JP2011254123A - Image processing device, image processing method, program, and recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct a gradation value without causing an disorder in gray balance or hue deflection in secondary color and without reducing color gamut as less as possible, even when solid density has changed to become lower than target density.SOLUTION: A gamma correction unit 1 corrects image data composed of patches used for calibration according to a setup correction table, and a printer output unit 2 performs halftone processing on gamma-corrected image data to convert it into color material amounts and outputs on a paper as a patch image 3. A color measuring unit 4 acquires density values by measuring colors of individual patches output on the paper with a colorimeter. A target density reducing unit 5 converts target density stored in a storage device 7 into corrected target density reflecting the acquired density values. A correction table setup unit 6 sets up a correction table so that single-color density meets the corrected target density.

Description

  The present invention relates to an image processing apparatus, an image processing method, a program, and a recording medium that execute calibration with high accuracy.

  In a color image forming apparatus that forms an image by superimposing multiple component colors, if the solid density (formation density upper limit) of any of the component colors is lower than the target density for calibration, calibration is performed using the gradation correction table. In principle, it is impossible to correct the corresponding component color whose solid density has been reduced to the target density even if the process is performed. Therefore, the technology that creates the gradation correction table by modifying the target density of the corresponding component color according to the actual solid density, and the target density other than the corresponding component color as well as the corresponding component color so that the gray balance is not lost. There is a technique for correcting and creating a gradation correction table.

  For example, in Patent Document 1, even when the maximum density that can be formed by the image forming apparatus is reduced, the user selects a tone adjustment process that emphasizes color balance in order to achieve a suitable tone without losing the tone. When instructed, a new basic gradation characteristic is generated while maintaining the CMYK ratio.

  However, there is a problem that the gray balance is lost or the hue of the secondary color is bent in the method of correcting the target density of only the corresponding component color, and in the method of correcting the target density other than the corresponding component color, There is a problem that the color gamut is narrowed as a whole, including hues that do not use.

The present invention has been made in view of the above problems,
The object of the present invention is to adjust the gradation value so that the gray balance is not lost and the hue is not bent in the secondary color and the color gamut is not narrowed as much as possible even when the solid density fluctuates and becomes lower than the target density. An object is to provide an image processing apparatus, an image processing method, a program, and a recording medium for correction.

  The present invention includes an image forming unit that forms an image composed of a plurality of component colors, a density upper limit value acquiring unit that acquires a density upper limit value of each component color of the image formed by the image forming unit as a formation density upper limit value, and a hue Gamma correction means for correcting gradation values by applying different parameters for each, and reduction control means for controlling the parameters so that the maximum gradation value of each component color is formed at the corrected target density The correction target density is a value obtained by reducing the target density at the maximum gradation value of each component color by reflecting the formation density upper limit value of the component color related to the formation of the hue color. And

  According to the present invention, a correction target density in which the target density is reduced is set to reflect the solid density (formation density upper limit value) for each hue, and the correction table is set so that the density of the single color (component color) matches the correction target density. Is applied to gamma correction, so even if the solid density fluctuates and falls below the target density, gray balance is not lost and hue distortion in the secondary colors does not occur, and the color gamut is as narrow as possible. The gradation value can be corrected so as not to occur.

  Furthermore, in contrast to the conventional example in which the secondary color fluctuates in the hue direction and shifts to both sides with respect to the original hue line, in the present invention, the fluctuation range is relatively small because it varies only in one direction of the saturation direction. Small and stable colors can be output.

The structure of Example 1 of this invention is shown. A patch image is shown. It is a figure explaining a target density reduction part. It is a figure explaining a correction table setting part. It is a figure explaining a gamma correction part. It is a figure explaining the effect by this invention. It is a figure explaining the conventional processing method. It is a figure explaining the effect by this invention. It is a figure explaining the effect by this invention. The structure of Example 2 of this invention is shown. It is a figure explaining the gamma pre-processing part classified by hue and the gamma pre-processing parameter setting part classified by hue. It is a figure explaining the function of gamma preprocessing parameter calculation.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the present invention, when setting the corrected target density reflecting the solid density obtained by colorimetry in calibration, the solid density at that point in time is set according to the target density for each hue of the tertiary color and the secondary color. The correction target density is set by obtaining the maximum value of the gradation range that can be output.

  FIG. 1 shows the configuration of an image processing apparatus according to Embodiment 1 of the present invention. In FIG. 1, 1 is a gamma correction unit, 2 is a printer output unit, 3 is a patch image, 4 is a colorimetry unit, 5 is a target density reduction unit, 6 is a correction table setting unit, and 7 is a storage device.

  The gamma correction unit 1 performs correction according to the set correction table on the image data composed of calibration patches, and the printer output unit 2 performs halftone processing on the gamma-corrected image data. The color material amount is converted and output as a patch image 3 on the paper surface. The color measurement unit 4 measures the color of each patch output on the paper surface using a colorimeter to obtain a density value. Alternatively, the density value is acquired by scanner reading. The target density reduction unit 5 corrects the target density stored in the storage device 7 to reflect the acquired density value, and converts it into a corrected target density. The correction table setting unit 6 sets the correction table so that the density of the single color (component color) matches the correction target density. The correction table applied to the gamma correction unit 1 at the time of patch output for calibration is a correction table set at the time of the most recently performed calibration.

  The gamma correction unit 1 always performs not only when a calibration patch image is output but also when a normal document image is normally output, and performs a gamma correction by applying a correction table created by calibration. Print out the image data after

  The monochromatic density and the target density may be substituted with parameters that can specify the color with one parameter. For example, it may be defined by ΔE between the Lab white value and the monochrome Lab value, that is, the distance from the paper white in the Lab space. In the case of a single color, if the variation in the hue direction is minute and can be ignored, the color can be roughly specified by ΔE.

  FIG. 2 shows a patch image 3, and the printer output unit 2 outputs the patch image 3 including C, M, and Y single-color gradation patches. The patch of the maximum gradation value of each single color is indispensable, and in this embodiment, the maximum gradation value is assumed to be 255.

  FIG. 3 is a diagram for explaining the target density reduction unit 5. The target density of C stored in the storage device 7 is expressed as Tc (n), the target density of M as Tm (n), and the target density of Y as Ty (n). n is a gradation value from 0 to 255. The corrected target concentrations are TRc (n), TRm (n), TRy (n). In the range of 0 ≦ n ≦ th, TRc (n) = Tc (n), TRm (n) = Tm (n), TRy (n) = Ty (n), that is, the density value is not corrected. “th” is a boundary value for correcting the density value, and is set in advance in consideration of the fluctuation range so that the gradation is not completely destroyed in the high density portion even when the solidest density is lowered. .

A method for obtaining the corrected target density in the range of th <n ≦ 255 will be described. If the colorimetric values of C, M, and Y are Dc (n), Dm (n), and Dy (n), the solid density is Dc (255), Dm (255), and Dy (255). From the solid density and the target density Tc (255), Tm (255), and Ty (255) of the maximum gradation value, Cmax, Mmax, and Ymax are obtained by Equation 1, and set as the corrected target density.
Cmax = Min (Dc (255), Tc (255))
Mmax = Min (Dm (255), Tm (255))
Ymax = Min (Dy (255), Ty (255)) Formula (1)

Next, the gradation values nc, nm, ny that satisfy Equation 2 are obtained.
Tc (nc) = Cmax
Tm (nm) = Mmax
Ty (ny) = Ymax Formula (2)

The maximum gradation values nk, nr, ng, and nb that can maintain the color balance in the gray hue, R hue, G hue, and B hue are obtained by Equation 3.
nk = Min (nc, nm, ny)
nr = Min (nm, ny)
ng = Min (nc, ny)
nb = Min (nc, nm) Equation (3)

  For example, when nm> ny, in the range of n ≦ ny, in principle, the target densities Tm (n) and Ty (n) can be output, and the color balance of the R hue is guaranteed, and the hue is No bending occurs. Expression 3 obtains the maximum value of the gradation range that can be output according to the target density with a solid density at each time point for each hue of the tertiary color and the secondary color.

  In FIG. 3, the set correction target density (Cmax, Mmax, Ymax) is shown at the corresponding coordinate position of the cube with the C, M, Y gradation values as axes, and three cubes (a), In (b) and (c), C corrected target concentrations (Cmax, Tc (nb), Tc (ng), Tc (nk)), and M corrected target concentrations (Mmax, Tm (nr), Tm (nb) ), Tm (nk)), and Y corrected target concentrations (Ymax, Ty (nr), Ty (ng), Ty (nk)).

  Monochromatic solid ((C, M, Y) = (255,0,0), (0,255,0), (0,0,255)), secondary solid ((C, M, Y) = ( 0, 255, 255), (255, 0, 255), (255, 255, 0)), the cubic color ((C, M, Y) = (255, 255, 255)) is the coordinates of the vertex of the cube Correction target density (Tc (nb), Tc (ng), Tc (nk), Tm (nr), Tm (nb), Tm (nk), Ty (nr)) , Ty (ng), Ty (nk)) are values that are reduced from the target concentration in accordance with nk, nr, ng, nb determined by Equation 3.

  FIG. 4 is a diagram for explaining the correction table setting unit 6. The correction table setting for C will be described as an example. The inside of a rectangular parallelepiped with a corrected target density C in FIG. 3 and a short C-axis direction represented by a solid line (a rectangular parallelepiped composed of eight vertices, Tc (nb), Tc (ng), Tc (nk), Cmax, Tc (th)) Corresponds to the gradation range in which the target density is reduced, and the correction tables corresponding to the coordinate positions of the four short sides along the C axis are set as γ1 to γ4. However, at least one of the four of γ1 to γ4 is substantially the same correction table.

  For example, in the case of nm <ny <nc, nk = nb = nm from Equation 3, so Tc (nk) = Tc (nb), and the short side (Tc (th) of the third color solid in FIG. A correction table γ3 corresponding to a line segment from a vertex having a value to a vertex having a value of Tc (nk), and a short side including a secondary color (Blue) solid (from a vertex having a value of Tc (th) to Tc The correction table γ4 corresponding to the line segment to the vertex having the value (nb) is substantially the same correction table.

  FIG. 4A is a graph showing the corrected target density TR (C1) corresponding to the gradation value C1 of the rectangular parallelepiped short side indicated by the solid line in FIG. 3, taking the case of nm <ny <nc as an example, and th In the range of <C1 ≦ 255, the correction target density corresponding to each short side is set, so that the line graph is branched.

  FIG. 4B is a graph showing the relationship between the gradation value C2 and the density Dc (C2) acquired by the colorimetric unit 4. Using these two graphs, a C correction table for converting the gradation value C1 of FIG. 4C to the gradation value C2 is set. The correction table represents a combination of C1 and C2 where TR (C1) = Dc (C2), and as illustrated, the combination of C1 and C2 where TR (C1) = Dc (C2) = a is C1a and C2a. Then, C2a is set to be output with respect to the input C1a. This correction table becomes a plurality of correction tables branched at the high density portion, reflecting the correction target density table of FIG.

  The M correction table and the Y correction table are also set in the same procedure as the C correction table. In the high density portion larger than th, a plurality of correction tables corresponding to the short sides of the solid rectangular parallelepiped at the correction target density of the corresponding color in FIG. 3 are set.

  FIG. 5 is a diagram for explaining the gamma correction unit 1. Γ1 to γ4 in FIG. 4C correspond to the short sides of the solid rectangular parallelepiped as shown in FIG. 5A. γ1 corresponds to a short side (line segment from a vertex having a value of Tc (th) to a vertex having a value of Cmax) including a monochrome Cyan solid ((C, M, Y) = (255, 0, 0)) Γ2 is a correction table from the vertex having the value of Tc (th) including the secondary color Green solid ((C, M, Y) = (255, 0, 255)). Γ3 is the short side (Tc (th)) including the tertiary color Gray ((C, M, Y) = (255, 255, 255)). Is a correction table corresponding to a line segment from a vertex having a value to a vertex having a value of Tc (nk), and γ4 is a secondary color Blue ((C, M, Y) = (255, 255, 0) ) Including a short side (a line segment from a vertex having a value of Tc (th) to a vertex having a value of Tc (nb)) It is a correction table that.

  In the gamma correction unit 1, when a gradation value of 0 ≦ C1 ≦ th is input, general gamma correction is performed according to the correction table of FIG. When a gradation value of th <C1 ≦ 255 is input, two-dimensional linear interpolation is performed in a C = C1 plane orthogonal to the C axis to obtain a conversion value.

FIG. 5B shows a plane of C = C1, where the output value for the input value C1 at γ1 is γ1 (C1), the output value for the input value C1 at γ2 is γ2 (C1), and the input value at γ3. The output value for the value C1 is γ3 (C1), and the output value for the input value C1 at γ4 is γ4 (C1). The value C2 ′ after gamma correction at the coordinate position (C, M, Y) = (C1, M1, Y1) existing in the plane is obtained by the linear interpolation calculation of Expression 4.
C2 ′ = {γ1 (C1) · (255−M1) · (255−Y1)
+ Γ2 (C1) · (255−M1) · Y1
+ Γ3 (C1) ・ M1 ・ Y1
+ Γ4 · M1 · (255−Y1)} / 255 ² formula (4)

  FIG. 6 illustrates the color gamuts and the hues of the single color and the secondary color in the first conventional example (a) and the present invention (b) when the solid density of only C of M, Y, and Y decreases. FIG.

  The color gamut in the L * a * b * three-dimensional color space is projected onto the two-dimensional plane consisting of the two axes a * b *, and the dotted lines are actually output in all C, M, and Y single colors. Color gamut and representative colors (R, G, B, C, M, Y) when the possible solid density (output density when output at the maximum gradation value) is equal to or higher than the target density set for each color Represents the hue.

  The solid line represents the case where the solid density of Y does not reach the target density. In the conventional example (a), the color gamut when the target density of the high density portion of Y is corrected so as to be reduced in accordance with the solid density of Y. And the representative color hue. The present invention (b) controls to reduce the target density according to the hue, and with respect to the Y hue, similarly to (a), the target density in the high density portion of Y is reduced to match the solid density of Y, and the G hue For Y, the target density in the high density portion of Y and C was reduced to match the solid density of Y, and for the R hue, the target density in the high density portion of Y and M was reduced to match the solid density of Y. In this case, in the G and R hues of the secondary colors, hue bending is prevented by correcting the target densities of C and M in accordance with the solid density reduction of Y.

  FIG. 7 is a diagram for explaining the color gamut and the hues of monochromatic and secondary colors in the second conventional example when the solid density of only Y of C, M, and Y is lowered.

  When the solid density of Y does not reach the target density in consideration of color balance (mainly gray balance) as in the technique described in the above-mentioned Patent Document 1, the target of the high density portion of C, M, and Y If the density is uniformly reduced in any hue according to the solid density of Y, the gray balance can be prevented from being lost and the hue of the secondary color can be prevented from being bent, but the color gamut is narrowed as a whole. Similarly, when the solid density of C and M other than Y decreases, the color gamut becomes narrow as shown in FIG.

  FIG. 8 illustrates the color gamut, the monochromatic color, and the secondary color hue in the first conventional example (a) and the present invention (b) when only M of C, M, and Y has a reduced solid density. FIG.

  FIG. 6 shows the case where the solid density of Y is lowered. As another example, FIG. 9 (the effect of the present invention) will be described for the case where the solid density of another color (M) is lowered. Shown in

  The solid line in FIG. 8A represents the hues of the color gamut and the representative color when the target density of the high density portion of M is corrected so as to be reduced in accordance with the solid density of M. The solid line of the present invention (b) controls the target density to be reduced according to the hue, and for the M hue, the target density in the high density portion of M is reduced according to the solid density of M as in (a). For the R hue, the target density in the high density part of M and Y is reduced according to the solid density of M, and for the B hue, the target density in the high density part of M and C is reduced according to the solid density of M. In this case, in the R and B hues of the secondary colors, the hue density is prevented by correcting the target density of Y and C in accordance with the M density reduction.

  FIG. 9 is a diagram for explaining the fluctuation direction and fluctuation width in the first conventional example (a) and the present invention (b) in the R hue. FIG. 6 and FIG. 8 are overlapped, and the portion corresponding to the high density portion (solid vicinity) of the R hue is enlarged and displayed.

  The direction of the arrow in FIG. 9A represents the variation direction in the conventional example in which only the target density of the color having a reduced solid density is corrected, and the length of the arrow represents the variation range. When the high density portion fluctuates in the hue direction and the solid density of Y is lowered, the hue is bent in the lower right direction, and when the solid density of M is fluctuated, the hue is bent in the upper left direction.

  The arrows in FIG. 9B represent the direction of change and the range of fluctuation in the present invention, and change only in the saturation direction even when the solid density of Y decreases or when the solid density of M changes. Compared to the conventional example that blurs on both sides of the original R hue line, it changes only in one direction, so there is an effect that the fluctuation range is relatively small and a stable color can be output.

  FIG. 10 shows a configuration of Embodiment 2 of the present invention. In the first embodiment, an example in which a corrected target density obtained by reducing the target density for each hue is obtained and a correction table for γ1 to γ4 is set is shown. However, in the second embodiment, only the correction table is γ1, and instead, An embodiment will be described in which a process for reducing the tone value according to the hue is added as a pre-process for the gamma correction, thereby performing substantially the same process as that for reducing the target density for each hue.

  Only differences from the first embodiment (FIG. 1) will be described. The target density reduction unit 5α obtains Cmax, Mmax, Ymax according to Equation 1 to obtain a corrected target density, and the correction table setting unit 6 outputs C, M, and C for outputting the maximum gradation value 255 at Cmax, Mmax, and Ymax. A correction table for Y is obtained (a correction table for γ1 in FIG. 4 is obtained). The gamma correction unit 1α includes a hue-specific gamma preprocessing unit 11 and a hue common gamma correction unit 12. The hue-specific gamma pre-processing unit 11 applies the parameters set by the hue-specific gamma pre-processing parameter setting unit 8 to reduce the gradation value according to the hue. The common hue gamma correction unit 12 performs correction according to the correction table set by the correction table setting unit 6.

  FIG. 11 is a diagram for explaining the hue-specific gamma preprocessing unit 11 and the hue-specific gamma preprocessing parameter setting unit 8.

  In the gamma pre-processing unit 11 for each hue, similarly to the reduction of the target density in FIG. 3, the gradation is not reduced when the input gradation value n is in the range of 0 ≦ n ≦ th, and the range of th <n ≦ 255. The gradation is reduced with. However, for C, M, and Y single colors, even if th <n ≦ 255, the gradation value is not output and the input gradation value is output as it is. When the gradation value of th <C1 ≦ 255 is input, the output gradation value is obtained by performing three-dimensional linear interpolation from the parameters of the vertex of the rectangular parallelepiped indicated by the solid line.

  In the gamma preprocessing parameter setting unit 8 for each hue, the C gamma preprocessing parameters fc (Tc (ng)), fc (Tc (nk)), fc (Tc (nb)), and the M gamma preprocessing parameters fc (Tc (ng)), fc (Tc (nk)), fc (Tc (nb)), and Y gamma preprocessing parameters fc (Tc (ng)), fc (Tc (nk)), fc (Tc) (Nb)). fc (), fm (), and fy () are functions for calculating gamma preprocessing parameters.

  FIG. 12 is a diagram for explaining the function fc () for calculating the gamma preprocessing parameter. The C corrected target concentration TR (C1) set by the target concentration reducing unit 5α is TR (C1) = C1 in the range of 0 ≦ C1 ≦ th, and TR (255) = in the range of th <C1 ≦ 255. The table is formed by connecting points of C1 = th and C1 = 255 with a straight line so as to be Cmax. Expressed as an equation, equation (5) is obtained.

  Since the hue common gamma correction unit 12 applies the correction table set by the correction table setting unit 6 according to this, the gradation value 255 input to the gamma correction unit 1α is finally printed out at the density C ′. Can be obtained by converting 255 into fc (C ′) by the gamma pre-processing unit 11 for each hue. A function for obtaining fc (C ′) when th <C ′ ≦ Cmax can be expressed by Expression (6). Functions fm () and fy () can be similarly expressed by Expression 7 and Expression 8.

  Since the gamma pre-processing parameter is a parameter for outputting according to the corrected target density shown in FIG. 3 in the end, a secondary whose C corrected target density is, for example, Tc (ng) in comparison with FIG. The color green solid gamma preprocessing parameter is fc (Tc (ng)) in FIG.

  According to the second embodiment, instead of holding nonlinear γ2 to γ4 in which conversion values are set for each gradation, gamma preprocessing parameters corresponding to secondary colors and three-dimensional solids are held, By performing the linear interpolation calculation in the range of th <n ≦ 255 in the gamma preprocessing unit 11, the same effect as that of the first embodiment can be obtained while reducing the parameters to be held.

  According to the present invention, a storage medium in which a program code of software for realizing the functions of the above-described embodiments is recorded is supplied to a system or apparatus, and a computer (CPU or MPU) of the system or apparatus is stored in the storage medium. This is also achieved by reading and executing the code. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiment. As a storage medium for supplying the program code, for example, a hard disk, an optical disk, a magneto-optical disk, a nonvolatile memory card, a ROM, or the like 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 an OS (operating system) operating on the computer based on an instruction of the program code. A case where part or all of the actual processing is performed and the functions of the above-described embodiments are realized by the processing is also included. Further, after the program code read from the storage medium is written into a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. A case where the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing and the functions of the above-described embodiments are realized by the processing is included. Further, the program for realizing the functions and the like of the embodiments of the present invention may be provided from a server by communication via a network.

DESCRIPTION OF SYMBOLS 1 Gamma correction part 2 Printer output part 3 Patch image 4 Colorimetry part 5 Target density reduction part 6 Correction table setting part 7 Storage device

JP 2005-109975 A

Claims (7)

  An image forming unit that forms an image composed of a plurality of component colors, a density upper limit value acquiring unit that acquires the density upper limit value of each component color of the image formed by the image forming unit as a formation density upper limit value, and a parameter that differs for each hue And a reduction control means for controlling the parameter so that the maximum gradation value of each component color is formed at the correction target density, and the correction target The density is a value obtained by reducing the target density at the maximum gradation value of each component color by reflecting the formation density upper limit value of the component color related to the formation of the hue color.   The gamma correction unit includes a hue-specific gamma pre-processing unit and a hue common gamma correction unit, and the hue-specific gamma pre-processing unit is a unit that applies the parameter to reduce a gradation value, and the hue common 2. The image processing apparatus according to claim 1, wherein the gamma correction means is a means for correcting the gradation value by applying one correction table to each component color regardless of the hue.   The parameter does not reduce the tone value of a hue color formed using one of a plurality of component colors, and the tone value of a hue color formed using two or more of a plurality of component colors. 3. The correction table according to claim 2, wherein the correction table is a correction table for correcting a gradation value so that a maximum gradation value of the component color is formed at the formation density upper limit value. Image processing apparatus.   In a hue color formed using two or more of a plurality of component colors, a gradation value having the formation density upper limit value as a target density is obtained for each component color to be used, and the gradation for each obtained component color 2. The image processing apparatus according to claim 1, wherein a target density at a minimum gradation value among the values is set as a corrected target density at the maximum gradation value of the hue color.   An image forming step for forming an image composed of a plurality of component colors, a density upper limit value acquiring step for acquiring the upper limit value of the density of each component color of the image formed by the image forming step as a forming density upper limit value, and different parameters for each hue And a reduction control step for controlling the parameters so that the maximum gradation value of each component color is formed at the correction target density, and the correction target The image processing method, wherein the density is a value obtained by reducing the target density at the maximum gradation value of each component color by reflecting the formation density upper limit value of the component color related to the formation of the hue color.   A program for causing a computer to realize the image processing method according to claim 5.   A computer-readable recording medium on which a program for causing a computer to implement the image processing method according to claim 5 is recorded.

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