Hue compensation apparatus for a color hard copy machine

A hue compensation apparatus outputs hue-compensated data of a color designated by an input color selection signal of red, green and blue, responsive to red, green and blue input signals. The hue compensation apparatus uses first, second and third lookup table circuits, a decoder, a polarizing circuit and a synthesizing circuit. The lookup table circuits output hue-compensated red, green and blue signals in response to input red, green and blue signals, respectively. The decoder decodes the input color selection signal, and the polarization circuit polarizes outputs from the first, second and third lookup table circuits according to an output of the decoder. The synthesizing circuit synthesizes the polarized output from the first, second and third lookup table circuits. The synthesizing circuit thus outputs a hue-compensated color signal whose color is designated by the color selection signal by synthesizing the polarized outputs from the polarization circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a hue compensation apparatus and, in 
particular, to a hue compensation apparatus for a color hard copy machine. 
2. Description of the Related Art 
Often, it is desirable to keep a copy of a CRT (cathode ray tube) display 
in the form of a photograph or a hard copy. For example, the CRT display 
may illustrate characteristic curve data in an experiment, figures in CAD 
(computer aided design) equipment, or TV displays. Color hard copy 
machines are developed to respond to the needs mentioned above. Many types 
of color hard copy machines exist; a sublimation-type color hard copy 
machine is described below. 
In the sublimation-type color hard copy machine, first, color signals R 
(red), G (green) and B (blue), which are derived from picture signals of 
pictures displayed on the CRT, are transformed to color signals Y 
(yellow), M (magenta) and C (cyan). Then, heat according to the signals Y, 
M and C is supplied to thermal print paper by printing heads for every 
color signal Y, M and C. A color hard copy of the picture displayed on the 
CRT is obtained by the above procedure. 
To properly reproduce the color of the picture displayed on the CRT, 
compensation as described below is required. That is, the difference 
between the characteristics of signals R, G, B and signals Y, M, C, or the 
non-linearity of color depth development to the thermal energy supplied to 
the thermal print paper, or the non-linearity of color development 
characteristics in the thermal print paper, or the coming out of a part of 
a color developed first when next color development is implemented in 
groups after first color development, need to be compensated. Thus, hue 
compensation to signals R, G, B need to be implemented to obtain an 
accurate color hard copy in relation to the original picture. 
FIG. 1 is a block diagram of one example of a hue compensation apparatus 
according to the prior art. Input terminals 10, 12 and 14 receive picture 
data R, G and B, respectively, to be printed. An input terminal 16 
receives a color selection signal which is described later. A lookup table 
circuit 18 is connected to input terminals 10, 12, 14, 16. One of signals 
Y, M and C, for example, Y, which is hue-compensated according to input 
signals R, G and B from input terminals 10, 12 and 14, is output from 
lookup table circuit 18. The color of output signals from lookup table 
circuit 18 is designated by the color selection signal from input terminal 
16. An output terminal 20 is connected to lookup table circuit 18 and 
outputs one of signals Y, M and C hue-compensated which is designated by 
the color selection signal from input terminal 16 as mentioned above. 
If signals input to input terminals 10, 12 and 14 are 8 bit signals, the 
capacity of memory in lookup table circuit 18 must be 16M bites (=2.sup.8 
.times.2.sup.8 .times.2.sup.8) per color. To print three colors, lookup 
table 18 needs 48M bites (=16M bites.times.3) of memory capacity. The 
signal for color selection input to input terminal 16 needs only 2 bits. 
In the embodiment of FIG. 1, the data must be stored in lookup table 
circuit 18 which outputs signals Y, M and C responsive to a pair of input 
signals R, G and B. Moreover, storing the data (which has 2.sup.24 
possible combinations) in lookup table circuit 18 is also cumbersome. 
FIG. 2 is a block diagram of another example of a hue compensation 
apparatus according to the prior art. In FIG. 2, input terminals 22, 24 
and 26 receive color signals R, G, B, respectively. An operation circuit 
28 is connected to input terminals 22, 24 and 26. Operation circuit 28 
performs a matrix operation as follows: 
##EQU1## 
Output terminals 30, 32 and 34 are connected to operation circuit 28, and 
receive the output signals R.sub.HC, G.sub.HC, B.sub.HC which are 
hue-compensated, respectively, from operation circuit 28. 
In the apparatus, hue-compensated signals R.sub.HC, G.sub.HC, B.sub.HC are 
obtained from output terminal 30, 32 and 34, respectively, according to 
the input signals R, G, B from input terminals 22, 24 and 26. 
The apparatus contains simple hardware and is low in cost. However, 
coefficients multiplied to input signals R, G, B are constant. Thus, the 
hue compensation method as described may not compensate for non-linear 
color development characteristics of the thermal print paper. Accordingly, 
there is a drawback that the hard copy picture may be different from the 
picture displayed on the CRT. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved hue compensation apparatus for a color hard copy machine which is 
low cost and also offers highly accurate hue compensation. 
In accordance with the present invention, the foregoing object is achieved 
by providing a hue compensation apparatus which outputs hue-compensated 
data of a color designated by an input color selection signal of one of 
red, green and blue according to red, green and blue input signals. The 
hue compensation apparatus of the present invention uses first, second and 
third lookup table circuits which output hue compensated red, green and 
blue signals in response to input red, green and blue signals, 
respectively. The hue compensation apparatus also employs a decoder which 
decodes the input color selection signal, polarization circuits which 
polarize outputs from the first, second and third lookup table circuits, 
respectively, and a synthesizer which synthesizes the polarized outputs 
from the polarization circuits. By employing polarization and synthesizing 
as described above, the present invention offers an advantage over the 
prior art by providing hue compensation for non-linear characteristics 
while maintaining a simple, low cost configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention will now be described in 
more detail with reference to the accompanying drawings. 
FIG. 3 is a block diagram of a hue compensation apparatus for a color hard 
copy machine in accordance with the present invention. Input terminals 40, 
42 and 44 receive color signals R (red), G (green) and B (blue), 
respectively. These signals R, G, B are derived from the separation (by a 
separator which is not shown) of the signal whose picture is displayed on 
a CRT (not shown). An input terminal 46 receives a color selection signal. 
The color selection signals designates the color to be hue-compensated. 
Thus, the color selected by the color selection signal designates changes 
from R to G, and from G to B. 
A lookup table circuit 48 is connected to input terminals 40 and 46. Lookup 
table circuit 48 non-linearly compensates the hue of the R signal from 
input terminal 40. A lookup table circuit 50 is connected to input 
terminals 42 and 46. Similar to lookup table 48, lookup table circuit 50 
non-linearly compensates the hue of the G signal from input terminal 42. A 
lookup table circuit 52 is connected to input terminals 44 and 46. Again, 
lookup table circuit 52 non-linearly compensates the hue of the B signal 
from input terminal 44. Lookup table circuits 48, 50 and 52 multiply input 
signals R, G and B by matrixes, such as the coefficient part of the 
equation (1), respectively. For example, an output signal R.sub.HC is 
obtained by implementation of an operation of 
(a.multidot.R)+(b.multidot.G)+(c.multidot.B). The values of the matrixes 
are varied according to the brightness of the input color signals R, G and 
B, and also according to the color designated by the color selection 
signal. Non-linear hue compensation is obtained by changing the 
coefficients of the matrices according to the amplitude of the input color 
signals and the color selection signal. 
Actually, output values obtained by multiplying the above-mentioned 
matrixes by the input color signals are stored in advance corresponding to 
the input signals in lookup table circuits 48, 50 and 52, respectively. 
Thus, to input color signals R, G and B, hue-compensated signals R.sub.HC, 
G.sub.HC, B.sub.HC are output according to the brightness of input color 
signals R, G and B, and the color selection signal. 
In the embodiment, data output from the lookup table circuit which outputs 
a hue-compensated signal of the color designated by the color selection 
signal is 8 bit data. Data outputs from the other two lookup table 
circuits are also 8 bit data. However, only the lower first to seventh 
bits are actually used as data, and the upper eighth bit is used as a 
polarization bit component since compound color compensation components 
are smaller than print data. (The 8 bit data are 2's compliment data.) 
A decoder 54 is connected to input terminal 46. Decoder 54 outputs control 
signals for controlling flag selection circuits 56, 58 and 60 (detailed 
later) according to the color selection signal from input terminal 46. 
Flag selection circuit 56 is connected to lookup table circuit 48 and 
decoder 54. Flag selection circuit 56 adds the polarity of addition or 
subtraction, which is used in a full adder circuit 62 (detailed later), to 
the output R.sub.HC from lookup table circuit 48. That is, if the polarity 
of addition (subtraction) in flag selection circuit 56 is added to the 
output signal R.sub.HC from lookup table circuit 48, the output signal 
from flag selection circuit 56 is added to (subtracted from) another 
signal in full adder circuit 62. 
Flag selection circuit 58 is connected to lookup table circuit 50 and 
decoder 54. Similarly, flag selection circuit 58 adds a polarity of 
addition or subtraction to the output signal G.sub.HC from lookup table 
circuit 50. Also, flag selection circuit 60 adds a polarity of addition or 
subtraction to the output signal B.sub.HC from lookup table circuit 52. 
Thus, flag selection circuits 56, 58, and 60 polarize the output of lookup 
tables 48, 50, and 52, respectively. 
Full adder circuit 62 is connected to flag selection circuits 56, 58 and 
60. Full adder circuit 62 synthesizes, by addition or subtraction, output 
signals R, G and B from flag selection circuits 56, 58 and 60 according to 
each polarity added in flag selection circuits 56, 58 and 60, 
respectively. An output terminal 64 is connected to full adder circuit 58 
and outputs final hue-compensated signals R, G and B. 
Flag selection circuit 56 (58, 60) is described more precisely below. FIG. 
4 is a block diagram of one example of the flag selection circuit 56 (58, 
60) shown in FIG. 3. An input terminal 70 is connected to decoder 54. 
Input terminals 72 through 86 are connected to lookup table circuit 48 
(50, 52). Input terminals 72 through 86 correspond to the eighth bit 
through the first bit of the output signal from lookup table circuit 48, 
(50, 52). 
A switch circuit 88 is connected to input terminal 72 and ground at its 
input terminal 88a and 88b, respectively. Switch circuit 88 is controlled 
by the input signal from input terminal 70. Switch circuit 88 is a switch 
to set the ninth bit of the output from flag selecting circuit 56 (58, 60) 
to `0`, or to the same value as the value of eighth bit of input signal 
(via input terminal 72) from lookup table circuit 48 (50, 52), according 
to the input signal from input terminal 70. That is, in the flag selection 
circuit connected to the lookup table circuit outputting the 
hue-compensated signal of the color designated by the color selection 
signal, switch circuit 88 is connected to terminal 88b. The ninth bit of 
the output from the flag selection circuit is `0`, and the eighth bit 
through the first bit of the output from the flag selection circuit are 
the same as from the lookup table circuit. In this case, `0` means the 
flag (polarity) of addition. 
Meanwhile, in the flag selection circuits connected to the other two lookup 
table circuits, switch circuit 88 is connected to the terminal 88a. The 
ninth bits of the outputs from the flag selection circuits are obtained 
from the values of the eight bits, respectively. As mentioned above, in 
the other two lookup table circuits, the eighth bit is a polarity bit. 
That is, the ninth bit is used as a flag giving polarity which designates 
addition or subtraction. 
The operation of the embodiment shown in FIGS. 3 and 4 is described below. 
First, the case in which the color selection signal from the input 
terminal 46 is designating red (which means that hue-compensated red data 
is output from full adder circuit 62) is described. 
Color signals R, G and B from input terminals 40, 42 and 44 are input to 
lookup table circuits 48, 50 and 52, respectively. Then, hue-compensated 
(where each non-linear element is compensated) signals R.sub.HC, G.sub.HC, 
B.sub.HC, according to the brightness of the input signals R, G and B and 
the color selection signal, are output and supplied to flag selection 
circuits 56, 58 and 60, respectively. 
In this case, data output from lookup table circuit 48 is 8 bit data such 
as `11111111` (=255). Data output from lookup table circuits 50 and 52 are 
also 8 bit data such as `11110110` (=-9), `00000011` (=3), respectively. 
In this case, as mentioned above, compound color compensation components 
(G and B in this case) are smaller than print data (R in this case). Thus, 
the first through seventh bits are used as data and the eighth bit is used 
as a polarity bit. 
To input signals R.sub.HC, G.sub.HC, and B.sub.HC supplied to flag 
selection circuits 56, 58 and 60, respectively, polarities are added. That 
is, in flag selection circuit 56, switch 88 is connected to terminal 88b 
(see FIG. 4) by the output from decoder 54. The ninth bit of the output 
from flag selecting circuit 56 is `0`, the eighth through the first bits 
from flag selection circuit 56 are the same as the output from lookup 
table circuit 48. The output from flag selection circuit 56 is 
`011111111`. 
Meanwhile, in flag selection circuits 58 and 60, switch circuit 88 is 
connected to terminal 88 a by the output from decoder 54. The ninth bits 
of the outputs from flag selection circuits 58 and 60 are the same as the 
eighth bits of the outputs from lookup table circuits 50 and 52, 
respectively. That is, the ninth bits are used as flags giving polarity, 
thereby designating (which designates) addition or subtraction. The 
outputs from flag selection circuits 58 and 60 are `111110110` and 
`000000011`, respectively. 
The output signal from flag selection circuits 56, 58 and 60 are added or 
subtracted in full adder circuit 62 according to the polarities of the 
output signals from flag selection circuits 56, 58 and 60. Actually, the 
output signals from flag selection circuits 56, 58 and 60 are only added 
because the outputs from flag selection circuits 56, 58 and 60 are 
compliments of 2. From output terminal 64, non-linearly hue-compensated 
signal R (`011111001` (=249=255-9+3)) is output. 
The color selection signal input from input terminal 46 is then changed to 
a signal designating green (G). Then, similar hue-compensation as when the 
signal designated red (R) is implemented. However, in this case, switch 88 
is connected to terminal 88b in flag selection circuit 58. In flag 
selection circuits 56 and 60, switch 88 is connected to terminal 88a. From 
full adder circuit 62, data for green used as print data is output which 
is hue-compensated. Naturally, in lookup table circuits 48, 50 and 52, 
different tables are used than when the color selection signal is red, as 
mentioned above. 
The color selection signal input from input terminal 46 is then changed to 
a signal designating blue (B) after outputting signal G hue-compensated 
from full adder circuit 62. Similar hue compensation as with signal R or G 
is implemented to signal B. However, in flag selection circuit 60, switch 
88 is connected to terminal 88b. In flag selection circuit 56 and 58, 
switch 88 is connected to terminal 88a. Data for blue used as print data 
is output which is hue-compensated from full adder circuit 62. Naturally, 
a different table from the one used when the color selection signal is red 
or green is used in lookup table circuits 48, 50 and 52. 
Additionally, output signals R, G and B from output terminal 64 are 
transformed to signals C, Y and M, respectively, to be used as print data 
as mentioned above. That is, a circuit (not shown) calculates (1-R) (=C), 
(1-G) (=M), (1-B) (=Y) to get signals C, Y and M. These signals C, Y and M 
are used as actual print data. 
In the above embodiment, lookup table circuits 48, 50 and 52 correspond to 
only one color, respectively. Thus, lookup table circuits 48, 50 and 52 
only require a memory capacity of (256.times.3) (=2.sup.8 .times.3)) 
bites, respectively. Further, data to be stored is simple to determine. 
Accordingly, the hue compensation apparatus of the present invention is 
low in cost. Moreover, using lookup table circuits 48, 50 and 52 also 
offers hue compensation for non-linear characteristics such as color 
development characteristics in thermal print paper. Thus, the accuracy of 
color reproduction in a color hard copy may be improved, while still 
employing a low cost matrix operation system. 
Modifications and variations of the present invention are possible in light 
of the above teachings. It is therefore to be understood that, within the 
scope of the appended claims, the present invention can be practiced in a 
manner other than as specifically described herein.