JP3140269B2 - Color space processing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a color space processing method for an image, and more particularly to a color space processing method for reading an image, converting the read image information into color space data, and performing color processing on the image.

[0002]

2. Description of the Related Art When making color photographic prints, color copies, various prints, or displaying a color image on a display, a gray balance can be obtained.
2. Description of the Related Art Adjusting an image or a screen to a desired color balance by adjusting a color density or a luminance is performed.

[0003] For example, in the production of a color photographic print or color copy, if the produced photographic print or copy has a reddish tint as a whole, it is printed again through a color filter absorbing red. ing. Also, when displaying a color image on the display,
When the entire screen is reddish, the output of the electron gun that excites the red phosphor among the three electron guns corresponding to the three primary colors of red (R), green (G), and blue (B) Is smaller than the others, it is possible to adjust the overall color balance.

However, these color processing methods are performed by adjusting an operation amount for color processing such as color density and luminance by adjusting three characteristic curves corresponding to the three primary colors.
In the case of color photographic printing and color copying, trial and error is performed based on skilled intuition and experience, and when a color image is displayed on a display, a complicated control circuit is required.

In recent years, instead of the above-described color processing method using mechanical and electrical operations, color information of pixels forming an image is converted into a numerical value and converted into a coordinate system called a color space. A color processing method (hereinafter, referred to as color space processing) has been proposed in which a mathematical operation is performed in a space, and the operation result is reduced to the original pixels to adjust the color of an image. In this color space processing method, an object is measured, and the color (object color) of the object is expressed by numerical data (coordinates) composed of chromaticity (saturation, hue) and lightness, and this numerical data is This is a method in which the original object color is changed and reproduced by calculating in a coordinate system (color space) consisting of chromaticity and lightness.

More specifically, the system comprises a series of processes shown in FIG. 3, in which an original 10 such as a photographic film or a printed matter is read by an input means 20 such as a scanner or a TV camera.
The color data (light quantity, luminance, etc. of each color light) for each pixel constituting the image of the document 10 is obtained, and the color data is subjected to an appropriate calculation by the processing system 30, and the calculation result is output to the printer 40 for printing. 50 is created or displayed on the CRT 70.

The image data read by the input means 20 is data read by an optical system such as a scanner or a TV camera (hereinafter referred to as optical data), and cannot be directly handled in a color space. . Therefore, the processing system 30 first converts the optical data into data suitable for processing in a color space, for example, a CI defined by the International Commission on Illumination (CIE).
The image data is converted into image data (XYZ) of the EXYZ color system.

In the color space processing, the image data (XYZ)
Is a uniform color space based on the reference white (W0), for example, C
IEL ^* a ^* b ^* converting the projection to the color space data (L ₁ a ₁ b ₁₎ in the uniform color space (hereinafter, referred to as forward conversion.), And then specify the color to be corrected, the color possessed by the color This is performed by increasing or decreasing each value of the coordinate components (L), (a), and (b) according to the difference between the space data and the processed color space data.

The new color space data (L ₂ a ₂ b ₂ ) thus obtained is converted into XYZ color system image data (X′Y′Z ′) again using the reference white (W0). Conversion (hereinafter referred to as inverse conversion), output conversion to output image data (color material density, dot% value, exposure amount, CRT voltage data, etc.), and output to the printer 40 or CRT 70, To create a spatially processed print 50,
For example, an image subjected to color space processing can be displayed on a screen.

In a series of steps in the color space processing, the amount of increase or decrease of each value of the coordinate components constituting the color space data is determined while viewing the image of the original 10 displayed on the CRT (monitor) 60 connected to the processing system 30. Adjusted and done. Since the coordinate components can be changed independently and continuously,
The image of the document can be corrected to a desired color balance.

[0011]

A conventional color space processing method
Is the same reference white (W0) for forward conversion and reverse conversion.
Since the conversion is performed as a reference, substantial color correction is required.
Positive is the color space data (L₁
a₁b₁) And the processed color space data (L_Twoa_Twob _Two)When
The coordinate component (L) by the difference (ΔLΔaΔb)
This is performed by increasing or decreasing (a) and (b).

However, the increase / decrease of the coordinate component has the same effect on the pixels other than the pixel having the chromaticity to be corrected, so that all the pixels constituting the image have the difference (ΔLΔ
a.DELTA.b) is translated in the uniform color space.
As a result, pixels that need to be corrected more than the difference amount are insufficiently corrected, while pixels that do not need to be corrected as much as the difference amount are excessively corrected, resulting in an unnatural color tone of the entire image.

In order to eliminate the parallel movement of all the pixels, it is necessary to repeat the above-described series of conversions for each pixel or each color to find an image having a good hue.
In practice it is impossible. Accordingly, the present invention has been made in view of the above circumstances, and has as its object to provide a color space processing method in which a processed image does not have an unnatural color.

[0014]

In order to achieve the above object, the present invention provides a color space processing method for processing image data in a color space based on a reference white. After converting the chromaticity of the color to be achromatic to the color space data as the reference white, the non-self-luminous output medium
Chromaticity corresponding to the color temperature of the light source used to observe the body
Is converted to the output image data as the reference white,
The output is made to a self-luminous output medium.

[0015]

According to another aspect of the present invention, there is provided a color space processing method for processing image data in a color space based on a reference white color. After converting the chromaticity of the color to be colored into color space data as the reference white, the chromaticity of the achromatic color on the non-self-luminous output medium irradiated with the observation light source is converted back to the output image data as the reference white, The output is made to the non-self-luminous output medium.

Further, in order to achieve the same object, the present invention relates to a color space processing method for processing image data in a color space based on a reference white color, wherein the image data is processed in a colorless manner. After converting the chromaticity of the color to be colored into color space data as reference white, the chromaticity corresponding to the color temperature of the self-luminous output medium is converted back to output image data as reference white, and the self-luminous output medium is converted. Output.

According to the means of the present invention, the amount of change in color for each pixel when converting image data to color space data, and the amount of color for each pixel when converting color space data back to output image data , The color processing can be performed without causing the processed image to have an unnatural tint, as compared with the conventional case where the colors of all pixels change uniformly.

The color processing method according to the present invention will be described below with reference to FIGS. As shown in FIG. 1, in a color processing method according to the present invention, an original 10 to be subjected to color processing is input to an input unit 2 such as a scanner or a TV camera.
0, the optical data of each pixel constituting the image of the document 10 is obtained, the optical data is subjected to an appropriate arithmetic processing by the processing system 30, and the arithmetic result is output to the printer 40 to create a print 50. And a series of steps for displaying an image on the CRT 70. Various operations in the calculation in the processing system 30 are performed while viewing the image of the document 10 displayed on the CRT (monitor) 60. The document 10 can be a photographic negative film, a reversal film, or various printed materials. In addition, the photograph
In the case of a negative film, the amount of information of the image is large and the image quality is good.
There is an advantage that it becomes.

The input means 20 is for taking an image of the object to be processed into the system, and may use, for example, a color scanner for printing. Further, it is also possible to photograph an object to be processed by using an imaging device such as a TV camera, a still camera, a CCD camera or the like, and take in a video signal as image data. Both the photometric value of the input means 20 and the video signal from the imaging device are optical data obtained by optically reading an object color to be processed.
This is mixed color data indicating the mixing ratio of the three primary colors of red (R), green (G), and blue (B). Since this optical data cannot be directly handled in a uniform color space described later, it is necessary to convert the optical data into data suitable for handling in a uniform color space.

Therefore, the processing system 30 converts the optical data into image data (XYZ) of the CIEXYZ color system specified by the International Commission on Illumination (CIE). The image data (XYZ) includes the original stimulus (X) corresponding to the RGB,
The conversion at this time is, for example, when the RGB is read using a color matching function of an RGB color system defined by CIE, the following conversion formula (1) is used. ).

[0022]

(Equation 1)

When the input means 20 is a TV camera, for example, in the case of an NTSC TV, the same conversion is performed using the following conversion equation (2).

[0024]

(Equation 2)

If the above color matching function cannot be realized,
It is necessary to determine the above conversion formula separately for each system. Next, in the converted image data (XYZ), a color in which the color after processing is set to be achromatic is set, and this is set as a reference white (W01) in a uniform color space, for example, CIEL ^* a ^* b.
^* Forward conversion of image data (XYZ) to uniform color space.
In setting the reference white (W01), it is also possible to collect a plurality of colors for which the color after processing is desired to be gray, and use the average value as the reference white.

The method of setting and inputting the reference white (W01) is as follows: while viewing the image of the document 10 displayed on the CRT (monitor) 60, the user designates a desired color on the image using a light pen or a mouse. Done. Also, CR
On the same screen of the T (monitor) 60, a color image in which hues such as color circles are arranged is displayed together with the image of the document 10, a color corresponding to a target color is selected from the color circle, and the color is written. It is also possible to specify with a pen or a mouse. Alternatively, when a target color is specified on the screen, the specified color may be displayed as a numerical value, and the numerical value may be input from a keyboard (not shown).

This forward conversion is performed using, for example, the following conversion formula (3).

[0028]

(Equation 3)

By this forward conversion, the image data (XYZ)
Is color space data (L ^* a ^* b ^* ), which is suitable for processing in a uniform color space. Uniform color space is CIEL ^*
In addition to the a ^* b ^* uniform color space, all color spaces based on the reference white, such as the CIEL ^* u ^* v ^* uniform color space, the L ^* C ^* H ^* uniform color space, and the color space based on the appearance model, are used. You can choose. When these uniform color spaces are selected, forward conversion is performed by appropriately changing the above-mentioned conversion formula (3) so as to correspond to each uniform color space.

Next, the color space data (L ^* a ^* b ^* ) obtained by the forward conversion is used as the reference color (W02), with the chromaticity of the achromatic color on the output object as the reference white (W02). The image data is inversely converted into image data of a system (in this case, the CIEXYZ color system). Here, the chromaticity of the achromatic color means the chromaticity of the color that the observer recognizes as an achromatic color among the colors expressed by the output medium,
In selecting an achromatic color, it is desirable to select a color having the highest lightness among colors recognized as an achromatic color, that is, a color which is generally recognized as white.

When the output target is a non-self-luminous output medium represented by a photographic material for photographic printing or electrophotographic paper, the chromaticity of the achromatic color becomes non-self-luminous when the observation light is irradiated. The chromaticity corresponds to the color temperature of the light reflected from the luminous output medium. For example, in the case of photographic materials for photo printing,
A value obtained by scanning a photographic print obtained by development processing in an unexposed state with a scanner under observation light can be adopted as achromatic chromaticity. Alternatively, the chromaticity corresponding to the color temperature of the light emitted from the observation light source can be adopted as the achromatic chromaticity.

On the other hand, when the output target is a self-luminous output medium represented by a CRT or the like, the chromaticity of the achromatic color is determined by the color temperature of white light among the light emitted from the self-luminous output medium, for example, the CRT. Is the chromaticity corresponding to the color temperature of the pixel recognized by the observer as white in the image displayed on the CRT screen. Preferably, a chromaticity corresponding to the color temperature of light used when obtaining the white balance of the CRT can be adopted.

The chromaticity value corresponding to the above color temperature can be easily obtained from an existing color temperature-chromaticity comparison table.
The chromaticity of these achromatic colors is based on the reference white (W02) at the time of inverse conversion.
Is input to the processing system 30. This inverse conversion is performed by the following conversion formula (4).

[0034]

(Equation 4)

By the above-described forward conversion and inverse conversion, the original 10
In the image, the color designated as the reference white at the time of forward conversion changes to an achromatic color, and the other colors change according to the difference from the reference white at the time of forward conversion and the difference from the reference white at the time of inverse conversion. As described above, in the color space processing method according to the present invention, even when a certain pixel or color is specified and the corresponding color space processing is performed, the color change amounts of the other pixels are not uniform. Therefore, even if some pixels and colors change greatly due to the color space processing of the designated pixel or color, the effect on the entire image is small compared to the conventional case where the color of all pixels changes, Unnatural color tone of the processed image is suppressed.

By the above-described color space processing, the image data (XYZ) of the original 10 becomes new image data (X'Y'Z ').
Is output to the printer 40 or the CRT 70. At this time, the output to the printer 40 or the CRT 70 is
The conversion is performed by converting the image data into output data so as to realize the image data (X′Y′Z ′). Printer 40
In addition to known photo printers, color copiers and various printing devices can be used, and prints 50 corresponding to each can be obtained.

Incidentally, in actual color space processing, a desired image may not be obtained by one forward conversion and one inverse conversion. In that case, the image data after the inverse conversion (X'Y'Z ')
Is displayed on the CRT 70, and while observing the image after the color space processing, the reference white (W
01) is reset, and image adjustment can be performed by performing the series of conversions.

FIG. 2 is a flow sheet for explaining a color space processing method when a photographic print 50 is created using a color reversal film as the document 10 in FIG. First, the color space processing is performed using the color reversal film 1
5 is read by a known color scanner 25, and the image data is input to a processing system 30. The image data at this time is optical data RGB in the description of FIG. 1, but here, the dye density of the image, for example, CMY dye density data 26 is obtained and input to the processing system 30 as image data.

In the processing system 30, first, the CMY dye density data 26 is converted into CIEXYZ color image data (XY
Z). This conversion is, for example, "N. Ohta, Pho
togra-phic Science and Engineering, pp. 487-494
, vol. 15, no. 6 (1971) ". Next, the converted image data (XY
In Z), set the color that you want to make the processed color achromatic,
Using this as a reference white (W01), image data (XYZ)
To a CIEL ^* a ^* b ^* uniform color space. The setting and input method of the reference white (W01) at this time can be performed in the same manner as described in FIG.
The image data (XYZ) read at 5 is converted into CRT light emission luminance data 27, and the image on the film 15 is converted to CRT 70.
This is performed by designating a target color or pixel on the screen of the CRT 70 with a light pen or a mouse. This forward conversion is performed using the conversion formula (3), and the image data (XYZ) becomes color space data (L ^* a ^* b ^* ), which is suitable for processing in a uniform color space.

Next, the color space data obtained by the forward conversion
(L^*a^*b^*) To the achromatic color
The inverse conversion is performed using the chromaticity as the reference white (W02). The opposite
The reference white (W02) at the time of conversion is the same as the description in FIG.
Is set in the same manner as described above and is input to the processing system 30. here
And the color space data (L^*a^*b^*) Is the inverse transformation
As an output method from the color space data (L^*a
^*b^*) Is converted to a CIEXYZ system using the conversion formula (4).
CMY dye density after conversion to image data (X'Y'Z ')
May be converted to data and output (code (1)),
Alternatively, color space data (L^*a^*b ^*) And CMY dye concentration
LUT (Look Up Table)
) Is created, and the CM is directly created based on this LUT.
Output as Y dye density data (code (2))
it can.

Note that the image data (X '
Y'Z ') can be converted to CMY dye density data by, for example, "N. Ohta, Journal of The Optical S".
oci-ety of America, vol. 62, no. 1, pp. 129-1
36 (1972) ". As the method of code (2), for example,
The methods described in JP-A-3-254863, JP-A-63-254864 and JP-A-63-254887 can be used.

The processing system 30 is inversely transformed as described above.
The exposure amount 32 of each color is adjusted based on the output image data 31 output from the printer, and a photographic print 50 is created based on the exposure conditions. In addition, an image subjected to color space processing as described in FIG. 1 can be displayed on the CRT 70. Furthermore, a desired image can be obtained by resetting the reference white (W01) at the time of forward conversion while observing the image after color space processing on the CRT 70, and repeating the series of conversions.

[0043]

Embodiment 1 An embodiment of the present invention will be described below with reference to the accompanying drawings. However, the present invention is not limited to only this embodiment. Color photographic film taken under tungsten light and developed (Fuji Photo Film Co., Ltd .: HG40
0) as a light-sensitive material (Fuji Photo Film: SUPE
(RFAV) was exposed and printed to produce a photographic print. The created photo print was orange on the whole.

The photographic print was scanned by a color scanner (SG1000, manufactured by Dainippon Screen Co., Ltd.) to obtain image data of three color densities of CMY, and the printed image was displayed on a monitor. The mouse is clicked on five points of the image on the monitor to be made achromatic, and their average chromaticity [(x, y) = (0.45,
0.41)] was used as a reference white color and forward-transformed to CIEL ^* a ^* b ^* uniform color space.

Thereafter, the obtained color space data is converted into the chromaticity [(x, y) = (x, y) = corresponding to the color temperature (D ₆₅ ) of the daylight light source by using the above-mentioned light-sensitive material (Fuji Photo Film: SUPER FAV).
(0.31, 0.33)] as the reference white,
The dye amount corresponding to each chromaticity was determined from the data, and a photographic print was created again based on the amount. The obtained photographic print had an image quality with a natural tint by adjusting the orange color.

For comparison, the same photographic print is scanned by the same color scanner, and the chromaticity [(x, y) = (0.31, 0.31) corresponding to the color temperature of the same observation light source as described above.
0.33)] as a reference white color and then forward-converted to a CIEL ^* a ^* b ^* uniform color space, and then look at (L), (a) and (b) with (ΔL = 0), (Δ
a = −19) and (Δb = −43), and again, the chromaticity [(x,
y) = (0.31, 0.33)] was used as the reference white color, and inverse conversion was performed to create a photographic print again.

In the obtained photographic print, although the orange color was corrected to some extent, this time, the portion where the orange color was corrected too much and became bluish, and the portion where the orange color remained due to insufficient correction were mixed. The image quality has become.

[0048]

Example 2 Using the same photographic print as in Example 1,
The average chromaticity obtained in Example 1 [(x, y) =
(0.45, 0.41)] and CIEL ^*
a ^* b ^* was converted into a uniform color space. Thereafter, the obtained color space data was subjected to a development process in an unexposed state, and the above-described light-sensitive material (Fuji Photo Film Co., Ltd .: SUPER FAV) was recognized as an achromatic color having high brightness under a fluorescent lamp, that is, a portion recognized as white. Is converted by using the chromaticity [(x, y) = (0.3779, 0.3882)] obtained by measuring the color of the color with a color scanner as a reference white, and the dye amount corresponding to each chromaticity is calculated from the data. The photograph print was made again based on the request.

The obtained photographic print was adjusted for orangeness in the same manner as in Example 1, and had a natural color image quality.

[0050]

Example 3 Using the same photographic print as in Example 1,
The average chromaticity obtained in Example 1 [(x, y) =
(0.45, 0.41)] and CIEL ^*
a ^* b ^* was converted into a uniform color space. Thereafter, the obtained color space data is converted into a chromaticity [(x, y) = (0.28) corresponding to a color temperature of 9300 K of a CRT as a self-luminous output medium.
3, 0.297)] as a reference white color and inversely converted to XY values, based on which the luminous intensity of the CRT is controlled to display an image.

The image on the CRT was adjusted to orange and had a natural color image quality.

[0052]

As described above, according to the color space processing method of the present invention, it is only necessary to specify the color to be processed on the monitor screen. The color processing can be performed by a simple operation as compared with the method of adjusting based on the tone conversion curve). Further, quick and accurate color processing can be performed without trial and error as in the related art.

Furthermore, an image having a natural color tone can be obtained as compared with a conventional color space processing method for processing in a uniform color space.

[Brief description of the drawings]

FIG. 1 is a diagram showing a flow sheet of a color space processing method according to the present invention.

FIG. 2 is a diagram illustrating a flow sheet of a color space processing method according to the present invention, and is a diagram illustrating a flow sheet of a method of creating a photographic print subjected to color space processing from a photographic film.

FIG. 3 is a diagram showing a flow sheet of a conventional color space processing method.

[Explanation of symbols]

 10 Document 15 Film 20 Input Means 25 Scanner 26 CMY Dye Density Data 27 CRT Emission Luminance Data 30 Processing System 31 Output Image Data 32 Each Color Exposure 40 Printer 50 Print 60 CRT (Monitor) 70 CRT

Claims (3)

(57) [Claims] 1. A color space processing method for processing image data in a color space based on a reference white, wherein the image data is processed using a chromaticity of a color whose processed color is desired to be an achromatic color as a reference white. After converting to spatial data, non-self
The color temperature of the light source used to observe the luminous output medium
The corresponding chromaticity is converted to the output image data as the reference white.
And outputting to the non-self-luminous output medium. 2. A color space processing method for processing image data in a color space based on a reference white, wherein the color of the image data is set to a reference white with a chromaticity of a color whose processed color is desired to be achromatic. After converting to spatial data, the observation light
A color space processing method, wherein the chromaticity of an achromatic color on a non-self-luminous output medium irradiated with a source is converted back to output image data as reference white and output to the non-self-luminous output medium. 3. A color space processing method for processing image data in a color space based on a reference white, wherein the image data is processed using a chromaticity of a color whose processed color is desired to be an achromatic color as a reference white. after converting to spatial data, self-onset
Color space processing method and outputting a chromaticity corresponding to the color temperature of the light type output medium back into an output image data as a reference white, before Symbol self-luminous output medium.