JP2774879B2 - Color correction circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color correction circuit used for full-color printing.

[0002]

2. Description of the Related Art Color printing uses the principle of subtractive color mixing. However, in practice, the addition rule and the multiplication rule, which are the basic principles of subtractive color mixing, often do not hold. Therefore, various color correction methods have been conventionally proposed.

The simplest method is a method of performing a 3 × 3 matrix operation represented by the following equation on input signals r, g, and b representing three colors of red, green, and blue, respectively.

[0004]

(Equation 1)

Here, R, G, and B are corrected signals, and a11
A33 are coefficients for color correction, and each coefficient is optimized by the least square method or the like. This method is called a masking method and can be realized by a simple circuit, and has been used for a long time.

[0006] Various methods have been proposed to improve this method. For example, in order to optimize each coefficient constituting the matrix according to the human visual characteristics, a method using a virtual color sample (Tajima: Transactions of the Institute of Electronics, Information and Communication Engineers, J68)
-D, p. 710 (1984)) and a method of optimizing the matrix for each hue (Ito: Information Society of Television Engineers, Information Input Study Group, January 25 (1991)). FIG. 5 shows a circuit for realizing the latter method. In this circuit, a hue determination circuit 501 determines a hue of a color represented by an input signal, and a coefficient generation circuit 502 outputs a matrix coefficient optimized in advance for each hue based on the determination result. 503 performs a matrix operation on the input signal using the matrix coefficient,
Perform color correction.

[0007]

SUMMARY OF THE INVENTION As a color correction circuit,
It is desirable that chromatic colors can be correctly reproduced without causing coloring of achromatic colors. Needless to say, the smaller the scale of the hardware, the better.

By the way, among the addition rules and the multiplication rules in color printing, the difference from the subtractive color mixing theory is particularly large in the addition rule. And the size of the shift is the secondary color (red, green,
The color of the tertiary color (white) is larger than that of the color blue (blue), and the shift is not linear.

Therefore, in order to perform good correction for both the secondary color and the tertiary color by the conventional method, it is necessary to make the number of terms of the matrix very large and perform nonlinear correction. A problem arises that the wear becomes very complicated.

In addition, the correction circuit shown in FIG. 5 cannot determine the secondary color and the tertiary color effectively, so that there is a problem that coloring occurs in an achromatic color.

An object of the present invention is to solve such a problem and to provide a color correction circuit which can reproduce chromatic colors correctly, does not produce achromatic colors, and has a small hardware scale.

[0012]

The color correction circuit according to the present invention comprises:
To achieve the above objective, determine the maximum value of the input signal.
Maximum value detection circuit and the minimum value for determining the minimum value of the input signal.
Value detection circuit and the minimum value detection circuit based on the output of the maximum value detection circuit.
Similarity judgment consisting of an arithmetic circuit for subtracting the output of the road
Constant circuit, the maximum value detection circuit and the minimum value detection circuit
Output difference, secondary or tertiary color of the color represented by the input signal
Similarity determination circuit that outputs a similarity indicating the degree of bias toward
And color interpolation based on the similarity output from the similarity determination circuit.
A coefficient generating circuit for outputting a positive coefficient;
A matrix that performs an operation between the input color correction coefficient and the input signal
And an arithmetic operation circuit.

[0013]

[Action] similarity determination circuit outputs a similarity level indicating the deviation degree on the secondary color or tertiary color colors represented by the input signal, the coefficient generation circuit based on the similarity output from the similarity determination circuit To output a correction coefficient. The matrix calculation circuit performs a matrix calculation between the correction coefficient from the coefficient generation circuit and the input signal. In the color correction circuit of the present invention, or similarity determination circuit
Or colors that represent the input signal by a degree of similarity et output is close to or tertiary color near the secondary color is shown, since different correction coefficient according to the degree of similarity <br/> is used, achromatic, Color correction suitable for each chromatic color is performed. Therefore, good color correction is performed on chromatic colors, while achromatic colors are not colored. Also, unlike the case where color correction is performed by increasing the number of terms in the matrix, the calculation for the correction is simple and the hardware is small.

[0014]

Next, embodiments of the present invention will be described in detail. FIG.
FIG. 3 shows a block diagram of a color correction circuit according to the present invention. In the figure, the similarity determination circuit 2 determines the degree of color bias represented by the input signals r, g, and b input to the input terminal 1, that is, how close the input signal is to the secondary color or the tertiary color. Is determined to be close to Specifically, the similarity I of the three signals is output as a value representing the degree of the secondary color or the degree of the tertiary color. The coefficient generating circuit 3 generates predetermined matrix coefficients for color correction, that is, data (1) to data (9), based on the similarity I. The matrix operation circuit 4 executes a matrix operation of the data and the input signal, and outputs the result to the output terminal 5 as corrected color signals R, G, and B.

The details of the similarity determination circuit 2 will be described below. The similarity determination circuit 2 determines how much the values of the input signals r, g, b are equal. That is, the similarity I is calculated based on the following equation.

I = max (r, g, b) -min (r, g, b) Here, max () and min () are functions that take the maximum value and the minimum value of the arguments in parentheses as their values, respectively. Represents Therefore, the smaller the difference between the signals r, g, and b, and the closer the color that they represent to the tertiary color, the smaller the value of the similarity I, and conversely, the greater the difference between the signals r, g, and b, and The closer to the secondary color, the greater the value of the similarity I.

The similarity determination circuit 2 has a circuit configuration as shown in FIG. 2, for example. The above-described maximum value and minimum value functions are realized by combining an appropriate comparator and a changeover switch. In the figure, r> g determination circuit 2
1, N> b determination circuit 22 and changeover switch 23,
Reference numeral 24 denotes a maximum value detection circuit 201, which includes an r <g determination circuit 25, an M <b determination circuit 26, and a switch 2
7, 28 constitute a minimum value detection circuit 202. r>
The g determination circuit 21 selects the larger one of the signals r and g, and outputs the selected signal as a signal N to the next stage N> b determination circuit 22 by controlling the changeover switch 23. The N> b determination circuit 22 selects the larger one of the signals N and b, and controls the changeover switch 24 to output the selected signal as the largest signal among the input signals r, g and b. On the other hand, the r <g determination circuit 25 selects the smaller one of the signals r and g and controls the changeover switch 27 to convert the selected signal to the signal M.
To the next stage M <b determination circuit 26. The M <b determination circuit 26 selects the smaller one of the signals M and b, controls the changeover switch 28, and outputs the selected signal as the minimum signal of the input signals r, g, and b. The subtractor 29 subtracts the minimum value signal from the changeover switch 28 from the maximum value signal input from the changeover switch 24, and outputs the result as the similarity I.

Next, the coefficient generating circuit 3 will be described. As shown in FIG. 3, the coefficient generating circuit 3 includes a plurality of ROMs in which data written in advance at each address is read based on the similarity I. The data written in the ROM is a correction coefficient used in the matrix operation circuit 4. This coefficient is determined in advance using a least squares method or the like. Here, ROMs are used by the number of coefficients, but if a ROM that can handle a large amount of data is used, the number can be reduced. In this embodiment, 9
ROMs are used. The similarity I is equal to each of ROMs 31 to 3
9, each ROM outputs data corresponding to the address as data (1) to (9).

As shown in FIG. 4, the matrix operation circuit 4 includes nine multipliers 401 to 409 and six adders 410
To 415. Multipliers 401 to 40
3 and the adders 410, 411 generate an output signal R by performing a matrix operation on the input signals r, g, b using the data (1) to (3) from the coefficient generation circuit 3, The multipliers 404 to 406 and the adders 412 and 413 perform a matrix operation on the input signals r, g, and b using the data (4) to (6) from the coefficient generation circuit 3 to output the output signal G. And multipliers 407 to 409
And adders 414 and 415 generate an output signal B by performing a matrix operation on the input signals r, g, and b using the data (7) to (9) from the coefficient generation circuit 3.

The input signals r,
g, b and data (1) to (3) are input, respectively.
Each multiplier calculates the product of the input signal and the data and outputs the result. The adder 410 includes multipliers 401 and 40
2 are added, and the adder 411 adds the output of the adder 410 and the output of the multiplier 403. As a result, an output signal R representing red after the color correction is obtained.

The input signals r,
g and b and data (4) to (6) are input, respectively.
Each multiplier calculates the product of the input signal and the data and outputs the result. Adders 412 are multipliers 404, 40
5, and the adder 413 adds the output of the adder 412 and the output of the multiplier 406. As a result, an output signal G representing green after color correction is obtained.

The multipliers 407 to 409 provide input signals r,
g and b and data (7) to (9) are input, respectively.
Each multiplier calculates the product of the input signal and the data and outputs the result. The adder 414 includes multipliers 407 and 40
9 and the adder 415 adds the output of the adder 414 and the output of the multiplier 409. As a result, an output signal B representing the corrected blue is obtained.

The output signals R, G,
B is output from terminal 5.

[0024]

As described above, in the color correction circuit of the present invention, the similarity determination circuit outputs the similarity indicating how close the color represented by the input signal is to the secondary color or the tertiary color.
Since different correction coefficients are used in accordance with the similarity , color correction suitable for each of achromatic and chromatic colors is performed. Therefore, good color correction is performed for chromatic colors, while
There is no achromatic color. Also, unlike the case where color correction is performed by increasing the number of terms in the matrix, the calculation for the correction is simple and the hardware is small.

[Brief description of the drawings]

FIG. 1 is a block diagram of a color correction circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram of a similarity determination circuit of FIG. 1;

FIG. 3 is a block diagram of a coefficient generation circuit of FIG. 1;

FIG. 4 is a block diagram of the matrix operation circuit of FIG. 1;

FIG. 5 is a block diagram of a conventional color correction circuit.

[Explanation of symbols]

Reference Signs List 1 input terminal 2 similarity determination circuit 3 coefficient generation circuit 4 matrix operation circuit 5 output terminal 21 r> g determination circuit 22 N> b determination circuit 23, 24, 27, 28 switch 25 r <g determination circuit 26 M <b Judgment circuit 29 Subtractor 31-39 ROM 201 Maximum value detection circuit 202 Minimum value detection circuit 401-409 Multiplier 410, 411, 412, 413, 414, 415 Adder

Claims (1)

(57) [Claims] 1. A maximum value detection for determining a maximum value of an input signal.
Circuit and minimum value detection circuit for determining minimum value of input signal
And the minimum value detection circuit from the output of the maximum value detection circuit.
Similarity judgment composed of an arithmetic circuit that subtracts the output of
A circuit comprising the maximum value detection circuit and the minimum value detection circuit.
An output difference between the road, and the similarity-size <br/> constant circuit for outputting a similarity indicating the degree of deviation of the secondary color or tertiary color colors represented by the input signal, the similarity determination circuit and characterized by comprising a coefficient generating circuit for outputting a color correction coefficient on the basis of the similarity to output, and a matrix operation circuit for performing an operation between the input signal and the color correction coefficient output from the coefficient generation circuit Color correction circuit.