JPH09219800A - Color image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize coincidence in color signal suitable for an observation state by detecting the observation states of an original image and a reproduction image and obtaining the degrees of adaptation states in observing the both pictures. SOLUTION: A scanner RGB signal is converted into a (X1, Y1 and Z1) signal in a color converting part 1. An observation condition correction processing part 5 converts a measured value (X1, Y1 and Z1) under a standard light source into (X1', Y1' and Z1') under an actual light source. The observation states are detected 8, an adaptation correcting part 9 executes a correction processing based on the adaptation degrees in accordance with the observation states and a correction value (X1", Y1" and Z1") is outputted. the color converting part 2 converts (X1", Y1" and Z1") into a monitor display signal RGB through the use of a color conversion parameter Pm so as to display it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image processing device for performing preferable color reproduction among various color image input / output devices.

[0002]

2. Description of the Related Art A color matching method using device-independent colors to easily and accurately reproduce colors between different color image input / output devices such as color scanners, color printers, and color monitors. Is being studied. This is to convert the color signal of the input device into a standard color signal that is irrelevant to the device, such as the tristimulus value recommended by the CIE, and then convert the converted standard color signal into a color signal suitable for the output device. It is a system that converts and outputs.

In such a matching system, a device-independent color signal is treated in the middle, and a signal suitable for human visual characteristics such as CIE 1931 XYZ signal is used as a standard signal, so that various devices can be used. In, it is possible to perform colorimetrically consistent color reproduction.

FIG. 4 shows a conventional color matching method. The color conversion unit 1 converts the scanner RGB signal into an XYZ standard signal using the color conversion parameter Ps, and the color conversion unit 2 converts the XYZ standard signal into a monitor RGB signal using the color conversion parameter Pm. When outputting to the printer, the color conversion unit 3 converts the XYZ standard signal into a printer output YMCK signal using the parameter Pp and outputs the YMCK signal.
When outputting the monitor display image with a printer, R
After converting the GB signal into the XYZ signal, the color conversion unit 3 performs YM
Convert to CK signal.

As the above-mentioned conventional method, for example, when the scanner RGB signal is converted into an XYZ signal by a matrix operation and is output to the printer, the XYZ signal is CMY.
There is also a color image forming apparatus that converts the signal into a signal and prints out a monitor display image in the same manner, which relays and outputs the XYZ signal (see Japanese Patent Application Laid-Open No. 1-103445). Further, a color correction method has also been proposed in which the standard signal is corrected by a predetermined function to easily achieve color matching under various observation light sources (Japanese Patent Application No. 7-1918).
No. 39).

[0006] However, human visual characteristics are easily affected by various observation environments, and it may not be sufficient to just match the standardized signals such as the XYZ tristimulus values. For example, when observing an image on a reflective original document under various light sources, even if the observation light source changes (that is,
It is known that even if the XYZ tristimulus values change, humans have a visual characteristic called color constancy, and therefore the appearance of an image does not change much.

Therefore, as a method of predicting the appearance of color when the observation light source changes, there is a chromatic adaptation prediction formula proposed by von Kries and Naya et al. There is an image input / output device that incorporates these prediction formulas into color conversion between a monitor and a reflection original to correct changes in the adaptation state during observation of an original image and observation of a reproduced image (see Japanese Patent Laid-Open No. 6-28437). ).
As a result, even when the original image and the reproduced image are observed under different observation light sources, it is possible to reproduce the colors with the same color appearance.

[0008]

However, the above-described conventional color conversion method assumes a state in which the observation light source of each image is perfectly adapted when observing the original image or the reproduced image. Therefore, the appearance of colors does not always match depending on the observation state. For example, when a reflective original is placed right next to the image displayed on the monitor for evaluation, there is almost no change in the adaptation state between the two. You can realize the appearance.

On the other hand, when an image is edited while observing the monitor for a long time and then output to a printer and the output image is observed under the illumination of a general fluorescent lamp, the monitor observation and the printer output image observation are performed. At times, the adaptation state of the observer changes, so it becomes necessary to correct the change in the adaptation state.

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to detect the observation state of an original image and a reproduced image to determine the degree of adaptation state when observing both images. Another object of the present invention is to provide a color image processing device that realizes matching of appearance of colors suitable for the observing state.

Another object of the present invention is to provide a color image processing apparatus which gives specific information of an observation state to be detected.

Further, for example, when observing a monitor having a color temperature of 9300K, when the monitor is first seen, the white of the monitor looks pale, but it seems that the color gradually becomes bluish over time. in this way,
Conventionally, an image of constant intensity has always been displayed on a monitor, even though the color appearance on the monitor changes with time. As a result, a phenomenon occurs that even if the user initially intended to specify a pale color, it will appear whitish over time. In other words, there is a problem that the impression of the displayed image varies depending on the observation time of the monitor.

Therefore, another object of the present invention is to provide a color image processing apparatus which always provides a constant color appearance regardless of the observation time.

Conventionally, in order to correct chromatic adaptation, it was necessary to use a white reference colorimetric value in the observation environment of the original image and the reproduced image. However, it is difficult to accurately grasp the reference white color of a reflection original that is output on paper because it largely depends on the observation state. In other words, the light that illuminates the original document is affected by the reflected light from the surrounding walls and the like in addition to the indoor illumination light, so it is not possible to know what the actual observation light source is. Therefore, for example, there is a method of obtaining a colorimetric value of paper in an actual observation environment by using a radiance meter or the like, but such a colorimeter is generally very expensive, so that it is not practical for general users. It cannot be an effective means.

Therefore, another object of the present invention is to provide a color image processing apparatus capable of easily detecting the white colorimetric value of paper under an actual observation environment by capturing the white colorimetric value of paper as a reference white color. To do.

[0016]

In order to achieve the above object, in the invention according to claim 1, means for outputting a first device signal corresponding to an original image, and second means for outputting the first device signal In the color image processing apparatus, which includes means for converting into a device signal and a means for reproducing an image based on the second device signal, means for detecting an observation state of the original image and the reproduced image, and And a means for performing chromatic adaptation correction based on the degree of adaptation.

According to a second aspect of the invention, the means for detecting the observation state detects the observation time of the original image and the reproduced image.

According to the third aspect of the invention, the image on the monitor is corrected and displayed again according to the observation time of the color monitor.

According to a fourth aspect of the present invention, the chromatic adaptation correction means converts the XYZ tristimulus values by a predetermined function based on the degree of adaptation and the reference white color of the first and second devices. It has a feature.

In a fifth aspect of the present invention, when the target device is a color monitor, the reference white color is the brightest color displayed on the monitor, and when the target device handles a reflective original, a large number of images displayed on the monitor are displayed. Of the color patches
The feature is that the reference white color is determined based on the display color of the patch closest to the white background of the document.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be specifically described below with reference to the drawings. FIG. 1 shows the configuration of an embodiment of the present invention. The figure shows a configuration for performing color conversion corresponding to environmental conditions, and shows an example in which a color scanner, a color monitor, and a color printer are connected. Other devices such as a still camera may be connected instead of the color scanner.

In this embodiment, the standard signal (CIE 1931
Color conversion units 1, 2, 3 for performing color conversion to XYZ, etc.,
The storage units 4, 6, 1 storing the color conversion parameters
1, an observation condition correction processing unit 5 for converting a standard signal according to an observation condition, an image information storage device 7 for storing display data on a monitor, an observation state detection device 8 for detecting an observation state, and an observation state It is composed of an adaptation correction unit 9 that performs an adaptation correction process according to the above, and a reference white data setting processing unit 10.

First, the case of inputting a document from the scanner and displaying an image on the monitor will be described. The scanner signal RGB is input to the color conversion unit 1. The color conversion unit 1 loads the color conversion parameter Ps from the storage unit 4, and uses Ps to convert the scanner RGB signal into an XYZ signal (X1, Y1, Z1) that is a standard signal. The color conversion method uses a matrix operation, a memory map interpolation method, or the like. Further, in order to improve the conversion accuracy of the scanner input signal and the XYZ signal, gamma conversion may be performed before color conversion. The XYZ signal is, for example, an XYZ signal when an original is observed under a light source having a color temperature D50.

Next, the observation condition correction processing unit 5 converts the colorimetric values (X1, Y1, Z1) under the standard light source into X under the actual light source.
It is converted into a YZ signal (X1 ′, Y1 ′, Z1 ′). Then, the image data after the observation condition correction processing is temporarily stored in the image information storage device 7 such as a hard disk or a memory.

Next, the adaptation correction unit 9 receives the data stored in the image information storage device 7 and performs a correction process according to the degree of adaptation to the monitor to obtain a correction value (X1 ",
Y1 ″, Z1 ″) is output. Finally, in the color conversion unit 2, the color conversion parameter Pm is loaded from the storage unit 11,
Pm is used to convert (X1 ″, Y1 ″, Z1 ″) into the monitor display signal RGB for display, which enables color matching according to the adaptation state of the observer.

In the above configuration, the image information after the observation condition correction processing is temporarily stored in the storage device, but the image signal may be directly sent to the adaptation correction section.

Similarly, when the monitor display image is output to the printer, the monitor RGB signals are X-rayed by the color conversion section 2.
YZ signals (X2 ″, Y2 ″, Z2 ″) are converted. This is the reverse conversion of the conversion from XYZ signals to monitor RGB signals described above. Next, (X2 ″, Y2 ″, Z2 ″) is adapted. X that is corrected according to the degree and is actually output by the printer
It is converted into a YZ signal (X2 ′, Y2 ′, Z2 ′) and temporarily stored in the image information storage device 7. (X2 ', Y2', Z
2 ′) represents an XYZ signal under an actual observation light source, and therefore this is X when the observation condition correction processing unit 5 observes the output image of the color printer under the observation light source of the color temperature D50.
Convert to YZ signals (X2, Y2, Z2).

In the color conversion unit 3, (X2, Y2, Z2) is CMYK of the printer (or CM in the case of a three color printer).
Y) color conversion. That is, the color conversion parameter Pp
Is XYZ and C of the print output under the observation light source of D50.
Only the parameter indicating the correspondence between the MYK signals is held. The color printed out looks like the color (X2, Y2, Z2) under the observation light source of D50, but this is (X2 ', Y) under the actual observation light source.
2 ', Z2'). Even in the above configuration, the output of the adaptation correction unit may be directly sent to the observation condition correction unit.

The observation condition correction processing section 5 in FIG. 1 has a function of converting an XYZ tristimulus value under a standard light source into an XYZ tristimulus value under an observation environment. , The previously proposed color image processing device (Japanese Patent Application No. 7-1
91839). The function of the previously proposed observation condition correction processing unit will be described below.

Generally, the color appearance (XYZ values) of an object under a certain observation light source is the spectral distribution characteristic P (λ) of illumination light, the spectral reflection characteristic R (λ) of the object, and the color matching function x (λ). , Y
It can be obtained by the following equation using (λ) and z (λ).

X = k∫visR (λ) · P (λ) · x (λ) dλ Y = k∫visR (λ) · P (λ) · y (λ) dλ Z = k∫visR (λ) · P (λ) · z (λ) dλ where ∫vis represents the integral in the visible wavelength range.

Therefore, the color appearance when the illumination light changes when the color matching function is regarded as constant is X '= k∫vis
R (λ) · P ′ (λ) · x (λ) dλ Y ′ = k∫visR (λ) · P ′ (λ) · y (λ) dλ Z ′ = k∫visR (λ) · P ′ ( λ) · z (λ) dλ.

The above equation is expressed as follows: X '= k∫vism (λ) R (λ) P (λ) x
(Λ) dλ Y ′ = k∫vism (λ) · R (λ) · P (λ) · y
(Λ) dλ Z ′ = k∫vism (λ) · R (λ) · P (λ) · z
It can be replaced with (λ) dλ.

Now, S (λ) = R (λ) · P (λ) · x
If (λ), then X ′ = k∫vism (λ) · S (λ) dλ Y ′ = k∫vism (λ) · S (λ) dλ Z ′ = k∫vism (λ) · S (λ) It becomes dλ.

Therefore, the conversion from XYZ to X'Y'Z 'is as follows: X' = ∫vism (λ) S (λ) dλ = Fx {∫visS (λ) dλ} = Fx (X) Y '= ∫vism (λ) · S (λ) dλ = Fy {∫visS (λ) dλ} = Fy (Y) Z ′ = ∫vism (λ) · S (λ) dλ = Fz {∫visS (λ) dλ} = Fz (Z) may be performed.

Looking at the above conversion formula, even if the value of X is the same, if S (λ) is different, the correspondence relationship of X'is also changed. Therefore, the conversion relationship cannot be defined in a strict sense. Shows. However, as a result of comparing the XYZ values under various observation light sources of various color patches, it was found that a large error does not occur even if the above conversion is approximated by a simple function. Therefore, the observation condition correction processing unit performs the above conversion based on the approximation function.

An example of the relationship between the light source and the tristimulus values is shown in FIG. The figure shows the result of comparing the XYZ tristimulus values when the color patch is measured in the light source mode of the reference light source D50. The horizontal axis represents the standard signal value at the reference light source (D50), and the vertical axis represents the standard signal value at the observation light source (D65). According to the comparison of the colorimetric values, the colorimetric values XYZ under the D50 light source and the colorimetric values X'Y'Z 'under the D65 light source can be approximated by the following relational expressions.

X ′ = ax · X Y ′ = ax · Y Z ′ = az · Z Therefore, in the previously proposed invention, the above-described linear function is used to correct the colorimetric value due to the change of the observation light source. Do it. It should be noted that a relatively low-order conversion equation such as a quadratic equation or a cubic equation is used in order to further improve accuracy.

Next, the operation of the adaptation correction unit, which is a feature of the present invention, will be described. Here, the basic chromatic adaptation correction method will be described based on the chromatic adaptation equation proposed by von Kries, but the same applies when other chromatic adaptation equations are used.

The von Kries chromatic adaptation equation is expressed by the following equation.

[0041]

[Equation 1]

Here, (M) represents a 3 × 3 matrix obtained by the chromaticity coordinates of the basic primary colors. [R
d, Gd, Bd] is the RGB of the reference light when observing the reproduced image.
The tristimulus values, [Rs, Gs, Bs], represent the RGB tristimulus values of the reference light when observing the original image. That is, if the reference light at the time of observing the monitor is the white of the monitor and the reference light at the time of observing the reflected original is the white of the paper, the above equation corresponds to a process of converting the white of the monitor into the white of the paper. This corresponds to the fact that humans feel the same as white, even though the white of the monitor and the white of the paper are colorimetrically different.

By the way, depending on the observation state of the observer,
I don't always feel that monitor white is white. That is, when the monitor begins to be observed, it is not very adapted to the monitor and is mostly adapted to ambient light. Therefore, the white color of the 9300K color temperature monitor is felt pale. However, with the passage of time, the monitor is adapted to the removal, and when it is completely adapted to the monitor, the bluish white color of the monitor becomes white.

Therefore, in the present invention, by adding a parameter of the degree of adaptation to the chromatic adaptation prediction formula, it is possible to reproduce colors in which the appearance of colors is more consistent. The chromatic adaptation equation will be expressed by the following equation.

[0045]

[Equation 2]

In the above equation, K is a parameter representing the degree of chromatic adaptation, K = 1 when in the adaptation state when observing the reproduced image, and K = when in the original image observation state.
It becomes 0. As a result, if the degree of color adaptation and the tristimulus value of the reference light are accurately known, it is possible to guarantee a highly accurate color appearance.

The adaptation correction unit of the present invention detects the observation state to obtain the degree of chromatic adaptation, and based on the degree of adaptation and the set reference white color, performs chromatic adaptation correction according to the above formula. It is characterized by doing.

First, how to determine the degree of chromatic adaptation will be described. Here, it is assumed that the state in which the user is accommodating in the room is considered to be the reference acclimatization state, and the state in which he or she is accommodating to the monitor is assumed to have undergone an adaptation change. In this case, in the above equation, [Rs, Gs, Bs] is the RG of the reference light in the room.
B basic tristimulus values are represented, and [Rd, Gd, Bd] represent RGB basic tristimulus values of monitor reference light. And
The adaptation degree K is regarded as changing according to the monitor observation time, and the observation state detection device 8 measures the monitor observation time Tm and calculates K based on Tm. For example, a camera may be installed above the monitor to detect whether there is an observer in front of the monitor, or the time during which the mouse or keyboard is operated may be measured. Then, K is converted by a conversion function having Tm as an input.

K = f (Tm) The function f is, for example, a function such as K = αTm / (αTm + 1) (α is a parameter).

In the above example, the degree of chromatic adaptation changes according to the observation time of the monitor, but it is also possible in principle to add the distance from the monitor to the viewpoint to the parameter. .

Since the degree of chromatic adaptation can be obtained by the method described above, chromatic adaptation correction can be performed by obtaining tristimulus values of the reference light when observing the original image and the reproduced image. The method will be described below.

As the tristimulus value of the reference light (or reference white) of the monitor, the tristimulus value of the monitor white is basically used. For example, the monitor white XYZ tristimulus values are R = G = B = 25
The XYZ tristimulus values at 5 are calculated by the color conversion unit 2 in FIG. From the XYZ tristimulus values [Rd,
Gd, Bd] is calculated by the following equation.

[0053]

(Equation 3)

However, [Xmw, Ymw, Zmw] represents the XYZ tristimulus values of the monitor white.

On the other hand, there are two methods for determining the reference light (or reference white) for the reflection original. One is a method of accommodating the lighting in the room, and the other is a method of accommodating the paper white of the reflection original. Considering the actual adaptation state, it is considered that neither of them is a very complicated adaptation, but here, the method of determining the white reference according to the above two methods will be described.

In the case of adapting to the room illumination, which is the first method, the reference white color can be set by obtaining the chromaticity of the illumination light. The chromaticity of room light can be measured using a radiance meter or the like, or can be selected from a list of illumination light stored in advance by designating the type of illumination.

In the second method, which is adapted to the paper white of the reflection original, the tristimulus value of the reflected light of the paper white is obtained. A radiance meter can be used to obtain the tristimulus value of paper white as in the case of illumination light, but the reference white can be set by a simpler method. That is, as shown in FIG. 3, some color patches are displayed in advance on the monitor screen, and the operator selects a patch having the same color as paper white. Then, the tristimulus value of paper white is obtained from the monitor RGB signals of the patch that is considered to have the same color. Conversion from RGB to tristimulus values can be realized by using the processing method of the color conversion unit 2 in FIG.

By the above method, it is possible to perform image conversion according to the degree of adaptation. By applying this method, it is possible to provide an image display device that always guarantees a constant color appearance. For example, when observing a monitor having a color temperature of 9300K, the white of the image looks pale. Therefore, the image displayed at that time also looks pale. However, as the adaptation to the monitor gradually progresses, the bluishness of the displayed image is removed. As described above, it is often inconvenient for the observer that the impression of the image varies depending on the observation time. Therefore, by changing the display image to paler depending on the observation time, it is possible to provide a function of continuing to display an image having the same impression as that at the beginning of observation. That is, by storing the display image in the case of adapting to the room illumination in the image information storage device of FIG. 1, performing the adaptation correction according to the monitor observation time, calculating the monitor display signal, and displaying it. realizable.

[0059]

As described above, according to the first aspect of the invention, the observation states of the original image and the reproduced image are detected, and the degree of color adaptation according to the observation states of the original image and the reproduced image is determined. Since the color adaptation correction process is performed on the basis, it becomes possible for the observer to reproduce colors.

According to the second aspect of the invention, since the observation time of the original image and the reproduced image is detected, it is possible to reproduce the color in consideration of the change in the color adaptation state due to the observation time.

According to the third aspect of the invention, since the image on the monitor is corrected and displayed again according to the observation time of the color monitor, the color appearance of the displayed image is always kept constant.

According to the fourth aspect of the invention, the XYZ tristimulus values are converted by a predetermined function based on the degree of chromatic adaptation, the reference white of the first device and the reference white of the second device. Since color adaptation correction processing is performed, it is possible to perform color reproduction in which the appearance of colors matches according to the observation environment.

According to the fifth aspect of the invention, when the target device is a color monitor, the reference white color is the brightest color of the monitor display color, and when the target device handles a reflective original, it is displayed on the monitor. Since the reference white color is determined based on the display color of the patch that is closest to the white background of the original among the large number of color patches, it is possible to set the white reference suitable for the observation environment.

[Brief description of drawings]

FIG. 1 shows a configuration of an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a light source and tristimulus values.

FIG. 3 is a diagram illustrating a reference white setting process.

FIG. 4 shows a conventional color matching method.

[Explanation of symbols]

 1, 2, 3 color conversion unit 4, 6, 11 color conversion parameter storage unit 5 observation condition correction processing unit 7 image information storage device 8 observation state detection device 9 adaptation correction unit 10 reference white data setting processing unit

Claims (5)

[Claims] 1. A means for outputting a first device signal corresponding to an original image, a means for converting the first device signal into a second device signal, and an image based on the second device signal. A color image processing apparatus having a means for reproducing is provided with means for detecting an observation state of the original image and the reproduced image, and means for performing color adaptation correction based on the degree of adaptation according to the observation state. Characteristic color image processing device. 2. The color image processing apparatus according to claim 1, wherein the means for detecting the observation state detects the observation time of the original image and the reproduced image. 3. The color image processing apparatus according to claim 2, wherein the image on the monitor is corrected and displayed again according to the observation time of the color monitor. 4. The chromatic adaptation correction means uses XY based on the degree of adaptation and the reference white color of the first and second devices.
The color image processing apparatus according to claim 1, wherein the Z tristimulus value is converted by a predetermined function. 5. When the target device is a color monitor, the reference white color is the brightest color displayed on the monitor, and when the target device handles a reflective original document, among the many color patches displayed on the monitor, 5. The color image processing apparatus according to claim 4, wherein the reference white color is determined based on the display color of the patch closest to the white background of the document.