JP2000287096A - Gamut compression method in color printer and color reproduction processing unit using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color reproduction processing method that can reproduce a color displayed on a monitor with fidelity. SOLUTION: This processing unit is provide with a monitor characteristic acquisition section 1, that converts original image data displayed on a monitor into three color spaces as lightness(L), chromaity(C) and hue(H) and outputs the result and with a gamut compression processing section 2 that compresses the lightness of the monitor into a lightness within that of a printer and maps the data to a point at which a color difference in the LC spaces for each hue is minimized for the compression.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a gamut compression method between different output devices such as a color printer and a color monitor, and a color reproduction processing device using the method.

[0001]

2. Description of the Related Art With the spread of personal computers (PCs) and digital still cameras (DSCs), opportunities for handling color images on PCs are increasing. Images taken by DSC and images obtained by performing various image edits on the images are enjoyed as electronic albums, and printed with a color printer.

[0002] When printing with a printer, the default image is used only as a PC monitor. When some kind of editing is applied to the image captured from the DSC, the work is performed while checking on the monitor, and if the image can be edited to a satisfactory image, printing is performed by a printer. Although it is preferable that the print result of the printer can accurately reproduce the image on the monitor, the color tone may actually differ considerably.

Therefore, there is a color management system (CMS) for compensating for a color difference between devices. CMS is
The correction is performed so that the color difference between the devices is reduced by using a common color space instead of the device-specific color expression such as RGB and YMC. Color reproduction by CMS is usually based on colorimetric color reproduction. This is based on the standard "white" or "light source color" in each condition.
(Commission International del'Eclairage) -193
Each color (X, Y, Z) value is calculated by the 1XYZ system, and if these values match, it is considered that "the reproduced colors match." In addition to the XYZ system, CIELab which is a uniform color space
A color space or a CIELUV color space is often used.

However, in the case of colorimetric color reproduction, a problem is a mismatch in the color gamut (color gamut). Generally, since the gamut of a monitor is wider than that of a printer, it is necessary to perform a process of moving a color outside the gamut of the printer into the gamut of the printer by some method. As this method, the following three types of methods are well known.

A. Convert to the color closest to the original color.

B. The color is converted to the color closest to the original color while maintaining the hue and brightness.

C. Convert to a color close to the original color while maintaining hue and saturation.

Since the gamut of the monitor does not match the gamut of the printer, colorimetric color reproduction cannot be realized in this state. The key is how to set the mapping relationship between different gamut. This conversion is called gamut conversion, and the compression from a wide gamut to a narrow gamut is called gamut compression.

[0009]

However, even if the gamut compression is performed as described above, there may be a case where the color of the monitor and the color of the printer are subjectively considerably different.

For colors outside the gamut range, there is a gamut compression method as shown in FIG. In this method, first, the brightness range of the monitor is compressed to the brightness range of the printer (see FIG. 3A). Thereafter, saturation compression is performed while maintaining the lightness after the compression (see FIGS. 3B and 3C). However, there is a problem that the color of the monitor has high saturation where the brightness is high, and a high saturation color such as a primary color becomes a low saturation color in the printer.

The present invention has been made to solve the above-mentioned conventional problems, and provides a color reproduction processing method capable of faithfully reproducing colors projected on a monitor.

[0012]

According to the present invention, in a gamut compression method for a monitor and a color printer, colors displayed on a monitor are converted into a three-color space of lightness (L), saturation (C), and hue (H). After the conversion, the brightness of the monitor is compressed to the brightness range of the printer, and then the mapping is performed by mapping each hue to a point where the color difference is minimized in the LC space.

In the present invention, it is preferable that the compression is performed by mapping to a point where the color difference becomes minimum in the LC space when the brightness is equal to or higher than the reference brightness, and the brightness is maintained and compressed when the brightness is equal to or lower than the reference brightness.

According to the present invention, there is also provided a monitor characteristic unit for converting original image data of a monitor into a three-color space of lightness (L), chroma (C), and hue (H) and outputting the converted data. And a gamut compression processing unit that performs compression by mapping to a point where the color difference is minimized in the LC space for each hue, and compresses the color in the brightness range of the printer.

It is preferable that the gamut compression processing unit maps the data to a point where the color difference becomes minimum in the LC space when the brightness is equal to or higher than the reference brightness, and performs compression while maintaining the brightness when the brightness is equal to or lower than the reference brightness.

With the above configuration, it is possible to map to a color having a low brightness and a high saturation, as compared with the saturation compression maintaining the brightness.

In the case where the chroma value of the monitor is outside the gamut of the printer, if all the colors are compressed so that the color difference is minimized, the brightness change may be conspicuous on an image in a portion where the gamut is close. Therefore, in the present invention,
When the brightness is equal to or higher than the reference brightness, the difference is mapped to a point where the difference is minimum, and compression is performed. When the brightness is equal to or lower than the reference brightness, gamut compression that maintains brightness is performed. With this configuration, unevenness in brightness change and the like are suppressed.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of a color reproduction processing apparatus using gamut conversion compression according to the present invention. The input values of this system are the original image data RGB
The output value is the print data R ", G", B "of the printer.

The image data RGB of the original image is given to the monitor characteristic acquiring unit 1, and the image data RGB of the original image is converted into the lightness L, which is the three attributes of the color defined by CIE, in the monitor characteristic acquiring unit 1. It is converted into saturation C and hue H.

The monitor characteristic acquiring section 1 is configured to execute the RG of the original image.
A gamma conversion unit 11 for performing gamma conversion according to the gamma value of the monitor using the B value, and a colorimetric value XYZ obtained by actually measuring the color of the gamma-converted R'G'B 'displayed on the monitor.
An XYZ conversion unit 12 for converting data into Lab data and a Lab conversion unit 13 and an LCH conversion unit 14 for converting data into Lab and LCH based on the conversion formula defined by CIE.

The gamma conversion section 11 performs gamma conversion represented by the following equation on the RGB values of the original image based on the gamma value of the monitor to be used.

R '= (R / 255) γ * 255 G' and B 'are obtained in the same manner.

The gamma-converted R'G'B 'is X
The YZ conversion unit 12 converts the stimulus into XYZ of tristimulus values defined by CIE. The XYZ conversion values are obtained by displaying a test chart composed of, for example, 420 colors on a monitor, measuring the XYZ values for each color, and measuring the RG values of the 420 colors.
The relationship between the R′G′B ′ value obtained by gamma-converting B and the XYZ value is calculated by using a multiple regression analysis to create a 3 × 3 matrix and using the equation shown in Equation 1 below.

[0024]

(Equation 1)

When comparing the XYZ calculation formula calculated by using the above equation (1) and the XYZ values obtained from the measured values, the color difference ΔE is within a sufficiently allowable range. For comparison, the XYZ values were converted to Lab, and the color difference ΔE represented by the following equation 2 was obtained.

[0026]

(Equation 2)

Therefore, in this embodiment, XY
The Z conversion unit 12 calculates R′G ′ based on the above equation (1).
B ′ is converted into an XYZ value and provided to the Lab conversion unit 13.

The Lab conversion unit 13 is provided with the XYZ conversion unit 1
The XYZ values from 2 are converted into Labs called a uniform color space, and the XYZ values are expressed as L,
Convert to a and b.

[0029]

(Equation 3)

Based on the L, a, and b converted by the Lab conversion unit 13, the LCH conversion unit 4 applies the following three equations to lightness L, saturation C, and hue H, which are three attributes of color. Based on the conversion.

[0031]

(Equation 4)

In the monitor characteristic acquisition unit 1, the RGB of the original image
, And the calculated LCH is provided to the gamut compression processing unit 2. As described above, the gamut differs between the monitor and the printer as shown in FIG.
In FIG. 2, white circles indicate the gamut of the monitor, and black triangles indicate the gamut of the printer.

As described above, since the two gamut are different, gamut compression is required. As will be described later, the gamut compression processing unit 2 according to the present invention compresses the brightness of the monitor to the brightness range of the printer, and then performs compression for each hue to a point at which the distance becomes minimum in the LC space. At this time, if the above compression method is applied to the part where the gamut is close,
Since the change in lightness is conspicuous on the image, the above-described method is applied to the lightness above a certain reference lightness, and gamut compression for maintaining the lightness below the reference lightness is used.

Here, the reference brightness is the brightness at which the gamut of the printer and the gamut of the monitor intersect, and the brightness at which the maximum chroma of the printer is obtained when the gamut of the printer is included in the monitor gamut. And

As described above, the gamut compression processing unit 2
Compress monitor gamut into printer gamut.

The color information (L ', C', H ') calculated by the gamut compression processing section 2 is converted by the print data conversion section 3 into print data (R ", G", B ") to be transferred to the print section. The print data conversion unit 3 converts the print data (R ", G", B) to be transferred to the print unit in order to reproduce the calculated color information (L ', C', H '). ") Is calculated. The print data conversion unit 3 prints a color chip of several hundred colors or more in advance, measures the color chip, and generates “(L ′, C ′, H ′) − (R”, G ",
B ") Conversion table", the print data is calculated.

Next, the processing of the gamut compression processing section 2 will be described in detail. As shown in FIGS. 3 and 4,
Gamut is different between monitor and printer. 3 and 4
, The solid line indicates the gamut of the printer, and the dashed line indicates the gamut of the monitor. FIG. 3 shows the shapes of red (R), yellow (Y) and green (G) printer gamut and monitor gamut. FIG. 4 shows the printer gamut and monitor of cyan (C), blue (B) and magenta (M). It is a gamut shape.

In the following description, the gamut of the monitor is adjusted to the gamut range of the printer with the hues of the monitor and the printer being the same. FIGS. 5 to 8 show the procedure simply. Representative models were selected for the shape of the printer gamut and the monitor gamut.

In the gamut compression processing device 2 according to the present invention, first, as shown in FIG. In brightness compression, the maximum brightness of the monitor at saturation (C) = 0 is compressed to the maximum brightness of the printer, and the minimum brightness of the monitor is compressed to the minimum brightness of the printer. As for the compression between the maximum brightness and the minimum brightness, the brightness compression was performed by mixing the linear compression and the S-shaped compression. That is, linear compression was performed in the high brightness range, and brightness compression was performed in the low-sloppy range by approximately halfway between linear compression and S-shaped compression.

As shown in FIG. 5A, the brightness is compressed.
When the brightness compression is performed, the brightness of the monitor is compressed as shown in FIG. Subsequently, the saturation of the monitor is compressed and mapped in the gamut of the printer. FIG. 5 illustrates the method and describes a typical case.

In the case of FIG. 5, in the lower half, the printer gamut is larger than the monitor gamut. In this case, the saturation value is used without performing the saturation compression. In the upper half, the monitor gamut is larger than the printer gamut. In this case, saturation compression is performed so as to be included in the gamut of the printer.

As a method of performing the gamut compression, according to the present invention, compression is performed by mapping to a point where the difference ΔE ′ on the LC plane is minimized. Here, ΔE ′ is defined as follows.

ΔE ′ = {(L1−L2) ² + (C1−C2) ² } ^1/2 By mapping in this way, as shown in FIG. 5, compared with the saturation compression maintaining the brightness, Will be mapped to colors with low saturation but high saturation.

When the chroma value of the monitor is outside the gamut of the printer and all the data are compressed by this method, as shown in FIG. It became noticeable.
Therefore, in the present invention, gamut compression is performed as shown in FIGS. As shown in FIG.
Above b, mapping is performed by mapping to a point where the difference ΔE ′ on the LC plane is minimized as described above. Then, as shown in FIG. 8, when the brightness is equal to or less than the reference brightness Lb, gamut compression in which brightness is maintained is used.

Here, the reference brightness is the brightness at which the gamut of the printer and the gamut of the monitor cross each other, and the brightness at which the maximum chroma of the printer is obtained when the gamut of the printer is included in the monitor gamut. And In this way, the gamut compression processing section 2 compresses the gamut of the monitor into the gamut of the printer.

As described above, by performing the gamut compression process and performing the color reproduction process, it was possible to print a color close to the color of the image projected on the monitor screen.

[0047]

As described above, according to the present invention, it is possible to suppress a phenomenon in which a change in brightness in the vicinity of the gamut causes image deterioration, and to perform printing with a color close to the monitor color.

[Brief description of the drawings]

FIG. 1 is a block diagram of a color reproduction processing device according to an embodiment of the present invention.

FIG. 2 illustrates a relationship between a gamut of a monitor and a printer.
It is a conceptual diagram which shows the ab plane in ab color space.

FIG. 3 is a diagram illustrating a relationship between a gamut of a monitor and a printer.
FIG. 3 is a conceptual diagram illustrating an LC plane in an ab color space.

FIG. 4 is a diagram showing L indicating a gamut relationship between a monitor and a printer.
FIG. 3 is a conceptual diagram illustrating an LC plane in an ab color space.

FIG. 5 is a conceptual diagram showing a gamut compression processing state according to the present invention on an LC plane in a Lab color space.

FIG. 6 is a conceptual diagram showing a gamut compression processing state according to the present invention on an LC plane in a Lab color space.

FIG. 7 is a conceptual diagram showing a gamut compression processing state according to the present invention on an LC plane in a Lab color space.

FIG. 8 is a conceptual diagram showing a gamut compression processing state according to the present invention on an LC plane in a Lab color space.

FIG. 9 is a conceptual diagram showing a conventional gamut compression processing state on an LC plane in a Lab color space.

[Explanation of symbols]

 1 monitor characteristics acquisition section 2 gamut compression processing section 3 print data conversion section

Claims (4)

[Claims] In a gamut compression method for a monitor and a color printer, a color displayed on a monitor is converted into a three-color space of lightness (L), saturation (C), and hue (H);
A gamut compression method for a color printer, comprising compressing the brightness of a monitor to a brightness range of a printer, and mapping each color to a point where the color difference is minimized in an LC space for each hue. 2. The method according to claim 1, wherein the compression is performed by mapping to a point where the color difference is minimum in the LC space when the lightness is equal to or higher than the reference lightness, and compressing while maintaining the lightness when the lightness is lower than the reference lightness. Gamut compression method for color printers. 3. A monitor characteristic section for converting the original image data of the monitor into a three-color space of lightness (L), chroma (C), and hue (H) and outputting the same; And a gamut compression processor for mapping each hue to a point where the color difference is minimized in the LC space and performing compression. 4. The gamut compression processing unit performs mapping by mapping to a point where the color difference is minimum in the LC space when the brightness is equal to or higher than the reference brightness, and performs compression while maintaining brightness when the brightness is equal to or lower than the reference brightness. A color reproduction processing device for a color printer according to claim 3.