JP3874658B2 - Contrast correction circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a contrast correction circuit, and more particularly to a contrast correction circuit suitable for use in a display device having a narrow dynamic range such as a plasma display panel (PDP).
[0002]
[Prior art]
In a display device with a narrow dynamic range such as a PDP, a contrast correction circuit for enhancing the contrast of a portion with a large amount of information in an image is often used in order to obtain a better image in terms of visual perception.
In such a conventional contrast correction circuit, a plurality of types of look-up tables having different contrast conversion characteristics are usually used for contrast conversion for enhancing the contrast of an image, and these tables are stored in a memory such as a ROM. It has been. For the selection of the lookup table, a result of integrating image data for one screen, a result of taking a histogram, or the like is used.
[0003]
[Problems to be solved by the invention]
When a conventional contrast correction circuit that performs contrast conversion of image data using a plurality of lookup tables as described above is used, a considerably good video can be obtained on most screens.
[0004]
However, there are cases where it is better not to make correction on a specific screen. Specifically, image data such as a dark screen of a movie corresponds to this. If there is a relatively bright image with a small area in such a screen, that is, an image with a large number of pixels at the black level, the area ratio (corresponding to the data amount) of the image composed of pixels at the black level. Therefore, in the conventional contrast correction circuit, a look-up table having contrast conversion characteristics effective on a dark screen is easily selected.
[0005]
However, in the case of a screen with a relatively bright image with a small area in an image with a large number of black-level pixels, the important information is rather a small area, and the dark screen for such a screen When the uptable is selected and contrast correction is performed, there is a problem that deterioration such as white crushing is likely to occur.
In addition, when there is a partially dark image on a white screen on a snowy landscape screen etc., there is a problem that it may be blacked out contrary to the above case, and it may be difficult to see important information .
The present invention has been made in view of such circumstances, and the contrast of an image to be noticed when an image having a uniform gradation occupies most of the screen and an image to be noticed occupies a small part of the screen. An object of the present invention is to provide a contrast correction circuit capable of appropriately correcting the above.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 is an integration means for integrating the brightness levels of the input video signals and calculating the sum of the brightness levels of the video signals of the entire screen; First storage means for storing table data indicating a plurality of types of contrast conversion characteristics corresponding to the brightness of the image, and brightness of all pixels when the brightness level of all pixels on the screen is maximum The reference value is compared with the integrated output of the integrating means using the sum of levels as a reference value, and the plurality of types are selected based on the comparison result or in consideration of the gradation of the video signal input to the comparison result. Selection means for instructing the first storage means to selectively output table data suitable for screen brightness among the table data, and contrast of the input video signal from the first storage means Output Second storage means storing a conversion table to be corrected by the table data, and dividing the gradation of the input video signal into a plurality of areas, and determining the frequency of gradation belonging to each of the divided areas Histogram detection means for detecting in pixel units and the detection output of the histogram detection means are fetched, and when there is a gradation whose frequency is equal to or higher than a preset threshold, it is determined that the gradation is uniform, and the uniform Control means for outputting data indicating the gradation determined as the gradation and the frequency of the gradation, and the selection means includes the gradation determined by the control means as the uniform gradation and the floor. When data indicating the frequency of the key is received, a value obtained by subtracting the product of the gray level determined as the uniform gray level and the frequency of the gray level from the output of the integrating unit is obtained on one screen as a reference value . The uniform gradation is determined Based on the value divided by the sum of the brightness levels when the brightness level of all the pixels in the image area excluding the image area occupied by the gradation is maximized, the brightness of the screen among the plurality of types of table data is calculated. The first storage unit is instructed to selectively output suitable table data.
[0007]
According to a second aspect of the present invention, in the contrast correction circuit according to the first aspect, the threshold value is defined in advance as an area ratio of a display area to be noted with respect to the entire display screen, and the background is uniform. In the case of a tone screen, the number of pixels corresponding to the area of the screen with the uniform gradation is set.
[0008]
The invention according to claim 3, in contrast correction circuit according to claim 1 or 2, wherein the control means per determines whether the uniform tone, previously determined target In the case where the gradation is not set, data indicating the frequency of the gradation is not output.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The configuration of the contrast correction circuit according to this embodiment is shown in FIG. The contrast correction circuit according to the present embodiment detects the frequency of a uniform and abundant gradation in a video signal, and if the amount exceeds a certain value, important information is transferred to other gradations. It is characterized in that it is judged that there is, and by operating a selection signal of table data indicating contrast conversion characteristics, white crushing on a dark screen, black crushing on a bright screen, and the like are improved.
[0011]
In FIG. 1, the contrast correction circuit includes an integration circuit 10, a histogram detection circuit 12, a selection signal generation circuit 14, a ROM 16, a RAM 18, and a control circuit 20.
The integration circuit 10 integrates the brightness levels of the video signal numbers input from the input terminals 100, 101, and 102, and calculates the sum of the brightness levels of the video signals of the entire screen.
[0012]
The histogram detection circuit 12 divides the gradation of the video signal (R, G, B) input from the input terminals 100, 101, 102 into a plurality of areas, and calculates the frequency of gradation belonging to each of the divided areas. It has a function of detecting and outputting in pixel units.
The selection signal generation circuit 14 compares the reference value with the integration output of the integration circuit 10 using the sum of the brightness levels of all the pixels when the brightness level of all the pixels on the screen is maximum as a reference value. Based on the comparison result or on the basis of the output of the control circuit 20, the screen of the plurality of types of table data is considered in consideration of the gradation of the video signal inputted from the input terminals 100, 101, 102 to the comparison result. A function of instructing the ROM 16 to selectively output table data suitable for the brightness of the image.
[0013]
The ROM 16 stores table data indicating a plurality of types of contrast conversion characteristics selected according to the brightness of the screen. The ROM 16 is selected by a selection signal output from the selection signal generation circuit 14. Output table data for contrast conversion characteristics.
The RAM 18 stores a conversion table for correcting the contrast of the video signal input from the input terminals 100 to 102 using the table data output from the ROM 16. The video signal whose contrast is corrected by the conversion table stored in the RAM 18 is output to the output terminals 200 to 202.
[0014]
The control circuit 20 takes in the detection output of the histogram detection circuit 12, determines that the gradation is a uniform gradation when there is a gradation whose frequency is equal to or greater than a preset threshold, and determines that the gradation is uniform. And a function of outputting data indicating the gradation and the frequency of the gradation.
Here, the threshold value is defined in advance as the area ratio of the display region to be noticed with respect to the entire display screen, and corresponds to the area of the uniform gradation screen when the background is a uniform gradation screen. Set to the number of pixels.
[0015]
In addition, the selection signal generation circuit 14 receives a reference value to be compared with the output of the integration circuit 10 when receiving data indicating the gradation determined by the control circuit 20 as a uniform gradation and the frequency of the gradation. A value obtained by subtracting the product of the gradation determined as the uniform gradation and the frequency of the gradation from the sum of the brightness levels of all the pixels when the brightness level of all the pixels of the screen is maximum, The reference value is divided by the sum of the brightness levels when the brightness level of all the pixels in the image area excluding the image area occupied by the uniform gradation determined in one screen is the maximum. It has a function of instructing the ROM 16 to selectively output table data that matches the brightness of the screen among a plurality of types of table data based on the value.
[0016]
Next, table data indicating a plurality of types of contrast conversion characteristics stored in the ROM 16 will be described with reference to FIG. FIG. 2 shows the contrast conversion characteristics of the image. In the figure, the contrast conversion characteristic indicated by the curve Q1 is a characteristic that is selected for a dark image having a small average value of the brightness level of the input video signal, and is indicated by the curve Q16. The contrast conversion characteristic is a characteristic that is selected for an image having a large average brightness level of an input video signal and having a bright overall image. In the figure, the characteristic is such that the contrast of the image data in the area surrounded by the broken line is enhanced.
[0017]
In the present embodiment, table data (numerical data) indicating 16 contrast conversion characteristics of the curves Q1 to Q16 obtained by dividing the curve Q1 and the curve Q16 equally is stored in the ROM 16 and inputted. Predetermined table data is selected according to the average value of the brightness level of the video signal.
The operation of the contrast correction circuit according to this embodiment configured as described above will be described.
[0018]
In FIG. 1, when R, G, B video signals (digital signals) are input from input terminals 100 to 102, the integration circuit 10 integrates the brightness level of the input video signal, and the entire screen image is displayed. The sum (average value) S0 of signal brightness levels is calculated. Here, assuming that the size of one screen is 852 (dots) × 480 (line) and one pixel is displayed with 8 bits for R, G, and B, the brightness level of all the pixels on the screen is maximum. The sum Sm of the brightness levels of all pixels is
Sm = 852 (dots) x 480 (lines) x 255 (8 bits: maximum value) x 3 (RGB) = 312854400
It becomes.
[0019]
The sum (average value) S0 of the brightness level of the video signal of the entire screen calculated by the integration circuit 10 is replaced by a selection signal for selecting table data by the selection signal generation circuit 14 as follows. The That is, the selection signal is a value obtained by dividing the output S0 of the integration circuit 10 by the total sum Sm (integrator output (total) ÷ 312854400), and takes a value of 0 to 1, but the contrast conversion characteristics Q1 to Q16 Allocate evenly according to the number of contrast conversion tables consisting of the table data shown. Here, a set of table data indicating each contrast conversion characteristic is referred to as a contrast conversion table.
[0020]
As described above, when there are 16 types of contrast conversion tables as described above, a selection signal for selecting the contrast conversion table is output to the ROM 16 in accordance with the calculation result in the selection signal generation circuit 14 as follows. That is,
When S0 / Sm = 0 to 0.0625, the contrast conversion table Q1 is
When S0 / Sm = 0.0625 to 0.125, the contrast conversion table Q2 is
When S0 / Sm = 0.875 to 0.9375, the contrast conversion table Q15 is
When S0 / Sm = 0.9375 to 1, selection signals for selecting the contrast conversion table Q16 are output to the ROM 16, respectively.
[0021]
As a result, the ROM 16 outputs table data having contrast conversion characteristics selected by the selection signal output from the selection signal generation circuit 14 to the RAM 18. The RAM 18 corrects the contrast of the video signals (R, G, B) input from the input terminals 100 to 102 based on the table data received from the ROM 16 by the conversion table, and the video signal (R, G, B) is output from the output terminals 200-202.
[0022]
In this way, an image in which the contrast of the dark part is emphasized when the entire screen is dark, and an image in which the contrast of the bright part is emphasized when the entire screen is bright can be obtained.
In most cases, a desired contrast correction can be performed by this method. However, as described above, adverse effects may occur. Specific examples thereof will be described with reference to FIGS.
[0023]
FIG. 3 shows a screen in the case where the background A1 is mostly black level gradation and there is a bright gradation image B1 partially close to white. In the case of the screen as shown in FIG. 3, the output of the integration circuit 10, that is, the sum (average value) of the brightness level of the video signal of the entire screen is a low value. When the control circuit 20 is not provided, a contrast conversion table suitable for a dark screen is selected, and the contrast of the dark portion is enhanced at the expense of the contrast of the bright portion in the screen.
[0024]
However, while the important video signal is rather in the bright part, the contrast of that part is reduced, causing a phenomenon such as white crushing.
Further, as shown in FIG. 4, contrary to FIG. 3, the background A2 is mostly white level gradation, and the screen in the case where there is a dark gradation image B2 partially close to black is bright. A contrast conversion table suitable for the screen is selected, and the contrast of the bright portion is enhanced at the expense of the contrast of the dark portion in the screen. Therefore, in the case of the screen shown in FIG. 4, a phenomenon such as black crushing is caused.
[0025]
Further, as shown in FIG. 5, in the screen where the background A3 is gray and there is a bright gradation image B31 that is partially close to white and a dark gradation image B32 that is partially close to black, A contrast conversion table corresponding to the gray of A3 is selected, and the contrast of the background A3 of the gray portion is enhanced at the expense of the contrast of the white portion and the black portion in the screen. Therefore, in the case of the screen shown in FIG. 5, both white and black may be crushed.
[0026]
When the video signals representing the screens shown in FIGS. 3 to 5 are input from the input terminals 100, 101, and 102, the histogram detection circuit 12 determines the gradation and frequency represented by the histograms shown in FIGS. Output the indicated data. 6 corresponds to the screen shown in FIG. 3, FIG. 7 corresponds to the screen shown in FIG. 4, and FIG. 8 corresponds to the screen shown in FIG.
In the present embodiment, the control circuit 20 determines the black level when the gradation of the video signal is 0 to 5, the gray level when the gradation is 125 to 1130, and the white level when the gradation is 250 to 255, for example. It shall be recognized.
[0027]
In the case of the screens shown in FIG. 3 to FIG. 5, the sum of the brightness levels of the video signal of the entire screen calculated by the integration circuit 10 is determined by the gradations that occupy the most area in the screen. The phenomenon described above will occur.
In order to avoid such a phenomenon, in the present invention, first, the sum total of the video signals is integrated by the integration circuit 10 and at the same time, the video signals (R, R) input from the input terminals 100, 101, 102 by the histogram detection circuit 12. The gradation of G, B) is divided into a plurality of areas, and the frequency of the gradation belonging to each of the divided areas is detected and output.
[0028]
The control circuit 20 outputs the frequency for each gradation to the selection signal generation circuit 14 when there is a gradation having a frequency exceeding the threshold set from the outside based on the output of the histogram detection circuit 12. That is, the control circuit 20 outputs data indicating the gradation D and the frequency H for correcting the calculation of the selection signal calculated by the selection signal generation circuit 14. The threshold value is a criterion for determining a uniform gradation when the frequency (area) of the histogram is exceeded.
[0029]
At this time, if the control circuit 20 is designated with a gradation range that is ignored from the outside, that is, a gradation range that is not subject to contrast correction, the control circuit 20 determines the frequency of the designated gradation. No data is output.
The threshold value is defined in advance as the area ratio of the display area to be noticed with respect to the entire display screen, and corresponds to the black screen area when the background is a screen with a uniform gradation, for example, a black screen. The number of pixels to be set may be set. For example, in the case of performing a small-screen display with a light gradation close to white with a black background (display area of 1/4 of the entire screen), the setting value as a threshold value set in the control circuit 20 is 852 × 480. When 3/4 of the display screen is a black screen, 852 × 480 × 3/4 = 306720.
[0030]
On the other hand, the selection signal generation circuit 14 calculates a selection signal by the following equation based on the gradation D and the frequency H output from the control circuit 20 when there is a gradation having a frequency exceeding the threshold.
S0 '= (output of the integration circuit 10 (sum) -D × H) (1)
Sm ′ = {852 (dots) × 480 (rows) −H} × 255 (8-bit maximum value) × 3 (RGB) (2)
Selection signal = S0 '/ Sm' (3)
[0031]
Here, D × H is the brightness of the total number of pixels (the number of dots) in the image area occupied by an image of uniform gradation to eliminate the influence on the image to be noticed when correcting the contrast in the screen. The sum of the levels, S0 ′ is the sum of the brightness levels in the image area to be noted in the screen.
Sm ′ is the sum of the brightness levels of all the pixels in the image area to be noticed when the brightness level of each pixel in the image area to be noticed is the maximum in the screen, that is, the image to be noticed. This is the sum (maximum value) of the brightness levels that can be taken in the area.
[0032]
As described above, the selection signal is calculated by the selection signal generation circuit 14, and the calculated selection signal is output to the ROM 16. As a result, the contrast conversion adapted to the image to be noticed when correcting the contrast of the video signal. Since the table data having the characteristics is selected, the influence of the gradation image region which is uniform and occupies a large area on the screen is eliminated, and the adverse effects described with reference to FIGS. 3 to 5 are prevented. Can do.
[0033]
According to the contrast correction circuit according to the present embodiment, when there is an image having a uniform gradation that occupies a large area in the screen when correcting the contrast of the video signal, the influence is eliminated. Since table data with more appropriate contrast conversion characteristics is selected for the image to be noticed, white spots on dark screens and blacks on bright screens are prevented when contrast is corrected. can do.
Further, even if the contrast of the video signal is intentionally processed, it is possible to prevent the adverse effects caused by the processing.
[0034]
【The invention's effect】
According to the first aspect of the present invention, the integration means for integrating the brightness levels of the input video signal and calculating the sum of the brightness levels of the video signal of the entire screen, and a plurality of units corresponding to the brightness of the screen. The first storage means storing table data indicating the contrast conversion characteristics of the seeds, and the sum of the brightness levels of all the pixels when the brightness levels of all the pixels of the screen are the maximum as a reference value The reference value is compared with the integrated output of the integrating means, and the brightness of the screen is selected from the plurality of types of table data based on the comparison result or considering the gradation of the video signal input to the comparison result. Selection means for instructing the first storage means to selectively output table data conforming to the table size, and table data output from the first storage means for contrast of the input video signal. Second storage means storing a conversion table to be corrected and the gradation of the input video signal is divided into a plurality of areas, and the frequency of the gradation belonging to each of the divided areas is detected in units of pixels. And detecting the histogram detection means, and determining that the gradation is uniform when there is a gradation whose frequency is equal to or greater than a preset threshold, and determining that the gradation is uniform. Control means for outputting data indicating the gradations and the frequency of the gradations, and the selection means determines the gradations determined by the control means as the uniform gradations and the frequencies of the gradations. When the data shown is received, the value obtained by subtracting the product of the gradation determined as the uniform gradation and the frequency of the gradation from the output of the integrating means is the uniform scale in one screen as a reference value. Image area occupied by tones determined as tones Select table data suitable for the screen brightness from among the plurality of types of table data based on the value divided by the sum of the brightness levels when the brightness levels of all the pixels in the image area are the maximum. To the first storage means.
[0036]
Therefore, when correcting the contrast of the video signal, if there is an image with a uniform gradation that occupies a large area in the screen, a more appropriate contrast for the image to be noted so as to eliminate the influence Since the table data having the conversion characteristics is selected, it is possible to prevent an adverse effect such as white crushing on a dark screen or black crushing on a bright screen when the contrast is corrected.
Further, even if the contrast of the video signal is intentionally processed, it is possible to prevent the adverse effects caused by the processing.
[0037]
According to the second aspect of the present invention, the threshold value is defined in advance as an area ratio of a display area to be noticed with respect to the entire display screen. The number of pixels corresponding to the screen area of uniform gradation is set.
Therefore, even on a screen with a uniform background, table data having a more appropriate contrast conversion characteristic is selected for an image to be noticed, so the influence of a background with a uniform gradation when the contrast is corrected Can be eliminated.
[0038]
According to the third aspect of the present invention, when determining whether or not the gradation is a uniform gradation, the control means determines this gradation when a gradation that is not subject to determination is set in advance. Since data indicating the frequency of the key is not output, the amount of data for contrast correction can be reduced and the processing load can be reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a contrast correction circuit according to an embodiment.
2 is an explanatory diagram showing contrast conversion characteristics indicated by table data stored in a ROM of the contrast correction circuit shown in FIG. 1;
FIG. 3 is an explanatory diagram showing a screen in a case where an image having a light gradation having a gray level close to white partially exists with the majority of the background being a black level gradation.
FIG. 4 is an explanatory diagram showing a screen in a case where an image having a dark gradation whose part of the background is mostly white level gradation and partially close to black exists.
FIG. 5 is an explanatory diagram showing a screen in a case where a gray background and a bright gradation image partially close to white and a dark gradation image partially black are present.
6 is a histogram showing the relationship between the gradation and the frequency of the video signal representing the screen shown in FIG. 3;
7 is a histogram showing the relationship between the gradation and frequency of the video signal representing the screen shown in FIG. 4;
8 is a histogram showing the relationship between the gradation and frequency of a video signal representing the screen shown in FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Integration circuit 12 ... Black level detection circuit 14 ... Selection signal generation circuit 16 ... ROM
18 ... RAM
20 ... Control circuit

Claims (3)

Integration means for integrating the brightness level of the input video signal and calculating the sum of the brightness levels of the video signal of the entire screen;
First storage means storing table data indicating a plurality of types of contrast conversion characteristics according to the brightness of the screen;
Based on the comparison result by comparing the reference value and the integration output of the integrating means with the sum of the brightness levels of all the pixels when the brightness level of all the pixels of the screen is the maximum as a reference value, Alternatively, the first storage means may selectively output table data suitable for screen brightness among the plurality of types of table data in consideration of the gradation of the input video signal in the comparison result. A selection means to direct;
Second storage means storing a conversion table for correcting the contrast of the input video signal by table data output from the first storage means;
Histogram detection means for dividing the gradation of the input video signal into a plurality of areas and detecting the frequency of gradation belonging to each of the divided areas in units of pixels;
The detection output of the histogram detection means is captured, and when there is a gradation whose frequency is equal to or higher than a preset threshold, it is determined that the gradation is uniform, and the gradation determined as the uniform gradation and the gradation Control means for outputting data indicating the frequency of gradation,
The selection means is determined to be the uniform gradation from the output of the integration means when receiving data indicating the gradation determined as the uniform gradation by the control means and the frequency of the gradation. The brightness level of all the pixels in the image area excluding the image area occupied by the uniform gradation in one screen, which is the reference value, obtained by subtracting the product of the gradation and the frequency of the gradation The first storage means is configured to selectively output table data suitable for screen brightness among the plurality of types of table data based on a value divided by the sum of brightness levels at the time when the brightness becomes maximum. It features and to Turkey contrast correction circuit to instruct.   In the threshold value, the area ratio of the display area to be noted with respect to the entire display screen is defined in advance, and when the background is a screen with a uniform gradation, the number of pixels corresponding to the area of the uniform gradation screen The contrast correction circuit according to claim 1, wherein the contrast correction circuit is set as follows. When determining whether or not the gradation is uniform, the control means does not output data indicating the frequency of the gradation when a gradation that is not subject to determination is set in advance. contrast correction circuit according to claim 1 or 2,.

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