KR19990013354A - Device and method of stretching the image - Google Patents

Abstract

Disclosed are a device and method for stretching an image. The device extracts an average of luminance levels below and below a predetermined luminance level in response to an active signal generated during the time the screen is displayed, corresponding to the luminance level distribution which is the level of the luminance signal of the image information currently input. Factor extraction means for using the extracted minimum and average values and extracting the threshold and the slope of the black region in response to the vertical synchronization signal, and an image for calculating the blackened luminance signal from the currently input luminance signal, the extracted threshold and the slope. The information calculating means, the selecting means for selectively outputting the blackened luminance signal and the currently input luminance signal in response to the selection signal, and the luminance level of the currently input luminance signal and a threshold value, and selecting according to the compared result. And a comparison means for outputting a signal.

Description

Device and method of stretching the image

The present invention relates to the stretching of an image, and in particular, an image stretching apparatus and method capable of stretching the black level of an image in consideration of the histogram of the image information or extending the black and color levels of the image using gamma correction. It is about.

If the data amount of the input video signal does not reach the amount of data that can be processed by the hardware that is already set, the data amount of the image signal is enlarged closer to the amount of data that the hardware can process, thereby increasing the resolution of the black region. In addition, when the video signal is not active in the black region, the entire screen appears to be white. In order to prevent such a phenomenon and increase the data level of the input video signal, black stretching is performed. In this case, when the bright area is extended, the data level of the video signal can be enlarged. However, since the black area has a more important effect, only the black area is extended. At this time, the black region is defined as a signal acting on the lower data amount of the total data amount of the video signal.

The black height can improve the range of change in the luminance level of the video signal by adding a non-linear conversion characteristic to the black region. Factors that determine the nonlinear transformation characteristics are the threshold value and the nonlinear characteristic slope of the black region in the image information.

On the other hand, with reference to the accompanying drawings of the conventional black stretching method will be described as follows.

1 is a graph for explaining a process of obtaining a conventional threshold and output image information, the horizontal axis represents the brightness level of the input image information, the vertical axis represents the brightness level of the output image information, respectively.

Referring to FIG. 1, the lowest and highest values of luminance levels of input image information are obtained, and the threshold values of the slope and the black region are determined using the minimum and maximum values. To this end, after low pass filtering the luminance level of the currently input image information, the threshold shown in FIG. 1 is obtained from the low pass filtered data as shown in Equation 1 below.

Threshold = lowest + α * (highest-lowest)

Where α is a value set by the user.

Therefore, as shown in FIG. 1, if the luminance level of the input image information is greater than or equal to the threshold, the luminance level of the input image information is output as it is as the luminance level of the output image information. As shown in FIG. 2, the luminance level of the output image information obtained by extending the input image information is obtained.

Luminance level of the output image information = slope * (luminance level of the input image information-threshold) + threshold

Since the above-described conventional black stretching method does not perform black stretching corresponding to the histogram of the luminance level of the input image information, in order to consider the distribution of the luminance levels of the image, the user accurately selects the slope and the threshold whenever the image information changes. There was a cumbersome problem to give.

In addition, since the above-described conventional black stretching method must perform black stretching in real time, there is a limitation in reducing the size of hardware, and there is a problem in that the color of the image cannot be extended because it is limited to the black stretching of the image. .

The first technical problem to be achieved by the present invention is to provide an image decompression device capable of extending the black level of the luminance signal in consideration of the distribution of the level of the luminance signal included in the image information.

Another object of the present invention is to provide an image decompression method performed by an image decompression device of the first technical problem.

Another object of the present invention is to provide an image decompression device capable of decompressing levels of luminance signals and chrominance signals of image information using gamma correction.

A fourth technical object of the present invention is to provide a method for stretching an image performed by an apparatus for stretching an image according to the third technical problem.

1 is a graph illustrating a conventional process of obtaining threshold and output image information.

2 is a block diagram illustrating an apparatus for stretching an image according to the present invention.

FIG. 3 is a flowchart for describing a method of blackening an image according to the present invention performed by the apparatus shown in FIG. 2.

FIG. 4 is a graph illustrating an apparatus and method for black stretching of an image according to the present invention, which is performed in the apparatus illustrated in FIGS. 2 and 3.

5 is a block diagram illustrating an apparatus for stretching an image using gamma correction according to the present invention.

FIG. 6 is a flowchart for describing a method of expanding an image using gamma correction according to the present invention performed by the apparatus shown in FIG. 5.

FIG. 7 is a graph for describing an apparatus and a method for expanding an image using gamma correction illustrated in FIGS. 5 and 6.

According to an aspect of the present invention, there is provided an apparatus for expanding an image, the luminance level corresponding to a luminance level distribution that is a level of a luminance signal of image information currently input and in response to an active signal generated during a time when a screen is displayed. A factor extracting means for extracting an average value of the lowest value of the and the luminance levels below a predetermined luminance level, using the extracted minimum value and the average values, and extracting a threshold and a slope of the black region in response to a vertical synchronization signal; Image information calculating means for calculating a black signal extended from the luminance signal, the extracted threshold value and the slope, and a selection for selectively outputting the black extended luminance signal and the currently input luminance signal in response to a selection signal; Means and the luminance level of the luminance signal currently input That corresponding to the comparison value, the comparison result by the comparison means is configured to output the selection signal is preferred.

According to an aspect of the present invention, there is provided a method of expanding an image, the method comprising: continuously determining whether an image is displayed on an active section, and in the case of the active section, a level of a luminance signal included in image information to be black stretched. Obtaining an average of the lowest value of the luminance level and luminance levels below a predetermined luminance level according to a distribution of luminance levels, continuously determining whether to apply a vertical blanking period, and in the case of the vertical blanking period, the average value Obtaining a threshold using the lowest value and the first predetermined value (where the first predetermined value is a positive number), and obtaining a slope using the threshold and the lowest value, and determining whether the luminance level is greater than or equal to the threshold value. Judging, and if the luminance level is greater than or equal to the threshold, the luminance level as a level of the blackened luminance signal. And determining the luminance level to be black extended as the luminance level extended by calculating the luminance level to be black extended when the luminance level is less than the threshold.

In order to achieve the third object, a color converting unit converts a luminance signal and a chrominance signal of image information into an RGB signal and stores the data stored in response to the RGB signal input as an address. A system for expanding an image according to the present invention, which includes a system having a look-up table for outputting image expansion and gamma-corrected data, corresponds to a luminance level distribution which is a level of a currently input luminance signal and is generated during a time when a screen is displayed. Extracting an average value of the lowest value of the luminance level and luminance levels below a predetermined level in response to an active signal, using the extracted minimum value and the average values, and extracting a threshold and a slope of a black region in response to a vertical synchronization signal In response to a result of comparing said threshold with a level of a signal to be gamma corrected from the means and an external source; Outputting the gamma correction value set as the data stored in the lookup table as the data stored in the lookup table, or outputting the gamma correction value obtained using the threshold and the slope from the signal to be gamma corrected as the data stored in the lookup table And a gamma correction value is stored in the address of the lookup table.

In order to achieve the fourth task, the image data is converted and converted into gamma-corrected data stored in response to the RGB signal input as an address and a color conversion unit for inputting a luminance signal and color difference signals of image information into an RGB signal. The image decompression method performed in a system having an output lookup table includes continuously determining whether an image is displayed on an active section, and in the case of the active section, corresponding to a distribution of luminance levels that are levels of the luminance signal. Obtaining an average value of the lowest luminance level and a luminance level below a predetermined luminance level; continuously determining whether to apply a vertical blanking period; and in the case of the vertical blanking period, the average value, the minimum value, and the first value. The threshold value is obtained using a predetermined value (wherein the first predetermined value is a positive number), and the minimum value and Obtaining a slope using a predetermined threshold, determining whether a level of a signal to be gamma corrected is greater than or equal to the threshold, and if a level of the signal to be gamma corrected is greater than or equal to the threshold, a gamma correction value that is set by itself is determined. Determining as the data to be stored in the up table and if the level of the signal to be gamma corrected is less than the threshold, storing a gamma correction value obtained using the threshold and the slope from the signal to be gamma corrected in the look up table It is preferred that this step consists of determining as said data to be.

Hereinafter, with reference to the accompanying drawings, the configuration and operation of the image stretching apparatus for the black kidney according to the present invention and the stretching method will be described as follows.

Fig. 2 is a block diagram for explaining an image decompression device according to the present invention, wherein the image decompression device 2, the color conversion unit 4, and a look up table (LUT) 6 ) And gamma curve generator 8.

The image decompression device 2 according to the present invention shown in FIG. 2 includes a first low pass filter 12, a minimum value extractor 14, an average value extractor 16, a second low pass filter 18, and a second low pass filter 18. Factor extractor 10, consisting of three low pass filter 20, first and second D flip-flops 26 and 28, threshold extractor 22, and gradient extractor 24, and subtractor 32 ), A multiplier 62 for implementing the comparator 52 for implementing the comparator 50 and the selector 60 for the image information calculating unit 30 including the multiplier 34 and the adder 36. do.

FIG. 3 is a flowchart illustrating a method for blackening an image according to the present invention performed by the apparatus shown in FIG. 2, wherein the thresholds and the slopes are calculated in an active period and a vertical blanking period (steps 76 to 86). And determining the luminance level of the black-extended luminance signal according to the comparison result of the luminance level of the luminance signal to be black-extended and the threshold (steps 88 to 92).

FIG. 4 is a graph for explaining a device and a method for black stretching an image according to the present invention, which is performed in the apparatus shown in FIGS. 2 and 3, and the horizontal axis represents the luminance level IN of the black stretched luminance signal. The vertical axis represents the luminance level OUT of the luminance signal which is extended in black and output.

The factor extractor 10 shown in FIG. 2 has an active value input through the input terminal IN1 corresponding to the luminance level Y distribution of the luminance signal currently input, and the average value of the luminance levels below the predetermined luminance level. extracts in response to the (active) signal and extracts the threshold and the slope of the black region in response to the vertical synchronization signal input through the input terminal IN2 using the extracted minimum and average values (76-86). step). Here, the active signal is generated during the time the screen is displayed, and the vertical synchronization signal is generated during the vertical blanking period.

To this end, first, the first low pass filter 12 of the factor extractor 10 performs low pass filtering on the input luminance signal Y to obtain a low frequency luminance signal (step 76). This is to reduce the influence of noise component in finding the lowest value.

After operation 76, the lowest and mean extractors 14 and 16 continuously determine whether the active section is active (operation 78). If the active period, the lowest value extractor 14 extracts the lowest value shown in FIG. 4 and the adjusted lowest values described below among the luminance levels of the luminance signal of the low pass component filtered by the first low pass filter 12, and averages the average value. The extraction unit 16 obtains an average of luminance levels equal to or lower than a predetermined luminance level of the highest value that the luminance level of the luminance signal of the low pass component output from the first low pass filter 12 may have (step 80). Here, the predetermined luminance level can be arbitrarily applied. For example, the predetermined brightness level can be set to the 1/2 brightness level of the highest value. At this time, the lowest value, the adjusted lowest value and the average value are obtained in response to the active signal so as to be applied only in the active video section in which the image is displayed.

After the 80th step, the second and third low pass filters 18 and 20 filter the adjusted lowest value and the lowest value and the lowest value of the lowest value and the average value respectively output from the lowest value extractor 14 and the average value extractor 16, respectively. (Step 82). This is to prevent errors caused by rapidly changing image information.

After step 82, it is determined whether or not the vertical return period (step 84). To this end, the first D flip-flop 26 inputs the output of the second low pass filter 18 to the data input terminal D, and responds to the vertical synchronization signal input during the vertical retrace period through the input terminal IN2. The output of the second low pass filter 18 is output to the threshold and slope extractors 22 and 24 through the constant output Q terminal. Similarly, the second D flip-flop 28 inputs the output of the third low pass filter 20 to the data input terminal D, and responds to the vertical synchronization signal input during the vertical retrace period through the input terminal IN2. 3 The output of the low pass filter 20 is output to the threshold extraction section 22 through the constant output (Q) terminal.

That is, in the vertical retrace period, the threshold extractor 22 is filtered by the second and third low pass filters 18 and 20, respectively, and inputs through the first and second D flip-flops 26 and 28, respectively. The threshold value is extracted by using the low-order component of the lowest value and the average value and the predetermined value (? ≥0) set by the user as shown in Equation 3 below, and the gradient extractor 24 extracts the following equation from the lowest value extractor 14: Using the adjusted lowest value and the threshold value 4 is calculated and output the slope as shown in the following equation (5) (step 86).

Threshold = mean + β * (mean-lowest)

Adjusted lowest = lowest (1-α)

Here, α is a predetermined value set by the user and is a positive number of 1 or less.

Slope = threshold / (threshold-adjusted lowest)

After operation 86, the comparator 50 determines whether the luminance level of the luminance signal Y is equal to or greater than the threshold (operation 88). If the luminance level of the luminance signal Y is greater than or equal to the threshold, the selector 60 outputs the level of the luminance signal Y currently input to the color converter 4 (step 90). That is, the comparator 52 inputs the level of the luminance signal Y to the negative input terminal, compares the threshold to the positive input terminal, and compares. When the level of the luminance signal Y is equal to or greater than the threshold, the level is '0'. The selection signal is output to the selection terminal S of the multiplexer 62, and when the level of the luminance signal Y is less than the threshold, the selection signal having the level '1' is output to the selection terminal S of the multiplexer 62. Therefore, the multiplexer 62 selects the luminance level Y of the image information input to the input terminal of '0' and outputs it to the color converter 4 in response to the selection signal having the level '0'.

However, if the level of the luminance signal Y is not greater than or equal to the threshold, the level of the blackened luminance signal extracted by the image information extraction unit 30 is selected and output to the color conversion unit 4 (step 92). To this end, the image information calculator 30 calculates the level of the blackened luminance signal using the threshold value and the slope extracted by the factor extractor 10 and the level of the luminance signal Y input. That is, the subtractor 32 of the image information calculator 30 subtracts the threshold from the luminance level of the input luminance signal Y, the multiplier 34 multiplies the subtracted value and the slope, and the adder 36 The multiplied value and the threshold value are added and output as the luminance level of the blackened luminance signal to the input terminal of '1' of the multiplexer 62. Therefore, the blackened luminance signal selected by the multiplexer 62 may be output to the color converter 4 in response to the selection signal of the '1' level in the comparator 52.

Meanwhile, the color converter 4 inputs the color difference signals Cr and Cb and the blackened luminance signal output from the multiplexer 62 to convert the RGB signal into an RGB signal, and converts the converted RGB into RGB signals. The signals are output to a look up table (LUT) 6. Here, the look-up table 6 inputs and stores data output from the gamma curve generator 8, and uses gamma-corrected R and G data stored with the RGB signals output from the color converter 4 as addresses. And B signals Rout, Gout, and Bout. At this time, the gamma curve generator 8 outputs the slope of the gamma correction curve set by itself to the lookup table 6 as data.

The above-described image stretching apparatus 2 according to the present invention shown in FIG. 2 obtains a minimum value and an average value in real time, a threshold value and a slope value are obtained during a vertical blanking period, and the black extended luminance signal is obtained in real time by a color converter ( Should be printed as 4). Therefore, adders 32 and 36, multiplier 34, comparator 52 and multiplexer 62 are required, so that multiplier 34 when the decompression device 2 of the image shown in FIG. 2 is implemented as a chip. ) Increases the chip area. In addition, the image decompression device 2 expands only the black level of the luminance signal, and the level of the chrominance signals cannot be expanded.

The configuration and operation of an image decompression device using gamma correction and a method of decompression thereof according to the present invention for solving this problem will be described as follows.

FIG. 5 is a block diagram illustrating an image decompression device using gamma correction according to an embodiment of the present invention, including a factor extractor 100, a color converter 102, and a look up table (LUT) 104. And a control unit 106.

FIG. 6 is a flowchart illustrating an image decompression method using gamma correction according to the present invention performed by the apparatus shown in FIG. 5, wherein the threshold and the slope are calculated in an active period and a vertical blanking period (140-140). And gamma correction value (steps 152 to 156) is determined according to the result of comparing the level of the signal to be gamma corrected from the outside with the threshold.

The factor extractor 100 shown in FIG. 5 corresponds to the level distribution of the luminance signal Y that is currently input and is generated during the time when the screen is displayed, and the luminance signal Y in response to the active signal input through the input terminal IN1. Extracts the average value of the lowest value of the level and the luminance levels below the predetermined level, extracts the threshold and the slope of the black region in response to the vertical synchronization signal input through the input terminal IN2 using the extracted lowest value and the average value. 106) (steps 140 to 150). Here, each of the 140th through 150th steps corresponds to each of the 76th through 86th steps shown in FIG. 3 and is the same, and thus description thereof will be omitted. In addition, the first low pass filter 112, the lowest value extracting unit 114, the average value extracting unit 116, the second low pass filter 118, and the third low pass filter 120 constituting the factor extraction unit 100. ), The D flip-flops 126 and 128, the threshold extractor 122 and the gradient extractor 124 are each a first low pass filter of the factor extractor 10 shown in FIG. 2 shown in FIG. 2. 12, the lowest value extractor 14, the average value extractor 16, the second low pass filter 18, the third low pass filter 20, the D flip-flops 26 and 28, the threshold extractor ( 22) and the gradient extractor 24, respectively, and perform the same function, so description thereof is omitted.

In addition, the color converter 102 and the lookup tables 104 shown in FIG. 5 correspond to the color converter 4 and the lookup tables 6 shown in FIG. 2 and perform the same function. That is, the color converter 102 inputs the luminance signal Y and the color difference signals CrCb of the image information to convert the RGB signal, and outputs the converted RGB signals as addresses of the LUT 104. In addition, the LUT 104 uses RGB data (Rout, Gout, and Bout) whose image is stretched and gamma-corrected in response to the RGB signal input from the color conversion unit 102 as the address. Output as.

FIG. 7 is a graph illustrating an apparatus and methods for stretching an image using gamma correction illustrated in FIGS. 5 and 6, wherein the horizontal axis represents an RGB signal to be gamma corrected, and the vertical axis represents an RGB signal to which the image is stretched and gamma corrected. Represent each. Here, reference numerals 182, 184, and 186 may each be one of Rout, Gout, or Bout.

On the other hand, after step 150, the controller 106 determines whether the level of the signal to be gamma corrected from the outside through the input terminal IN3 is greater than or equal to the threshold output from the threshold extractor 122 (step 152).

If the level of the signal to be gamma corrected through the input terminal IN3 is equal to or greater than the threshold value, a gamma correction value set by itself is output to the LUT 104, and the LUT 104 outputs the RGB from the color conversion unit 102. In response, the gamma correction value input and stored from the control unit 106 may be output as the image extension and gamma correction value (step 154). Therefore, as shown in FIG. 7, when the level of the signal to be gamma corrected through the input terminal IN3 is greater than or equal to the threshold value 180, it can be seen that the preset gamma correction value is output as the image extension and gamma correction value. .

However, if the level of the signal to be gamma corrected through the input terminal IN3 is less than the threshold 182, the controller 106 uses the following thresholds and inclinations obtained as shown in Equations 3 and 5, respectively. The gamma correction value obtained as shown in 6 is output to the lookup table 104 (step 156).

Gamma Correction = (Value to be Gamma Corrected-Threshold) * Slope + Threshold

Therefore, as shown in FIG. 7, if the level of the signal to be gamma corrected through the input terminal IN3 is less than the threshold 182, the gamma correction value calculated by the controller 106 is used as the image extension and gamma correction value. You can see the output.

The predetermined luminance level described in the description of FIG. 5 may be arbitrarily applied, as applied to the apparatus shown in FIG. That is, the predetermined luminance level may be set to 1/2 the luminance level of the highest level among the levels of the luminance signal.

As a result, as described above, although the image stretching apparatus shown in FIG. 2 does not have to perform the calculations of Equations 3 and 5 in real time before converting the YCrCb into an RGB signal, the black stretching corresponding to the image extended equation 6 Since the luminance signal must be generated in real time, it requires adders 32 and 36, multiplier 34, comparator 32 and multiplexer 62, while the extension of the image according to the invention shown in FIG. The apparatus does not need to process Equation 6 in real time as it performs color stretching as well as black stretching after converting YCrCb to an RGB signal, thus addingers 32 and 36, multiplier 34, comparator 32 and multiplexer (62) do not need.

As described above, the image stretching apparatus and method according to the present invention performs black stretching in consideration of the histogram of the image information, so that there is no hassle that the user must set the slope and the threshold value every time the image information changes, and the sharp change occurs. Errors due to the image can be prevented, and the gamma correction is used to extend the luminance and color signal levels of the image at the same time, so that a clearer image can be displayed and the image does not need to be expanded in real time. The effect is simple.

Claims (25)

Extracts and extracts an average of luminance levels below a predetermined luminance level and a minimum value of the luminance level in response to an active signal generated during a display time corresponding to a luminance level distribution that is a level of a luminance signal of currently input image information. Factor extraction means for utilizing the minimum and the averaged values and extracting a threshold and a slope of a black region in response to a vertical synchronization signal; Image information calculating means for calculating a luminance signal extended black from the luminance signal, the extracted threshold value, and the slope which are currently input; Selecting means for selectively outputting the black extended luminance signal and the currently input luminance signal in response to a selection signal; And And comparison means for comparing the luminance level with the threshold of the luminance signal currently input and outputting the selection signal in accordance with the result of the comparison. The method of claim 1, wherein the factor extraction means A minimum value extraction means for extracting the lowest value of the luminance level in response to the active signal and multiplying the minimum value by a first predetermined value (where the first predetermined value is a positive number less than or equal to 1) to output the adjusted lowest value; ; Average value extraction means for extracting the average value of the luminance levels below the predetermined luminance level in response to the active signal; In response to the vertical synchronizing signal, the value obtained by subtracting the lowest value from the average value is multiplied by a second predetermined value (where the second predetermined value is a positive number greater than or equal to 0) and the multiplied value is added by adding the average value. Threshold extracting means for outputting a value as the threshold; And And inclination extracting means for subtracting the adjusted lowest value from the threshold, dividing the threshold by the subtracted value, and outputting the divided value as the slope in response to the vertical synchronizing signal. . The method of claim 2, wherein the factor extraction means And a first low pass filter for filtering a low pass component of the luminance signal currently input and outputting the low pass filtered luminance signal to the lowest value extracting means and the average value extracting means. The method of claim 2 or 3, wherein the factor extraction means A second low pass filter for filtering the low pass component of the lowest point and the adjusted lowest value output from the lowest value extracting means, and outputting the filtered values to the threshold extracting means and the gradient extracting means; And And a third low pass filter for filtering the low pass component of the mean value outputted from the mean value extracting means and outputting the filtered mean value to the threshold extracting means. The apparatus of claim 1, wherein the predetermined luminance level is 1/2 level of a highest luminance level of the currently input luminance signal. The method of claim 1, wherein the video information calculating means Subtraction means for subtracting the threshold value from the luminance signal currently input; Multiplication means for multiplying the result by the result subtracted from the subtraction means; And And an addition means for adding the result multiplied by the multiplication means and the threshold value to output the black extended luminance signal. (a) continuously determining whether the screen is an active section on which the screen is displayed; (b) obtaining an average of luminance levels below a predetermined luminance level and a minimum value of the luminance level corresponding to a distribution of luminance levels that are levels of luminance signals included in image information to be black-extended in the active period; (c) continually determining whether to apply a vertical blanking period; (d) In the vertical blanking period, a threshold value is obtained using the average value, the lowest value, and a first predetermined value, wherein the first predetermined value is a positive number, and the slope is used using the threshold value and the minimum value. Obtaining; (e) determining whether the luminance level is above the threshold; (f) if the luminance level is greater than or equal to the threshold, determining the luminance level as a level of the blackened luminance signal; and (g) if the luminance level is less than the threshold, calculating the luminance level to be black expanded as the threshold and the inclination to determine the luminance level to be black extended. The method of claim 7, wherein the stretching of the image is performed. Low pass filtering the luminance signal and proceeding to step (a), In the step (b), the average value of the luminance levels below the predetermined luminance level and the minimum value is calculated according to the luminance level distribution of the low pass filtered luminance signal. The method of claim 7, wherein the stretching of the image is performed. Low pass filtering the average value and the minimum value and proceeding to step (c); In the step (d), the threshold value is obtained using the low pass filtered average value, the lowest value, and the first predetermined value. 10. The method of claim 9, wherein the step (d) calculates the slope using the low pass filtered lowest value and the threshold value. The method of claim 7 or 9, wherein step (d) (d1) subtracting the lowest value from the average value; (d2) multiplying the subtracted value by the first predetermined value: and and (d3) adding the multiplied value with the average value to obtain the threshold value. The method of claim 11, wherein step (d) (d4) after step d3, multiplying the lowest value by a second predetermined value (where the second predetermined value is a positive number of 1 or less) to obtain an adjusted lowest value; (d5) subtracting the adjusted lowest value from the threshold; And (d6) dividing a threshold value by a value subtracted in the step (d5) to obtain the slope. The method of claim 7, wherein step (g) (g1) subtracting the threshold from the luminance level of the luminance signal: (g2) multiplying the slope by the value subtracted in the step (g1); And and (g3) adding the value multiplied by the step (g2) and the threshold to determine the brightness level of the black extended luminance signal. A color converter for inputting luminance signals and chrominance signals of image information into R.B. (RGB) signals and data stored in response to the RGB signal input as an address are used as image extension and gamma corrected data. In the image stretching apparatus included in the system having a look-up table to output, Extracting the lowest value of the luminance level and an average value of luminance levels below a predetermined level in response to an active signal generated during a display time corresponding to a luminance level distribution that is a level of a luminance signal currently input; Factor extracting means for using the average values and extracting a threshold and a slope of a black region in response to a vertical synchronization signal; And In response to a result of comparing the level of the signal to be gamma corrected from the outside with the threshold value, a gamma correction value set by itself is output as the data stored in the lookup table, or the threshold value and the And control means for outputting the gamma correction value obtained using the slope as the data stored in the lookup table, And the gamma correction value is stored at the address of the lookup table. 15. The method of claim 14, wherein said factor extracting means A minimum value extraction means for extracting the lowest value of the luminance level in response to the active signal and multiplying the minimum value by a first predetermined value (where the first predetermined value is a positive number less than or equal to 1) to output the adjusted lowest value; ; Average value extracting means for extracting the average value of the luminance levels below the predetermined level in response to the active signal; In response to the vertical synchronizing signal, the value obtained by subtracting the lowest value from the average value is multiplied by a second predetermined value (where the second predetermined value is a positive number greater than or equal to 0) and the multiplied value is added by adding the average value. Threshold extracting means for outputting a value as the threshold; And And inclination extracting means for subtracting the adjusted lowest value from the threshold, dividing the threshold by the subtracted value, and outputting the divided value as the slope in response to the vertical synchronizing signal. . The method of claim 15 wherein said factor extracting means And a first low pass filter for filtering low-pass components of the luminance signal and outputting the low-pass filtered luminance signal to the lowest value extracting means and the average value extracting means. The method of claim 15 wherein said factor extracting means A second low pass filter for filtering the low pass component of the lowest point and the adjusted lowest value output from the lowest value extracting means, and outputting the filtered values to the threshold extracting means and the gradient extracting means; And And a third low pass filter for filtering the low pass component of the mean value outputted from the mean value extracting means and outputting the filtered mean value to the threshold extracting means. The apparatus of claim 14, wherein the predetermined luminance level is 1/2 level of a highest luminance level of the currently input luminance signal. A color converter for inputting luminance signals and chrominance signals of image information into R.B. (RGB) signals and data stored in response to the RGB signal input as an address are used as image extension and gamma corrected data. In the image stretching method performed in a system having a look-up table to output, (a) continuously determining whether the screen is an active section on which the screen is displayed; (b) in the active period, obtaining an average of luminance levels below a predetermined luminance level and a minimum value of the luminance level corresponding to a distribution of luminance levels that are levels of the luminance signal; (c) continually determining whether to apply a vertical blanking period; (d) In the vertical blanking period, a threshold is obtained using the average value, the lowest value, and a first predetermined value (where the first predetermined value is a positive number), and the slope is obtained using the minimum value and the threshold value. Obtaining; (e) determining whether a level of a signal to be gamma corrected is greater than or equal to the threshold; (f) if the level of the signal to be gamma corrected is greater than or equal to the threshold, determining a gamma correction value that is set as the data to be stored in the lookup table; And (g) if the level of the signal to be gamma corrected is less than the threshold, determining a gamma correction value obtained from the signal to be gamma corrected using the threshold and the slope as the data to be stored in the lookup table. Stretching method of the image, characterized in that. The method of claim 19, wherein the stretching method of the image Low pass filtering the luminance signal and proceeding to step (a), And (b) calculating the average value of the lowest values and luminance levels below the predetermined luminance level according to the distribution of the luminance levels of the low pass filtered luminance signal. The method of claim 20, wherein the stretching method of the image Low pass filtering the average value and the minimum value and proceeding to step (c); In the step (d), the threshold value is obtained using the low pass filtered average value, the lowest value, and the first predetermined value. 22. The method of claim 21, wherein step (d) calculates the slope using the threshold and the low pass filtered minimum. 22. The method of claim 19 or 21, wherein step (d) (d1) subtracting the lowest value from the average value; (d2) multiplying the subtracted value by the first predetermined value: and and (d3) adding the multiplied value with the average value to obtain the threshold value. 23. The method of claim 19 or 22, wherein step (d) (d4) after the step (d3), multiplying the lowest value by a second predetermined value (where the second predetermined value is a positive number of 1 or less) to obtain an adjusted lowest value; (d5) subtracting the adjusted lowest value from the threshold; And (d6) dividing a threshold value by a value subtracted in the step (d5) to obtain the slope. 20. The method of claim 19, wherein step (g) (g1) subtracting the threshold from the signal to be gamma corrected: (g2) multiplying the slope by the value subtracted in the step (g1); And and (g3) adding the multiplied value and the threshold value in the step (g2), and determining the added value as the gamma correction value.