JP2000069305A - Color correction method for color image input output device and recording medium with color correction program recorded therein - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an accurate color correction method over the entire range of a color input output characteristics for a color image input output device. SOLUTION: Color input output characteristics of a color image input output device are measured, a range where an output value is not lost even when an input value is decreased (a low level saturation area 301), a range where an output value is not increased even when an input value is increased (a high level saturation area 305), and respective range (a non-saturation area 303) are respectively detected and separate input output characteristics (302, 304, 306) are obtained for each range, and color is corrected for each range of each color component, based on the input output characteristics obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image input / output device which corrects the color input / output characteristics inherent to the color image input / output device so that the same color can be input and output as much as possible. The present invention relates to a color correction method and a recording medium storing the color correction program.

[0002]

2. Description of the Related Art When a color image is input by a color image input device such as a digital still image camera, a color image scanner, and a video camera, the color input characteristics of each color image input device are different. May be slightly different from the actual color. Also, color CRT
Even in color image output devices such as displays, color liquid crystal displays, color plasma displays, and color printers, the color output characteristics of each color image output device are different, so the colors in the output color image differ from the original colors. It may be slightly shifted.

Conventionally, as a color correction method for such a color image input / output device, a method of performing correction called γ correction for each color component is generally used. For ease of explanation, here, the color is a combination of values indicating the intensity of the three primary colors of red (R), green (G), and blue (B), that is, (R, G,
B), and each of R, G, and B takes a value of 0 to 1. The combination (R, G, B) of the values of the three primary colors is called color data, and only red, only green, and only blue in the color data are called red, green, and blue components, respectively.

Hereinafter, a conventional color correction method will be specifically described by taking a color image input device such as a digital still image camera as an example.

First, a color image including a color whose color data is known is prepared, and the color data in the image is represented by (Ri, G
i, Bi) (i is a color data number, i = 1, 2,...)
, N, N is the total number of color data). When this color image is input from a color image input device, it is assumed that the input result is (ri, gi, bi) due to the color input characteristics of the device. (Ri, Gi, Bi) and (ri, gi, bi)
It is assumed that there is a relationship of Equation 1 between.

[0006]

(Equation 1) Here, αr, βr, γr,..., Γb are constants.

Here, the originally correct color data (Ri, G
i, Bi) and the corresponding color data (ri, gi, bi) (i is the number of the color data) as input results, the coefficients αr,
.beta.r, .gamma.r,..., .gamma.b are obtained by using a least square method or the like. That is, for the red component, coefficients αr, rr that minimize the sum of the squares of the differences between the result of conversion of the original color data Ri as shown in equation 2 by equation 1 and the input result color data ri βr and γr are determined. The same applies to the green component and the blue component.

[0008]

(Equation 2)

Next, the coefficients αr, βr, γ thus obtained
Using r,..., γb, by transforming Equation 1 to Equation 3 and giving ri, Ri is obtained. gi, Gi,
The same applies to bi and Bi, but is omitted here.

[0010]

(Equation 3)

When the color image of the original color data Ri is input by the color image input device, the input result is ri.
Ri is obtained, and color data in a correct color image is obtained, so that color correction can be performed on the red component. That is, the color input characteristic of the digital still image camera is approximated by Expression 1, and the color input characteristic (Expression 1) is canceled by converting the color data of the input result by the reverse characteristic (Expression 3). Color can be corrected. The same applies to the green component and the blue component.

[0012]

Although the above-described conventional method can perform a certain degree of correction, it is difficult to accurately perform color correction depending on the color input / output characteristics of a color image input / output device. there were. For example, some digital still image cameras and the like exhibit color input characteristics as shown by a solid line (201) in FIG. That is, there are devices having color input / output characteristics such that the output is slightly saturated with respect to the input when the value of the color data is small or large. Even when color correction is performed on such a color image input / output device by the above-described conventional method, it has been difficult to accurately perform color correction over the entire range of color data. This is because the conventional method described above expresses the color input / output characteristics of the color image input / output device with one conversion formula such as Expression (1) over the entire range of color data.
The color input / output characteristic of this device is, for example, a broken line (202) in FIG.
This is because color correction cannot be performed accurately even if color conversion is performed using the reverse characteristic of the approximated color input / output characteristic as a result.

The present invention solves such a problem, and FIG.
For a color image input / output device with color input / output characteristics such that the output is slightly saturated with respect to the input, an accurate color correction method and its color correction program are recorded over the entire range of color data. It is an object of the present invention to provide a recording medium that has been used.

[0014]

According to the present invention, the color input / output characteristics of a color image input / output device are measured, and the range of the input color data is set such that the output does not decrease even if the value of the input color data is reduced. (Low level saturation range), range of input color data values that does not increase output even if the value of input color data is increased (high level saturation range), range of other input color data (non-saturation range) And color correction is performed using a different color correction method for each of the three ranges. That is, for example, as shown in FIG. 3, for the same color input / output characteristics as 201 in FIG. The output characteristics are approximated, and similarly for 303 (unsaturated region), 30
In 4, the color input / output characteristics are approximated in 306 with respect to 305 (high-level saturation region). As a result, the color input / output characteristics can be accurately approximated over the entire range of the input color data. Therefore, by converting the input color data with the opposite characteristics, highly accurate color correction can be realized.

[0015]

FIG. 1 is a schematic block diagram showing an example of a hardware configuration for realizing a color correction method according to the present invention. In FIG. 1, reference numeral 10 denotes a color image input / output device to be subjected to color correction, and reference numeral 20 denotes data such as a keyboard and a display used for inputting color data corresponding to a color mark in addition to inputting commands and data and displaying the data on a monitor. I / O device, 3
Reference numeral 0 denotes a color input / output characteristic acquisition processing unit that obtains color input / output characteristics of the target color image input / output device 10. A color correction processing unit that performs color correction, 50 is a color correction parameter memory that holds input / output characteristic correction parameters, and 60 is a bus that connects these units. Here, the functions of the color input / output characteristic acquisition processing unit 30 and the color correction processing unit 40 are actually realized on a so-called CPU using a program and an internal memory.

Hereinafter, an object is divided into a color image input device and a color image output device, and in each case, an embodiment of the color correction processing according to the present invention will be described.

[Color Correction of Color Image Input Device] FIGS. 4 and 5 show a flowchart of one embodiment of the color input characteristic correction process of the color image input device according to the present invention. FIG. 4 shows a process for obtaining a color input characteristic correction parameter of the color image input device. FIG. 5 shows a color correction of the color image input device using the color input characteristic correction parameter obtained in the process of FIG. This is the process of performing

Before starting the processing, first, a color chart image (color patch) used for obtaining a color input characteristic correction parameter of a color image input device is prepared. This is created by selecting tens to hundreds of colors from a color chart whose original color data such as a JIS standard color chart is known. At this time, experiments and the like are performed in advance, and the type and number of colors are determined so that the color input characteristics can be corrected as accurately as possible. Fig. 6 shows an example of a color chart image
Shown in If the required color cannot be included in one color patch image, the image is divided into a plurality of color patch images.

The color data of the color chart, including the JIS standard color chart, is given by numerical values (X, Y, Z) represented by a color expression method generally called a CIEXYZ color system. here,
This (X, Y, Z) is converted into color data (L) represented by a color expression method called CIE (L ^* , a ^* , b ^* ) uniform perceived color space.
^* , A ^* , b ^* ).
(X, Y, Z) to (L ^* , a ^* , b ^* ) can be converted by Equation 4 (for example, edited by The Japan Society of Color Science,
"New Color Science Handbook", published by The University of Tokyo Press,
1980, 7th edition, page 142).

[0020]

(Equation 4)

Here, (X0, Y0, Z0) is the color data of the light used as the reference white light, and is generally a C light source or a D65 light source. For example, in the case of a C light source used as a reference white color in the NTSC color television system or the like, the following values are used.

X0 = 98.072 Y0 = 100.000 Z0 = 118.225 The original color data of the color in the color chart image represented by CIE (L ^* , a ^* , b ^* ) is expressed by (Li, Ai, Bi).
Here, i is the number of the color in the color chart, i = 1, 2,.
, N. N is the total number of colors in the color chart.

The prepared color chart image (color patch) is input by a color image input device, and color data (l ^* , a ^* , b ^* ) represented by CIE (L ^* , a ^* , b ^* ) of each color drawn in the input image.
i, ai, bi) are obtained (step 401).

In general, the color of a color image input by a digital camera or a scanner is represented by (R, G, B) as described in the related art. This (R, G, B) can be easily converted to CIE (L ^* , a ^* , b ^* ). Specifically, when the C light source is set to the reference white,
From (R, G, B), according to the following equation 5, X = 0.6767R + 0.1736G + 0.2001B Y = 0.2988R + 0.5868G + 0.1144B Z = 0.0661R + 1.1150B (Formula 5) Color data of CIEXYZ color system (X, Y, Z), which can be further converted from (X, Y, Z) to (L ^* , a ^* , b ^* ) by Equation 4 (for example, "Handbook of New Color Science", published by The University of Tokyo Press, 1980, 7th edition, p. 936).

Next, the color data (li, a
i, bi) original color data (Li, Ai, Bi)
Is input (step 402). This is performed, for example, by preparing a color data table in which values of color data corresponding to colors of a color chart are described, and inputting a value from a keyboard or the like while the user looks at the color data table. Alternatively, color data corresponding to the color of the color chart is stored in advance in a ROM or the like, and
Color data may be read from an OM or the like.

Next, the original color data (Li, Ai, B
The relationship between i) and the input color data (li, ai, bi) is examined, and for each color component, a low level saturation range, a high level saturation range,
An unsaturated region is detected (step 403).

The CIE (L ^* , a ^* , b ^* ) a * component (hereinafter referred to as the a component; the L ^* and b ^* components are also referred to as the L component and the b component, respectively) will be specifically described as an example. I do. The value of the a component (Ai) of the original color data and the value of the a component (ai) of the input color data for that color are plotted on a graph as shown in FIG. Next, when the minimum value of the a component Ai of the original color data is AL and the a component of the input color data corresponding to this AL is aL, a predetermined threshold value is set to T, and Expression 6 is satisfied. Find the largest Aj, find the range from AL to Aj (AL ≦ Ai <Aj) for Ai, and a for ai
Let aj (aL ≦ ai <aj) from L be a low-level saturation range. (Aj-aL) / (Aj-AL) <T (Equation 6)

Further, the maximum value among the a component Ai of the original color data is AH, and the maximum value of the input color data corresponding to the AH is a
When the component is aH, a predetermined threshold value is T,
The minimum Ak that satisfies Equation 7 is searched for, and the high-level saturation range is set for Ai from Ak to AH (Ak <Ai ≦ AH) and for ai from ak to aH (ak <ai ≦ aH). (AH-ak) / (AH-Ak) <T (Equation 7)

The range excluding the low-level saturation range and the high-level saturation range, that is, the range from Aj to Ak (Aj ≦ Ai ≦ Ak) for Ai, and the range from aj to ak for ai
(Aj ≦ ai ≦ ak) is defined as an unsaturated region.

Similarly, for the L component and the b component, the ranges of the low level saturation range, the high level saturation range, and the non-saturation range are obtained.

Next, for each color component, the color input characteristics are estimated for each of three ranges of a low-level saturation region, a high-level saturation region, and a non-saturation region (step 404).

This is because, for example, by using a method called γ correction described in the background art, the relationship of equation 8 is established between the a component Ai of the original color data and the input color data ai in the low level saturation region. And the coefficients αaL, βaL, γ
aL is obtained by the least square method or the like.

[0033]

(Equation 5)

When the coefficients αaL, βaL, and γaL are determined, the color input characteristic of the color image input device is canceled by converting the a component ai of the input color data by Expression 9 obtained by modifying Expression 8. Can be obtained.

[0035]

(Equation 6)

The same applies to the L component and the b component, and the coefficients for those components are represented by αiL and βi, respectively.
L, γiL and αbL, βbL, γbL.

The color input characteristics of the high-level saturated region and the non-saturated region are estimated by the same formula. The coefficients of the conversion equation corresponding to equation 8 in the high-level saturation region are αlH, βlH, γl
H, αaH, βaH, γaH, αbH, βbH, γbH
And ΑM, βl
M, γlM, αaM, βaM, γaM, αbM, βb
M and γbM.

As a result, as shown in FIG. 8, for each color component, a range of a high-level saturated range, a non-saturated range, and a low-level saturated range and a coefficient of a color input characteristic in the range are determined. This is the color input characteristic of the input device.

Next, the low-level saturation range, high-level saturation range, and non-saturation range and color input characteristics (correction parameters) for each color component obtained in step 404 are stored in a memory (step 405). Specifically, each numerical value shown in FIG. 8 is stored.

Next, with reference to FIG. 5, a description will be given of a color correction process when the color image input device is operated.

First, the color input characteristic correction parameter acquired in the color input characteristic acquisition processing shown in FIG. 4, specifically, each numerical value shown in FIG. 8, is read (step 501).

Next, the actual color data (l, a, b) input by the color image input device is read (step 502).

Next, steps 503, 504, 505,
At 506, the color correction of the L component of the color data (l, a, b) is performed.
In steps 507, 508, 509, and 510, the color correction of the a component is performed, and in steps 511, 512, 513, and 514, the b correction is performed.
Performs component color correction.

For example, the correction of the component a is performed as follows. First, in step 507, the color data (l, a, b)
Is a low level saturation range (aL ≦ a ≦ aj), a non-saturation range (aj <a <ak), or a high level saturation range (ak ≦
a? aH) is checked, and a color correction method for the a component is selected according to the result. In this embodiment, the coefficient used for the color correction of the component a is a low-level saturation area coefficient (α
aL, βaL, γaL), coefficients for the unsaturated region (αaM, β
aM, γaM), high-level saturation region coefficients (αaH, βa
H, γaH). Next, the color correction of the component a is performed using the selected color correction method, that is, the color correction coefficient. For example, when aL ≦ a ≦ aj, the color correction coefficients (αaL, βaL, γaL) in the low-level saturation region are selected, and the value of the red component corrected by Expression 9 is obtained (step 508). Steps 509 and 510 are the same.

Finally, the color-corrected color data is output (step 515). That is, the color input characteristics of the color image input device are corrected by using the corrected color data as the color data input by the color image input device instead of the color data input directly by the color image input device. , A color close to the true color data can be obtained.

In this embodiment, the color data is represented by CIE (L ^* ,
a ^* , b ^* ) for color correction.
Instead of (L ^* , a ^* , b ^* ), a color expression method (R, G, B) used in the description of the related art or a color expression method based on the CIEXYZ color system can be used.

[Color Correction of Color Image Output Device] The color correction of the color image output device can be similarly realized by the following slight changes. That is, the color output characteristics of the target color image output device are acquired by the flowchart shown in FIG. 8 instead of the color input characteristic acquisition processing flowchart of FIG.

First, a color chart for examining the color output characteristics is prepared, and in step 901 the original color data (Li, A) of the color drawn in the color chart as in step 402 of FIG.
i, Bi) (i is a color number, i = 1, 2,..., N.
N is the number of colors in the color chart) using a keyboard or the like. Also in this case, the original color data drawn in the color chart may be input from a ROM or the like in which color data corresponding to the color of the color chart is stored in advance.

Next, in step 902, a color image including the true color data (Li, Ai, Bi) input in step 801 is output by the image output device, and the colors in the output color image are measured. The color data (li, ai, bi) is input by measuring with a measuring device capable of obtaining accurate color data, such as a meter.

Next, steps 903, 904, 905
Then, on the basis of the true color data (Li, Ai, Bi) and the measured color data (li, ai, bi), the same processing as in steps 403, 404, and 405 in FIG. , Color output characteristics (color output characteristic correction parameters) of a low level saturation region, a high level saturation region, and a non-saturation region. next,
In step 905, the obtained color output characteristic correction parameters are stored in the memory.

When the image output device is actually operated, a color correction process shown in FIG. 5 is performed based on the obtained color output characteristics. That is, all the color data in the color image to be output is color-corrected according to the processing procedure of FIG. 5, and the color data after the color correction is output by the color image output device. A change in color due to color output characteristics can be canceled in advance and a correct color can be output.

In this embodiment, the color data is represented by CIE (L ^* ,
a ^* , b ^* ), but CIE (L ^* , a ^* , b)
Instead of ^* ), a color expression method (R, G, B) used in the description of the related art or a color expression method using the CIEXYZ color system can be used.

Although the embodiment of the present invention has been described above, the processing procedures shown in FIGS. 4, 5 and 9 correspond to a computer-readable recording medium such as a floppy disk or a memory card as a color correction program. It can be recorded on a compact disk (CD-ROM) and provided.

[0054]

As described above, according to the present invention,
In a color image input / output device, an input or output range where the color input / output characteristics are saturated can be detected, and appropriate color correction can be performed in each of a saturated region and a non-saturated region. More accurate color correction is possible in the entire range of.

Inexpensive digital cameras have poor color reproducibility
Especially, from colors with gradually lower saturation (color vividness)
Inputting colors that change to high colors with a digital camera
And the saturation of the color of the input result is saturated in the high saturation part.
There is a tendency to go around. Digital with such color input characteristics
Color camera as described in the prior art.
Even if color correction is performed by the correct method, the above-mentioned saturation
Did not perform accurate color correction. On the other hand, if the color is (L^*,
a^*, B^*), The color saturation is almost a^*Or
b^*It is represented by the size of Therefore, the aforementioned embodiment
As described above, a^* Or b^*Input / output relationship
Color correction using another formula etc.
Digital cameras with poor color reproduction
Color correction can be performed more accurately than technology.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of a hardware configuration for implementing a color correction method of the present invention.

FIG. 2 is a diagram illustrating color input / output characteristics of a color image input / output device.

FIG. 3 is a diagram showing color input / output characteristics when the present invention is applied.

FIG. 4 is a flowchart of one embodiment of a color input / output characteristic acquisition process according to the present invention.

FIG. 5 is a flowchart of an embodiment of a color correction process according to the present invention.

FIG. 6 is a diagram illustrating an example of a color chart.

FIG. 7 is a diagram showing a relationship between original color data and input color data.

FIG. 8 is a diagram showing an example of a color input characteristic correction parameter obtained by the present invention.

FIG. 9 is a flowchart of one embodiment of a color output characteristic acquisition process according to the present invention.

[Explanation of symbols]

 Reference Signs List 10 color image input / output device 20 data input / output device 30 color input / output characteristic acquisition processing section 40 color correction processing section 50 color correction parameter memory 301 low level saturation area 303 non-saturation area 305 high level saturation area 302, 304, 306 Output characteristic curve

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshihiro Ochiai 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Co., Ltd. No. Nippon Telegraph and Telephone Corporation F-term (reference) LB00 LB02 MA01 NA03

Claims (2)

[Claims] 1. A color correction method for correcting input color data or color data to be output according to color input / output characteristics of a color image input / output device, wherein the color input / output characteristics of the color image input / output device are measured. For each of the color components, a range in which the change in the output value with respect to the change in the input value is smaller than a predetermined rate of change, and other ranges are detected, and separate input / output characteristics are obtained for each of those ranges. A color correction method for a color image input / output device, wherein color correction is performed separately for each range according to input / output characteristics. 2. A recording medium storing a color correction program for correcting color data input or output in accordance with color input / output characteristics of a color image input / output device, comprising: A process for measuring the color input / output characteristics of the device, a process for detecting, for each color component, a range in which a change in the output value with respect to a change in the input value is smaller than a predetermined rate of change, and a process in which the other range is detected. A color process, comprising: a process for obtaining input / output characteristics separately for each component and each range; and a process for separately performing color correction for each range based on the obtained input / output characteristics for each color component. A recording medium that stores a color correction program for an image input / output device.