JPH11262033A - Gradation correction method in color image output device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct respective one-dimensional gradation correction LUT for red, green, blue RGB data to generate a desired color, even in the case that a target gradation corresponding to print conditions (ink, print paper, color printer itself) which is to be simulated is changed in a color image output device that generates a print proof (color proof sheet). SOLUTION: A target gradation (target colorimetric value) is set so that gray balance is taken on the condition of R=G=B in input image data RGB (S11). A color patch which uniformly shares the input image data RGB is outputted from a color image output device and the color is measured (S12, S13), and a three-dimensional LUT used for converting a colorimetric value L'a'b' into an RGB value is generated (S14). A gray chart is outputted from the input image data RGB under the condition that R=G=B (S16) and the color is measured (used for a measured colorimetric value) (S16). RGB values respectively corresponding to the measured colorimetric values and the target colorimetric values are obtained from the three-dimensional LUT by means of a volume interpolation calculation (S19), and each difference of the obtained RGB values is added to each one-dimensional LUT for the RGB data to obtain each corrected one-dimensional LUT for the RGB data.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
Based on GB (red, green, blue) image data, a color output by a color image output device that generates colors in three hues of CMY (cyan, magenta, yellow) can be developed in a desired color. The present invention relates to a gradation correction method in a color image output device.

[0002]

2. Description of the Related Art For example, there is a color image output device such as a color printer which forms a color image by forming three color materials of CMY at predetermined gradations. FIG. 9 shows a schematic configuration of a color image output device 1 of this type. In the color image output device 1, input image data RGB of three hues is subjected to gradation conversion by an LUT 5 having one-dimensional LUTs (look-up tables) 2 to 4 for gradation correction, and then supplied to an exposure unit 6. .

[0005] The exposure unit 6 outputs R, R, and B signals in accordance with three-color output image data RGB after gradation correction by the one-dimensional LUTs 2 to 4.
The laser diodes that emit light of colors G, B, and B are driven, and each laser beam L is applied to the film F to form a latent image on the film F. With respect to the film F on which the latent image is formed, By performing a predetermined developing process, it is possible to obtain a film F on which an image composed of three hues of CMY is formed as a visible image.

[0004] Such a color image output device 1 is used, for example, as a proofer (referred to as a printing proofer) for a color printing machine. The reason for using a color image output device as a proofer for printing is that before creating an actual color print using a color printing machine that uses a rotary press, etc.
This is to create a proof print on which a color image for proofing is formed (referred to as a color print proof). A printing proofer does not require a printing plate process required by a color printing machine. This is because a color print (a hard copy on which a color image is formed) can be easily created a plurality of times in a short time.

That is, by simulating the color of a color print produced by a color printing machine to be used from now on by proof printing of the color image output device 1, it is possible to easily confirm the color in a process prior to actual printing. Because you can.

Incidentally, this type of color image output apparatus 1
In, the gradation correction characteristics (also referred to as gradation characteristics) of the one-dimensional LUTs 2 to 4 (stored in the memory) which are incorporated in advance corresponding to some printing conditions (conditions of ink, paper, and the printing press itself). ) Is the printing condition (desired printing condition) of the printing press that each user intends to actually use.
Therefore, when trying to create a print proof according to the desired printing conditions, depending on the desired printing conditions, the dynamic range of the density and the temperature change within the dynamic range are not considered. It is necessary to correct (correct) the gradation characteristics of the one-dimensional LUTs 2 to 4 in order to optimize the division.

Therefore, for example, the C on the film F is input to the input image data RGB in accordance with a desired printing condition.
The target gradation (target density gradation) of each color of MY is the target gradation (target density gradation) Dc0, Dm0, Dy as shown in FIG.
When set to 0, the standard printing conditions are set in advance so that the input image data RGB will match these target gradations Dc0, Dm0, and Dy0 by the one-dimensional LUTs 2 to 4 constituting the gradation correction LUT 5. It is necessary to correct (correct) the gradation characteristics of the standard one-dimensional LUTs 2 to 4 incorporated correspondingly.

In this case, according to the gradation correction method in the conventional color image output apparatus 1, first, as shown in the flowchart of FIG. 11, each of the image data R, G, and B constituting the input image data RGB is respectively processed to a predetermined level. Increase by key,
It is supplied to the exposure unit 6 through standard one-dimensional LUTs 2 to 4 (which are originally incorporated in the color image output device 1),
A single color patch of each color of CMY is output onto the film F by the laser light L of each color of RGB (step S1), and the densities Dc, Dm, and Dy of each single color patch are measured (step S1).
2).

Next, the measured densities Dc, Dm, Dy and the target gradations Dc0, Dm0, Dy0 shown in FIG. 10 are compared for each patch having a predetermined gradation, and a difference is output (step S3). Then, it is determined whether or not this difference is within a desired difference (step S4).

However, the target printed matter and the output patch have different spectral characteristics due to the difference in the coloring material, and even if the density values match, the apparent tint will be different. Therefore, the determination in step S4 is negative, and the respective RGB values of the input image data RGB of the one-dimensional LUTs 2 to 4 and the output image data RG are determined by trial and error according to the difference obtained in step S4.
The one-dimensional LUTs 2 to 4 are corrected by correcting the correspondence (conversion relation) of each RGB value of B (step S5).

Then, by repeating the processing from step S1 to step S5 until the determination in step S4 is satisfied, the measured densities Dc, Dm, Dy and the target gradation Dc
One-dimensional LUTs 2 to 4 after gradation correction in which 0, Dm0, and Dy0 are values within a desired range are obtained.

[0012]

However, in the above-described gradation correction method in the conventional color image output apparatus 1, the gradation characteristics of the one-dimensional LUTs 2 to 4 (input / output correspondence).
Is corrected (trial-and-error) by trial and error, so that the one-dimensional LUTs 2 to 4 are corrected according to the difference,
Using the corrected one-dimensional LUTs 2 to 4, print out a single color patch, measure the density for each printout,
Since it is necessary to repeat the operation of comparing with the target gradation many times, a large amount of time is required, and the determination of the correction amount based on the difference itself requires a high degree of skill.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and makes it possible to easily perform correction (correction) of gradation correction means of a color image output apparatus in response to a change in printing conditions and the like. It is an object of the present invention to provide a gradation correction method in a color image output device.

[0014]

According to the present invention, a conversion relationship between output colorimetric values of a color image output device and output image data of a one-dimensional gradation correction means is obtained in advance, and each hue of input image data is obtained. A gray chart is output from the color image output device under the condition that the gradation values are equal to each other, and the gray chart is measured to obtain a measured colorimetric value. A value of output image data for each hue corresponding to each of the tone colorimetric values is obtained. At this time, if the measured colorimetric value is not the value on the measurement grid point in the conversion relation, the value of the output image data is obtained by interpolation calculation. Among the values of the output image data for each hue corresponding to the obtained measured colorimetric value and the colorimetric value of the target tone, respectively, the one-dimensional tone correction means for each hue is determined by the difference amount for each hue. Correct the correction amount.

In this case, the difference corresponding to the correction amount of the gradation correction means can be obtained directly or by interpolation calculation based on the conversion relationship from the output colorimetric value to the output image data. Compared with the technique of the above, it is possible to perform the correction of the gradation correcting means in a very short time so as to adapt to the change of the printing condition and the like, and the skill is not required.

When the above-mentioned conversion relation is obtained, in the vicinity of gray, the output image data is changed more finely for each hue, and the measurement grid points for interpolation are made finer, so that the gradation correction means can be corrected more accurately. (Modify).

[0017]

An embodiment of the present invention will be described below with reference to the drawings. In the drawings referred to below, the same reference numerals are given to those corresponding to those shown in FIGS.

FIG. 1 shows a schematic configuration of a color image output apparatus 10 such as a color printer to which the method of the present invention is applied. The color image output device 10 includes an exposure unit 6 that outputs R, G, and B laser beams L, respectively.
have. The donor film F, which is cut into a predetermined length by an internal cutter (not shown), is transferred from the donor film F, which is a photosensitive material wound on a roll, to the exposure unit 6.

A dampening solution is applied to the donor film F on which a latent image has been formed by being exposed and recorded by the laser beam L from the exposure unit 6, and a roll-shaped image receiving paper G is cut into a predetermined length. The image receiving papers G are opposed to each other and are attached to each other.

The adhered material is heated by a heating roller 8 in a heat developing / fixing section, whereby the development proceeds,
The dye on the donor film F is transferred to and fixed on the receiving paper G,
The transfer of the image is completed. Thereafter, the used donor film F and the image receiving paper G are peeled off, and a high-quality color print (also denoted by a symbol G) on which an image Img having three hues of CMY is formed is completed.

In this case, the image Im on the color print G
g corresponds to the three-color image data RGB supplied from the look-up table (LUT) 5 as a gradation correction unit to the exposure unit 6. The image data RGB supplied to the input side of the exposure unit 6 are input into three hues by LUTs 2 to 4 which are LUTs as one-dimensional gradation correction means created in advance based on standard printing conditions. The image data RGB is converted data.

In such a color image output device 10,
As described above, printing conditions (also called desired printing conditions or target printing conditions) different from the printing conditions (standard printing conditions) of the one-dimensional LUTs 2 to 4 stored in the memory of the color image output apparatus 10 in advance. ) To proof (proofing)
In order to match the target printing condition, the gradation of the standard LUTs 2 to 4 pre-installed to optimize the dynamic range of the density and the division of the density change of the dynamic range It is necessary to correct (correct) the conversion characteristics.

Hereinafter, in order to meet the target printing conditions,
A gradation correction method according to this embodiment for determining the correction amounts of the LUTs 2 to 4 will be described with reference to the flowchart of FIG.

First, as shown in FIG. 3, a target gradation (target density gradation) Dt corresponding to a target printing condition corresponding to a color printing machine to be actually used is determined (step S11). Here, the target gradation Dt is the output image data RGB (in the following description, in principle, when the output image data RGB is referred to as LUT2
4, and the input image data RGB means the image data input to the LUTs 2-4. ) Are set so that the gray balance can be obtained on the color image output device 10 under the condition (R = G = B) that the respective RGB values (the gradation values of the respective hues of RGB) are equal. Color value (target colorimetric value) L
^* 0a ^* 0b ^* 0.

The conversion relationship between the colorimetric value and the density value can be determined, for example, by measuring a plurality of patches having different densities using a colorimeter and a densitometer, respectively. Then, a density value for an arbitrary colorimetric value L ^* a ^* b ^* can be obtained by volume interpolation.

The target gradation Dt is actually measured, for example, by measuring the relationship between the dot% value and the density of a printed matter of a color printing machine for which a print proof is to be created, and converting the dot% value into the output image data RGB value. It can be obtained by conversion. At the target gradation Dt in FIG. 3, the density value exists when the value of the output image data RGB is near R = G = B = 0.
This is based on the paper color of the printed matter (the background color of the book paper).

Next, in order to create a look-up table (LUT) representing the conversion relationship between the colorimetric values output from the color image output device 10 and the RGB values of the output image data RGB, first, the one-dimensional LUTs 2 to 4 Is a so-called through state, a color print G having color patches in which the values of the respective hues of the RGB of the output image data RGB are evenly distributed.
Is output (step S12). For example, RG of output image data RGB
When the gradation value of the B each color is 8-bit gradation taking values from 0 to 255, each nine stages of a value to each of the RGB colors were uniformly varied gradation width 3 every ^2, total 9 A color chart G 'having ³ = 729 color patches is output. At this time, in the vicinity of gray where R = G = B, a color chart having a color patch whose number of divisions is doubled is output. This is because in the vicinity of gray where human visual discrimination is excellent, the interpolation grid is made finer, and the conversion accuracy of the LUT representing the conversion relationship from the output colorimetric values to the output image data RGB is increased.

Next, the color patches of the output color chart G 'are measured by the colorimeter 20 (step S1).
3) For each color patch, a three-dimensional LUT representing the conversion relationship between the RGB values of each hue of the output image data RGB and the colorimetric values L ^* a ^* b ^* is created (step S14).

FIG. 4, 9 ^three because combination becomes complicated five three-dimensional representation is omitted manner as ^three combinations LUT12
3 shows the configuration of the measurement grid point. The measurement grid points represented by small circles correspond to points having colorimetric values L ^* a ^* b ^* actually measured by the colorimeter 20. Note that, as described above, in the vicinity of R = G = B, since the grid points (colorimetric points) are measured finely, the three-dimensional LUT 1 shown in FIG.
In the cube representing 2, each cubic lattice including a line connecting the origin of the RGB coordinates and the vertex farthest from the origin has finer lattice points. For example, if the description is made two-dimensionally using the image data R and the image data G, as shown schematically in FIG. The color values L ^* a ^* b ^* are measured.

Next, using the one-dimensional LUTs 2 to 4 of the RGB hues before correction (correction) in which the gray balance is adjusted under standard printing conditions, the input image data R
Under the condition that the values of the respective hues of GB are equal (R = G = B), RG
A gray chart (R = G = B = 0) which is a color print G when the B value is changed simultaneously, for example, in 17 steps
Is output from the color image output device 10 (see FIG. 1) G ″ (see FIG. 1) in which a gray scale is formed including the ground color of the color print G (step S1).
5).

Next, this gray chart G ″ is represented by each R =
Colorimetry is performed by the colorimeter 20 for each combination of G = B to obtain a colorimetric value (referred to as a measured colorimetric value) L ^* a ^* b ^* (step S16).

Next, as shown in FIG. 6, each R = G
= Measured colorimetric value L for each combination of B^*a ^*b^*And step S
The colorimetric value L of the target gradation determined in 11^*0a^*0b^*0 and
Are compared (step S17). In FIG. 6,
The characteristic indicated by the symbol Dt is the target gradation (target darkness) shown in FIG.
(Dot gradation) Dt.

The colorimetric value L ^* 0a ^* 0b ^* 0 of this target gradation
And the measured colorimetric values L ^* a ^* b ^* and of the difference ΔL ^* Δa ^* Δb ^*
Is within a desired range (step S
18). If the printing conditions have changed, the first determination is not made.

Therefore, the third order obtained in step S14
Using the original LUT 12, the colorimetric value L of the target gradation^*0a^*
0b^*0 for each RGB value of the output image data.
Constant color value L^*a^*b^*Of each of the output image data corresponding to
The GB values are obtained (step S19), and the obtained R values are obtained.
A difference between GB values is obtained (step S20). in this case,
Each RGB value refers to the three-dimensional LUT 12, and the
Colorimetric value L^*0a^*0b ^*0 and measured colorimetric value L^*a^*b
^*The interpolation grid (cubic grid) surrounding each
The RGB values of each grid point forming the interstitial are read, and FIG.
As shown in the figure, the volume interpolation processing unit consisting of a computer etc.
14, the colorimetric value L of the target gradation is obtained by the volume interpolation calculation.^*0
a^*0b^*0 and measured colorimetric value L^*a^*b^*To each
The corresponding RGB values of the three hues can be obtained. This
Here, the colorimetric value L of the target gradation^*0a ^*0b^*Corresponding to 0
The obtained RGB values are defined as R0G0B0, and the measured colorimetric values L^*a
^*b^*R1G1B1 is the RGB value obtained corresponding to
Then, the difference between the RGB values is ΔR (= R0−R1), ΔR
G (= G0-G1) and ΔB (= B0-B1)
Can be.

Next, the LUTs 2 to 4 are corrected (corrected) based on the difference ΔRΔGΔB (step S21). This correction calculation is very simple, and the difference amount ΔRΔGΔB is calculated by using the current LUT2 to LUT2, respectively.
4 may be added to the respective correction amounts. More specifically, for example, based on the R value, as shown in FIG. 8, the input image data R is converted into output image data R1 by an LUT2 which is a previously-determined gradation correction means. Since it is preferable that the image data is converted into image data R0 corresponding to the colorimetric value L ^* 0a ^* 0b ^* 0 of the target gradation, the difference ΔR (= R0)
It is only necessary to replace the new LUT 2 'with the addition of -R1) with the LUT 2 shown in FIG.

As described above, the LUTs 2 to 4 are subjected to the color correction values L ^* 0a of the target gradation by the correction processing once by calculation.
Since it can be corrected to LUTs 2 to 4 that can be converted to ^* 0b ^* 0, LUs corresponding to changes in printing conditions
The correction of T2 to T4 can be performed very easily.

In order to further increase the accuracy, the processes from steps S14 to S17 are repeated again using the LUTs 2 to 4 corrected in this manner. LUTs 2 to 4 can be corrected.

As described above, according to the above-described embodiment,
Conventionally, an adjustment operation, which is a correction (correction) operation of the one-dimensional LUTs 2 to 4 for tone correction of RGB colors, which has been conventionally performed by trial and error, can be mechanically performed. This achieves an effect that even a person who has not experienced adjustment of 4 can create the one-dimensional LUTs 2 to 4 in which the color developed efficiently becomes gray in a short time.

It should be noted that the present invention is not limited to the above-described embodiment, but can adopt various configurations without departing from the gist of the present invention.

[0040]

As described above, according to the present invention, it is possible to optimize the dynamic range of the density of the color image output device required in response to the change of the printing conditions and the like and to divide the density change within the dynamic range. Comparing with the conventional technology that adopts an inefficient adjustment method that converges by trial and error, it does not require skill, and it is dramatically shorter. It is possible to easily correct the gradation of the gradation correction means with time.

[Brief description of the drawings]

FIG. 1 is a schematic overall configuration diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a flowchart illustrating a procedure for correcting a one-dimensional LUT.

FIG. 3 is a diagram showing a target gradation.

FIG. 4 shows a three-dimensional LU for changing a colorimetric value to an RGB value.
It is a schematic diagram of T.

FIG. 5 is an explanatory diagram showing an example in which a division grid interval is small in the vicinity of gray of a three-dimensional LUT.

FIG. 6 is a diagram illustrating a difference between a measured colorimetric value after a change in output conditions and a target gradation.

FIG. 7 is a diagram provided for explaining a method of using a three-dimensional LUT.

FIG. 8 is a block diagram provided for specific description of a one-dimensional LUT correction process.

FIG. 9 is a block diagram provided for explaining a conventional technique.

FIG. 10 is a diagram used to explain a correction process according to the prior art for a one-dimensional LUT.

FIG. 11 is a flowchart provided to explain a correction process according to the prior art for a one-dimensional LUT.

[Explanation of symbols]

1, 10: color image output device 2-4: one-dimensional LUT 6: exposure unit 8: heating roller 12: three-dimensional lookup table 14: volume interpolation processing unit 20: colorimeter

Claims (2)

[Claims] 1. A method for converting a gradation of each hue of input image data into a gradation of each hue of output image data.
A gradation correction method of a one-dimensional gradation correction unit for each hue in a color image output device having a two-dimensional gradation correction unit, wherein a color is provided under the condition that gradation values of each hue of the output image data are equal. A step of setting a colorimetric value of a target gradation for the output image data so that a gray balance can be obtained on an output image of the image output device; and a combination in which the output image data is changed substantially uniformly for each hue. Color chart with color patches
Using the color image output device to perform colorimetry and obtaining a conversion relationship from the colorimetric values to the output image data; and inputting using a previously obtained one-dimensional gradation correction means for each hue. Under the condition that the gradation value of each hue of the image data is equal,
Outputting a gray chart from the color image output device,
The gray chart is measured to obtain a measured colorimetric value, and each hue value of the output image data corresponding to the measured colorimetric value and each of the output image data corresponding to the target tone colorimetric value. The hue value is obtained by referring to the conversion relationship, and the correction amount of the one-dimensional tone correction means for each hue, which is obtained in advance, is obtained from the difference between the corresponding hue values among the obtained values. Correcting the color image, and the method of correcting the gradation in the color image output device. 2. The method according to claim 1, wherein in the step of obtaining the conversion relationship from the colorimetric values to the output image data, a color comprising a combination of the output image data substantially uniformly changed for each hue. Color chart with patches,
When outputting from the color image output device, in the vicinity of gray, a color chart having a color patch composed of a combination in which output image data is more finely changed for each hue is output. A gradation correction method in a device.