JP2001251524A - Image processor and image processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resolve the problem where a color close to gray is affected by grating pints of chromatic colors and is deflected to the chromatic color side to tinge a monochromatic image or an achromatic area of a color image in color conversion processing dependent upon interpolation operation, using a conversion table. SOLUTION: Input image data has the color converted to data for print by three-dimensional interpolation operation using a conversion table 4 (S3), and it is discriminated as to whether the color represented by input image data is an achromatic color or a color approximating an achromatic color (S4 to S6), and data for print obtained in the step S3 is corrected, on the basis of a pint Ig obtained by projecting the color represented by input image data to the achromatic base and data obtained from the conversion table 4 with respect to input image data, having an achromatic color or a color approximating an achromatic color (S8).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image processing apparatus and method, and more particularly, to an image processing apparatus and method for generating image data for forming an image on a medium using a plurality of color materials.

[0002]

2. Description of the Related Art A color conversion process used in a color printer or the like is performed by an interpolation operation using a three-dimensional table representing a relationship between color data in a certain color space and color data formed by a color material such as ink. Done.

[0003]

In order to improve the accuracy of the color conversion process in the vicinity of gray, a tetrahedron surrounding an input color is determined in a three-dimensional table, and interpolation is performed using four grid points of the tetrahedron. Body interpolation is known. This is because when performing an interpolation operation near gray, the gray axis (achromatic axis) connecting the black point and the white point is one side of the tetrahedron, so the color on the gray axis is the achromatic color at both ends of the gray axis. This is because the interpolation calculation is performed from the two grid points. Further, since the color near the gray axis and the color continuous from the gray axis are obtained by the same tetrahedral interpolation, the continuity of the color is also guaranteed. However, the solids that can be used for interpolation are limited to those having a gray axis as a side, such as a tetrahedron or an oblique triangular prism.

On the other hand, in cubic interpolation having eight grid points,
Since the points on the gray axis are obtained from all eight lattice points, they are affected by chromatic six lattice points other than achromatic two lattice points. Further, in a cube in which only one grid point is on the gray axis, the color near gray shifts to the chromatic color side due to the influence of the seven chromatic grid points, causing a problem in which a monochrome image is colored (hereinafter referred to as “color shift”). ").

Similarly, in interpolation using a solid having no gray axis on a side, such as in the case of triangular prism interpolation having six grid points, the color near gray is affected by the chromatic grid points. There is a problem in that the monochrome image is colored to the side and the achromatic region of the color image is colored.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems individually or collectively, and has as its object to improve the accuracy of color conversion processing and / or color continuity in and around the gray axis. .

Another object of the present invention is to alleviate the restrictions on the interpolation solid.

[0008]

The present invention has the following configuration as one means for achieving the above object.

An image processing apparatus according to the present invention comprises a conversion means for converting input image data into data for image formation by three-dimensional interpolation using a conversion table, and a color represented by the input image data. Determining means for determining whether or not the color is an achromatic color or a color close to an achromatic color; and a point obtained by projecting a color represented by the input image data on the achromatic color axis with respect to the input image data of the achromatic color or a color close to the achromatic color. And correction means for correcting the image forming data obtained by the conversion means based on the data obtained from the conversion table.

According to the image processing method of the present invention, the input image data is color-converted into data for image formation by three-dimensional interpolation using a conversion table, and the color represented by the input image data is achromatic or Determining whether the color is close to an achromatic color, and for the input image data of the achromatic color or a color close to the achromatic color, projecting a color represented by the input image data onto an achromatic color axis; and The data for image formation obtained by the color conversion is corrected based on the data obtained from.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image processing apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

[0012]

[First Embodiment] FIG. 1 is a block diagram showing a configuration example of the present embodiment.

In FIG. 1, image data 1 to be printed is
This is index image data having index values corresponding to RGB values. A color palette indicating the relationship between the RGB values and the index values is usually attached to the image data 1 as header data. The input image data 1 is not limited to the index image data, but may be full-color RGB image data.

The conversion table 2 stored in the ROM or the like is a table obtained from the characteristics of the printing unit 5 and its color materials, and defines the color material values when printing a predetermined grid point coordinate group in the RGB space. This is a table in which data to be stored is stored.
FIG. 2 is a diagram showing a configuration example of the conversion table, in which CMYK values corresponding to RGB values are stored.

An image processing unit 3 composed of a one-chip microprocessor or the like uses the conversion table 2 to convert the image data 1 into data for printing. The image processing unit 3 includes a gray correction unit 4 that performs a gray correction process described later.

The printing unit 5 prints a visible image corresponding to the print data output from the image processing unit 3 on a medium using, for example, four CMYK inks.

FIG. 3 shows an image processing unit 3 and a gray correction processing unit.
14 is a flowchart illustrating an example of a processing procedure of No. 4;

The image processing unit 3 acquires the image data 1 to be printed in step S1, and obtains the RGB value of the corresponding color palette from the index value of the image data 1 in step S2.

Next, in step S3, for each pixel data (RGB value) of the image data 1, the corresponding CMYK value Qc is obtained by cubic interpolation using the conversion table 2.
The interpolation operation to be used, as shown in equation (1), divides the cube into eight rectangular parallelepiped from a total of nine grid points and eight RGB values of the cube, and calculates the CMYK values of their volumes and grid points. A weighted average is calculated. Qc = {Σ _{i = 0} ⁷ Pi × Vi} / Σ _{i = 0} ⁷ Vi… (1) where Pi: C, M, Y, K values of grid points Vi: Volume of rectangular parallelepiped

By the way, for example, if the interval between grid points is 17 steps in each RGB axis, the distance R between the gray axis and the nearest grid point is
Is represented by equation (2). ^{R = √ {17 2 - 17} 2/3} = 17√ (2/3) ≒ 13.88 ... (2)

Therefore, if the distance from the gray axis exceeds R
The GB value is determined to be out of the gray correction target. By this determination, the color corresponding to the grid point of the conversion table 2 is determined in step S
The printing is performed without being affected by the gray correction processing of the fourth and subsequent steps, and the adverse effect of reducing the accuracy of the color corresponding to the lattice point can be eliminated.

Next, in step S4, a projection point at which the RGB values are projected on the gray axis is determined. However, the projection point is a point at which a perpendicular is drawn from the RGB value to the gray axis. RGB coordinates I
If c (r, g, b), the coordinates of the projection point Ig (x, x, x), x = (r + g + b) / 3
become.

Next, in step S5, the distance d between the two points Ic and Ig is determined. The distance d is obtained by the equation (3) from the coordinates of three points including the origin and the theorem of three squares. d = √ {(r ² + g ² + b ² )-(x ² + x ² + x ² )}… (3)

Next, in step S6, the distance d is compared with the distance R, and if d ≦ R, the process proceeds to step S7. If d> R, steps S7 and S8 are jumped to proceed to step S9.

In step S7, the interpolation value Qg of the projection point Ig is calculated by setting the data of the chromatic lattice points of the cube surrounding the RGB values to (C, M, Y, K) = (0, 0, 0, 0). I do. Then, in step S8,
The weighted average of the two points Qc and Qg obtained in steps S3 and S7 is calculated by equation (4), and the result is used as the CMYK value Qc. Equation (4) is for the case where the interval between the lattice points is set to 17 in each of the RGB axes. Qc = Qg × {1-d / 17 × √ (3/2)} + Qc × d / 17 × √ (3/2) = Qg + (Qc-Qg) × d / 17 × √ (3/2) … (4) where 0 ≦ d ≦ 17√ (2/3)

Next, in step S9, the obtained CMYK values of Qc are binarized (or quantized) by using pseudo halftone processing such as an error diffusion method, and converted into print data. The printing data is sent to the printing unit 5 in accordance with the image forming operation of the printing unit 5, and a visible image is formed on the medium. In addition,
If the printing unit 5 is a multi-valued printer such as a color laser beam printer, it is not necessary to use the pseudo halftone processing, and the obtained CMYK value Qc can be used as it is, or the CMYK value subjected to the necessary filter processing can be output. I just need.

The processing in steps S4 to S8 is a gray correction processing performed mainly by the gray correction unit 4.
Other processes are processes performed mainly by the image processing unit 3.

As described above, according to the present embodiment, when color conversion processing is performed by three-dimensional interpolation using a conversion table, for a color near gray, the CMYK corresponding to that color is used.
Achromatic CMYK values and their colors projected on the gray axis
Values (gray correction values), weighted averages of these CMYK values based on the distance from the gray axis to obtain the final CMYK values. Can be improved. Therefore, it is possible to improve the color reproducibility of an achromatic color and a region in the vicinity thereof when forming a monochrome image and a color image on a medium.

Further, the solid used for the interpolation is not limited to the one including the gray axis as a side of the solid, but can be applied to any solid interpolation, and the tint improvement near the gray and the color continuity are achieved. You.

[0030]

Second Embodiment Hereinafter, an image processing apparatus according to a second embodiment of the present invention will be described. Note that, in the present embodiment,
Components that are substantially the same as those in the first embodiment are given the same reference numerals, and detailed descriptions thereof are omitted.

In the first embodiment, in the RGB space
Projection point Ig perpendicular to the gray axis from the RGB value coordinate point Ic
The example in which the gray correction is performed by the weighted average using the CMYK values Qg described above (see FIG. 4). However, the color of Ig point is RGB of Ic point
It is not the closest to an achromatic color. If you want a color that is visually closer, use a uniform color space such as L * a * b *.
It is desirable to find a point corresponding to the Ig point. However, processing all data in a uniform color space is computationally expensive.

Therefore, in the second embodiment, the lightness (for example, L *) is visually dominant in the lightness (or luminance) and the color difference, and the condition that the lightness (for example, L *) is close to gray is satisfied. A point on the gray axis having the same lightness is used as an Ig point.

When L * a * b * colorimetry conditions are illumination light D65 and a viewing angle of 10 degrees, and L * is represented by an XYZ value, equation (5) is obtained. L * = 116 (Y / 100.0) ¹ /3-16… (5)

When the Y value is represented by an RGB value, the expression (6) is obtained. Y = 0.30R + 0.59G + 0.11B… (6)

The fact that L * is equal means that the Y values are equal. Therefore, as shown in FIG. 5, the point corresponding to Ic (r, g, b) is Ig
(y, y, y), y = (0.30r + 0.59g + 0.11b).

In step S4 shown in FIG. 3, if the projection point Ig is obtained by the above-described method and the subsequent processing is executed, gray correction for obtaining a visually closer color near the gray axis is executed.

[0037]

[Modification] In each of the above embodiments, a linear example in which the ratio change of the weighted average is simple has been described (see FIG. 6). This is an advantage in that the load is the least in calculation and color continuity can be ensured.

However, since the change of the CMYK value is not smooth at the boundary where the processing is completely switched at the end of the processing area, when gradation or the like is printed, the ratio of the dark ink rapidly changes. The boundaries may be visible as density and / or color steps.

In order to solve this, CMYK is applied between the gray axis (Qc 100%), which is the processing switching point, and the end of the processing area (Qc 0%).
Weighted averaging is performed so that the change in the value is smooth, that is, the differential value of the change is continuous.

As a ratio graph, as shown in FIG.
It is desirable that the inclination be substantially zero between the gray axis and the end of the processing area. As a result, the amount of change in dark ink at the processing boundary can be reduced, and the continuity of density and / or color can be improved.

[0041]

[Other Embodiments] Even if the present invention is applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), an apparatus (for example, a copying machine) Machine, facsimile machine, etc.).

Further, an object of the present invention is to provide a storage medium (or a recording medium) recording a program code of software for realizing the functions of the above-described embodiments to a system or an apparatus, and to provide a computer (a computer) of the system or the apparatus. It is needless to say that the present invention can also be achieved by a CPU or an MPU) reading and executing the program code stored in the storage medium. In this case, the program code itself read from the storage medium implements the functions of the above-described embodiment, and the storage medium storing the program code constitutes the present invention. Also,
When the computer executes the readout program code, not only the functions of the above-described embodiments are realized, but also the operating system (O) running on the computer based on the instructions of the program code.
Needless to say, S) and the like perform part or all of the actual processing, and the processing realizes the functions of the above-described embodiments.

Further, after the program code read from the storage medium is written into the memory provided in the function expansion card inserted into the computer or the function expansion unit connected to the computer, the program code is read based on the instruction of the program code. Needless to say, the CPU included in the function expansion card or the function expansion unit performs part or all of the actual processing, and the processing realizes the functions of the above-described embodiments.

When the present invention is applied to the storage medium, the storage medium stores program codes corresponding to the above-described flowchart shown in FIG.

[0045]

As described above, according to the present invention,
It is possible to improve the accuracy of color conversion processing and / or continuity of colors in and around the gray axis.

In addition, the limitation on the interpolation solid can be relaxed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration example of an embodiment;

FIG. 2 is a diagram showing a configuration example of a conversion table shown in FIG. 1;

FIG. 3 is a flowchart illustrating an example of a processing procedure of an image processing unit and a gray correction processing unit illustrated in FIG. 1;

FIG. 4 is a view for explaining gray correction in the first embodiment;

FIG. 5 is a view for explaining gray correction according to the second embodiment;

FIG. 6 is a diagram illustrating an example of a simple linear change in the weighted average ratio;

FIG. 7 is a diagram illustrating an example of a change in the ratio of a weighted average in which the inclination is substantially zero between the gray axis and the end of the processing area.

Continued on front page F-term (reference) 2C262 AA24 AC19 BA01 BA20 BC01 BC11 BC15 BC19 5B057 AA11 BA29 CA01 CA08 CA12 CA16 CB01 CB07 CB08 CB12 CB16 CE17 CE18 CH07 CH08 DA17 DB03 DB06 DB09 DC03 5C077 MP08 PP32 PP33 PP36 PP37 PP38 PP38 PP46 PP37 TT02 5C079 HB01 HB03 HB08 HB12 LA03 LA28 LB01 MA04 MA11 NA03

Claims (8)

[Claims] 1. A conversion means for converting input image data into data for image formation by a three-dimensional interpolation operation using a conversion table, wherein the color represented by the input image data is achromatic or achromatic. Determining means for determining whether or not the color is close; for the input image data of the achromatic color or a color close to the achromatic color, a point where a color represented by the input image data is projected onto an achromatic axis; and the conversion table An image processing apparatus comprising: a correction unit configured to correct image forming data obtained by the conversion unit based on data obtained from the image processing unit. 2. The image processing apparatus according to claim 1, wherein the correction unit obtains image forming data of the projection point using the conversion table, and calculates a weighted average according to a distance between a color represented by the input image data and the projection point. 2. The image processing apparatus according to claim 1, wherein the correction is performed. 3. The image processing apparatus according to claim 2, wherein the slope of the weighting function according to the distance in the weighted average is a continuous function. 4. The determination means determines a color of image data having coordinates closer to the achromatic axis than a grid point of the conversion table closest to the achromatic axis as an achromatic color or a color close to an achromatic color. 4. The image processing device according to claim 1, wherein: 5. The image processing apparatus according to claim 1, wherein the projection point is a point obtained by drawing a perpendicular from the coordinates of the target image data to the achromatic axis. apparatus. 6. The image processing apparatus according to claim 1, wherein the projection point has the same lightness as the target image data. 7. A color conversion of input image data into image forming data by a three-dimensional interpolation operation using a conversion table, wherein a color represented by the input image data is an achromatic color or a color close to an achromatic color. For the input image data of the achromatic color or a color close to the achromatic color, a point where the color represented by the input image data is projected onto the achromatic color axis, and based on data obtained from the conversion table, And an image processing method for correcting image forming data obtained by the color conversion. 8. A medium on which a program code for image processing is recorded, wherein the program code converts at least input image data into data for image formation by three-dimensional interpolation using a conversion table. And a code for determining whether the color represented by the input image data is an achromatic color or a color close to an achromatic color; and inputting the input image data of the achromatic color or a color close to an achromatic color. A point where the color represented by the image data is projected onto the achromatic axis, and a code for correcting the image forming data obtained by the color conversion based on the data obtained from the conversion table. Characteristic recording medium.