Image analysis – Color image processing – Color correction
Reexamination Certificate
2000-09-28
2004-02-24
Wu, Jingge (Department: 2623)
Image analysis
Color image processing
Color correction
C358S518000
Reexamination Certificate
active
06697522
ABSTRACT:
This application is based on application No. 11-276036 filed in Japan, the contents of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to image processing apparatuses and particularly to an image processing apparatus that corrects chromatic aberration caused by an optical system in digital processing of image information obtained through optical reading of an original document image.
2. Description of the Related Art
Regarding conventional digital copying machines and the like, color deviation (chromatic aberration) of an optical system occurs on edges in the main scanning direction, due to chromatic aberration of a lens employed in a reading unit.
FIG. 10
illustrates the principle of chromatic aberration of a lens. It is supposed here that black lines are printed on an original document
10
at respective positions x
1
, x
2
and x
3
in the main scanning direction.
Referring to
FIG. 10
, light rays reflected from the black line at the position x
2
are passed straight through the center of a lens and then focused on a CCD (charge-coupled device) and accordingly the reflected light rays are focused at the same position. In other words, chromatic aberration of the lens does not affect the reflected light rays. The CCD can then read the reflected light rays as black data including three colors of R (red), G (green) and B (blue) having the same barycenters and peak values.
On the other hand, light rays reflected from the black line at the position x
1
or x
3
are directed from the edge region of the lens and focused on the CCD, and therefore affected by the chromatic aberration of the lens. Specifically, light rays of RGB incident on the lens are refracted according to respective wavelengths and focused at different positions so that respective barycenters and peak values of these three colors differ from each other. In particular, as shown in the lower part of
FIG. 10
, long-wavelength light (R) condenses inward on the CCD while short-wavelength light (B) condenses outward on the CCD.
FIG. 11
shows chroma relative to addresses in the direction of main scanning for explaining the above chromatic aberration. As seen from
FIG. 11
, the chroma relative to edge parts and therearound such as x
1
and x
3
in the main scanning direction is higher because of the influence of chromatic aberration while the chroma relative to the central part like x
2
which is not affected by the chromatic aberration is lower as compared therewith.
The chromatic aberration as discussed above causes no problem for an image having a relatively even density distribution such as a color patch. However, for an image where density abruptly changes such as a character image, the chromatic aberration generates color deviation on the edge portion thereof. Especially, on the edge of a black character, an erroneous judgement due to the chromatic aberration results in blurring of color around the character, partial loss of the character, and the like.
A lens of high quality is thus required in a PPC (plain paper electric copying machine) employing a color CCD, however, such a lens cannot satisfactorily meet requirements. Specifically, improvement of lens performance is accompanied by increase in size of a lens system, resulting in increase in size of an entire machine including an optical system of a scanner. Further, there is a considerable difference in quality and performance between lens parts. A method is then necessary for correcting the chromatic aberration finally by an image processing system.
A method is now described of correcting chromatic aberration that has been employed in an image processing system of a conventional art. The chromatic aberration is generally corrected by mixing data on pixels adjoining each other using chromatic aberration correction coefficients as represented by the following equations:
R
(
n
)=
a
1(
n
)×
R
(
n−
1)+
a
2(
n
)×
R
(
n
)+
a
3(
n
)×
R
(
n+
1),
G
(
n
)=
G
(
n
),
B
(
n
)=
a
3(
n
)×
B
(n−1)+
a
2(
n
)×
B
(
n
)+
a
1(
n
)×
B
(
n+
1),
where n represents the position of a target pixel relative to a reference position of the main scanning, and a1(n), a2(n) and a3(n) represent correction coefficients for the target pixel which is the nth pixel.
FIG. 12
is a block diagram illustrating a general method of correcting chromatic aberration according to the conventional art. Referring to
FIG. 12
, in order to correct chromatic aberration according to the conventional art, data on RGB read by a reading unit
101
is corrected by a chromatic aberration correcting unit
109
using correction coefficients (a1, a2, a3) calculated by a correction coefficient calculating unit
103
.
If predetermined values are used respectively as the correction coefficients (a1, a2, a3), correction would be accomplished for a state different from the actual state of chromatic aberration since manufactured lenses have different qualities and performances. It is thus necessary to determine the actual state of chromatic aberration for each machine and then determine correction coefficients a1(n), a2(n) and a3(n) which are appropriate for each machine. However, measurement of the chromatic aberration for each machine is inefficient in terms of production efficiency and the like. Accordingly, a method is actually employed as explained below.
FIG. 13
illustrates pseudo-calculation of correction coefficients by correction coefficient calculating unit
103
in FIG.
12
. Referring to
FIG. 13
, RGB data read by reading unit
101
is transmitted first to chromatic aberration correcting units
1301
,
1302
and
1303
in respective blocks where three sets of chromatic aberration correction coefficients ([a11, a12, a13], [a21, a22, a23], [a31, a32, a33]) calculated at the time of lens design are used to correct chromatic aberration for each pixel ([R
1
, G
1
, B
1
], [R
2
, G
2
, B
2
], [R
3
, G
3
, B
3
]).
Chroma data (MAX(R,G,B)−MIN(R,G,B)) is then calculated for each block (M
1
, M
2
, M
3
) by a chroma calculating unit
1305
and thereafter the minimum one (Y) of them is determined by a MIN unit
1307
. RGB data ([RY, GY, BY]) is finally determined by a selector
1309
that is data obtained by correction of chromatic aberration associated with the minimum value Y.
In this way, RGB data of any block that allows the chroma data to be minimum for respective colors is selected to calculate chromatic aberration correction coefficients in a pseudo-manner for correcting chromatic aberration. In other words, instead of using fixed chromatic aberration correction coefficients calculated for each machine, optimum chromatic aberration correction coefficients are selected from three sets of chromatic aberration correction coefficients which are calculated in advance. Then, the optimum chromatic aberration correction coefficients are used to accomplish correction of chromatic aberration.
Since the influence of chromatic aberration is noticeable on the edge part of a black character and is inconspicuous on the remaining part thereof, such a chromatic aberration correction has been considered to be satisfactory in the practical use. Accordingly, this method has been regarded as the one which saves labor of measuring chromatic aberration for each machine and thus achieves an easier and more appropriate correction of chromatic aberration.
However, according to this conventional art, an image processing apparatus always selects, from predetermined chromatic aberration correction coefficients, those correction coefficients which provide the minimum chroma. Therefore, depending on image data, the selected correction coefficients may be different from those suitable for the actual chromatic aberration. Consequently, a problem occurs that a thin line of a single color for example RGB cannot be reproduced.
FIG. 14
shows an image of an original document having a ladder pattern
McDermott & Will & Emery
Minolta Co. , Ltd.
Wu Jingge
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