JPH07162715A - Digital gamma correcting method - Google Patents

Abstract

PURPOSE:To optimize corrected lock-up data by introducing correction intensity based on the relation between a reference frequency value and a counted frequency. CONSTITUTION:This method is composed of an AD conversion part 1 converting a supplied video signal 15, a gamma correction part 2 performing a gamma correction, a DA conversion part 3 converting digital data after a correction into a video signal 16, a luminance gradation frequency counting part 4 counting the data to be outputted by the AD conversion part 1 according to luminance gradation classes, a correction intensity generation part 5 generating correction intensity for every luminance gradation class based on the frequency counted for every luminance gradation class and a set reference value, an arithmetic part 7 generating correction lock-up data by introducing a function showing the relation of the correction intensity for every luminance gradation class, the histogram for every luminance gradation class of an original image and the histogram for every luminance gradation class after a conversion, a first memory 8 storing correction look-up data, a second memory 9 storing the frequency for every luminance gradation class, a third memory 10 storing correction intensity and a control part 11 controlling each part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital gamma correction method applied to a liquid crystal projector device or the like, and more specifically, the contrast ratio of an image to be displayed is improved by flattening the gradation distribution state of the image. Regarding the technology to make.

[0002]

2. Description of the Related Art Originally, gamma correction is a correction performed on the sending side of a television signal in order to correct the light emission luminance of a CRT television. The relationship between the input voltage E of the cathode ray tube and the light emission luminance L is not linear, and can be approximated by the relational expression of the above expression: L = K × (E to the power of γ). When L and E are plotted on a logarithmic scale, a straight line is obtained, and γ is its slope. In a color picture tube, γ is 2.2. Therefore, it is necessary to perform correction in order to reproduce the gradation and chromaticity of the image receiving screen in proportion to that of the subject. In the NTSC system, in order to reduce the load on the image receiving side, γ = 1 /
After applying the correction of 2.2, the display of the cathode ray tube is γ =
It is set to 1. Since the relationship between the input voltage and the light emission brightness of the liquid crystal projector device is different from that of the cathode ray tube, it is necessary to perform appropriate correction on the liquid crystal projector device side. In addition to the basic correction contents for accurately reproducing the gradation and chromaticity described above, correction such as edge enhancement may also be performed on the liquid crystal projector device side so that the displayed image becomes a picture that is easy to see. In particular, improving the contrast ratio has been an important technique for displaying so-called easy-to-see pictures. The gamma correction method for improving the contrast ratio implemented in the conventional liquid crystal projector apparatus is realized by a method of flattening the gradation class frequency distribution. For example, as shown in FIG. 2 and FIG. 3, when the gradation level distribution diagram 2 (a) of the image is expressed in 256 steps with 8 bits of the total brightness gradation for each gradation class, A is calculated for every 64 gradations. (0 to
63 gradations), B (64 to 127 gradations), C (128 gradations)
To 191 gradations) and D (192 to 255 gradations) four classes A to D, and the image brightness is divided into the four classes for each field to generate frequency distribution data. 2B, the required reference power value (α) is compared with the power of each class. As a result, the class exceeding the reference value α has a judgment value of “1” and is less than the reference value α. The class has a judgment value of "0" in Fig. 2 (C). Then, when the same judgment value is “1”, the correction is not performed, and when the same judgment value is “0”, the correction is performed. By preparing and referring to the table, the characteristics of the original image
(B) is converted and flattened, and the converted characteristic is shown in FIG. 3 (c).
Was getting However, in the case where the frequency of each class is close to the reference frequency value, as shown in FIG. 4, the characteristic of a in the figure shows that the reference value α is in the highest gradation class (192 to 255) D class.
Since it is less than, the judgment value is '0'. In the characteristic of b in the figure, the judgment value is '1' because the reference value α is exceeded in the highest gradation class D. Therefore, in the image of the frequency distribution shown in the example of FIG. 4, the determination value is likely to change, and as a result, the gamma correction table selected is switched each time, so the flattened frequency distribution after conversion changes, and the final There was a problem that the display image that was corrected to flickers.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides a digital gamma correction method capable of improving the contrast ratio while reducing the flicker of an image generated by the conventional digital gamma correction. The purpose is to do.

[0004]

To achieve the above object, an AD converter for converting a video signal into digital data, and an AD converter for holding the supplied gamma correction lookup data and based on the gamma correction data. The gamma correction unit that gamma-corrects the digital data output by the device, the DA conversion unit that converts the corrected digital data into a video signal, and the frequency of the digital data output by the AD conversion unit is counted for each luminance gradation class. A correction intensity that generates a correction intensity indicating the degree of gamma correction for each brightness gradation class based on the brightness frequency counting unit and the frequency counted by the brightness frequency counting unit for each brightness gradation class and the set reference value. The relationship between the generation unit, the correction intensity for each brightness gradation class, the histogram for each brightness gradation class of the original image, and the histogram for each converted brightness gradation class A calculation unit that generates gamma correction lookup data based on the function, a first memory that stores the generated gamma correction lookup data, and a luminance gradation class counted by the luminance frequency counting unit. The second memory stores the frequency, the third memory stores the correction intensity, and the control unit that controls each unit.

[0005]

With the above configuration, the histogram for each luminance gradation class of the original image is corrected and flattened by the correction intensity generated by the correction intensity generator. In that method, if the counted frequency value is close to the reference value, a small amount of correction is performed on the histogram of the original image, and the degree of correction (correction amount) is set according to the ratio of the counted frequency value from the reference value. Increase and correct.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A digital gamma correction method according to the present invention will be described in detail below with reference to the drawings. FIG. 1 is an example of a block diagram for implementing a digital gamma correction method according to the present invention. As shown in the figure, reference numeral 1 is an AD converter that converts the supplied video signal 15 into, for example, 8-bit digital data. 2 stores the gamma correction data in the form of a look-up table and updates the contents of the data table with the sent data, for example, SRAM (Static Random Access).
ss Memory), the 8-bit digital data output from the AD conversion unit 1 is used as an address input, and the correction data of the table stored in the corresponding address is output to perform gamma correction. This is a gamma correction unit. Reference numeral 3 denotes a DA converter that converts the corrected digital data into the video signal 16. 4 is the A
It is a brightness frequency counting unit configured to count digital data output from the D conversion unit 1 for each brightness gradation class, for example, a counter provided for each of the four gradation classes A to D. Reference numeral 5 denotes a correction intensity generation unit that generates a correction intensity indicating a degree of gamma correction for each brightness gradation class based on the frequency counted by the brightness frequency counting unit 4 for each brightness gradation class and a set reference value. is there. 7 introduces a function showing the relationship between the correction intensity for each luminance gradation class, the histogram for each luminance gradation class of the original image, and the histogram for each luminance gradation class after conversion, and gamma based on the function is introduced. It is a calculation unit that generates correction lookup data. Reference numeral 8 is a first memory that stores the gamma correction lookup data that is generated. 9
Is a second memory for storing the frequency of each brightness gradation class counted by the brightness frequency counting section 4. Reference numeral 10 is a third memory that stores the correction intensity. Reference numeral 11 is a control unit that controls each unit. The first memory 8 to the third memory 10
Each memory of, for example, RAM (Random Acc
ess Memory).

The operation of the digital gamma correction method according to the present invention will be described with reference to FIG. The video signal 15 supplied from the AD conversion unit 1 is quantized by, for example, 8 bits for each field (1 V), and the luminance frequency counting unit 4 is quantized.
Is, for example, A (0 to 63 gradations), B (64 to 127 gradations), C (128 to 191 gradations), D (192 to 255).
It operates so as to obtain a histogram FIG. 5B having luminance frequency distribution data in which frequencies are counted for each of four luminance classes of (gradation). The luminance gradation frequency distribution characteristic of the original image is shown in FIG. In the correction strength generation unit 5, the frequency reference value α shown in the histogram FIG.
Then, the correction strength P is calculated by the following equation. P = PS + (1-PS) × β / α ... where PS is the standard correction intensity when the frequency value β is zero.
Further, when the frequency value β is 0, the correction strength is maximum, the correction strength decreases as the frequency value β increases, and when the frequency value β matches the reference value α (P = 1), the correction is not performed ( Minimum). In FIG. 5C, the luminance classes A to C have the correction intensity P = 1 and no correction is performed. The correction intensity of the brightness class D is P = p obtained from the relationship of the above equation.
The calculator 7 calculates gamma correction lookup data based on the following equation. g (j) is the histogram of the image after conversion, f (i) is the histogram of the original image, P is the correction intensity, and m is the number of luminance gradation classes. For example, in the arithmetic unit 7,
Gamma correction lookup data having the input / output characteristics shown in (d) is generated. The gamma correction lookup data generated by the arithmetic unit 7 is stored in the first memory 8 and also sent to the gamma correction unit 2 to be stored in, for example, SRAM to update the data table. Gamma correction unit 2
Performs conversion by supplying, for example, 8-bit digital data supplied from the AD conversion unit 1 to the address of the SRAM and reading the corresponding data. For example, the histogram f (i) of the original image shown in FIG.
Is converted with the gamma correction lookup data of FIG. 5D to obtain the histogram g (i) of the flattened converted image shown in FIG. 5F. The angle a shown in FIG. 5D is given by f (i) / g (i).

An extended example using a plurality of correction intensities will be described below with an example in which the correction intensities are two. It should be noted that a description that overlaps with the case of using one correction intensity described above will be omitted. When the original image has the luminance gradation frequency distribution characteristics as shown in (a) of FIG. 6, the frequencies of the luminance classes A and D are β2 and β1, respectively (FIG. 6B). The respective correction intensities P are calculated from the following formulas, and are P2 and P1 as shown in FIG. Note that PS ′ is the standard correction strength when the correction strength is two. The gamma correction lookup data generated by the calculation unit 7 is calculated by the following equation, and has the characteristic of FIG. g (j) is the converted histogram, f (i) is the original image histogram, P is the correction intensity, and m is the number of luminance gradation classes. However, the angle a shown in FIG. 6D is a = f
It is given by (i) / g (i). The histogram f (i) of the original image shown in FIG. 6C is flattened by the gamma correction lookup data having the characteristics shown in FIG.
The histogram g (i) of the converted image shown in (f) is obtained.

[0009]

As described above, the present invention provides a digital gamma correction method capable of improving the contrast ratio while reducing the flicker of an image generated by the conventional digital gamma correction. Therefore, there is an advantage that it is possible to realize a liquid crystal projector device having a high contrast ratio and high image quality without causing a flicker phenomenon.

[Brief description of drawings]

FIG. 1 is an example of a block diagram for implementing a digital gamma correction method according to the present invention.

FIG. 2 is an explanatory diagram for determining whether or not to perform correction, using a reference frequency as a criterion for each gradation class.

FIG. 3 is a diagram illustrating an operation of flattening a histogram of an image using gamma correction lookup data.

FIG. 4 is a diagram showing an example in which correction is performed depending on a state of an image histogram and when correction is not performed.

FIG. 5 is a diagram for explaining the operation of the digital gamma correction method according to the present invention.

FIG. 6 is a diagram for explaining the operation of another embodiment of the digital gamma correction method according to the present invention.

[Explanation of symbols]

 1 AD conversion unit 2 Gamma correction unit 3 DA conversion unit 4 Luminance frequency counting unit 5 Correction intensity generation unit 7 Calculation unit 8 First memory 9 Second memory 10 Third memory 11 Control unit 15 Video signal 16 Video signal after gamma correction

Claims (6)

[Claims] 1. A for converting a video signal into digital data
A D conversion unit, a gamma correction unit that holds the supplied gamma correction lookup data and gamma-corrects the digital data output by the AD conversion unit based on the gamma correction data, and converts the corrected digital data into a video signal. DA
A conversion unit, and a brightness frequency counting unit that counts the frequency of the digital data output from the AD conversion unit for each brightness gradation class,
A correction intensity generation unit that generates a correction intensity indicating a degree of gamma correction for each luminance gradation class based on a frequency counted by the luminance frequency counting unit for each luminance gradation class and a set reference value; A function showing the relationship between the correction intensity for each gradation class, the histogram for each luminance gradation class of the original image, and the histogram for each converted luminance gradation class is introduced, and gamma correction lookup data is generated based on this function. A first memory for storing the generated gamma correction lookup data, a second memory for storing the frequency of each brightness gradation class counted by the brightness frequency counting section, and a first memory for storing the correction intensity. It consists of three memories and a control unit that controls each unit, and uses the gamma correction lookup data generated based on the correction intensity to generate a histogram for each luminance gradation class of the original image. A digital gamma correction method that improves the contrast ratio of an image by flattening it. 2. The correction intensity is between the maximum value and the minimum value in a relation that when the value of the ratio between the counted frequency value and the frequency reference value is 0, it becomes the maximum value and when it is 1, the minimum value is obtained. 2. The digital gamma correction method according to claim 1, wherein at least one correction intensity value is set to be present in each. 3. The histogram for each converted luminance gradation class corresponds to the functional expression showing the relationship between the histogram for each luminance gradation class of the original image and the histogram for each converted luminance gradation class. For each luminance gradation class, a term obtained by multiplying the numerator obtained by multiplying the histogram value of the original image of the luminance gradation class by the correction intensity value and the corresponding histogram value of the original image of the luminance gradation class by the correction intensity value The digital gamma correction method according to claim 1, wherein the denominator is formed by adding all. 4. The gamma correction lookup data based on the plurality of correction intensities is provided when the correction intensities have values other than 1 and a plurality of the same correction intensities exist at the same time. Item 2. The digital gamma correction method according to Item 1. 5. The digital gamma correction method according to claim 1, wherein the gamma correction unit is composed of an SRAM. 6. The digital gamma correction method according to claim 1, wherein the brightness frequency counter is provided with a counter for each gradation class.