JPH06350943A - Picture processing circuit - Google Patents

Abstract

PURPOSE:To display a picture with optimum luminance and gradation at all times by correcting luminance gradation in response to a luminance distribution of an input picture. CONSTITUTION:A picture signal from an input section 1 is converted into a digital signal at an A/D converter section 2. The digital signal is applied to a comparator 3, in which the signal is respectively compared with plural required reference values and a counter 4 counts number of picture elements of a picture signal by each reference number and an arithmetic operation circuit 5 makes arithmetic operation based on the count. Data from the arithmetic operation are converted into an analog signal at a D/A converter section 6, the data are amplified up to a required signal level at buffers 7, 7', 7'' and the result is fed to a D/A converter section 8 as a reference voltage. The D/A converter section 8 converts the digital signal from the A/D converter section into an analog picture signal based on the reference voltage to provide an output of the picture signal via an output section 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing circuit, and more particularly to an image processing circuit which corrects gradation according to a distribution of luminance gradation of a video signal.

[0002]

2. Description of the Related Art In a display device using a liquid crystal display panel (LCD) or a plasma display panel (PDP) as a display body, an image signal input according to the gradation display characteristic of the display device is referred to as gamma (hereinafter referred to as γ). (Abbreviated) correction is made so that it is displayed on the screen with an appropriate density gradation. In this case, no matter how the brightness distribution of the input image is, it is usually always corrected with the same coefficient,
In the case of a display device such as the LCD or PDP in which it is relatively difficult to obtain contrast, for example, in a bright part of the image in the case of an entirely bright image, or in a dark part of the image in the case of an entirely dark image, images of these parts are There is a problem that the brightness gradation is compressed and displayed as an image with poor contrast. In addition, as a measure for this, when the brightness gradation is corrected according to the brightness of the image, for example, when the displayed image is a moving image, the gradation of the image is changed before and after the gradation at which the correction coefficient is switched. There is a problem that flicker is noticeable.

[0003]

SUMMARY OF THE INVENTION In consideration of the above-mentioned problems, the present invention continuously corrects the brightness gradation by correcting the brightness gradation correction characteristic in accordance with the brightness distribution of the input image. The optimum brightness correction is performed on the image, and thus, for example, the brightness gradation of the part with low contrast of the bright part of the overall bright image, or the image of the dark part of the overall dark image The object of the present invention is to provide such a correction that the brightness gradations of the low-contrast part of the image are corrected so that they have contrast.

[0004]

In order to solve the above problems, the present invention solves the above problems by an A / D converter for converting an input image signal into a digital signal, and a luminance level of each pixel of an image from the A / D converter. A comparator that compares a key number with a plurality of reference values and outputs each reference value, a counter that counts data of each reference value from the comparator, an arithmetic circuit that calculates a signal from the counter, and an image A lookup table for converting the signal into the required luminance gradation distribution,
A first converter for converting each signal from the arithmetic circuit converted based on a look-up table into an analog signal.
The D / A conversion section and the second D / A conversion section which converts the image signal from the A / D conversion section into an analog signal and outputs the analog signal by using the signal from the first D / A conversion section as a reference level for analog conversion. An image processing circuit is provided.

[0005]

With the above arrangement, the image processing circuit according to the present invention detects the luminance gradation of each pixel of the input image, counts the number of pixels for each preset gradation division, and detects the input image signal. The distribution of the brightness gradation is obtained, and the brightness gradation is corrected so as to increase the gradation difference between the gradations of the pixels in the brightness gradation region having a high distribution frequency.

[0006]

Embodiments of the image processing circuit according to the present invention will be described in detail below. FIG. 1 is a block diagram of essential parts of an embodiment of an image processing circuit according to the present invention. In the figure, reference numeral 1 is an input unit for inputting an analog image signal. An A / D converter 2 digitally converts the image signal from the input unit 1.
Reference numeral 3 denotes a comparator, which divides each pixel of the image from the A / D conversion unit 2 by this reference value with reference to a plurality of preset reference values. A counter 4 counts the number of pixels divided by the comparator 3 for each division. Reference numeral 5 denotes an arithmetic circuit which calculates data from the counter 4. 6 is D / A
The converter converts the data from the arithmetic circuit 5 into an analog signal. Reference numerals 7, 7'and 7 "are buffers for amplifying the signals from the D / A conversion unit 6 to the required signal levels respectively. Reference numeral 8 is a D / A conversion unit and buffers 7, 7'and 7".
Is used as a reference voltage, and the digital signal from the A / D converter 2 is converted into an analog image signal. An output unit 9 generates the image signal from the D / A converter 8 to a required signal level and outputs it.

FIG. 3 is a block diagram of essential parts of another embodiment of the image processing circuit according to the present invention. In the figure, 11 is a look-up table, which is provided with data for converting an image signal into a required luminance gradation distribution, converts the data from the arithmetic circuit 5 based on this reference data, Send to. The other reference numerals are the same as those in FIG.

Next, the operation of the image processing circuit according to the present invention will be described with reference to FIGS. When the luminance gradation distribution of the input image is concentrated in the area of medium luminance gradation as shown in FIG. 2A, for example, most of the pixels of the displayed image have medium density (gray). Therefore, there is almost no image in the bright region and the dark region, and the image has no contrast and is not powerful. The gradation difference between the gradations of each pixel in the middle luminance gradation area of this image is widened and corrected so as to be distributed in the low gradation area and the high gradation area as shown in FIG. It produces a powerful image.

The frequency distribution of the density gradation shown in (c) of FIG. 2 is that of the image signal shown in (a) of FIG.
Gradation frequency is a, 64 to 127 Gradation frequency is b, 128 to 191
It is shown that the gradation frequency is c and the gradation frequency of 192 to 255 is d. The image signal having such a frequency distribution is reduced in the number of pixels in a region of 64 to 127 gradations and 128 to 191 gradations having a high frequency distribution, and 0 to 63 gradations and 192 to 255 gradations are correspondingly reduced.
By increasing the frequency of gradation, the image signal as shown in FIG. 2B is converted. FIG. 2D is an example of a correction coefficient for this conversion. This correction coefficient is applied to pixels between 64 grayscales and 127 grayscales with 127 grayscales, which is the central value of density grayscales, as a boundary. Decrease the number of gradations (density) to move to the range of 0 to 63 gradations, or increase the number of gradations of pixels between 128 to 191 gradations to move to the range of 192 to 255 gradations, With these, the image signal is converted into the image signal having the density distribution shown in FIG.

Therefore, in the circuit shown in FIG. 1, the image signal from the input section 1 is converted into a digital signal by the A / D conversion section 2. This digital signal is applied to the comparator 3, and the comparator 3 uses the A / D converter 2
However, for example, in the case of converting an image signal into 8-bit data, each pixel of the digitally converted image data has 0 to 63 gradations, 64 to 127 gradations, 128 to 191 gradations, 192 to 25 gradations.
In order to classify into 4 gradations of 5 gradations, each reference voltage corresponding to 63 gradations, 127 gradations and 191 gradations is applied to the reference value input terminal.

Then, by the comparator 3, the A /
The gradation number of each pixel from the D conversion unit 2 is compared with these reference values, and 63 gradations or less (a in FIG. 2C) and 127 gradations or less (the same,
a + b), 191 gray levels or less (the same, a + b + c), and 25
5 gradations or less (the same, a + b + c + d = total number of pixels) are divided and output. The data from the comparator 3 is applied to the counter 4, the number of pixels is counted for each section, the calculation circuit 5 calculates based on this count value, and the calculated value is input to the D / A conversion unit 6 and the analog signal (voltage value ).
This analog signal is input to the buffer 7, 7 ′ or 7 ″, amplified to a required signal level, and applied as a reference voltage to the D / A conversion unit 8. The D / A conversion unit 8 analogizes this reference voltage. As a standard when converting,
The digital signal from the A / D conversion unit 2 is converted into an analog image signal, which is generated and output to a required signal level via the output unit 9.

In the circuit shown in FIG. 3, the data from the arithmetic circuit 5 is converted into a required luminance gradation distribution by the look-up table 11 and then applied to the D / A conversion section 6. This is because in the case of the circuit of FIG. 1, the reference voltage of the D / A converter 8 is applied based on the distribution of the brightness gradation of the input image signal. The luminance gradation distribution of the output image signal is shown in FIG.
As shown in (a), the conversion is performed on the assumption that the pixels are distributed substantially uniformly from the dark part to the bright part of the image.

However, in a general image such as a television broadcast, a blackout of an image (a state where there is almost no gradation difference in a dark portion)
Alternatively, when the brightness and contrast are adjusted so as to be displayed on the screen in a state where there is no blown-out highlights (a state where there is almost no gradation difference in the bright part), the brightness gradation distribution of the image signal is
As shown in (b), it has a characteristic that the number of pixels in the high-luminance region and the low-luminance region gradually decreases with a peak in the medium-luminance region. Therefore, the look-up table 11 is provided with conversion data weighted for such a luminance gradation distribution characteristic, and the look-up table 11 is used for conversion. As a result, in the circuit example of FIG. 1, the characteristic shown by the dotted line in FIG. 4C is converted into the characteristic shown by the solid line (substantially straight line). The characteristic is such that the number of pixels in the luminance region and the low luminance region gradually decreases.

In the above description, the data from the counter 4 is directly input to the arithmetic circuit 5 for arithmetic operation. However, since the arithmetic circuit 5 directly arithmetically operates the data from the counter 4, it corresponds to the number of counters 4. 3 arithmetic circuits are required. In this case, if a latch circuit corresponding to each counter 4 is provided in front of the arithmetic circuit 5 to latch data, and the arithmetic circuit 5 sequentially operates the data from the latch circuit one by one, All you have to do is to provide one.

[0015]

As described above, according to the image processing circuit of the present invention, the image having the optimum luminance gradation is always corrected according to the distribution state of the luminance gradation of the input image. In addition, this correction is performed so that the correction amount continuously changes with respect to the number of gradations, so that in the case of a moving image, as in the past, the image has a flicker of light and shade before and after the correction coefficient is switched. Problems such as conspicuousness are also solved.

[Brief description of drawings]

FIG. 1 is a block diagram of a main part of an embodiment of an image processing circuit according to the present invention.

FIG. 2 is a diagram showing an example of a density gradation distribution for explaining the operation of an embodiment of the image processing circuit according to the present invention.

FIG. 3 is a block diagram of a main part of another embodiment of the image processing circuit according to the present invention.

FIG. 4 is a diagram showing an example of density gradation distribution and the like for explaining the operation of another embodiment of the image processing circuit according to the present invention.

[Explanation of reference numerals] 1 input unit 2 A / D conversion unit 3 comparator 4 counter 5 arithmetic circuit 6 D / A conversion unit 7 buffer 8 D / A conversion unit 9 output unit 11 lookup table

Claims (2)

[Claims] 1. An A / D conversion unit for converting an input image signal into a digital signal, and the number of luminance gradations of each pixel of the image from the A / D conversion unit are compared with a plurality of reference values, and each of the reference values is compared. A comparator that outputs separately, a counter that counts the data for each reference value from the comparator, an arithmetic circuit that calculates the signal from the counter, and a first D / A that converts the signal from the arithmetic circuit into an analog signal. Image processing including a conversion unit and a second D / A conversion unit that converts the image signal from the A / D conversion unit into an analog signal and outputs the analog signal using the signal from the first D / A conversion unit as a reference level for analog conversion. circuit. 2. A look-up table for converting an image signal into a required luminance gradation distribution is provided, and a signal from the arithmetic circuit is converted based on the look-up table, and then the first D / A conversion unit. The image processing circuit according to claim 1, wherein the image processing circuit is input to the image processing circuit.