Method and apparatus for processing color images having high maximum saturation

A color image processing apparatus comprises: a circuit to provide a color image signal comprising a luminance signal and two color difference signals; a converter to convert the two color difference signals into a hue signal and a saturation signal; a compressing circuit to independently compress the dynamic range of one or both of the luminance signal and the saturation signal a memory to store the hue signal and the output of the compressing circuit; and a color printer to reproduce the color image on the basis of the hue signal, and signals stored in the memory. The compressing circuit performs the compression on the basis of a frequency distribution of the input signal levels. With this apparatus, the color balance can be automatically adjusted by use of correction ROMs of a small memory capacity.

FIELD OF THE INVENTION 
The present invention relates to a color image processing method and an 
apparatus for processing a color image signal. 
BACKGROUND OF THE INVENTION 
For example, in a conventional color video printer or the like, the Cy 
(cyan), M (magenta), and Ye (yellow) signals are mainly processed. FIG. 2 
shows an example of the processes typically involved. The A/D converted R, 
G and B signals are LOG converted into the Cy, M, and Ye signals. Further, 
these signals are subjected to the masking correction and become Cy', M', 
and Ye' signals. These signals are input to a head driver and the heads 
are driven, so that a color image is printed. The masking correction is 
performed by the following matrix arithmetic operation. 
##EQU1## 
However, this method has a drawback that the good color reproduction is not 
obtained. Namely, if the logarithm conversion is simply performed, there 
is a case in which the concentration becomes a value above 3.0 because of 
the difference of the dynamic range of the input image signal and the 
difference of the reproducing range of the ink of the output. In such a 
case, the concentration obviously exceeds the maximum concentration of the 
print. On the other hand, a method of correcting the concentration range 
by the gamma conversion process is used. However, this method has a 
drawback that the saturation changes and the reproduced color image 
differs from the original image. 
Even in a color display apparatus as well as the foregoing color printer, 
as shown in FIG. 13, a frame buffer is necessary for each image signal in 
order to display a color image. In this case, a problem occurs with 
respect to which kind of image signal is stored in the frame buffer. 
Namely, in the conventional example of FIG. 13, the R, G and B signals are 
converted by the matrix conversion into the luminance signal Y and two 
color difference signals (R-Y and B-Y). The Y, R-Y, and B-Y signals are 
stored and thereafter, the necessary image processes are performed. For 
example, in the case of displaying the color image by CRT device (not 
shown), those signals are again converted into the R, G and B signals and 
output. The storage of the color image signal in, e.g., the frame buffer 
or the like causes a problem. If the number of pixels cannot be reduced in 
the interest of saving the memory capacity, the bit number in the 
direction of depth of the color image signal cannot help decreasing. 
However, hitherto, for example, when the number of bits in the depth 
direction of each of two color difference signals is set to six, there is 
a problem such a difference occurs between the original image and the 
reproduced color image because of the decrease in number of bits. To avoid 
such problem, eight bits are needed to obtain good color reproduction. Due 
to such situation, since the number of memories cannot be reduced, the 
memory capacity and the cost increase. 
On the other hand, hitherto, two kinds of methods have been generally used 
in order to obtain good color balance in the image. 
(1) The color balance is adjusted before photographing. 
(2) The photographed image is corrected. 
Method (1) corresponds to "the white balance switch" of a video camera. A 
white paper or the like is photographed prior to starting the 
photographing operation and the white balance is set using the "white" 
image as a reference. Method (2) is widely used in the printing field and 
the like. However, it largely depends on the feeling and experience of the 
craftsman. 
Therefore, in the case of method (2), hitherto, it is impossible to 
automatically set the color balance. 
In the case of digitizing, hitherto, for example, the least significant bit 
is omitted or the data is merely compressed a regular interval. In the 
case of digitizing a regular interval, unless there is a limitation of the 
capacity of a ROM, the color reproducibility is improved more and more as 
the interval is made increasingly. However, there is a certain limitation 
of the ROM capacity as mentioned above. 
When the saturation distribution of the color image signal from the actual 
natural image is examined, it will be understood that it is a rare case 
that the color image signals are uniformly distributed in color space, and 
in most cases, the color image signal falls within a region below the half 
value of the maximum saturation as shown in FIG. 20. 
Since it is considered that the influence on the color reproducibility of 
the whole image by the color of a certain saturation corresponds to the 
number of pixels having the saturation, the influence of the pixels in the 
portion having a large saturation with a small distribution is relatively 
small. Therefore, in the case of constituting the masking ROM by the 
digitization of the equivalent interval, the portion corresponding to the 
high saturation in the ROM occupies a constant capacity although it is 
hardly used for the masking. Therefore, this portion is the vain portion 
(i.e., not efficiently used). Namely, in order to obtain the high color 
reproducibility, the portion of a relatively low saturation in which many 
pixels are distributed must be finely digitized. Thus, the capacity of the 
ROM increases and the vain degree (size of the vain portion) also 
increases. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a color image 
processing method and apparatus which can eliminate the foregoing 
drawbacks of the conventional techniques. 
Another object of the invention is to provide a color image processing 
method and apparatus in which the memory capacity is reduced without 
deteriorating the color image quality. 
Still another object of the invention is to provide a color image 
processing method and apparatus in which, regardless of the dynamic range 
of the input color image signal, good color reproducibility can be 
derived. 
Still another object of the invention is to provide a color image 
processing method and apparatus in which the color image signal is output 
as an image signal which is close to the memory color which the human 
being feels preferable. 
Still another object of the invention is to provide a color image 
processing method and apparatus in which even in the case of a color 
correction processing table having less capacity, higher color 
reproducibility is maintained. 
Still another object of the invention is to provide a color image 
processing method and apparatus which can automatically adjust the color 
balance. 
The above and other objects and features of the present invention will 
become apparent from the following detailed description of the preferred 
embodiments, taken with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments according to the invention will be described in 
detail hereinbelow with reference to the drawings. 
(Outline of the first embodiment) 
As a color processing system for realizing the good color reproduction, an 
image processing system for processing by the luminance and color 
differences has been known. FIG. 1 shows an embodiment in which a 
constitution of a fundamental processing block was applied to a color 
video printer. After the R, G and B signals are A/D converted, they are 
stored in image memories 5, 6, and 7, respectively. These signals are 
matrix-operated upon by a matrix operation unit 8 and converted into a 
luminance signal Y and color difference signals R-Y and B-Y. 
These luminance and color difference signals are subjected to predetermined 
converting processes, which will be explained hereinafter, by conversion 
tables 9 and 10 and converted into luminance (Y'), hue (H), and saturation 
(C). The luminance conversion table 9 is provided to obtain the luminance 
Y' by normalizing the luminance Y in order to match the dynamic range of 
the input image signal with the reproduction range of the output (e.g., 
ink). The normalization and compression are performed by the conversion 
from Y to Y'. The color difference conversion table 10 is provided to 
compress the dynamic range in the direction of the saturation C so as not 
to lose color reproducibility. In this embodiment, as shown in FIG. 1, 
both luminance conversion and the color difference conversion are 
performed. However, when considering the viewpoints of holding good color 
reproducibility and comprising the dynamic range, even if only the 
luminance conversion or color difference conversion (not both) is 
independently performed, the similar effect can be obtained as will be 
clearly understood from the description hereinafter. 
The luminance (Y'), hue (H), and saturation (C') are converted into the 
recording chrominance signal components of Cy (cyan), M (magenta), and Ye 
(yellow) by a color correction conversion table 11. Further, these 
components are subjected to the masking correction to correct the uneven 
color components of the ink and input to a head driver 12 and printed. The 
outline of the first embodiment is as explained above. The first 
embodiment will be further described in detail hereinbelow. 
(Conversion into the luminance and color difference) 
After the R, G and B signals were A/D converted, they are stored in the 
memories 5 to 7 and matrix-operated upon on the basis of the following 
matrix arithmetic operation of 3.times.3 matrix: 
##EQU2## 
The reason why the image signal is stored in the memory is mainly because 
the luminance conversion, which will be explained hereinafter, needs the 
image of one picture plane in order to examine the dynamic range in the 
image signal in one picture plane of the input. Therefore, in the 
embodiment of FIG. 1, the luminance conversion, which will be explained 
hereinafter, is performed after the R, G and B signals stored in the 
memories 5 to 7 are converted into the luminance signal Y and color 
difference signals R-Y and B-Y. If the high speed process is further 
needed, the memories 5 to 7 are provided at the post stage of the matrix 
operation unit 8. The input R, G and B signals are directly 
matrix-operated upon and converted into the luminance signal and color 
difference signals. These signals are stored in the memories 5 to 7. 
The freeze (storage into the memories) of the image signals in the memories 
as mentioned above is performed at a timing when a freeze switch 21 on a 
console 20 is turned on. 
(Conversion (gradation conversion) of the luminance signal) 
The conversion of the luminance signal in the luminance conversion table 9 
is performed to reduce the difference between the input dynamic range and 
the reproducible range of the output as mentioned above. However, when 
this difference is compensated, it is desirable that the input dynamic 
range not be narrowed anymore than necessary. Therefore, in this 
embodiment, the dynamic range of the luminance signal Y of the input image 
signal is examined, thereby selecting the conversion characteristic which 
is optimum to the input. 
The dynamic range regarding the luminance in the image stored in the memory 
by the freeze switch 21 is detected by a CPU 2. Namely, the image data is 
sequentially read out of the memories 5 to 7 and a luminance histogram as 
shown in FIG. 3 is obtained, thereby detecting the dynamic range. The 
histogram is stored in a RAM 1. In this case, since it takes a long time 
if all pixels in the memory 5 are scanned, it is possible to thin out the 
pixels and to scan the pixels, for example, every five pixels in 
accordance with the sampling (picture plane of the luminance Y) as shown 
in FIG. 9A. Since the image data of one picture plane must be scanned 
within 1/30 second, it is necessary to use memories which can operate at a 
high speed as the ROM and as memories which constitute the A/D converter, 
conversion table, and the like (if such implementation is desired). 
The dynamic range of the luminance is determined by a highlight point 
(H.sub.p) and a dark point (D.sub.p) in the histogram. It is defined that 
the highlight point (H.sub.p) denotes the luminance of the point which is 
1% away from the bright luminance portion and the dark point (D.sub.p) 
represents the luminance of the point which is 1% away from the dark 
luminance portion. 
In the luminance conversion table 9, it is possible to perform the 
conversion of a few kinds of gradation conversion characteristics as shown 
in FIG. 4. Either one of those conversion characteristics is selected in 
the following manner. Assuming that the highlight point (H.sub.p) and dark 
point (D.sub.p) in FIG. 3 have the values near H.sub.p1 and D.sub.p1 in 
FIG. 4, respectively, the conversion characteristic connecting H.sub.p1 
and D.sub.p1 in FIG. 4 is the optimum conversion characteristic which does 
not lose the dynamic range of the input image. On the contrary, in the 
case of the characteristic connecting H.sub.p4 and D.sub.p1, the color 
reproducibility in the higher luminance portion will be lost. The 
luminance conversion table 9 having such a few kinds of characteristics is 
constituted by, e.g., a ROM or the like and the number of conversion 
tables can be increased or decreased in accordance with the capacity. It 
is sufficient to properly address the ROM on the basis of the values of 
the highlight point (H.sub.p) and dark point (D.sub.p) of the histogram. 
(Feature of the luminance conversion) 
In this manner, the dynamic range in the direction of the luminance is 
properly compressed (what is called the normalizing process is performed) 
and the accurate output image which cannot be obtained by the conventional 
unitary log conversion is reproduced. Namely, the gradation correction 
suitable for the output ink is performed. On the other hand, since the 
luminance converted luminance signal Y' is compressed, if it is necessary 
to store the image signal in the frame memory or the like, the memory 
capacity can be saved by use of the luminance signal Y' after the 
luminance conversion rather than the luminance signal Y after the matrix 
operation 8. As explained above, the invention has such an excellent 
feature. 
On the other hand, if only the luminance is compressed without compressing 
the dynamic range in the direction of the saturation, which will be 
explained hereinafter, the saturation C is preferably held and good color 
reproduction is realized. Further, since the table conversion is used, an 
increase in circuit scale or the like can be prevented and the like. 
In the foregoing embodiment, an explanation has been made with respect to 
the example of the video printer having the frame memory. However, if a 
line memory is used, the invention can be also embodied without the frame 
memory. 
(Conversion of the color difference.fwdarw.saturation/hue) 
As shown in FIG. 5, the dynamic range A in the direction of the saturation 
obtained with the R, G and B input signals is considerably wider than the 
reproduction range B in the saturation direction obtainable with the ink 
of the Cy, M, and Ye signals, so that the input image signals need to be 
compressed in the saturation direction. For this purpose, in this 
embodiment, after the color difference signals R-Y and B-Y are converted 
into the saturation signal C and hue signal H by the color difference 
conversion table 10, the compression in the saturation direction is 
performed, thereby obtaining the saturation signal C' from the saturation 
signal C. First, the color difference signals are converted into the hue 
and saturation signals in accordance with the following equations: 
##EQU3## 
where, when R-Y=0, H=0. 
##EQU4## 
The results of the above arithmetic operations in the combinations of the 
respective values of R-Y and B-Y are stored in the conversion table 10. 
However, as mentioned above, the dynamic range in the saturation direction 
of the R, G and B input signals is wider than the reproduction range in 
the saturation direction of the inks of the Cy (cyan), M (magenta), and Ye 
(yellow) signals, so that even if the compression is performed, the color 
reproducibility in printing will not deteriorate excessively. The simplest 
compression characteristic is shown in FIG. 6. The portion of a high 
saturation is clipped on the basis of the fact that the ordinary images 
existing in the natural world are mostly concentrated in the low 
saturation region. In the diagram, C' is obtained by further converting 
the following saturation C by the conversion table 10 as follows: 
##EQU5## 
There occurs a problem that after all of the input saturation signals C 
larger than the output saturation signal C'.sub.max are compressed to 
C'.sub.max by the above conversion, the difference of the saturation in 
this portion cannot be expressed. In this case, it is also considered to 
use such a conversion characteristic as shown in FIG. 7 in order to 
prevent such a problem. However, when this characteristic is used, the 
saturations of all pixels decrease and good color reproduction cannot be 
derived. Therefore, it is considered to use a non-linear characteristic as 
shown in FIG. 8. With this characteristic, the saturations of most of the 
pixels do not decrease, namely, the saturation reproducibility is not 
lost. Moreover, the saturation signal C' can be formed without losing the 
difference of the saturation in the portion of the high saturation. This 
is because the ordinary natural images are concentrated in the region of a 
relatively low saturation. 
(Normalization of the saturation) 
The normalization which has been mentioned in the luminance conversion may 
be also used for the compression of the saturation in order to compress to 
the more proper dynamic range. Namely, a histogram of the saturation is 
obtained from the memories 6 and 7. It is assumed that the histogram is as 
shown in, e.g., FIG. 10A. Assuming that the maximum saturation derived 
similarly to the case of the luminance compression is C.sub.H and the 
minimum saturation is C.sub.L, the optimum compression/conversion 
characteristic is selected from a plurality of characteristics as shown in 
FIG. 10B on the basis of the values of those maximum and minimum 
saturations. In this way, the optimum color reproduction can be obtained. 
However, the selection of the proper conversion characteristic from the 
histogram is likely to cause the processing speed to be reduced; 
therefore, the high speed memory device or the like is needed. Examples of 
the sampling of the color difference signals to obtain the histograms at a 
high speed are shown in FIG. 9B (picture plane of the color difference 
R-Y) and FIG. 9C (picture plane of the color difference B-Y). 
(Color correction conversion) 
The resultant Y', H, and C' signals are input to the color correction 
conversion table 11 and converted into Cy', M', and Ye' signals. The 
values which are stored into the conversion table are calculated 
fundamentally by the conversion opposite to the foregoing conversion in 
accordance with the following equations: 
EQU (R-Y)'=C'.times.Cos H 
EQU (B-Y)'=C'.times.Sin H 
EQU Further, 
EQU R'=(R-Y)'+Y' 
EQU G'={0.3(R-Y)'+0.11(B-Y)'}+Y' 
EQU B'=(B-Y)'+Y' 
EQU Next, 
EQU Cy=-log R' 
EQU M=-log G' 
EQU Ye=-log B' 
The following matrix approximation is performed to correct the uneven color 
components of the ink: 
##EQU6## 
where a.sub.11 to a.sub.33 are constants. 
(Feature of the saturation compression) 
According to the foregoing embodiment, not only the difference between the 
saturations of the input and output is effectively compensated but also 
the maximum amount of the saturation signal is reduced. Therefore, even in 
the case of the same number of bits being assigned, the fine digitization 
can be performed and the efficiency rises. Namely, the storing efficiency 
by the compression increases. 
Although the example of the video printer has been described in the 
embodiment, the invention can be also applied to other systems regarding 
the correction of the color image such as a constituting system in the 
printing and the like. On the other hand, in this embodiment, after the R, 
G and B signals are A/D converted, they are converted into the Y, R-Y, and 
B-Y signals by matrix operation. However, it is also possible to 
constitute the system in such a manner that the R, G and B signals are 
matrix converted into the Y, R-Y, and B-Y signals before the A/D 
conversion, and then these signals are A/D converted into digital signals 
and these digital signals are used. 
(Color image memory apparatus) 
The embodiment of FIG. 1 has not only a feature the that the color 
reproduction can be properly performed but also a feature such that the 
image signal can be compressed as mentioned above. Therefore, it will be 
understood that this also solves, the problem described above in reference 
to the conventional systems as occurring when the number of bits in the 
depth direction of each of two color difference signals is e.g., six, a 
difference occurs between the original image and the reproduced color 
image." In other words, even if memory is saved, the saved amount is 
compensated for by the compression and no problem occurs in the color 
reproduction because of the foregoing reason. 
FIG. 11 shows an embodiment which is constituted from the viewpoint of the 
saving of the memory. A large difference from the embodiment of FIG. 1 
relates to memories 30 to 32. The memories 30 to 32 are, for example, the 
frame memories (frame buffers) to output an image. Hitherto, it has been a 
problem to reduce the memories in this portion as mentioned above. 
However, memory can be saved by the compression of the luminance signal or 
saturation signal in this embodiment. 
There are a few methods of compressing the luminance signal as mentioned 
above. However, since the memory 5 is essentially necessary to obtain the 
histogram, the memory 5 will be unnecessary if the characteristics as 
shown in, e.g., FIGS. 6 and 7 are fixedly used. Because of the similar 
reason, the memories 6 and 7 are also unnecessary when such a compression 
that the histogram of the saturation is unnecessary is performed. 
In this embodiment, the number of pixels has been set to the standard 
number (640.times.480) of the NTSC system. However, the number of pixels 
can be also reduced. In particular, if the color correction conversion 
table 11 in FIG. 1 or 11 is constituted as shown in FIG. 12, the color 
data can be remarkably reduced. 
As described above, according to the foregoing embodiment, by storing the 
image signals as the formats of luminance, hue, and saturation, the 
compression can be performed, so that the capacity of the memory section 
can be decreased without deteriorating the image quality. 
In addition, since the conversion is performed in accordance with the 
dynamic range of the input color image signal, in particular, the 
luminance signal, even in the case of the input color image signal of any 
dynamic range, good color reproduction can be derived. 
(Embodiment of the correction of the skin color) 
An explanation will now be made with respect to an embodiment such that in 
the color correcting process, when the hue is close to the hue of the 
stored color of a predetermined color, the hue is output as a value which 
is closer to the hue of the stored color of the predetermined color. 
FIG. 14 is a block diagram of a color video printer of the embodiment. This 
embodiment includes a masking circuit 114 which receives the color 
difference signals R-Y and B-Y and an HC conversion ROM 113 disposed at 
the front stage thereof as main components. As the outline of the whole 
operation, the input signals of Y, R-Y, and B-Y are A/D converted and 
stored into image memories (110, 111, 112). A CPU 115 takes out the Y, 
R-Y, and B-Y signals of the pixel to be printed from the image memories 
(110, 111, 112). The color difference signals R-Y and B-Y taken out are 
converted into the hue signal H and saturation signal C by the HC 
conversion ROM 113. Data such that the color difference signals R-Y and 
B-Y are used as address inputs and converted into the hue signal H and 
saturation signal C is stored in the HC conversion ROM 113. The Y, H, and 
C signals are transmitted through the masking circuit 114 and converted 
into the Cy' (cyan), M'(magenta), and Ye' (yellow) signals and further D/A 
converted and printed. 
The HC conversion ROM 113 converts the color difference signals R-Y' and 
B-Y' into the luminance signal Y and saturation signal C. In this case, 
such a digitization as shown in FIG. 15 is executed when converting into 
the H (hue) and C (saturation) signals. Namely, the digitization is 
performed in a manner such that the plane of the R-Y and B-Y signals is 
divided into the regions surrounded by circles and straight lines and the 
values in the regions are represented by points ".". In this case, 
##EQU7## 
where, when R-Y=0, H=0, and 
##EQU8## 
The amplitude from a reference axis corresponds to a value the hue H and 
the radius of a circle corresponds to a value the saturation C. In FIG. 
15, H of the output shows the case of four bits and C shows the case of 
three bits. The hatched portion 100 shows the region in which the colors 
near the skin color are distributed. 
In the diagram, h.sub.0 denotes a hue of the preferable skin color of the 
stored color, and h.sub.1 and h.sub.2 represent ranges of the hues which 
ought to be sensed as the skin color by the digitization of FIG. 15. The 
digitization as shown in FIG. 15 is nothing but the process such that the 
values of hue (H) are taken every equal angle and the values of saturation 
(C) are taken at regular intervals and these values are set representative 
values. Therefore, no consideration is made to the visual sensation of the 
human being in the case of such a digitization. 
The skin color to which the human being pay the largest attention is 
reproduced by use of the masking circuit 114 having such a constitution as 
shown in FIG. 17. The masking circuit 114 comprises: an HC decoder 120 for 
converting the H and C signals to the corrected H' and C' signals as shown 
in FIG. 16; a converter 121 to reversely convert the Y', H', and C' 
signals into the Y', R-Y', and B-Y' signals; an RGB converter 122 to 
further convert the Y', R-Y', and B-Y' signals into the R, G, and B 
signals; a converter 123 to convert the R, G, and B signals into the Cy 
(cyan), M (magenta), and Ye (yellow) signals; a masking matrix circuit 124 
to perform the ordinary masking processes; and the like. 
The HC decoder 120 will be first described. When it is assumed that the HC 
conversion ROM 113 outputs the H signal of h bits and the C signal of k 
bits, the hue and saturation to the region other than the skin-colored 
region 100 are 
EQU 360.degree..times.(h/2.sup.n) and 
EQU C.sub.max .times.C/(2.sup.k -1) 
where C.sub.max denotes the maximum saturation. The HC decoder 120 does not 
perform any process in such a region 100. 
On the other hand, assuming that the HC conversion ROM 113 generated the 
image signals included in the skin-colored region 100 to the HC decoder 
120 (as the signals of h bits and k bits), the HC decoder 120 outputs the 
hue signal as h' which is closer to h.sub.0 as shown in FIG. 16 when h 
lines between h.sub.1 and h.sub.2. Further, when also considering the 
saturation in the skin-colored region 100, only the output of the HC 
conversion ROM 113 in the range where h falls between h.sub.1 and h.sub.2 
and 2.sup.k .ltoreq.C.sub.0 is corrected. The HC decoder 120 to convert 
the input as shown in FIG. 15 into the output as shown in FIG. 16 can be 
easily constituted as a ROM. 
In this manner, by setting the hue of the image signal which is considered 
to be the skin color into the value near the hue of "the preferable skin 
color", the skin-colored region which is generally distributed can be 
collected to the region of "the preferable skin color" and the 
reproducibility of the skin color can be improved. 
The resultant Y, H, and C signals are input to the converter 121 and 
converted in the following manner: 
EQU (R-Y)'=C.times.Cos H 
EQU (B-Y)'=C.times.Sin H 
Further, the following R, G, and B signals are output from the RGB 
converter 122: 
EQU R=(R-Y)'+Y' 
EQU G=-{0.3(R-Y)'+0.11(B-Y'}+Y' 
EQU B=(B-Y)'+Y' 
The following conversion is performed by the converter 123: 
EQU Cy=-log R' 
EQU M=-log G' 
EQU Ye=-log B' 
Then, the following matrix conversion is performed by the masking matrix 
circuit 124 to correct the uneven color components of the ink: 
where a.sub.11 to a.sub.33 are constants. 
The HC conversion ROM 113 and HC decoder 120 can be also constituted by one 
ROM. Although the foregoing embodiment has been described with respect to 
the video printer, in the case cf outputting to a CRT device or the like, 
a more preferable skin color can be reproduced if the output of the RGB 
converter 122 is displayed by the CRT. Although the embodiment has been 
described with regard to the skin color, the invention can be also applied 
to other specified color, e.g., sky color or the like. 
As described above, according to the present embodiment, by approaching the 
present hue in a predetermined range in the image signals to the hue of a 
desired stored color, the color which the human being will deem preferable 
can be reproduced. In particular, the skin color can be effectively 
reproduced. 
(Digitization) 
A non-linear digitizing method of the color image signals will now be 
described. 
FIG. 18 is a diagram showing a fundamental constitutional concept of an 
embodiment in which the luminance signal Y and color difference signals 
R-Y and B-Y are input as color image signals and masking processed. When 
the luminance signal Y and color difference signals R-Y and B-Y are used 
as the color image signals, the luminance Y is unnecessary for the 
saturation. Therefore, the luminance signal Y is not input to an address 
conversion table 201. The address conversion table 201 consists of, e.g., 
a ROM or the like and the color difference signals are input as addresses 
in the ROM. Data whose low-saturation portion has been finely digitized 
and a high-saturation has been roughly digitized is stored in advance in 
the ROM. The ROM has the functions to perform the digitization and the 
conversion of the address space. The address conversion table 201 outputs 
the color image signals R-Y' and B-Y' which are the converted addresses 
and whose values were changed. In this case, even when the signals each 
consisting of six bits are input to the address conversion table 201 and 
the signals each consisting of five bits are output therefrom, the 
digitization is not simply performed at regular intervals as in the 
conventional apparatus. Therefore, the accuracy does not simply 
deteriorate to 1/4. 
FIG. 19 is a block diagram of a color video printer to which the invention 
is applied. This printer comprises: a masking ROM 214 which receives the 
Y, R-Y, and B-Y signals; and an address conversion ROM 213 arranged at the 
front stage thereof. As the outline of the whole operation, the Y, R-Y, 
and B-Y input signals are A/D converted and stored into image memories 
(210, 211, 212). A CPU 215 reads cut the Y, R-Y, and B-Y signals of the 
pixel to be printed from the image memories 210 to 212. The R-Y and B-Y 
signals are transmitted through the address conversion ROM 213 and 
digitized into the signals each consisting of less bits. Next, the Y, R-Y, 
and B-Y signals are transmitted through the masking ROM 214 and converted 
into the Cy (cyan), M (magenta), and Ye (yellow) signals and further D/A 
converted and printed. 
When the digitization is performed at regular intervals as in the 
conventional apparatus, the content of the address conversion ROM 213 (or 
address conversion table 201) is as shown in FIG. 21A. The digitization 
(i.e., the address conversion) of the (color difference) image signals 
denotes that the plane which is expressed by R-Y and B-Y is represented by 
points ".multidot.". At this time, the cross point of the R-Y and B-Y axes 
has the saturation 0 and the saturation increases as one moves outside, 
away to the outside from this point. 
FIG. 20 shows a histogram of the saturations of the pixels of the color 
image which ordinarily exists in the natural world. In such a natural 
image, the number of pixels having the saturations above a certain 
saturation value (C.sub.n) rapidly decreases. Therefore, in the case of 
the digitization space of regular intervals shown in FIG. 21A, only the 
space of the masking ROM shown in FIG. 21B is obtained. In the ROM space 
of FIG. 21B, the portions of the saturations above the saturation value 
C.sub.n relatively become vain as mentioned before. 
A method according to the embodiment is shown in FIGS. 22A and 22B. It is 
the fundamental concept of the digitization of the embodiment that the 
pixel distribution according to the saturation is plotted in the color 
difference (R-Y, B-Y) space, and the digitization interval is changed at a 
long interval of a few stages in accordance with the distribution density. 
FIG. 22A shows a digitization space corresponding to FIG. 21A in the 
embodiment. The digitization interval is set to two stages using the 
saturation value C.sub.n as a reference. The portion of a low saturation 
is digitized at a short interval. The region of a high saturation is 
digitized at a long interval. FIG. 22B shows an ROM space in the address 
conversion ROM 213. The sampling point in FIG. 22A corresponds to the 
sampling point in FIG. 22B in a one-to-one corresponding manner. Thus, as 
shown in FIG. 22B, the area in the portion surrounded by C.sub.n and 
-C.sub.n is widened as compared with that in FIG. 21A. The fine masking 
process is performed to the image signal of a low saturation and the color 
reproducibility is improved by only the amount of increased area. The 
reason why the intervals in FIG. 22B are equal is because, in general, the 
addresses must be input into the ROM at regular intervals. 
The address conversion ROM 213 will be further described with reference to 
FIG. 22A. In the diagram, the region obtained when the maximum saturations 
of R-Y and B-Y were set to C.sub.max is shown by a straight line. The 
region of the saturation C.sub.n which is half of C.sub.max is indicated 
by a broken line. Assuming that the number of bits of each of the outputs 
R-Y and B-Y of the address conversion ROM 213 is n, the number of sample 
points to be digitized is m (=2.sup.n .times.2.sup.n). When it is assumed 
that K points among the m points are contained in the region of a low 
saturation in the area surrounded by the broken line, the number of 
sampling points to be included in one side of the area in the broken line 
is .sqroot.K. Therefore, when the region between -C.sub.n and +C.sub.n is 
divided by .sqroot.K at regular intervals, it is sufficient to assign the 
intervals in the vertical and horizontal directions at every other 
2.multidot.C.sub.n /.sqroot.K. In the region other than the area in the 
broken line, the number of sample points is n.sup.2 -K and the area is 
3.multidot.C.sub.max.sup.2. Therefore, it is sufficient to arrange in both 
of the vertical and horizontal directions at every other 
.sqroot.3.multidot.C.sub.max /.multidot.m-k. 
Although the digitization in the square region has been performed in FIG. 
22A, in the case of the saturation, there is also considered a method 
whereby the region of a low saturation is set to a circle and a larger 
number of bits are assigned to this circular region as compared with that 
in the other regions as shown in FIG. 23 by paying due attention to the 
fact that the saturation increases like a circle from the cross point of 
the R-Y and B-Y axes. 
As described above, the address conversion ROM which receives the Y, R-Y, 
and B-Y signals is arranged at the front stage of the masking ROM, and a 
larger number of bits are assigned to the region of a low saturation, so 
that the natural image can be extremely efficiently digitized. 
Although the masking process has been described as an example in the 
foregoing embodiment, the color correcting process is not limited to the 
masking process. The input color image signals are not limited to the 
luminance signal and color difference signals but may be the signals of 
the RGB system or other color expression system. When the distribution of 
the pixels is complicated, the number of digitization stages is not 
limited to two but may be set to a large number of stages. In this case, 
the fine color reproduction can be realized with less memory capacity. 
As described above, according to the embodiment, an attention is paid to 
the deviation of the pixel distribution to the saturations of the input 
image signals, the fine color correcting process is performed to the image 
of a low saturation consisting of a large number of pixels, and the rough 
color correcting process is executed to the image of a high saturation 
consisting of less number of pixels. Therefore, the memory capacity of the 
color correction processing table can be efficiently used. 
(Correction of the color balance) 
An embodiment in which the color balance is automatically corrected will 
now be described. 
The correction of the color balance in this embodiment is performed by 
paying an attention to the highlight point (the pixel of the highest 
luminance) in the image. Namely, in the case of the image whose colors 
were well balanced, the protability such that the color difference of the 
highlight point is "0", namely, "white" is high. Therefore, the color 
balance correction in the color image processing apparatus in this 
embodiment will be summarized as follows. 
(1) Since the color difference amount of the highlight point pixels in the 
image obtained is considered to be the "deviation" of the color balance, 
this color, difference amount is set to the value .DELTA.E to correct the 
color balance. 
(2) With respect to all pixels, the .DELTA.E.times.luminance/maximum 
luminance is subtracted from the color difference amount of each pixel and 
the resultant difference is used as the color image signal after the 
correction, thereby adjusting the color balance. 
FIG. 25A shows a color stereogram of the image whose color balance is 
deviated. FIG. 25B shows a color stereogram of the image after the color 
balance was corrected. The color differences (R-Y, B-Y) of the pixel 
having the maximum luminance (Y) are considered to represent the 
"deviation" of the color balance. Therefore, when the color correction is 
performed by regarding the color difference amounts as the correction 
amounts .DELTA.E (.DELTA.R.sub.Y, .DELTA.B.sub.Y), the corrected image as 
shown in FIG. 25B is obtained. When the correction amounts are calculated, 
the correction amounts corresponding to the color differences of an 
arbitrary pixel in the image are set to the values such that .DELTA.E was 
proportionally distributed by the luminance/maximum luminance 
(Y/Y.sub.max) as shown in FIG. 25A. In the following embodiment, two 
examples of the method detecting .DELTA.E (.DELTA.R.sub.Y, .DELTA.B.sub.Y) 
will be explained. (Outline of the apparatus in the embodiment) 
FIG. 24 is a block diagram of a color image processing apparatus in the 
embodiment. The outline of the image processing apparatus shown in FIG. 24 
is as follows. 
The A/D converted color image signal Y and color difference signals R-Y and 
B-Y are stored in an image memory 302. A CPU 308 reads out the data from 
the image memory 302 and detects the correction amounts .DELTA.E by either 
the method shown in FIG. 26 or the method shown in FIGS. 27A and 27B. The 
correction amounts (.DELTA.R.sub.Y, .DELTA.B.sub.Y) are respectively input 
to correction ROMs 303 and 304. The luminance signal Y and color 
difference signals R-Y and B-Y are also input to the correction ROMs 303 
and 304, respectively. The outputs of the ROMs 303 and 304 and the 
luminance signal Y are input to a masking circuit 305 and converted into 
the Cy, M, and Ye signals. Then, these signals are D/A converted and 
printed by a head driver 307. The correction of the color balance by the 
correction ROM is shown in FIG. 24 and will be explained in detail 
hereinafter. An explanation will be also made hereinafter with respect to 
an embodiment such that the correction of the color balance to each of the 
pixels constituting the image is executed by the CPU 308 in a software 
manner. The program for a procedure as shown in FIG. 26 and the like is 
stored in a RAM 309. 
(Determination of .DELTA.E) 
In the method of determining .DELTA.E shown in FIG. 26, the color 
difference signals of the pixel having the maximum luminance (Y.sub.max) 
are set to .DELTA.E. First, in step S1, the pixels of the highlight points 
are found out by comparing the luminance Y of each pixel in the image 
memory. In step S2, the color difference amounts R-Y and B-Y of the pixels 
are regarded as the "deviation" of the color balance and set to the 
correction amounts .DELTA.E (.DELTA.R.sub.Y, .DELTA.B.sub.Y). 
FIG. 27A shows the concept of another method of obtaining .DELTA.E. FIG. 
27B shows a procedure for this method. According to the foregoing method 
of obtaining .DELTA.E, the values for .DELTA.E are obtained from one point 
of the maximum luminance. Therefore, there is a fear that the correction 
amount might become unstable. To prevent this problem, about ten sample 
points of the pixels having the highest luminance are collected and the 
average of the color difference amounts of these pixels is calculated to 
obtain the correction amounts, thereby stably and effectively correcting 
the color balance. 
FIG. 27B is a flowchart for the process to determine the correction amount 
.DELTA.R.sub.Y and .DELTA.B.sub.Y. First, ten highlight points are 
searched from the image in step S10. However, in this case, it takes a 
long time to search ten highlight points from all of the pixels. 
Therefore, as shown in FIG. 27A, when the input image consists of 
640.times.480 pixels, the pixels are sampled every other four pixels in 
the vertical and horizontal directions in the region of (121 to 
520).times.(86 to 295) inside of this image and ten highlight points are 
searched. Even by such a thinned-out sampling, almost the same result as 
that in the case of searching them from all of the pixels can also be 
derived. 
When the color difference amounts R-Y and B-Y of ten highlight points are 
(R-Y).sub.1, (R-Y).sub.2, . . . , (R-Y).sub.10 and (B-Y).sub.1, 
(B-Y).sub.2, . . . , (B-Y).sub.10, .DELTA.E (.DELTA.R.sub.Y, 
.DELTA.B.sub.Y), become 
EQU .DELTA.R.sub.Y ={(R-Y).sub.1 +(R-Y).sub.2 +. . . +(R-Y).sub.10 }/10 
EQU .DELTA.B.sub.Y ={(B-Y).sub.1 +(B-Y).sub.2 +. . . +(B-Y).sub.10 }/10 
Thus, the more effective correction amounts can be obtained as compared 
with the case of deciding the correction amounts from one highlight point. 
(Correction of the color balance) 
The maximum correction amounts are derived in this manner. An embodiment in 
the case where the CPU 308 performs the correction of the color balance in 
a software manner will now be explained. In the correction of the color 
balance, the correction amounts .DELTA.R.sub.Y and .DELTA.B.sub.Y are used 
as the maximum values and the color balance is variably corrected in 
proportion to the luminance of each component pixel of the image. This is 
because if the correction amounts .DELTA.R.sub.Y and .DELTA.B.sub.Y are 
merely subtracted from the color differences R-Y and B-Y of each component 
pixel of the image, the correction is performed too much as the luminance 
of the pixel becomes low. Therefore, less correction amounts are 
subtracted for the pixels having a low luminance with regard to all of the 
pixels as explained in FIG. 25A by performing the following correction. 
EQU R-Y'.rarw.(R-Y)-.DELTA.R.sub.Y .times.Y/Y.sub.max 
EQU B-Y'.rarw.(B-Y)-.DELTA.B.sub.Y .times.Y/Y.sub.max 
Y, R-Y, and B-Y denote the image signals of each of the pixels constituting 
the image. Y.sub.max denotes the maximum luminance. FIG. 28 shows a 
procedure for the correction of the color balance. By the foregoing 
method, the image whose color balance was deviated in the image memory as 
shown in FIG. 25A is corrected as shown in FIG. 25B. 
As described above, the resultant data obtained by multiplying the 
correction amounts obtained from the highlight points with the 
luminance/maximum luminance is subtracted from the color difference 
amounts of each component pixel of the image, so that the color balance 
can be effectively corrected at a high speed by the simple calculations. 
The color balance correction amounts linearly change in accordance with 
the luminance in the foregoing embodiment. However, the invention may be 
also applied to non-linear correction amounts by applying the weights 
corresponding to the luminances. 
(Color balance correction table) 
The correction ROMs 303 and 304 constituting the correction table for the 
correction of the color balance will now be explained. These ROMs intend 
to realize the high-speed process by executing the operations in step S21 
in FIG. 28 in a hardware manner in place of the CPU 308. 
As shown in FIG. 24, with respect to R-Y, the Y signal, R-Y signal, and 
signal of the correction amount .DELTA.R.sub.Y are input to the ROM, so 
that the corrected R-Y' signal is output. Similarly, the B-Y' signal is 
output with respect to the B-Y signal. The correction amount 
.DELTA.R.sub.Y and .DELTA.B.sub.Y are previously calculated on the basis 
of the following equations and stored. 
EQU R-Y'.rarw.(R-Y)-.DELTA.R.sub.Y .times.Y/Y.sub.max 
EQU B-Y'.rarw.(B-Y)-.DELTA.B.sub.Y .times.Y/Y.sub.max 
The processes can be performed at a high speed by executing the table 
conversion in this manner. FIG. 24 shows a block diagram of the color 
video printer using the table conversion. For example, with respect to 
R-Y, only the R-Y signal, .DELTA.R.sub.Y signal, and Y signal are input to 
the ROMs 303 and 304 shown in FIG. 24. The Y.sub.max signal is not input. 
This is because the value of Y.sub.max is fixed in order to reduce the 
number of input bits to the ROMs. For example, when the luminance Y 
consists of eight bits, the value of Y.sub.max is fixed to, e.g., "255". 
The values obtained by performing the foregoing equations using Y.sub.max 
=255 are stored into the ROMs. The correction accuracy does not 
deteriorate even by this method and, accordingly, the scale of each ROM 
can be reduced by only the amount corresponding to the decrease in number 
of input bits. 
By use of the table conversion on the basis of three input signals of 
correction amount (.DELTA.E), luminance, and color difference amount, it 
is possible to realize the processing apparatus which can effectively 
correct the color balance at a high speed with a simple constitution. 
FIG. 29 shows a modified example of the correction circuit. In this 
example, in order to reduce the scale of the ROM, only the calculations of 
.DELTA.R.sub.Y .times.Y/Y.sub.max and .DELTA.B.sub.Y .times.Y/Y.sub.max in 
the foregoing equations are executed in the ROM and the subtractions are 
performed in subtracters 322 and 323. 
As described above, according to the embodiment, the color difference 
amounts of the pixel of a predetermined luminance are regarded as the 
deviation of the color balance and the color correction is performed in 
accordance with the luminance on the basis of this deviation. Therefore, 
it is possible to provide the color image processing apparatus which can 
correct the color balance by a constant algorithm. If the predetermined 
luminance is set to the maximum luminance, the color balance can be more 
effectively corrected. 
As described above, according to the invention, the useful color image 
process can be realized. 
The present invention is not limited to the foregoing embodiments but many 
modifications and variations are possible within the spirit and scope of 
the appended claims of the invention.