JP2000287097A - Gamut compression method in color printer and color reproduction processing unit using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the occurrence of a color difference caused by gamut conversion on the basis of calorimetric color reproduction. SOLUTION: This color reproduction processing unit is provided with a monitor characteristic acquisition section 101, that converts original image data for a monitor into calorimetric color information data and provides an output, a gamut compression conversion table 110 that selects a color equal to or subjectively felt closest to a color among several colors, typically selected by a subject in advance from print colors of a printer and stores the selected color in pairs and in cross reference with color information of the monitor and the printer, and a gamut compression section 104 that reads conversion information of two representative colors, whose hue is close to that of monitor colors from the table 110 and applies conversion processing to the conversion information of the selected colors in an equivalent way by the gamut conversion.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field 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.

[0002]

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.

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. It is preferable that the print result of the printer can accurately reproduce the image on the monitor, but in actuality, the color tone may be considerably different.

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 color uniform 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, monitor gamut is wider than printer,
A process is required to move the color outside the gamut of the printer into the gamut of the printer in some way. 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 and the gamut of the printer do not match, 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.

[0008]

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.

One of the causes is that the CIELAB color space and the CIELV color space, which are called uniform color spaces, are not actually uniform, but have distortion and the like. For example, blue on a monitor is quite saturated, which is located outside the gamut of the printer. Therefore, applying gamut compression to compress saturation while maintaining hue and brightness,
Print with a printer. However, this color is reproduced in a color that does not seem to be the same hue as blue on the monitor.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and has as its object to reduce color differences occurring in gamut compression based on colorimetric color reproduction.

[0011]

A gamut compression method for a color printer according to the present invention is characterized in that a color that is subjectively determined to be the same as or closest to the color displayed on a monitor of several colors typically selected by the subject in advance. A table is prepared in which the colors selected by the printer are selected, and the selected result is recorded as a pair of the color information of the monitor and the color information of the printer. The conversion information is referred to from the table, and conversion processing is performed so as to be equivalent to these conversion relationships.

A color reproduction processing apparatus according to the present invention further comprises a monitor characteristic acquisition unit for converting the original image data of the monitor into colorimetric color information data and outputting the colorimetric color information data; A gamut compression conversion table that selects from the printer's print color the color that is perceived to be equal to or closest to the color displayed above, and that the selected result is recorded as a pair of monitor color information and printer color information. And a gamut compression unit that reads conversion information of two representative colors whose hue is close to the color of the monitor from the table, and performs a conversion process so as to be equivalent to the gamut conversion relationship. .

The gamut compression section inputs three-attribute (LCH) information of a color reproduced on a monitor, reads conversion information of two representative colors whose hues are close to the color of the monitor from the table, and reads the conversion information of the monitor. The conversion is performed so as to be equivalent to the relationship between the two colors, and three attribute (LCH) information of the color that is subjectively determined to be closest in the gamut range of the printer is output.

As described above, since the subjective gamut compression conversion table of the representative color is prepared by the subject in advance, the gamut compression which the subject feels most preferable can be performed, and the printing satisfactory to the user can be performed.

[0015]

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 subjective gamut conversion compression according to the present invention. The input value of this system is the original image data RG
B, the output values are the print data R ', G', B 'of the printer.
It is.

The monitor characteristic acquisition unit 101 includes a LAB color space conversion unit 102 and an LCH color space conversion unit 103.
The AB color space conversion unit 102 receives the RGB of the image data and receives L *, a in the CIELAB color space, which is a uniform color space.
*, B *. The LCH color space conversion unit 103
The AB color space conversion unit 102 converts the L *, a *,
Based on b *, the three attributes of color are converted into lightness L, saturation C, and hue H. Here, the LCH is converted based on the following Equation 1.

[0017]

(Equation 1)

Since the characteristics of the LAB conversion section 102 differ depending on the monitor used, the output value also differs depending on the monitor.

The subjective gamut compression section 1 according to the present invention
Reference numeral 04 denotes a unit for compressing the gamut of the monitor into the gamut of the printer. The H'C 'calculating unit 105 and the L' calculating unit 1
06. The calculation units 105 and 106 refer to a representative color gamut compression conversion table 110 based on information preliminarily selected by the subject as described later,
Calculate each value. This subjective gamut compression unit 1
04, the three attributes LC of the color reproduced on the monitor
The H value is input, and the colors (L ′, C ′,
H ′) is output.

The color information (L ′, C ′, H ′) calculated by the gamut compression section 104 is output to the print data conversion section 107 by the print data conversion section 107.
The print data (R ', G',
B ′). In the print data conversion unit 107, print data (R ', R') to be transferred to the print unit in order to reproduce the calculated color information (L ', C', H ').
G ′, B ′) are calculated. This print data conversion unit 107
Prints a color chip of several hundred colors or more in advance, measures it, and prepares L'C'H'-R 'based on the measured value.
The print data is calculated by referring to the G′B ′ print conversion table 111.

Next, the processing of the gamut compression section 104 will be described in detail.

First, the subjective gamut compression conversion table 110 of the representative color referred to by the gamut compression section 104 will be described.

In this embodiment, the subjective gamut conversion table 110 stores R (red), G
(Green), B (blue), Y (yellow), M (magenta), and C (cyan). The conversion table is created by printing all colors that can be represented by a printer and using the printed colors as a color batch. Next, one of the selected representative colors is projected on the monitor, and one color batch that is felt closest to the monitor is selected. This operation is performed for all representative colors. For each representative color, the colors (L, C, H) on the monitor and the colors (L ', C', H ') of the selected printer color batch are stored in pairs.
Table 1 shows an example of the representative color gamut compression conversion table 110 created by such an operation.

[0024]

[Table 1]

The above operation is performed by a plurality of subjects in advance.
A few selected colors are projected on the monitor, and the colors equivalent to these colors or the colors that are subjectively felt as the closest colors are selected from the printing colors of the printer, and the results are averaged, and the colors are averaged. Color information, lightness L, saturation C, hue H
And color information of the printer, lightness L ', saturation C', and hue H '.

In the above example, in order to eliminate bias,
An average of the results of a plurality of subjects is used to prepare a subjective gamut conversion table 110 of representative colors. Of course, one subject can select several representative colors (L, C, and C) on the monitor. H) and the colors of the printing colors (L ', C', H ') selected by the printer that the subject felt to be closest to this color (L', C ', H')
May be used.

Next, the processing operation of the subjective gamut compression unit 104 will be described. Here, it is assumed that the subjective gamut compression table 110 of the color information (L, C, H) already displayed on the monitor and the representative color described above is prepared.

First, the processing performed by the H'C 'calculation unit 105 in FIG. 1 will be described with reference to FIG. FIG. 2 shows an ab plane in the Lab color space, and Ym, R in FIG.
m is the monitor's yellow and red information, Yp, Rp is the printer's yellow and red information, with reference to the above-described subjective-subjective gamut conversion table 110 of the representative color,
It is a plot. Hereinafter, the lightness L of the color Ym is described as Lym, the saturation C is described as Cym, and the hue H is described as Hym.

Now, the color of the original image, that is, the color on the monitor is set to Xm (Lxm, Cxm, Hxm), and the color reproduced by the printer to be obtained from this is Xp (Lxp, Cxp, Hyp).
And

From the coordinates of Xm in the figure, it can be seen that this color has a hue between yellow and red, and has a value close to red. This relationship should be the same for printing colors on a printer. As described above, the yellow Ym of the monitor and the Yp of the printer have been determined to be nearly similar colors by the subject, and the red Rm
And Rp. Therefore, the hue is H on the monitor.
The colors existing in rm to Hym may be reproduced in the range of hues Hrp to Hyp when reproduced by a printer. This can be expressed by equation (1).

[0031]

Hxp = (Hyp · β + Hrp · α) / (α + β) (1) where α = | Hym−Hxm | and β = | Hrm−Hx
m |.

The hue Hxp determined by the above equation (1) becomes the hue H 'to be reproduced by the printer.

Next, a method of calculating the saturation C 'will be described with reference to FIG. 2, the same symbols as those in FIG. 2 are used in the same meaning as described above.

In FIG. 3, Zm has the same hue as Xm and is located at the intersection with line YmRm of triangle YmORm, and Zp has the same hue as Xp and is located at the intersection with triangle YpORp.

Here, the triangle YmORm can be regarded as the color reproduction range from the monitor hues Y to R,
Rp can be said to be the color reproduction range from hue Y to R of the printer. That is, gamut compression can be realized by mapping the triangle YmORm to the triangle YpORp. Therefore, the saturation (C) compression of the original image Xm is performed by setting the ratio between the line segment ZmO and the line segment XmO of the triangle YmORm to the triangle YmORm.
What is necessary is just to calculate the saturation (C) of Xp such that the ratio between the line segment ZpO of pORp and the line segment XpO is maintained. This can be expressed as the following equation (2).

[0036]

(3) {Crm + (Cym−Crm)} · β / (α + β): Cxm = {Crp + (Cyp−Crp)} · β / (α + β): Cxp (2)

When formula (2) is summarized for the saturation of Xp, formula (3) is obtained.

[0038]

Cxp = [Cxm · [Crp + (Cyp-Crp)] · β / (α + β)] / [[Crm + (Cym-Crm)] · β / (α + β)] (3)

Cxp obtained here is H ′ in FIG.
This is the output value C 'of the C' calculation unit 105.

Next, a method of calculating the lightness L 'will be described with reference to FIG.
This will be described with reference to FIG. In these figures, the same symbols as those in FIGS. 2 and 3 have the same meanings as described above. FIG. 4 is a three-dimensional view of the CIELAB color space. Wm and Km are white and black on the monitor, and Wp and Kp.
Are white and black on the printer. Ym, Rm, Yp, R
p is the same as that in FIG.

Now, vertices Wm, Lym, Ym, Lrm, R
Assuming that the solid consisting of m and Km is cut by the same hue plane of the hue Hxm value of the original image, a triangle A consisting of vertices Wm, Lm0, and Km in FIG. 5 is obtained, which is a color reproducible with the hue Hm on the monitor ( Lightness) range.

Similarly, assuming that the solid consisting of the vertices Wp, Lyp, Zp, Lrp, and Kp in FIG. 4 is cut along the same hue plane of the hue Hxp value of the original image as the color reproduction range of the printer, the vertices Wp, A triangle B consisting of Lp0 and Kp is obtained, which is a color (brightness) range that can be reproduced with the hue Hp in the printer.

Here, the relationship between the monitor brightness and the printer brightness is converted as shown in FIG. That is, compression is performed so that each vertex of the triangle A becomes each vertex of the triangle B. Specifically, in the triangle A, the color of the original image is a point of Lxm in the figure. When the saturation is Cxm and the lightness is Lxm, (Lxm
−Lm1) / (Lm2−Lm1) is defined by triangles B and X
That is, the conversion may be performed so as to be applied to the saturation relationship with p. Therefore, the printer color Lx is obtained from the following equation (4).
What is necessary is just to obtain p.

[0044]

Lxp = (Lxm−Lm1) / (Lm2−Lm1) · (Lp2−Lp1) + Lp1 (4) In the above-described example, the original color was a hue and a color between Y and R. May be calculated using the representative color closest to the hue of the original image on the negative and positive sides.

When an image using the gamut compression technique of this embodiment and a sample using the colorimetric gamut compression are subjectively evaluated, the gamut compression of the present invention is closer to blue and cyan colors. The result was obtained.

In this embodiment, the saturation compression described with reference to FIG. 3 is performed. However, there is no problem even if a method of compressing with a certain coefficient when Xp is obtained from Xm is used.

[0047]

As described above, according to the present invention, the color difference between the image output from the monitor and the image output from the printer becomes smaller as compared with the sample using the colorimetric gamut compression. Furthermore, the method of preparing the subjective gamut compression conversion table of the representative color by the subject in advance has been described. In other words, it can be said that the gamut compression which the subject feels most preferable is performed. In other words, by making it possible for a user who performs printing to select this work, a printer that can perform printing satisfactory to the user can be configured.

[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 is a conceptual diagram showing an ab plane in a LAB color space for describing a processing operation of hue calculation according to the present invention.

FIG. 3 is a conceptual diagram showing an ab plane in a LAB color space for explaining a processing operation of a saturation calculation according to the present invention;

FIG. 4 is a conceptual diagram three-dimensionally showing an LAB color space for describing a brightness calculation processing operation according to the present invention;

FIG. 5 is a conceptual diagram in a LAB color space for explaining a brightness calculation processing operation of the present invention.

FIG. 6 is a conceptual diagram in a LAB color space for describing a brightness calculation processing operation according to the present invention;

[Explanation of symbols]

 Reference Signs List 101 monitor characteristics acquisition unit 104 subjective gamut compression unit 107 print data conversion unit 110 gamut compression conversion table

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5B057 CA01 CA08 CA12 CB01 CB08 CB12 CC01 CE18 CH07 5C077 LL19 MP08 NP01 PP32 PP33 PP35 PP36 PP37 PQ23 TT02 5C079 HB01 HB02 HB06 HB08 HB11 LB02 MA04 MA17 NA03 PA03

Claims (3)

[Claims] In a gamut compression method for a monitor and a color printer, a color which is subjectively considered to be equal to or closest to a color displayed on a monitor of several colors typically selected by a subject in advance is determined from a print color of the printer. A table is prepared in which the selected result is recorded as a pair of monitor color information and printer color information, and the conversion information of two representative colors that are close in hue to the monitor color during the color reproduction process is stored in the table. More specifically, a gamut compression method for a color printer, wherein a conversion process is performed so as to be equivalent to these conversion relationships. 2. A monitor characteristic acquiring unit for converting original image data of a monitor into colorimetric color information data and outputting the same, and a color equal to or displayed on a monitor of several colors typically selected in advance by a subject. A gamut compression conversion table in which the color that is subjectively perceived as closest is selected from the printer's print color and the selected result is recorded as a pair of the monitor's color information and the printer's color information, and the monitor's color and hue are close A color reproduction processing apparatus for a color printer, comprising: a gamut compression unit that reads conversion information of two representative colors from the table and performs conversion processing so as to be equivalent to the conversion relationship. 3. The gamut compression unit inputs three-attribute (LCH) information of a color reproduced on a monitor, reads conversion information of two representative colors having colors close to the color of the monitor from the table, 3. A three-color attribute (LCH) information of a color which is subjectively determined to be closest in the gamut range of the printer by converting the data so as to be equivalent to the relationship between the two colors of the monitor. 3. A color reproduction processing device in a color printer according to claim 1.