversion means. A color value conversion device characterized by the above.