Color correcting apparatus

In a color correcting apparatus, transformation errors in an entire color space can be uniformly reduced in response to a characteristic of an input/output appliance, and a high precision color correction can be achieved. A first reference table is formed by storing input color signals produced by reading a reference color chip by using a color image scanner in correspondence with measurement values produced by measuring the reference color chip by employing a calorimeter. A calculation is made of a transformation coefficient for transforming input color signals into measurement values based on the input color signals close to lattice points defined in a color space, and also the measurement values corresponding thereto. The color data of the respective lattice points are corrected by using the transformation coefficient. Then, a second reference table is formed by storing the color data of the lattice points in correspondence with the corrected color data of the lattice points. The color correction process operation is performed for the image data entered from the image input apparatus with employment of the second reference table.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is related to a color correcting apparatus for 
correcting a color reproduction characteristic of a color input/output 
appliance, and for converting a color image signal into color image data 
used to form a color image. 
2. Description of the Related Art 
In color input/output appliances, corrections are made in accordance with 
color reproduction characteristics of the respective appliances, and 
thereafter color signals are inputted, or outputted. FIG. 10 is an 
explanatory diagram for explaining input signal processing operation by a 
conventional color image scanner. In this color image scanner, an input 
process operation 8 is carried out by a color CCD (charge-coupled device). 
The color CCD color-separates a color image of an original to output the 
color-separated color image as R (red), G (green), B (blue) signals. In a 
shading correction process operation 9, the respective levels of the 
entered R, G, B signals are regulated. Furthermore, in a line correction 
process operation 10, color shifts (deviation) are corrected which are 
caused by positional differences in the respective R, G, B color CCDs with 
respect to the original. In addition, in a color correction process 
operation 11, a color correction is carried out so as to correct color 
difference from the image of the original, which are caused by the 
characteristics of the lamps of the color image scanner and of the color 
CCDs. In the color correction process operation 11, the inputted R, G, B 
signals are transformed by way of a transformation matrix of 3-row by 
3-column, so that corrected R', G', B' signals are produced. The 
transformation matrix (transformation coefficient) of 3-row by 3-column is 
employed so as to correct the color differences between the image and the 
original, which are caused by the characteristic of the color image 
scanner. It should be noted that the R', G', B' signals correspond to the 
NTSC-R, G, B signals ruled in a CRT (cathode-ray tube). The 
color-corrected R', G', B' signals are outputted to an image processing 
apparatus connected to the color image scanner. 
As explained above, in the case that the color correction is carried out 
with employment of the linear interpolation by a single transformation 
matrix, or a single transformation formula, the color correction would 
become difficult when the characteristic of the color input/output 
appliance is nonlinear. There is such a problem that when the color 
correction is carried out with employment of a single transformation 
formula, this color correction cannot be performed in such a manner that 
the transformation errors become uniform over the entire color space. 
The present invention has an object to provide a color correcting apparatus 
capable of uniformly reducing transformation errors in an entire color 
space in response to a characteristic of an input/output appliance, and 
also capable of executing a color correction in high precision. 
SUMMARY OF THE INVENTION 
A color correcting apparatus, according to an aspect of the present 
invention, is featured by such a color correcting apparatus for correcting 
color data of an image inputted from an image input apparatus for reading 
a color image in accordance with a reading characteristic of said image 
input apparatus, comprising; calculating means for calculating a 
transformation coefficient used to transform input data of a reference 
color into normal data based upon the input data of the reference color 
and the normal data indicative of a reference color of a reference color 
chip, said input data of the reference color being produced by reading the 
reference color chip having the reference color located near a lattice 
point defined within a color space by employing said image input 
apparatus; lattice point correcting means for correcting the color data at 
the lattice point of said color space by using said transformation 
coefficient; and color correcting means for correcting the color data of 
the image inputted from said image input apparatus with employment of the 
corrected color data of the lattice point. 
In accordance with the color correcting apparatus of the present invention, 
the correction coefficient is set to each of the color data about the 
lattice point of the color space. As a result, the color correction errors 
can be reduced over the entire color space. As a consequence, it is 
possible to realize the high precision color correction. 
Furthermore, in accordance with the color correcting apparatus of the 
present invention, the correction coefficient is set to each of the color 
data about the lattice point of the color space. As a result, the color 
correction errors can be reduced over the entire color space. As a 
consequence, it is possible to realize the high precision color 
correction. 
Furthermore, in accordance with the color correcting apparatus of the 
present invention, the transformation coefficients are newly calculated 
only when the characteristic change caused by the aging change of the 
image input apparatus exceed a certain range. As a consequence, it is 
possible to omit such a process operation for sequentially calculating the 
transformation coefficient due to a very unstable characteristic of the 
image input apparatus. Thus, the color correction can be effectively 
performed. 
Moreover, in accordance with the color correcting apparatus of the present 
invention, the correction coefficients can be set every preselected region 
within a color space. The color correction can be done in high precision 
in response to the characteristic of the image input apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to drawings, various embodiments of the present invention 
will be described. 
FIG. 1A schematically shows an arrangement of a color image input/output 
apparatus according to a first embodiment of the present invention. This 
color image input/output apparatus is arranged by a color image scanner 1 
and a computer 2. The color image scanner 1 reads a color image of an 
original to produce a color input signal, and then supplies this color 
input signal to the computer 2. The computer 2 acquires the color input 
signal, color-corrects the color input signal derived from the color image 
scanner 1 in such a manner that the color image of this color input signal 
is matched to the color image of the original, and also displays the 
color-corrected color image on a color display 3. 
A color correcting apparatus for correcting the colors of the color input 
signal is constituted by a software under control of a CPU (central 
processing unit) of the computer 2. FIG. 2 is a flow chart for describing 
operations of the color correcting apparatus. Referring now to FIG. 2, the 
color correction process operation executed in the color correcting 
apparatus will be explained. 
First of all, a reference color chip 4 as indicated in FIG. 1B is prepared. 
A plurality of color chips 4a are formed on the reference color chip 4, 
and these color chips 4a are selected in such a manner that several sorts 
of colors can be uniformly arranged over the entire space. This reference 
color chip 4 is read by the color image scanner 1, so that input color 
signals (Rsi, Gsi, Bsi) of the reference color chip 4 are produced, to 
which the color correction process operation is not performed. In the 
input color signal, symbol "i" is integers of "0" to "n" corresponding to 
the color chips 4a, and symbol "n" represents a total number of color 
chips 4a. 
Next, measurement values (Rmi, Gmi, Bmi) are entered into the computer 2, 
which are produced by measuring the respective color chips 4a of the 
reference color chip 4 by a calorimeter. In this computer 2, the input 
color signals (Rsi, Gsi, Bsi) entered from the color image scanner 1, and 
also the measurement values (Rmi, Gmi, Bmi) produced by the calorimeter 
are stored with establishing a corresponding relationship between these 
input color signals and measurement values to thereby form a first 
reference table 5. FIG. 3 schematically indicates a structure of this 
first reference table 5. It should be noted that the measurement values 
correspond to the NTSC-R, G, B values (step S1). 
When the first reference table 5 is formed, a process operation for forming 
a second process table by executing the below-mentioned process operation 
is commenced. 
FIG. 4 schematically illustrates a concept of a color space. A color space 
corresponds to a three-dimensional space for representing a color, namely 
R (red), G (Green), and B (blue) are plotted as the respective coordinate 
axial components in an orthogonal coordinate system, and coordinate 
positions indicate the R, G, B colors, respectively. In FIG. 4, symbol 
".smallcircle." indicates a coordinate point of color data of a lattice 
point 6, and symbol ".times." denotes a coordinate point 7 of input color 
signals (Rsj, Gsj, Bsj) of the reference color chip 4. Thus, color data 
(Ri, Gi, Bi) corresponding to the lattice points 6 set to predetermined 
positions within the color space are firstly entered into a second 
reference table (step S2). 
Next, the input color signals (Rsj, Gsj, Bsj) of the first reference table 
5 are sequentially derived, and then distances "d" between the input color 
signals (Rsj, Gsj, Bsj) of this reference color chip 4 and the color data 
(Ri, Gi, Bi) of the lattice point are calculated based on the formula (1) 
((Rsi-Ri).sup.2 +(Gsj-Gi).sup.2 +(Gsj-Gi).sup.2).sup.1/2. Thus, more than 
four sets of the input color signals (Rsi, Gsi, Bsi) of the reference 
color chip in which more than four distances "d" are ordered from the 
smallest value are retrieved. 
In this case, more than 4 sets of input color signals (Rsj, Gsj, Bsj ) of 
the reference color chip are retrieved, taking account of the fluctuations 
in the reading characteristic of the color image scanner 1 (step S3). 
Since the above-described process operation is carried out, coordinate 
points 7 of more than 4 sets of input color signals (Rsj, Gsj, Bsj) for 
the reference color chip are retrieved around the coordinate point 6 of 
the color data (Ri, Gi, Bi) of the respective lattice points. 
Furthermore, based upon the retrieved input color signals (Rsj, Gsj, Bsj) 
and the measurement values (Rmi, Gmi, Bmi) corresponding thereto, a 
calculation is made of transformation coefficients from the input color 
signals (Rsj, Gsj, Bsj) into the measurement values (Rmi, Gmi, Bmi) 
corresponding thereto by using the method of least squres. The 
transformation coefficients may be obtained as a transformation matrix of 
3-row.times.3-column (step S4). 
Moreover, the transformation matrix calculated at the above step is applied 
to the lattice point data (Ri, Gi, Bi) so as to calculate transformed 
values R'i, G'i, B'i. These transformed lattice point data R'i, G'i, B'i 
are saved in the second reference table in correspondence with the 
original lattice point data (Ri, Gi, Bi) (step S5). 
In addition, a check is made as to whether or not the process operations 
defined from the above-described steps S2 to S5 are carried out with 
respect to all of the lattice point data (step S6). When the process 
operations for all of the lattice point data are not yet completed, the 
color correction process operation is returned to the previous step S2 and 
then the process operations defined from this step S2 upto the step S5 are 
repeatedly performed. Conversely when the process operations for all of 
the lattice point data are accomplished, the color correction process 
operation is advanced to the following steps. 
Since the above-described process operations are executed, the formation of 
the second reference table used to color-correct the image data inputted 
from the color image scanner 1 is accomplished. As a result, an image 
input process operation from the color image scanner 1 is commenced. 
Into the color correcting apparatus, input color signals (rii, gii, bii) of 
a color image read by the color image scanner 1 are entered (step S7). 
Next, the entered input color signals (rii, gii, bii) are transformed into 
output color signals (roi, goi, boi) of NTSC-RGB by way of the 
three-dimensional reference table interpolating method with employment of 
the second reference table (step S8). 
In addition, another check is made as to whether or not the transformation 
process operation is performed with respect to all of the color image 
data. If this transformation process operation is not yet accomplished, 
then the color correction process operation is returned to the step S7 at 
which the process operation of the step S8 is repeatedly performed. 
Conversely when all of the process operations are completed, this color 
correction process operation is accomplished (step S9). 
With execution of the above-described process operations, the color 
correction process operation can be carried out in high precision over the 
color space with respect to the image data entered from the color image 
scanner 1. 
Referring now to drawings of color spaces, the above described color 
correction sequence will be explained. FIG. 5 is a diagram for 
schematically indicating a color space (a) made by reading the reference 
color chip, and another color space (b) made by measuring the reference 
color chip by the calorimeter. In this drawing, symbol ".times.", 
indicates color data read by the color image scanner, and symbol ".DELTA." 
represents color data measured by the perceived color chip. FIG. 6 is a 
diagram for schematically indicating a color space (a) of lattice point 
data used in a color correction, and another color space (b) of correction 
data. In this drawing, symbol ".smallcircle." shows ideal lattice point 
data of the color image scanner, and symbol ".quadrature." indicates 
lattice point data produced by correcting the ideal lattice point data of 
the color image scanner. FIG. 7 is a diagram for showing a color space (a) 
produced by reading an actual image by the color image scanner, and 
another color space (b) of the corrected output data. In this drawing, 
symbol ".circle-solid." shows color data read by the color image scanner, 
and symbol ".box-solid." indicates corrected color data. It should be 
understood that FIG. 5(a), FIG. 6(a), and FIG. 7(a) indicate the same 
dimensional color spaces, and further FIG. 5(b), FIG. 6(b), FIG. 7(b) show 
the same dimensional color spaces. 
In general, since a reading characteristic of a color image scanner 
functioning as a color image input appliance is different from an output 
characteristic of a printer functioning as an image output appliance, it 
is required to correct the reading characteristic of this color image 
scanner. In this embodiment, a scanner reading signal is corrected by the 
measurement value of the calorimeter defined by CIE. 
To this end, as indicated in FIG. 5, both the color data (Rsi, Gsi, Bsi) 
produced by reading the reference color chip by the color image scanner, 
and the color data (Rmi, Gmi, Bmi) produced by measuring the reference 
color chip by the calorimeter are expanded on the color space. A 
corresponding relationship among the color data of the respective color 
spaces is formed as the first reference table. 
Next, more than 4 sets of color data (Rsi, Gsi, Bsi) indicated by symbol 
".times.", which are located apart from the lattice point data (Ri, Gi, 
Bi) shown in FIG. 6(a) from the close positions are extracted from the 
color space of FIG. 5(a). Then, the color data (Rmi, Gmi, Bmi) indicated 
by symbol ".DELTA." and corresponding to these color data (Rsi, Gsi, Bsi) 
are extracted from the first reference table. Then, transformation 
coefficients from the respective color data (Rsi, Gsi, Bsi) to the color 
data (Rmi, Gmi, Bmi) are calculated by using the method of least squares. 
In FIG. 6, the above-explained transformation coefficients are multiplied 
with the respective lattice point data indicated by symbol ".smallcircle." 
of FIG. 6(a) to thereby obtain the corrected lattice point data indicated 
by symbol ".quadrature." of FIG. 6(b). Then, the lattice point data of 
FIG. 6(a) are used to form the second reference table in correspondence 
with the lattice point data of FIG. 6(b). As a result, the color 
transformation based on the reference color chip of FIG. 5 is applied to 
the color transformation of the lattice point data shown in FIG. 6, which 
constitutes a basis in such a case that the actual image will be 
color-transformed later. 
Finally, in FIG. 7(a), the actual image is read by the color image scanner, 
and the above-explained second reference table is applied to the 
respective color data shown by symbol ".circle-solid." so as to correct 
these color data by way of the three-dimensional reference table 
interpolation method. Then, as indicated in FIG. 7(b), the finally 
color-corrected data are outputted. 
Embodiment 2 
The above-explained color correcting apparatus, according to the first 
embodiment, is arranged by that every time the reference color chip 4 is 
read by the color image scanner 1, the content of the first reference 
table 5 is updated so as to execute the color correction. In contrast, a 
color correcting apparatus, according to a second embodiment, is arranged 
by that only when a value produced by reading the reference color chip 4 
by the color image scanner 1 is shifted from a preselected range, the 
content of the first reference table 5 is updated to execute the color 
correction. 
FIG. 8 is a flow chart for describing operations of this color correcting 
apparatus according to the second embodiment. In this second embodiment, a 
description will now be made of different color correction operations from 
those of the color correcting apparatus according to the first embodiment. 
First, the reference color chip 4 is read by the color image scanner 1 to 
thereby acquire input color signals (Rsi', Gsi', Bsi') (step S11). 
Next, a calculation is made of a difference between the input color signals 
(Rsi', Gsi', Bsi') newly read at the above-described step S11, and the 
input color signals (Rsi, Gsi, Bsi) of the reference color chip of the 
first reference table (see FIG. 3), which have been previously read by the 
color image scanner 1 to be stored in the storage apparatus of the 
computer 2. Then, a judgment indicated in formula (2) 
.vertline.Rsi'-Rsi.vertline.&gt;.DELTA.Rc 
.vertline.Gsi'-Gsi.vertline.&gt;.DELTA.Gc 
.vertline.Bsi'-Bsi.vertline.&gt;.DELTA.Bc is carried out based on this 
difference (steps S12, S13). 
In the case that the difference between both the input color signals is 
smaller than judging values (.DELTA.Rci, .DELTA.Gci, .DELTA.Bci), the 
color correction process operation is advanced to a step S15. Conversely, 
when this difference is larger than the judging values, the input color 
signals saved in the first reference table 5 are substituted by the newly 
read input color signals (step S14). 
A check is made as to whether or not the above-described process operation 
is performed with respect to all of the input color signals of the 
reference color chip. When this process operation is not yet executed for 
all of the input color signals, the color correction process operation is 
returned to the step S13 at which the above-explained process operation is 
repeatedly performed. When all of the process operations are completed, 
the update process operation of the first reference table 5 is completed. 
Thereafter, the process operations defined from the step S2 to the step S9 
shown in FIG. 3 are executed. 
In the process operations defined from the step S11 to the step S15, very 
small fluctuations produced during the reading operation by the color 
image scanner 1 are allowed. As a result, this color correcting apparatus 
is not excessively brought into such an unstable operation that very small 
fluctuations are produced every time the image is read by the color 
scanner 1, so that the color correction process operation can be 
effectively performed. 
Embodiment 3 
A color correcting apparatus, according to a third embodiment of the 
present invention, is arranged in such a manner that the color space shown 
in FIG. 4 is subdivided into a plurality of regions, a transformation 
coefficient from an input color signal into a measurement value is 
calculated with respect to each of these subdivided regions, and then the 
resultant transformation coefficients are used to execute the color 
correction. FIG. 9 is a flow chart for describing operations by the color 
correcting apparatus according to the third embodiment. 
First, the reference color chip 4 is read by the color image scanner 1 to 
thereby acquire input color signals (Rs, Gsi, Bsi) of the reference color 
chip 4. Moreover, measurement values (Rmi, Gmi, Bmi) are acquired by 
measuring the reference color chip 4 by the calorimeter to thereby form 
the first reference table 5 (see FIG. 3). 
Subsequently, the color space is subdivided into a plurality of regions. 
The color correction process operations defined from the step S2 to the 
step S5 shown in FIG. 2 are carried out with employment of the input color 
signals (Rsi, Gsi, Bsi) and the measurement values (Rmi, Gmi, Bmi), 
corresponding to the respective subdivided regions, which are derived from 
the first reference table 5. As a result, the second reference table is 
calculated with respect to each of the subdivided regions obtained by 
subdividing the color space (step S22). 
Next, input color signals (ri, gi, bi) of a color image is acquired from 
the color image scanner 1 (step S23). 
Then, the transformation coefficients corresponding to the judged region 
are derived from the second reference table, and the input color signals 
of the pixels are converted to execute the color correction (step S25). 
Next, a check is made as to whether or not the transformation process 
operation for all of the color image data is accomplished. If this 
transformation process operation is not yet completed, then the color 
correction operation is returned to the step S23 at which the 
above-described process operation is repeatedly performed. If this process 
operation is accomplished, then the color process operation for the color 
image is ended (step S26). 
As described above, since the color space is subdivided into a plurality of 
subregions and the transformation coefficients for color corrections are 
set to each of the subdivided regions, the color correction can be carried 
out in high precision in response to the perceiver color. 
It should be noted that the above-explained color correcting apparatuses 
according to the embodiments 1 to 3 may be arranged by employing not only 
the software, but also the hardware. 
In accordance with the present invention, since the transformation 
coefficients are set in accordance with the reading errors over the entire 
color space, the color corrections can be achieved in high precision.