Image analysis – Color image processing – Color correction
Reexamination Certificate
1999-03-08
2003-02-18
Wu, Jingge (Department: 2623)
Image analysis
Color image processing
Color correction
C358S520000, C345S601000
Reexamination Certificate
active
06522778
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of converting colorimetric values measured by a first colorimeter into colorimetric values measured by a second colorimeter.
The present invention also relates to a color image output apparatus which is capable of correcting, at desired colorimetric values, colorimetric values measured by a built-in colorimeter which is incorporated in a color image output apparatus or colorimetric values measured by an external colorimeter connected on-line to the color image output apparatus to correct colors in three hues C, M, Y that are outputted by the color image output apparatus based on image data in three hues R, G, B inputted to the color image output apparatus.
2. Description of the Related Art
There are known color image output apparatus for producing color images by developing colors in given gradations with colorants in three hues C, M, Y.
FIG. 10
of the accompanying drawings schematically shows such a color image output apparatus
1
. In the color image output apparatus
1
, as shown in
FIG. 10
, inputted image data in three hues R, G, B, i.e., device-dependent image data R, G, B, are converted in gradation by a lookup table (LUT) unit
5
having gradation-correcting one-dimensional LUTs
2
,
3
,
4
, and the gradation-converted image data R, G, B are supplied to an exposure unit
6
.
In the exposure unit
6
, three laser diodes (not shown) are energized by the gradation-converted image data R, G, B from the LUT unit
5
to emit respective laser beams L in R, G, B. The laser beams L in R, G. B are applied to a film F to form a latent image on the film F. The latent image formed on the film F is then developed into a visible color image whose colors are expressed by three hues C, M, Y.
The color image output apparatus
1
is used as a proofer for a color printing press, for example. The color image output apparatuse
1
is used as a proofer because a color proof with a color image needs to be made for proofreading before an actual color print is produced by a color printing press such as a rotary press and also because such a proofer is capable of easily producing a plurality of color proofs, i.e., hard copies with color images thereon, in a short period of time as it does not require press plates which would be needed by color printing presses.
Therefore, the colors of a color print to be produced by a color printing press which will be used can be simulated with a color proof produced by the color image output apparatus
1
, so that they can easily be confirmed prior to being actually printed.
The gradation-correcting one-dimensional LUTs
2
,
3
,
4
, which are stored in a memory in the color image output apparatus
1
, are prepared so as to meet standard printing conditions including inks, sheets, and printing press conditions. However, it is impossible for the gradation-correcting characteristics (also referred to as “gradation characteristics”) of the one-dimensional LUTs
2
,
3
,
4
to be in full accord with the desired printing conditions of the printing press that is going to be actually used. For generating a color proof depending on the desired printing conditions, therefore, it is necessary to correct the gradation characteristics of the one-dimensional LUTs
2
,
3
.
4
depending on the desired printing conditions.
Specifically, if target gradations (target density gradations) for the hues C, M, Y on the film F with respect to the inputted image data R, G, B are set respectively to target gradations (target density gradations) Dc
0
, Dm
0
, Dy
0
depending on the desired printing conditions, as shown in
FIG. 11
of the accompanying drawings, then it is necessary to correct the standard gradation characteristics of the one-dimensional LUTs
2
,
3
,
4
which have been stored in advance to meet the standard printing conditions, so that the target gradations Dc
0
, Dm
0
, Dy
0
will be achieved from the inputted image data R, G, B by the one-dimensional LUTs
2
,
3
,
4
.
According to a gradation correcting process employed in the conventional color image output apparatus
1
, as shown in
FIG. 12
of the accompanying drawings, the image data R, G, B are incremented by respective given gradations, and then supplied through the standard one-dimensional LUTs
2
,
3
,
4
that have originally been incorporated in the color image output apparatus
1
to the exposure unit
6
, which emit laser beams L in R, G, B to output monochromatic patches in the respective hues C, M, Y on the film F in step S
1
. Then, the patches in C, M, Y are measured for respective densities Dc, Dm, Dy thereof in step S
2
.
The measured densities Dc, Dm, Dy are then compared with the respective target gradations Dc
0
, Dm
0
, Dy
0
shown in
FIG. 11
, and density differences therebetween are outputted in step S
3
. Thereafter, it is decided whether the density differences fall in a desired difference range or not in step S
4
.
Since the desired printing conditions are usually different from the standard printing conditions, the density differences usually do not fall in the desired difference range in step S
4
. The association (conversion relationship) between the image data R, G, B inputted to the one-dimensional LUTs
2
,
3
,
4
and the image data R, G, B outputted from the one-dimensional LUTs
2
,
3
,
4
is corrected on a trial-and-error basis depending on the density differences for thereby correcting the one-dimensional LUTs
2
,
3
,
4
in step S
5
. Hereinafter, the association (conversion relationship) means, e.g., both of the relationship converting the inputted image data into the outputted image data and relationship inversely converting the outputted image data into the inputted image data.
The loop of steps S
1
-S
5
is repeated until the density differences fall in the desired difference range in step S
4
. When the density differences fall in the desired difference range in step S
4
, i.e., when the measured densities Dc, Dm, Dy differ from the respective target gradations Dc
0
, Dm
0
, Dy
0
within the desired difference range, the one-dimensional LUTs
2
,
3
,
4
are properly corrected in gradation.
The above gradation correcting process employed in the conventional color image output apparatus
1
, however, is time-consuming for the reason that since the gradation characteristics, which represent the association between the inputted and outputted image data, of the one-dimensional LUTs
2
,
3
,
4
, are corrected on a trial-and-error basis, patches in C, M, Y need to be printed and measured for respective densities Dc, Dm, Dy each time the one-dimensional LUTs
2
,
3
,
4
are corrected depending on the density differences. In addition, the user of the color image output apparatus
1
has to be highly skilled in determining how much the one-dimensional LUTs
2
,
3
,
4
need to be corrected, i.e., corrective quantities for the one-dimensional LUTs
2
,
3
,
4
, on the basis of the density differences.
The above problems may be solved by a process which will be described below. Inasmuch as the process is a novel process, it will be described briefly below and will subsequently be described in more detail with respect to an embodiment of the present invention.
According to this process, first, target gradations (referred to as “target colorimetric values”) are established such that a gray balance will be achieved by the color image output apparatus
1
when the image data R, G, B outputted by the color image output apparatus
1
are equal to each other, i.e., R=G=B (stated otherwise, when the inputted image data R, G, B pass through the one-dimensional LUTs
2
,
3
,
4
without being changed thereby i.e., the inputted and outputted image data of these LUTs
2
,
3
,
4
are in a 1:1 correspondence). Then, the color image output apparatus
1
outputs a color chart having color patches which represent equal image data R, G, B, respectively. The color patches are then colorimetrically measured, and a three-dimensional color-matching lookup table (here
Fuji Photo Film Co. , Ltd.
Wu Jingge
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