JP2819953B2 - 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, and more particularly to a color correction circuit capable of improving the color reproducibility of a single-chip video camera using one CCD as an image pickup device.

[0002]

[Technical background] Hue is a word that indicates the "classification of color type" of the perceived color of the object surface, and is red (R), orange (Or), and yellow.
(Ye), green (G), blue (B), purple (V), and the like, which are scales of visual attributes that are characterized based on color designations. Lightness is the “degree of brightness” of the perceived color of the object surface, and is a measure of the visual attribute that determines the magnitude of the luminous reflectance. Saturation is the “degree of vividness” of the perceived color of the object surface, and is achromatic (Neu
It is a measure of the visual attribute of the distance from the (tral color). The achromatic color is a color that does not cause a series of hue perceptions from black to gray to white, and is a chromatic color except for the achromatic color.

In the Munsell color system, the lightness scale is V = 10 for ideal white and V = 0 for black, and the brightness is divided into 10 parts so that the interval between them is perceptually equal. The scale of the saturation is C = 0 for an achromatic color, and as the saturation increases, C =
.. Are scaled so that the numerical values are increased to 1, 2, 3,. Further, in this color system, the perceptual difference corresponding to the brightness difference ΔV = 1 is intended to be substantially equal to the perceptual difference corresponding to the saturation difference ΔC = 2. Therefore, in the Munsell color chart collection actually created,
(Value) The color chart is selected such that the difference is 1 and the chroma (saturation) difference is 2.

[0004]

2. Description of the Related Art Recently, the International Electrotechnical Commission (IEC / TC84)
In single-panel video cameras (hereinafter “V
Discussion of a standard method for measuring the performance of cameras) has also begun and is progressing. Regarding color reproducibility, uniform chromaticity coordinates (C) defined by the CIE (International Commission on Illumination) and Japanese Industrial Standards have been determined from measurements using a conventional vectorscope.
(IE UCS chromaticity diagram). This is because there is a demand for more precise numerical measurement and expression of colors.

There have been many studies on the improvement of the color reproduction (error) of broadcast cameras. However, there are research examples on the improvement of the color reproduction (error) of other V cameras such as a single-plate V camera such as a video movie. In order to improve the color reproducibility of a single-panel V-camera, there has been no conventional correction of a Y (luminance) signal, and most correction of only a color difference signal such as RY and BY has been performed. is there.

[0006]

In a commercial V camera, a spectral prism is used to separate color light from a subject into three primary colors of R, G, and B. However, a single-panel V camera is known. As described above, a CCD image sensor used as an image pickup device has Cy (cyan), Mg (magenta),
Ye (Yellow) and G (Green) have four filters equally,
A colorization method (complete color difference signal line sequential method) using such an all-color transmission complementary color filter is employed. The sum of the light transmitted through the four filters is used as a luminance (Y) signal.

When photographing with a V-camera using such an image sensor, if the subject is a bright (highly saturated) color,
FIG. 3 shows the characteristics of the image sensor and the well-known RGB linear color correction.
As shown in FIG. 4 and FIG. 4, not only the saturation but also the brightness (Y level) of the measured value (△) tends to shift to a higher value than the reference value (○). The higher the saturation, the more it will be perceived as "darker color", whereas the higher the Y (luminance) level, the more likely it will be that the color will be perceived as "light" on a monitor screen such as a CRT. There is a disadvantage that the image is reproduced and displayed thinner than the actual color.

In FIGS. 3 and 4, the vertical axis represents the luminance (Y
Signal) level, and the horizontal axis is the chroma signal level. Also,
HUE 5R, 10RP, etc. are hues (numbers) scaled in the Munsell system, and the reference is a tristimulus value that is the same as the tristimulus value XYZ of the color chart used for photographing, on an ideal monitor screen. The input signal to the monitor, as shown, is obtained by calculation. The tristimulus value XYZ is a basic color, as is well known, and is a means for numerically expressing a color obtained from the spectral radiation characteristics of the light source and the reflection characteristics of the object.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has an encoder 2 for generating a chroma signal C from color difference signals RY and BY, and an envelope detector for the chroma signal. Detector 3 that performs
A nonlinear converter 5 for nonlinearly converting the output of the envelope detector 3; a coefficient unit 6 for multiplying the output of the nonlinear converter 5 by a predetermined constant;
And a subtractor 7 for subtracting the output level of the luminance signal from the luminance signal in response to an increase in the level of the chroma signal. A color correction circuit characterized by increasing an output level is provided. 2. The color correction circuit according to claim 1, wherein the coefficient of said coefficient unit is varied according to the hue detected by the hue detector.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the color correction circuit of the present invention will be described.
This will be described with reference to FIG. FIG. 1 is a block diagram of a color correction circuit 1 according to the present invention, in which 2 is an encoder, 3 is an envelope detector, 4 is an LPF (low-pass filter), 5 is a non-linear converter, and 6 is a coefficient unit. , 7 are subtractors. The LPF 4 can be omitted, but if the LPF 4 is provided, a stable signal with little undulation can be obtained for the reason described later.

The color difference signals RY are input from input terminals In1 and In2, respectively.
And BY are supplied to the encoder 2 to generate a chroma signal C, which is output from a terminal Out1 and supplied to a well-known color signal processing circuit, while being supplied to an envelope detector 3 to provide an envelope of the chroma signal C. Perform detection. Thereby, the amplitude level of the chroma signal C is extracted, and unnecessary high-frequency components such as color subcarriers are removed by the LPF 4.
Then, a nonlinear conversion optimal for correction is performed according to the characteristics shown in FIGS. As shown by the curve in the block 5 of FIG. 1, the nonlinear converter 5 does not change the reproducibility of the gradation near the achromatic color, and increases the output as the saturation increases in the high saturation portion. Although it has input / output characteristics, it is not limited to such a curve, and a certain effect can be obtained even if the characteristics are approximated by a straight line close to the slope of this curve.

The output of the non-linear converter 5 is multiplied by a predetermined coefficient by a coefficient unit 6 to adjust the level, and then supplied to a negative input terminal of a subtractor 7. The terminal In3 is connected to the positive input terminal of the subtractor 7.
Since the luminance signal Yh is supplied, an error of the signal Yh is corrected by performing subtraction with the output of the coefficient unit, and the luminance signal Yh substantially corrected to the original Y level.
n 'is output from the terminal Out2. As a result, the color reproducibility on the monitor screen is greatly improved, and a brightly colored subject is reproduced with a color that is considerably close to the original color.

Next, a second embodiment of the color correction circuit of the present invention will be described with reference to the block diagram of FIG.
In FIG. 2, reference numeral 8 denotes a variable coefficient unit using a VCA (Variable Voltage Amplifier) or the like, 9 denotes a hue detector, and in FIG.
The same components as those of the circuit 1 of the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.

As is clear from the comparison of FIG. 2 with FIG. 1, a variable coefficient unit 8 is used in place of the coefficient unit 6 and a hue detector 9 is provided so that the coefficient can be changed according to the hue. I have. This is apparent from a comparison between FIG. 3 and FIG.
Since the shift of the Y level is slightly different depending on the hue, the hue is detected by the hue detector 9 and a coefficient suitable for each hue is added to the output of the nonlinear converter 5. As a result, a luminance signal Yn ′ very close to the original Y level is output from the terminal Out2 from the output terminal Out2 regardless of the color of any hue.

The hue detection is performed by using both color difference signals RY and B
This is performed by checking the magnitude of the level of −Y and the difference between the two levels. However, the present invention is not limited to this. For example, the hue may be detected based on the output of the encoder 2.

[0016]

Since the color correction circuit of the present invention is constructed as described above, a vivid color can be almost exactly reproduced as a dark color. Further, if the coefficient of the coefficient unit is configured to be variable according to the hue, there is an excellent feature that any color of the hue can be reproduced with high accuracy.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing a second embodiment of the circuit of the present invention.

FIG. 3 shows a chroma (hue 5) in a single-panel video camera.
R is a characteristic diagram illustrating a phenomenon that a luminance signal level is increased by an increase in a signal level.

FIG. 4 shows a chroma (hue 10) in a single-panel video camera.
RP) is a characteristic diagram illustrating a phenomenon in which a luminance signal level increases due to an increase in a signal level.

[Explanation of symbols]

1, 10: color correction circuit, 2: encoder, 3: envelope detector, 4: LPF (low-pass filter), 5: nonlinear converter, 6: coefficient unit, 7: subtractor, 8: variable coefficient unit 9 ...
Hue detector.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 9/77 H04N 9/07

Claims (2)

(57) [Claims] An encoder for generating a chroma signal from a color difference signal.
And over da, an envelope detector for performing envelope detection of the chroma signal, a nonlinear converter for nonlinear converting the output of the envelope detector, the coefficient unit for multiplying a predetermined constant to the output of the non-linear transformer, the luminance signal Tan'itashikibi provided with a subtracter from the level subtracting the output level of the coefficient unit
A color correction circuit for use in a video camera,
Subtract from the luminance signal level as the signal level increases
A color correction circuit for increasing the output level of the coefficient unit . 2. A hue detector for detecting coefficients of said coefficient unit.
Characterized by being variable according to the hue
The color correction circuit according to claim 1.