JP2003078776A - Image processing method, apparatus and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To excellently reproduce a color image displayed on a monitor by means of a printer by taking account of a visual difference between a light source color and an object color. SOLUTION: In the case of converting a monitor color signal into a printer color signal, the hue of the printer color signal corresponding to the hue of the monitor color signal is derived on the basis of a hue angle correlation table in a light source color and an object color in order to correct the visual difference between the light source color and the object color, and color gamut compression processing is applied to the monitor color signal on the basis of color gamut information in the hue of the obtained printer color signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to converting monitor color signals into printer color signals.

[0002]

2. Description of the Related Art In order to properly reproduce a color image displayed on a color monitor with a color printer, the following procedure is generally used. First, the monitor color signal is converted into a signal in the color gamut compression color space. Next, color gamut compression processing for converting this signal into a color within the color gamut of the printer is performed. Further, the color gamut-compressed signal is converted into a printer color signal and output.

The color gamut compression color space is generally a colorimetric value space, typically a CIE / L ^* a ^* b ^* color space and a CIE / L ^* C ^* h color obtained by converting this into a polar coordinate system. Space is used. (The CIE / L * ^C * ^h of L ^* is the lightness corresponding to the brightness, the C ^* is chroma corresponding to saturation, h is the hue corresponding to the color of the system correspond respectively.) Further, a method of converting a color signal in a corrected color space in which the distortion of hue (especially in the blue region) in the CIE / L ^* C ^* h color space is corrected has also been proposed.

As a method of color gamut compression, when the color gamut of the printer is smaller than the color gamut of the monitor, CIE / L ^* C ^* h
In a color space, or a color space modified from this, in the equal hue plane including the input color signal, a continuous color signal (input range) in the monitor color gamut including the input color signal and a continuous color signal in the printer color gamut ( Output range), and the input color signal is converted by a predetermined conversion method for converting the color signal on the input range into the color signal on the output range.

An example of a specific method for obtaining the input range and the output range will be described with reference to FIG. FIG. 13 is an equal hue plane including the input color signal Pi, where P0 is a predetermined compression fixed point,
P1 indicates a point on the outermost color gamut of the monitor, and P2 indicates a point on the outermost color gamut of the printer. In FIG. 13, the color signal on the line segment P0-P1 is the input range, and the color signal on the line segment P0-P2 is the output range.

A method of converting an input color signal into an output color signal from the input range and the output range will be described with reference to FIG. The figure schematically shows an example of the proposed conversion method.

In the conversion based on 1401 in FIG. 14, the input color signal is converted into the output color signal while maintaining the gradation. Further, in the conversion based on 1402, the color signal within the color gamut of the monitor is output as it is, and the color outside the color gamut is clipped to the outline of the color gamut. Further, in the conversion based on 1403, the low chroma color signal is output as it is, and the high chroma color signal is largely compressed and output.

As described above, in the conventional color gamut compression processing, the CIE / L ^* C ^* h color space, or CIE /
In the corrected color space in which the distortion of the L ^* C ^* h color space is corrected,
A method of converting a color signal under the constraint of constant hue is used.

[0009]

However, the appearance of the color is different between the light source color and the object color, and the hues of the same color are different. For example, in a color-matching experiment between a monitor and a printer, the colorimetric value of a print that matches the color chart displayed on the monitor is a color obtained by rotating the hue counterclockwise, The colorimetric value of a print that is color-matched to another color chart was a color obtained by rotating the hue clockwise. The phenomenon of different color matching points is not caused by the distortion of color matching lines in the color space, but by the difference in color appearance between the light source color and the object color. It was a problem.

The present invention has been made in view of the above problems, and an object thereof is to reproduce a color image displayed on a monitor more favorably by a printer in consideration of a difference in appearance between a light source color and an object color. .

[0011]

In order to achieve the above object, the image processing method of the present invention provides a hue angle correspondence table for a light source color and an object color for correcting the difference in appearance between the light source color and the object color. Holds the hue of the printer color signal corresponding to the hue of the monitor color signal on the basis of the hue correspondence table, based on the color gamut information of the printer in the hue of the printer color signal obtained, based on the monitor color signal The color gamut compression processing is performed on the other hand.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below with reference to the drawings.

(First Embodiment) FIG. 1 is a diagram showing an example of a schematic configuration of an image processing apparatus to which the present invention is applied. The image processing apparatus includes a monitor 11, an image input unit 12, an image processing unit 13, an image output unit 14, and a printer 15.
The color image data displayed on the monitor 11 is input through the image input unit 12, converted into the color gamut compressed color space by the image processing unit 13, and then subjected to a predetermined color gamut compression process, and the image output unit. It is output to the printer 15 via 14.

The image processing unit 13 includes an input side color space conversion unit 1.
31, a color gamut compression processing unit 132, and an output side color space conversion unit 133. The input side color space conversion unit 131 converts the color image data in the color space of the monitor 11 input via the image input unit 12 into a color gamut compressed color space. In the color gamut compression processing unit 132, color gamut compression processing is performed on the color image data in the color gamut compression color space supplied via the input side color space conversion unit 131 so that the color gamut within the color gamut reproducible by the printer 15 is obtained. Convert to data. In the output side color space conversion 133, after converting the color image data in the color gamut compression color space that has been subjected to the color gamut compression processing by the color gamut compression processing unit 132 into the color image data in the color space of the printer 15, The image is output to the printer 15 via the image output unit 14.

FIG. 2 is a view showing the arrangement of the color processing apparatus of this embodiment. As shown in the figure, the color processing device according to the present embodiment includes a color signal input unit 201, an input color signal holding unit 202,
Monitor gamut holding unit 203, printer gamut holding unit 204,
Hue conversion information holding unit 205, input range calculation unit 206,
The input range holding unit 207, the output hue calculation unit 208, the output hue holding unit 209, the output range calculation unit 210, the output range holding unit 211, the output color signal calculation unit 212, the output color signal holding unit 213, and the color signal output unit 214. Consists of.

The monitor color gamut holding unit 203 and the printer color gamut holding unit 204 respectively store monitor color gamut information and printer color gamut information. The hue conversion information holding unit 205 stores a correction table that associates the hue angles of the monitor and the printer in consideration of the difference in appearance between the light source color and the object color. The input color signal is stored in the input color signal holding unit 202 via the color signal input unit 201. The input range calculation unit 206 obtains the end point of the input range corresponding to the input color signal from the color signal stored in the input color signal holding unit 202 and the monitor color gamut information stored in the monitor color gamut holding unit 203, The result is stored in the input range holding unit 207. Output hue calculator 208
Calculates the output hue corresponding to the input color signal from the color signal stored in the input color signal holding unit 202 and the correction table stored in the hue conversion information holding unit 205, and stores the result in the output hue holding unit 209. To do. Output range calculator 210
Is an input color signal from the color signal stored in the input color signal holding unit 202, the printer color gamut information stored in the printer color gamut holding unit 204, and the output hue stored in the output hue holding unit 209. The end point of the corresponding output range is obtained, and the result is stored in the output range holding unit 211. The output color signal calculator 212 receives the input color signal holder 2 based on the endpoints of the input range stored in the input range holder 207 and the endpoints of the output range stored in the output range holder 211.
The input color signal stored in 02 is subjected to predetermined conversion, and the converted output color signal is stored in the output color signal holding unit 213.
The output color signal stored in the output color signal holding unit 213 is output via the color signal output unit 214.

<Color Gamut Information Holding Unit> An example of the color gamut information stored in the monitor color gamut holding unit 203 and the printer color gamut holding unit 204 will be described with reference to FIGS. The monitor gamut holding unit 203 and the printer gamut holding unit 204 store monitor gamut information and printer gamut information, respectively.

The color gamut information is obtained by determining a point P on the outermost contour of the color gamut and a predetermined fixed point P'in the equi-hue plane of the hue angle h shown in FIG.
Let k be the length of the line segment that connects to and L
^* A table that holds the values of k in discrete values of h and φ, as shown in FIG. 11, where φ is the angle formed with a constant straight line (lc). The values of k at arbitrary h and φ are obtained by interpolation with reference to this table.

<Determination of Color Gamut> CIE / L ^* a ^* b ^* coordinate (L, a, b) in the color space is hφ according to the following equation.
It can be converted into coordinates (h, φ, k) in the k color space.

H = tan ⁻¹ (b / a) (Equation 13) φ = tan ⁻¹ ((L−L0) / C) (Equation 14) k = √ {C ² + (L−L0) ² } ( Equation 15) However, the coordinate of P ′ is [L0, 0.0, 0.0] C = √ (a ² + b ² ) (Equation 16) Using the above conversion equation, the hφk coordinate of an arbitrary color signal P can be calculated. By determining and comparing with the printer color gamut information, it is possible to determine whether the point P is inside or outside the color gamut. Specifically, if the value kp of k at the point P (hp, φp, kp) is smaller than the value k at hp, φp obtained from the printer gamut information, the point P is within the gamut and the gamut If it is larger than the value of k obtained from the information, it is determined that the point P is outside the color gamut.

<Hue Conversion Information Holding Unit> An example of the correction table stored in the hue conversion information holding unit 205 will be described with reference to FIG. As shown in FIG. 12, the correction table is a table that holds a discrete monitor hue angle hm and a printer hue angle hp corresponding thereto. This correspondence table is created based on the hue angle calculated by obtaining a set of printer output colors corresponding to the monitor display colors by color matching experiments and calculating from the colorimetric values of both. Further, the printer hue angle at an arbitrary monitor hue angle is obtained by interpolation with reference to this table.

<Color Gamut Compression Method> FIG. 3 is a flowchart showing the procedure of the color gamut compression method in this embodiment. In step S301, the input color signal is stored in the input color signal holding unit 202 via the color signal input unit 201. In step S302, the input range calculator 20
Reference numeral 6 obtains the end point of the input range from the input color signal stored in the input color signal holding unit 202, the monitor color gamut information stored in the monitor color gamut holding unit 203, and the coordinates of the compression fixed point,
It is stored in the input range holding unit 207. Step S303
In, the output hue calculation unit 208 obtains an output hue from the input color signal stored in the input color signal holding unit 202 and the hue correction table stored in the hue conversion information holding unit 205, and the output hue holding unit 209 To store. Step S
In 304, the output range calculation unit 210 causes the input color signal stored in the input color signal holding unit 202, the output hue stored in the output hue holding unit 209, and the printer color gamut holding unit 204 to store the input color signal. The end point of the output range is obtained from the color gamut information and the coordinates of the compression fixed point, and is stored in the output range holding unit 211. In step S <b> 305, the output color signal calculation unit 212, based on the end point of the input range stored in the input range holding unit 207, the end point of the output range stored in the output range holding unit 211, and the coordinates of the compression fixed point. The color signal stored in the input color signal holding unit 202 is subjected to predetermined conversion, and the output color signal after conversion is stored in the output color signal holding unit 213. In step S306, the output color signal stored in the output color signal holding unit 213 is output to the printer via the color signal output unit 214.

<Method for obtaining input range and output range>
A method for calculating the end points of the input range and the end points of the output range will be described in detail with reference to FIGS. 4 and 5. FIG. 4 is a diagram in which the equal hue planes of the hue of the input color signal (hm in FIG. 5) and the output hue stored in the output hue holding unit 209 (hp in FIG. 5) are overlapped, and the horizontal axis represents the saturation, The vertical axis represents the brightness. In FIG. 4, Ci is the hue hm of the input color signal.
At the outermost color gamut of the monitor, Co is the outermost color gamut of the printer at the output hue hp, point Pi is the input color signal, and point P0 is the compression fixed point in the color gamut compression, for example {C ^* , L ^* } = It is {0.0, 50.0}. The end point of the input range is the intersection point P1 between the straight line connecting the point P0 and the point Pi and the outermost color gamut Ci of the monitor. The end point of the output range is the intersection point P2 between the straight line connecting the point P0 and the point Pi and the outermost color gamut Co of the printer. Here, P1 is a point on the hue hm of the input color signal, and P2 is a point on the output hue hp.

An example of the procedure for obtaining the input range is shown in the flowchart of FIG. This shows the step S302 in FIG. 3 in detail. In step S601, a color signal vector P is obtained from the input color signal vector Pi stored in the input color signal holding unit 202 and the compression fixed point vector P0 by the following formula.

P = ((ΔE (Pi, P0) + S) / ΔE (Pi, P0)) (Pi−P0) (Equation 1) However, the color signal vector is L in the CIE / L ^* a ^* b ^* space. ^When the values of ^* , a ^* , and b ^* are used as elements and T1 and T2 are color signals, ΔE (T1, T2) is a distance function given by the following Expression 2.

T1 = [L1, a1, b1], T2 = [L2, a2, b2] ΔE (T1, T2) = √ {(L1-L2) ² + (a1-a2) ² + (b1-b2) ² } (Formula 2) Here, S is a variable and the initial value is 0. Step S6
In 02, it is determined whether the color signal vector P is within the color gamut of the monitor or out of the color gamut based on the color gamut information stored in the monitor color gamut holding unit 203. If it is within the color gamut, the process proceeds to step S603, If so, the process proceeds to step S605. In step S603, the color signal vector P is stored in the color signal vector variable Pp, and the process proceeds to step S604. In step S604, the step variable S is updated by Expression 3, and the process proceeds to step S601.

S = S + ΔS (Equation 3) Here, ΔS is a predetermined step constant, and the smaller the ΔS, the more accurately the input range can be obtained. In step S605, the color signal vector variable Pp is stored in the input range holding unit 207.

An example of the procedure for obtaining the output range is shown in the flowchart of FIG. This shows the step S304 in FIG. 3 in detail. In step S701, the input color signal vector Pi (Li, ai, bi) stored in the input color signal holding unit 202, the output hue (ho) stored in the output hue holding unit 209, and the compression fixed point vector P0 (L0, L0, a0, b0) and the color signal vector Pi '(L', a ', b') is calculated by the following equation.

L ′ = Li (Equation 4) a ′ = C cos (ho) (Equation 5) b ′ = C sin (ho) (Equation 6) where C = √ (ai ² + bi ² ) (Equation 7) In step S702, a color signal vector P'is obtained from the color signal vector Pi 'and the compression fixed point vector P0 by the following formula.

P = ((ΔE (Pi ′, P0) + S) / ΔE (Pi ′, P0)) (Pi′−P0) (Equation 8) Here, S is a variable and the initial value is 0. Step S7
In 03, it is judged whether the color signal vector P is within the color gamut of the printer or out of the color gamut based on the color gamut information stored in the printer color gamut holding unit 204, and if it is within the color gamut, step S70.
8, the process proceeds to step S704 if it is outside the color gamut. In step S704, the step variable S is updated by Expression 9.

S = S-ΔS (Equation 9) Here, ΔS is a predetermined step constant, and the output range can be more accurately obtained by reducing ΔS. In step S705, the color signal vector P
Ask for. In step S706, based on the color gamut information stored in the printer color gamut holding unit 204, it is determined whether the color signal vector P is within or outside the color gamut of the printer. If there is, go to step S704. In step S707, the color signal vector P is stored in the output range holding unit 211. In step S708, the color signal vector P is stored in the color signal vector variable Pp. In step S709, the step variable S is updated by Expression 3. In step S710, the color signal vector P is obtained by the equation 8. In step S711, based on the color gamut information stored in the printer color gamut storage unit 204, it is determined whether the color signal vector P is within or outside the color gamut of the printer. If there is, it proceeds to step S712. In step S712, the color signal vector variable Pp is stored in the output range holding unit 211.

<Method for Obtaining Output Hue> The procedure for obtaining the output hue is shown in the flowchart of FIG. This shows step S303 in FIG. 3 in detail. In step S801, the hue angle hm of the input color signal is obtained based on the input color signal stored in the input color signal holding unit 202. In step S802, the printer hue angle hp corresponding to the monitor hue angle hm is obtained by interpolation with reference to the correction table stored in the hue conversion information holding unit 205. In step S803, hp is stored in the output hue holding unit 209.

<Color Signal Conversion Method> An example of an input color signal conversion method will be described with reference to FIG. In FIG. 9, the horizontal axis is the input value, the vertical axis is the output value, S1 is the distance between the compression fixed point P0 and the end point P1 of the input range, Si is the distance between P0 and the input color signal Pi, and S2 is the distance between P0 and the output range. The distance of the end point P2, So is the distance between P0 and the converted output color signal Po. The conversion of the color signal is performed by the following formula based on the conversion function of FIG. In the conversion function of FIG. 9, values up to 80% of S2 are output as they are, and values higher than that are linearly compressed and output.

When Si ≦ 0.8xS2 So = Si (Equation 10) When Si> 0.8xS2 So = (0.2xS2) / ((S1- (0.8xS2)) x (Si-0.8xS2 ) + 0.8 × S2 (Equation 11) Using the above So and the distance function ΔE of Equation 2, the output color signal P
o is calculated by Equation 12.

[0035]     Po = (So / ΔE (P0, P2)) × P2 + ((S2-So) / ΔE (P 0, P2)) xP0 (Equation 12)

(Other Embodiments) <Color Signal Conversion Method> The color signal conversion method is not limited to the method shown in the above embodiment, and the method of FIG. 14 described in the prior art or another conversion method is used. You may.

<Compression Fixed Point> Although the compression fixed point in the above embodiment is one point on the achromatic color axis (C ^* = 0.0), it is not limited to this and may be a point which differs depending on the input color signal.

<Distance Function> The distance function ΔE is not limited to the method in the above embodiment, but may be the CIE / 1994 color difference formula, C
Other color difference formulas such as MC color difference formula, BFD color difference formula and MLAB color difference formula may be used.

<Color Gamut Inside / Outside Judgment> The color gamut inside / outside judgment is not limited to the method in the above-mentioned embodiment, and other methods such as a sandwiching method may be used.

<Color Gamut Information> The color gamut information is not limited to the parameters in the above embodiment, and may be held in any other form that allows the inside / outside of the color gamut to be determined for any color signal.

<Others> Even when the present invention is applied to a system composed of a plurality of devices (for example, host computer, interface device, league, printer, etc.), a device composed of one device (for example, copying machine, It may be applied to a facsimile machine).

Further, an object of the present invention is to supply a storage medium storing a program code of software for realizing the functions of the above-described embodiment to a system or apparatus, and to supply a computer (or CPU) of the system or apparatus.
Needless to say, this is also achieved by the MPU) reading and executing the program code stored in the storage medium.

In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention.

As a storage medium for supplying the program code, for example, a floppy (registered trademark) disk, a hard disk, an optical disk, a magneto-optical disk, C
A D-ROM, a CD-R, a magnetic tape, a non-volatile memory card, a ROM, etc. can be used. Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also the OS (operating system) running on the computer based on the instructions of the program code.
Needless to say, this also includes the case where the above-mentioned processes perform a part of the actual processing and the functions of the above-described embodiments are realized by the processing.

Further, after the program code read from the storage medium is written in the memory provided in the function expansion board inserted into the computer or the function expansion unit connected to the computer, based on the instruction of the program code, It goes without saying that a case where the CPU or the like included in the function expansion board or the function expansion unit performs some or all of the actual processing and the processing realizes the functions of the above-described embodiments is also included.

[0046]

As described above, according to the present invention,
It is possible to correct the hue unmatching due to the difference in appearance between the light source color and the object color, and it is possible to better reproduce the color image displayed on the monitor by the printer.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of an image processing apparatus to which the present invention is applied.

FIG. 2 is a configuration diagram of an embodiment.

FIG. 3 is a flowchart showing a procedure of color gamut compression in the embodiment.

FIG. 4 is a diagram showing how to obtain an input range and an output range in the embodiment.

FIG. 5 is a diagram showing hues of an input range and an output range in the embodiment.

FIG. 6 is a flowchart showing an input range setting procedure in the embodiment.

FIG. 7 is a flowchart showing an output range setting procedure in the embodiment.

FIG. 8 is a flowchart showing an output hue setting procedure in the embodiment.

FIG. 9 is a flowchart showing an input / output conversion function in the embodiment.

FIG. 10 is a diagram showing parameters of color gamut information in the embodiment.

FIG. 11 is a diagram showing an example of color gamut information in the embodiment.

FIG. 12 is a diagram showing an example of hue conversion information in the embodiment.

FIG. 13 is a schematic diagram showing a method of gamut compression toward a fixed point on an achromatic axis.

FIG. 14 is a diagram showing a general input / output conversion function.

Continued front page    F-term (reference) 2C262 AB11 BA01 BA09 BA18 BC01                       BC19 CA07 CA08 CA10                 5B057 CE11 CE17 CE18 CH07 CH11                       DB02 DB06 DB09                 5C077 MP08 PP15 PP35 PP36 PP37                       PQ22 PQ23 TT02                 5C079 HB06 HB08 LA12 LA26 LB02                       MA01 MA04 NA03 PA03

Claims (6)

[Claims] 1. An image processing method for converting a monitor color signal into a printer color signal, which holds a hue angle correspondence table between a light source color and an object color for correcting a difference in appearance between the light source color and the object color, The hue of the printer color signal corresponding to the hue of the monitor color signal is obtained based on the hue correspondence table, and the gamut compression is performed on the monitor color signal based on the printer gamut information in the obtained hue of the printer color signal. An image processing method characterized by performing processing. 2. The color gamut compression processing is based on the color gamut information of the monitor in the hue of the monitor color signal and the color gamut information of the printer so that gradations outside the color gamut of the printer are also stored. The image processing method according to claim 1, wherein a color signal is converted. 3. An image processing method for converting a monitor color signal into a color signal corresponding to a color gamut of a printer and outputting the color signal, wherein a correction condition for correcting a difference in appearance between a light source color and an object color is held, An image processing method, wherein the monitor color signal is converted into a color signal corresponding to a color gamut of the printer using the correction condition. 4. The color processing apparatus according to claim 3, wherein the correction condition is a hue angle correspondence table for light source colors and object colors. 5. A program for realizing an image processing method for converting a monitor color signal into a printer color signal, wherein the hue in the light source color and the object color for correcting the difference in appearance between the light source color and the object color. An angle correspondence table is held, the hue of the printer color signal corresponding to the hue of the monitor color signal is obtained based on the hue correspondence table, and based on the color gamut information of the printer in the hue of the obtained printer color signal, the monitor A program for implementing color gamut compression processing on color signals. 6. An image processing apparatus for converting a monitor color signal into a printer color signal, and a table for holding a correspondence between hue angles of a light source color and an object color for correcting a difference in appearance between the light source color and the object color. A means for obtaining the hue of the printer color signal corresponding to the hue of the monitor color signal based on the hue correspondence table; and a means for obtaining the hue of the printer color signal for the monitor color signal based on the printer color gamut information in the hue of the printer color signal obtained. And a color gamut compression unit that performs color gamut compression processing.