JP2010079236A - Backlight brightness controlling apparatus and image display apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a backlight brightness controlling apparatus and an image display apparatus which can suppress occurrence of brightness bleeding. <P>SOLUTION: In the image display apparatus, the backlight 12 which illuminates a liquid crystal panel (image display panel) 11 is segmented into a plurality of areas. A frequency distribution processing section 14 calculates the amount of dark pixels from frequency distribution of gradation values of pixels in an image based on an image signal. A backlight controlling section 13 calculates a lower limit value of brightness in the plurality of areas included in the backlight 12 so as to increase the brightness when the amount of dark pixels is smaller, adjusts the brightness in each area individually according to the gradation value of each part of the image, and adjusts the brightness to the lower limit value of the brightness in an area where the brightness is lower than the lower limit value. A difference of brightness between areas which illuminate a dark part of the image in the backlight 12 is reduced, thereby suppressing the occurrence of brightness bleeding caused by the difference of the brightness between the areas. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a video display device configured using a liquid crystal panel and a backlight, and more particularly to a backlight luminance control device and a video display device that adjust video quality by controlling the luminance distribution of the backlight. .

  Conventionally, a liquid crystal display has been widely used as a video display device such as a computer monitor device or a television receiver. The liquid crystal display is configured by using a liquid crystal panel and a backlight that illuminates the liquid crystal panel from the back, and displays an image by controlling transmission or blocking of light from the backlight in each part on the liquid crystal panel. Although many fluorescent tubes are used as the light source of the backlight, those using a light emitting diode (LED) as the light source have been developed. Patent Document 1 discloses a video display device that uses an LED as a light source of a backlight.

A backlight using LEDs is configured by arranging a large number of LEDs in a plane and is disposed on the back surface of a liquid crystal panel. Further, the backlight is divided into a plurality of areas where the positions of the liquid crystal panels to be illuminated are different, and the video display device can individually control the luminance in each area of the backlight. Each area of the backlight includes a plurality of LEDs and illuminates a part of the liquid crystal panel. Usually, an image displayed by the image display device includes a relatively bright bright portion and a relatively dark dark portion. The portion of the LCD panel displaying the bright part of the image needs to be illuminated with a high-brightness backlight, while the portion of the LCD panel displaying the dark part of the image has a backlight brightness of Low brightness may be used. Therefore, by adjusting the brightness in each area of the backlight individually, the brightness in the area where the bright part of the image is displayed in the liquid crystal panel is increased and the dark part is displayed. It is possible to reduce the luminance in the area that illuminates the portion and to save power while maintaining the necessary luminance.
JP 2007-219234 A

  As described above, in the video display device that individually controls the luminance in each area included in the backlight, the area that illuminates the portion displaying the dark portion of the video in the liquid crystal panel has low luminance, and the liquid crystal The area that illuminates the portion of the panel displaying the bright portion of the image has high brightness. By the way, since a portion illuminated by one area of the backlight has a certain size, an image portion in which a bright portion and a dark portion are mixed may be illuminated by one area of the backlight. In such an area, it is necessary to illuminate the liquid crystal panel with high brightness in order to display a bright part of the video. For this reason, the dark part close to the bright part in the image is illuminated by the backlight with high luminance. That is, even in the same image, the dark part has a dark part illuminated with high brightness and a dark part illuminated with low brightness, and the dark part illuminated with high brightness is illuminated with low brightness. Brighter than dark areas. Therefore, on the video, a phenomenon occurs in which a portion darker than the bright portion and brighter than the other dark portions is generated around the bright portion. This phenomenon appears as if the brightness of the bright part has oozed out to the surroundings, and this phenomenon is hereinafter referred to as brightness bleed.

  Luminance blur appears prominently when an observer observes a halftone image displayed by the image display device from an oblique direction. This is due to the viewing angle characteristics of the liquid crystal.Low-brightness images are less dependent on the viewing angle of the liquid crystal, so brightness blur is less likely to occur. It tends to occur. Since luminance blur lowers the quality of the video, in order to improve the quality of the video displayed by the video display device, it is necessary to suppress the occurrence of luminance blur especially when displaying a halftone video. However, Patent Document 1 does not describe a technique for suppressing the occurrence of luminance blur.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a difference in luminance in an area where a dark portion of an image is illuminated among a plurality of areas included in a backlight. It is an object of the present invention to provide a backlight luminance control device and a video display device that can suppress the occurrence of luminance blur by reducing the brightness.

  The luminance control device according to the present invention illuminates the luminance in a plurality of areas obtained by dividing a planar backlight that illuminates a video display panel that displays an image based on a video signal, in each area in the video display panel. A luminance control device that individually controls the luminance according to the gradation value of the video displayed in the portion to obtain the frequency distribution of the gradation values of the pixels included in the video based on the video signal; Means for obtaining a dark pixel amount indicating an amount of pixels whose gradation value is equal to or less than a predetermined first reference value from the frequency distribution obtained by the means; and a lower limit value of luminance in the area, the magnitude of the dark pixel amount And a means for calculating a value that monotonously decreases in accordance with the brightness, and adjusting a brightness in an area in which the brightness according to the gradation value of the video is less than the lower limit value to the lower limit value among the plurality of areas of the backlight Means.

  An image display device according to the present invention includes an image display panel that displays an image based on an image signal, a backlight that emits light in a planar shape and illuminates the image display panel, and a plurality of areas that divide the backlight. And a means for individually controlling the brightness of the video display panel to a brightness corresponding to a gradation value of a video displayed in a portion illuminated by each area in the video display panel. Means for acquiring a frequency distribution of gradation values of pixels included in the image, and means for obtaining a dark pixel amount indicating an amount of pixels having a gradation value equal to or less than a predetermined first reference value from the frequency distribution acquired by the means And means for calculating a lower limit value of luminance in the area to a value that monotonously decreases in accordance with the magnitude of the dark pixel amount, and luminance in accordance with the gradation value of the image among the plurality of areas of the backlight Is less than the lower limit. Characterized in that it comprises a means for adjusting the luminance in the lower limit value in.

  In the present invention, in the video display device in which the backlight for illuminating the video display panel is divided into a plurality of areas, the dark pixel amount is obtained from the frequency distribution of the gradation values of the pixels in the video based on the video signal, Calculate the lower limit of brightness in multiple areas included in the backlight so that the smaller the dark pixel amount is, the brightness in each area of the backlight is individually determined according to the gradation value of each part of the video And the luminance in the area where the luminance corresponding to the gradation value is less than the lower limit value is adjusted to the lower limit value. The minimum luminance of a plurality of areas included in the backlight is increased, and a difference in luminance is reduced between an area that illuminates both a bright part and a dark part of an image and an area that illuminates only a dark part.

  The video display device according to the present invention is configured to calculate, from the frequency distribution, a means for calculating an average value of gradation values, and a coefficient that monotonously increases according to the magnitude of the average value calculated by the means, the calculated lower limit value Means for correcting the lower limit value by multiplying by.

  In the present invention, the video display device corrects the lower limit value by calculating the average value of the gradation values from the frequency distribution of the video, and multiplying the lower limit value by a coefficient that decreases as the average value of the gradation values decreases. To do. In an image with a low average gradation value and overall darkness, the lower limit of brightness in multiple areas included in the backlight is reduced, the brightness in the area that illuminates the dark part of the image is reduced, and black Is suppressed.

  The video display device according to the present invention is configured to obtain, from the frequency distribution, a bright pixel amount indicating a pixel amount having a gradation value equal to or greater than a predetermined second reference value greater than the first reference value; The apparatus further comprises means for correcting the lower limit value by multiplying the calculated lower limit value by a coefficient that monotonously increases in accordance with the obtained amount of the bright pixel amount.

  Further, in the present invention, the bright pixel amount is calculated from the frequency distribution of the video, and the lower limit value of luminance in a plurality of areas included in the backlight is obtained by multiplying the lower limit value by a coefficient that increases as the bright pixel amount increases. Correct it. Since the coefficient increases as the bright pixel ratio increases, when an image having a large bright pixel ratio and a large number of bright pixels is displayed, the minimum luminance of the area increases, and the occurrence of luminance blur is effectively suppressed.

  The video display device according to the present invention is a means for measuring the ambient brightness, and by multiplying the calculated lower limit value by a coefficient that monotonously increases according to the magnitude of the ambient brightness measured by the means. And a means for correcting the lower limit value.

  In the present invention, the video display device measures the brightness of the surroundings, and multiplies the lower limit by a coefficient that increases as the measured brightness increases, thereby lowering the lower limit of luminance in a plurality of areas included in the backlight. Correct the value. Since the coefficient increases as the ambient brightness increases, the minimum brightness of the backlight increases when the image is displayed in an environment with bright external light, while making the contrast ratio less noticeable. The occurrence of brightness blur is suppressed.

  The video display device according to the present invention includes: a specifying unit that specifies an area in which the luminance according to the video gradation value is equal to or higher than the lower limit value from the plurality of areas of the backlight; Means for limiting an area whose luminance is adjusted to the lower limit value among areas where the luminance according to the value is less than the lower limit value to an area within a predetermined distance from the area specified by the specifying means; It is characterized by.

  Further, in the present invention, the video display device determines an area in which the luminance less than the lower limit value among the plurality of areas included in the backlight is adjusted to the lower limit value from the area where the luminance corresponding to the video is equal to or higher than the lower limit value. Limited to areas within distance. The number of areas where the luminance increases is limited, and the increase in power consumption is suppressed.

  In the present invention, the difference in brightness between the area that illuminates both the bright and dark portions of the image and the area that illuminates only the dark portion of the plurality of areas included in the backlight is reduced. The occurrence of luminance blur due to the difference in luminance between the areas that illuminate the dark portion inside is suppressed, and the quality of the video displayed by the video display device is improved. In addition, by setting the lower limit of luminance so that the darker pixel amount in the video is smaller, the luminance blur is more strongly suppressed as the video is brighter, and the luminance blur can be effectively suppressed. . For dark images with a large dark pixel ratio, it is difficult for luminance blurs to occur. Therefore, by reducing the lower limit value, the power of the image display device can be saved within a range that does not deteriorate the image quality. The invention has an excellent effect.

Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof.
(Embodiment 1)
FIG. 1 is a block diagram showing an internal configuration of a video display device according to the first embodiment of the present invention. The video display device includes a tuner 21 that receives a broadcast wave using an antenna (not shown) and demodulates the input data, and an input unit 22 that receives input data from an external device (not shown) such as a recording device or a tuner device. ing. The input data includes video data, audio data, electronic program guide (EPG) data, and the like, and the video display device of the present invention displays a video based on the video data. In addition, the video display device may be configured to handle digital data as input data, or may be configured to handle analog data.

  The tuner 21 and the input unit 22 are connected to a selector 23 that selects which of the tuner 21 and the input unit 22 inputs input data. The selector 23 selects either the tuner 21 or the input unit 22 as necessary as an input source of the input data, and the input data is input from the selected tuner 21 or the input unit 22. A separator 24 is connected to the selector 23, and the separator 24 separates the input data input to the selector 23 into a plurality of types of data such as video data, audio data, and EPG data. The separation unit 24 is connected to the decoder 25, and the decoder 25 decodes various data separated from the input data by the separation unit 24. The decoder 25 is connected to the audio processing unit 26, and the decoder 25 inputs the decoded audio data to the audio processing unit 26. The sound processing unit 26 performs processing for outputting sound through a speaker (not shown) according to the sound data.

  The decoder 25 is connected to the video processing unit 15 and inputs the decoded video data to the video processing unit 15. The video processing unit 15 executes processing for generating a video signal for displaying one frame of video from the video data input from the decoder 25. For example, the video processing unit 15 performs video processing such as matrix processing for calculating tone values of each color, enhancement processing for enhancing video, color adjustment, tint adjustment, and white balance adjustment as necessary, thereby Generate a signal. The liquid crystal panel 11 is connected to the video processing unit 15, and the video processing unit 15 inputs a video signal to the liquid crystal panel 11. The liquid crystal panel 11 is a video display panel according to the present invention, and displays a video based on a video signal.

  Further, a frequency distribution processing unit 14 is connected to the video processing unit 15, and a backlight control unit 13 is connected to the frequency distribution processing unit 14. The frequency distribution processing unit 14 and the backlight control unit 13 are the luminance control device 1 according to the present invention. The video processing unit 15 inputs the video signal to the frequency distribution processing unit 14. The frequency distribution processing unit 14 acquires a frequency distribution of gradation values of pixels included in the video based on the video signal, and performs a process of obtaining a dark pixel ratio described later from the acquired frequency distribution. A backlight 12 is connected to the backlight control unit 13. The frequency distribution processing unit 14 inputs the video signal and the dark pixel ratio to the backlight control unit 13, and the backlight control unit 13 performs a process of controlling the operation of the backlight 12 based on the video signal and the dark pixel ratio. .

  FIG. 2 is a schematic diagram showing the configuration of the backlight 12. The backlight 12 uses LEDs as a light source and is configured by arranging a plurality of LEDs in a two-dimensional shape, and has a planar shape as a whole. In FIG. 2, LEDs are indicated by circles. When a plurality of LEDs arranged in a two-dimensional manner emit light, the backlight 12 emits light in a planar shape. The backlight 12 is divided into a plurality of areas that respectively illuminate different portions on the liquid crystal panel 11. In FIG. 2, the boundary of the area in the backlight 12 is shown with the broken line. As shown in FIG. 2, each area includes a plurality of LEDs. The backlight control unit 13 determines the luminance of each area of the backlight 12 based on the gradation value of each pixel indicated by the video signal, and the level of the video displayed on each part in the liquid crystal panel 11 illuminated by each area. The brightness is individually controlled according to the tone value. That is, the backlight 12 is configured by arranging a plurality of areas in which the brightness can be individually controlled in a plane. In addition, the video processing unit 15, the frequency distribution processing unit 14, and the backlight control unit 13 are incorporated in a single video processor.

  Next, details of processing executed by the video display device of the present invention will be described. FIG. 3 is a flowchart showing a procedure of processing performed by the video display apparatus according to the first embodiment. The input data is input, the video processing unit 15 generates a video signal and inputs the video signal to the frequency distribution processing unit 14, and the frequency distribution processing unit 14 generates a video for one frame based on the video signal from the video signal. The frequency distribution of the gradation values at each pixel included in is acquired (S11). The video signal includes information representing the gradation value at each pixel included in the video as a numerical value, and the video for one frame based on the video signal corresponds to the video for one screen. In step S11, the frequency distribution processing unit 14 acquires the frequency distribution by counting the pixels whose gradation values are the respective values. For example, when an image for one frame is composed of 1000 × 2000 pixels, and the gradation value of each pixel is represented by any numerical value from 0 to 255, the frequency distribution is within 1000 × 2000 pixels. This represents how many pixels each have a gradation value of 0 to 255.

  FIG. 4 is a characteristic diagram showing an example of a histogram showing the frequency distribution as a diagram. The horizontal axis in the figure indicates the gradation value at the pixel included in the video, and is a numerical value in the range of 0 to 255 or the like. The vertical axis in the figure indicates the frequency, and corresponds to the number of pixels in which the gradation value is the respective value in the video. The frequency distribution processing unit 14 may acquire the frequency distribution from the entire video for one frame, or samples pixels from the video for one frame according to a predetermined rule, and the frequency distribution of gradation values at the sampled pixels You may earn. The frequency distribution processing unit 14 may acquire the frequency distribution directly from the video signal for one frame, acquire the frequency distribution from the video signal for one field, and add the frequency distribution for two fields. In this way, the luminance frequency distribution in one frame of video may be acquired.

  Next, the frequency distribution processing unit 14 calculates the number of dark pixels, which is the number of pixels whose gradation value is equal to or less than a predetermined dark reference value, from the acquired frequency distribution (S12). The dark reference value is a predetermined reference value for determining a dark pixel that is a pixel that has a relatively small gradation value and appears dark in an image, and is stored in the frequency distribution processing unit 14 in advance. The dark reference value corresponds to the first reference value in the present invention. As the dark reference value, a value that is larger than the minimum value of the gradation value but is visible on the dark screen is used. FIG. 4 shows the dark reference value. A pixel whose gradation value is equal to or less than the dark reference value is a dark pixel, and an area of a portion where the gradation value is equal to or less than the dark reference value in the histogram illustrated in FIG. 4 is the number of dark pixels. In step S12, the frequency distribution processing unit 14 integrates a portion where the gradation value is equal to or less than the dark reference value in the histogram, or counts the number of pixels whose gradation value is equal to or less than the dark reference value. Calculate the number of dark pixels.

  Next, the frequency distribution processing unit 14 calculates a dark pixel ratio indicating the ratio of dark pixels included in the image by dividing the calculated number of dark pixels by the number of pixels of the entire image (S13). The number of pixels of the entire image may be calculated each time by counting all the pixels included in the image for one frame or integrating the histogram, and is stored in advance in the frequency distribution processing unit 14. A predetermined value may be used. The dark pixel ratio corresponds to the dark pixel amount in the present invention. The frequency distribution processing unit 14 inputs the video signal input from the video processing unit 15 and the calculated dark pixel ratio to the backlight control unit 13.

  The backlight control unit 13 performs a process of calculating lower limit values of luminance in a plurality of areas included in the backlight 12 according to the input dark pixel ratio (S14). The lower limit value of the brightness calculated in step S12 is a value equal to or greater than the minimum brightness value that can be realized by the backlight 12, and the lower limit of the brightness in each area of the backlight 12 that is actually controlled by the backlight control unit 13 is defined. The value to be The backlight control unit 13 stores the relationship between the dark pixel ratio and the lower limit value of the brightness in the form of a function or a numerical table. In step S14, the lower limit value of the brightness according to the dark pixel ratio is determined according to the stored contents. Calculate

  FIG. 5 is a characteristic diagram showing the relationship between the dark pixel ratio and the lower limit value of luminance. The horizontal axis in the figure indicates the dark pixel ratio, the minimum value is 0, and the maximum value is 1. The vertical axis in the figure indicates the lower limit value of luminance corresponding to each dark pixel ratio. In a state where the dark pixel ratio is 1, which is the maximum value, the lower limit value of the luminance is the minimum value that can be realized by the backlight 12. For example, this value is a value of brightness 0 when all the LEDs included in the area of the backlight 12 are turned off. Until the dark pixel ratio decreases from 1 to a predetermined threshold value b, the lower limit value of the luminance is the minimum value. Until the dark pixel ratio decreases from the threshold value b to the predetermined threshold value a, the lower limit value of the luminance changes linearly, and the lower limit value of the luminance increases as the dark pixel ratio decreases. In a state where the dark pixel ratio is the threshold value a, the lower limit value of the luminance is a predetermined maximum value, and the lower limit value of the luminance is the maximum value until the dark pixel ratio decreases from the threshold value a to zero. Overall, the relationship between the dark pixel ratio and the lower limit of luminance is such that the lower limit of luminance decreases monotonously according to the magnitude of the dark pixel ratio, and the lower limit of luminance increases as the dark pixel ratio decreases. It has become a relationship. As the maximum value of the lower limit of luminance, for example, an amount by which the brightness of the liquid crystal panel 11 varies when the viewing angle is changed may be used.

  Note that the relationship between the dark pixel ratio and the lower limit of luminance shown in FIG. 5 is an example. If the lower limit of the luminance monotonously decreases with respect to the dark pixel ratio, the relationship other than the relationship shown in FIG. Relationship may be. For example, the relationship between the dark pixel ratio and the lower limit of luminance may be a smooth function. Further, for example, the lower limit value of the luminance may take a value that jumps off at a predetermined dark pixel ratio value.

  Next, the backlight control unit 13 adjusts the luminance in each area of the backlight 12 according to the gradation value at each pixel in the video indicated by the input video signal, and the luminance becomes less than the lower limit value. A process of adjusting the luminance in the area to the lower limit value is performed (S15). The backlight control unit 13 stores the relationship between the gradation value in the pixel included in each part of the video and the luminance in the area that illuminates the part displaying the part in the liquid crystal panel 11. For example, the luminance in the area is determined with respect to the maximum value of the pixels included in a part of the image illuminated in one area. In step S15, the backlight control unit 13 obtains the luminance in each area according to the gradation value at each pixel in the video indicated by the video signal according to the stored content, and the obtained luminance value is equal to or greater than the lower limit value. For a certain area, the luminance in the area is adjusted to the calculated luminance, and for the area where the calculated luminance value is less than the lower limit value, the luminance in the area is adjusted to the lower limit value.

  Each area of the backlight 12 emits light with brightness controlled by the backlight control unit 13, and the liquid crystal panel 11 adjusts the amount of light transmission in the part corresponding to each pixel in the video according to the video signal. The video display device displays video. The video display device ends the process of displaying one frame of video. The video display device executes the processes of steps S11 to S15 every time one frame of video is displayed.

  FIG. 6 is a schematic diagram showing a part of the screen and the brightness distribution of the screen of the video display device of the present invention. The upper part of FIG. 6 shows a part of the screen on which the video is displayed, and the boundary of the area of the backlight 12 that illuminates each part on the liquid crystal panel 11 that displays the video is indicated by a broken line. 121, 122, and 123 in the figure indicate specific areas. In the drawing, a bright portion which is a relatively bright portion in the video is shown, and a portion other than the bright portion is a dark portion darker than the bright portion. The lower part of FIG. 6 shows the brightness distribution at each position on the screen. Since the bright part of the image is displayed at the position illuminated by the area 121, the brightness is high. Further, at the position where the area 122 illuminates, a part is a bright part of the video, so that the luminance of the area 122 is larger than the minimum value that can be realized by the backlight 12. Since the dark portion of the image is displayed at the position illuminated by the area 123, the luminance of the area 123 has conventionally been the minimum value, which is lower than the lower limit value calculated by the present invention. The dark part in the image illuminated in the conventional area 123 has a brightness corresponding to the minimum value of the luminance of the backlight 12. In the present invention, the luminance of the area 123 is adjusted from the conventional luminance to the lower limit value. FIG. 6 shows an example in which the lower limit value is the same value as the brightness of the area 122. For this reason, the brightness of the dark part of the image illuminated in the area 123 is larger than the brightness of the conventional dark part, and the brightness corresponds to the lower limit value that is the brightness of the area 123. In the example of FIG. 6, since the luminance is the same in the area 123 that illuminates the dark part in the video and the area 122 that illuminates the part where the bright part and the dark part are mixed, the brightness of the dark part included in the video The brightness is the same, and no brightness blur occurs.

  As described above in detail, in the video display device of the present invention, a lower limit value that defines the lower limit of the luminance in each area of the backlight 12 is determined, and conventionally the luminance in an area where the luminance is less than the lower limit value is set as the lower limit value. adjust. Thereby, compared with a prior art, the brightness | luminance difference is reduced by the area which illuminates both the bright part and dark part of an image | video, and the area which illuminates only a dark part. Therefore, it is possible to suppress the occurrence of a luminance blur in which the dark portion near the bright portion becomes brighter than the other dark portions due to the difference in luminance between the areas that illuminate the dark portion in the video, and the video display device displays. The video quality is improved.

  In the present invention, the video display apparatus sets the lower limit value so that the lower limit value of the luminance in the area becomes larger as the dark pixel ratio indicating the ratio of dark pixels included in the video is smaller. Brightness blur is less likely to occur when the video is dark, but brightness blur is likely to occur with halftone images, so the lower the dark pixel ratio, the larger the lower brightness limit, so the brighter the video. The occurrence of luminance blur can be suppressed, and the occurrence of luminance blur can be effectively suppressed. Further, since dark blur hardly occurs in a dark image with a large dark pixel ratio, power consumption of the video display device can be reduced within a range in which the quality of the video is not deteriorated by reducing the lower limit of the brightness.

(Embodiment 2)
In the first embodiment, although the lower limit value of the luminance is obtained according to the dark pixel ratio of the video, when the lower limit value of the luminance is set to be large for an image with low brightness of the entire video, Black floating occurs and the quality of the video is reduced. In the second embodiment, a mode in which the lower limit value of the luminance of the backlight 12 is obtained according to the brightness of the entire video will be described. The internal configuration of the video display apparatus according to the second embodiment is the same as that of the first embodiment, and the description thereof is omitted.

  FIG. 7 is a flowchart showing a procedure of processing performed by the video display apparatus according to the second embodiment. The video processing unit 15 generates a video signal and inputs the video signal to the frequency distribution processing unit 14. The frequency distribution processing unit 14 uses each pixel included in one frame of video based on the video signal from the video signal. The frequency distribution of the gradation values is acquired (S21). Next, the frequency distribution processing unit 14 calculates an average value of gradation values in the video for one frame represented by the video signal from the acquired frequency distribution (S22). For example, the frequency distribution processing unit 14 integrates a value obtained by multiplying the gradation value by the number of pixels over all the gradation values based on the frequency distribution, and divides the integrated value by the total number of pixels, thereby obtaining an image. The average value of the gradation values of the pixels at is calculated. Next, the frequency distribution processing unit 14 calculates the number of dark pixels included in the video from the acquired frequency distribution (S23), and calculates a dark pixel ratio indicating the ratio of dark pixels included in the video (S24). After step S24 is completed, the frequency distribution processing unit 14 inputs the video signal, the average value of the calculated gradation values, and the calculated dark pixel ratio to the backlight control unit 13.

  The backlight control unit 13 performs a process of calculating lower limit values of luminance in a plurality of areas included in the backlight 12 according to the input dark pixel ratio (S25). Next, the backlight control unit 13 calculates a correction coefficient for correcting the lower limit value according to the average value of the gradation values input from the frequency distribution processing unit 14 (S26). The backlight control unit 13 stores the relationship between the average value of the gradation values and the correction coefficient in the form of a function or a numerical table, and in step S26, the backlight control unit 13 corresponds to the average value of the gradation values according to the stored contents. Calculate the correction factor.

  FIG. 8 is a characteristic diagram showing the relationship between the average value of the gradation value of the video and the correction coefficient. In the figure, the horizontal axis indicates the average value of the gradation values, and the vertical axis indicates the correction coefficient corresponding to the average value of the gradation values. In a state where the video is bright overall and the average value of the gradation values is large, the correction coefficient is 1 which is the maximum value. The correction coefficient is a maximum value of 1 until the average value of the gradation values decreases to a predetermined threshold value d. The correction coefficient changes linearly until the average value of the gradation values decreases from the threshold value d to the predetermined threshold value c, and the correction coefficient decreases as the average value of the gradation values decreases. When the average value of the gradation values is the threshold value c, the correction coefficient is a predetermined minimum value, and when the average value of the gradation values is less than or equal to the threshold value c, the correction coefficient is the minimum value. Overall, the relationship between the average value of the gradation value and the correction coefficient is such that the correction coefficient increases monotonically according to the average value of the gradation value, and the smaller the average value of the gradation value, the more the correction is made. The coefficient is also reduced.

  Note that the relationship between the average value of the gradation value and the correction coefficient shown in FIG. 8 is an example. If the correction coefficient monotonically increases with respect to the average value of the gradation value, the relationship shown in FIG. Other relationships may be used. For example, the relationship between the average value of the gradation values and the correction coefficient may be a smooth function. Further, for example, the correction coefficient may take a jump value with an average value of predetermined gradation values as a boundary.

Next, the backlight control unit 13 performs a process of correcting the lower limit value of the luminance calculated in step S25 using the calculated correction coefficient (S27). Specifically, the backlight control unit 13 calculates the corrected lower limit value by multiplying the lower limit value before correction by the correction coefficient as in the following equation.
(Lower limit value after correction) = (Lower limit value before correction) x Correction coefficient

  Next, the backlight control unit 13 adjusts the luminance in each area of the backlight 12 according to the gradation value at each pixel in the video indicated by the input video signal, and the luminance becomes less than the lower limit value. A process of adjusting the luminance in the area to the lower limit value is performed (S28). Each area of the backlight 12 emits light with brightness controlled by the backlight control unit 13, and the liquid crystal panel 11 adjusts the amount of light transmission in the part corresponding to each pixel in the video according to the video signal. The video display device displays video. The video display device ends the process of displaying one frame of video. The video display device executes the processes of steps S21 to S28 every time one frame of video is displayed.

  As described above in detail, in this embodiment, the lower limit of luminance in a plurality of areas of the backlight 12 is obtained by multiplying the lower limit of luminance by a correction coefficient that decreases as the average value of the gradation values of the pixels decreases. Correct the value. The smaller the average value of the gradation value, the smaller the correction coefficient. The brightness of becomes smaller. As a result, the brightness of the dark part included in the video is reduced, and the occurrence of black float can be suppressed. In addition, in the case of a video with a large average gradation value and a bright overall image, the lower limit value of the luminance in the area is larger, so that occurrence of luminance blur is effectively suppressed. In the case of a bright image as a whole, even if black floating occurs, it is not noticeable. Therefore, in the present embodiment, the occurrence of luminance blur is suppressed in a bright image where black floating is not conspicuous, while the occurrence of black floating is suppressed in a generally dark image, and the video displayed by the video display device Improve the quality.

(Embodiment 3)
In Embodiment 1, the lower limit value of the luminance in the plurality of areas of the backlight 12 is set smaller as the dark pixel ratio of the video is larger. By the way, an image includes an image having a small number of intermediate gradation values and many dark pixels and bright pixels. In the first embodiment, luminance blur is likely to occur when the dark pixel ratio of the video is large, and luminance blur is more noticeable when there are many bright pixels in the video, and the quality of the video is reduced. In the third embodiment, a mode in which the lower limit value of the luminance of the backlight 12 is obtained according to both the amount of dark pixels and bright pixels is shown. The internal configuration of the video display apparatus according to Embodiment 3 is the same as that of Embodiment 1, and the description thereof is omitted.

  FIG. 9 is a flowchart illustrating a procedure of processing performed by the video display apparatus according to the third embodiment. The video processing unit 15 generates a video signal and inputs the video signal to the frequency distribution processing unit 14. The frequency distribution processing unit 14 uses each pixel included in one frame of video based on the video signal from the video signal. The frequency distribution of the gradation values is acquired (S31). Next, the frequency distribution processing unit 14 calculates the number of dark pixels included in the video from the acquired frequency distribution (S32), and calculates a dark pixel ratio indicating the ratio of dark pixels included in the video (S33).

  Next, the frequency distribution processing unit 14 calculates the number of bright pixels, which is the number of pixels whose gradation value is equal to or greater than a predetermined bright reference value, from the acquired frequency distribution (S34). The bright reference value is a predetermined reference value for determining a bright pixel which is a pixel having a relatively large gradation value and appears bright in the video, and is stored in the frequency distribution processing unit 14 in advance. The bright reference value is larger than the dark reference value and corresponds to the second reference value in the present invention. As the bright reference value, a value that is larger than the dark reference value and smaller than the upper limit of the gradation value but is visible on a bright screen is used. FIG. 10 is a characteristic diagram showing a histogram to which a bright reference value is added. A pixel whose gradation value is equal to or higher than the bright reference value is a bright pixel, and an area of a portion where the gradation value is equal to or higher than the bright reference value in the histogram shown in FIG. 10 is the number of bright pixels. In step S34, the frequency distribution processing unit 14 integrates a portion where the gradation value is equal to or higher than the bright reference value in the histogram, or counts the number of pixels whose gradation value is equal to or higher than the bright reference value. Calculate the number of bright pixels.

  Next, the frequency distribution processing unit 14 calculates the bright pixel ratio indicating the ratio of the bright pixels included in the video by dividing the calculated number of bright pixels by the number of pixels of the entire video (S35). The bright pixel ratio corresponds to the bright pixel amount in the present invention. After step S35 is completed, the frequency distribution processing unit 14 inputs the video signal and the calculated dark pixel ratio and bright pixel ratio to the backlight control unit 13.

  The backlight control unit 13 performs a process of calculating lower limit values of luminance in a plurality of areas included in the backlight 12 according to the input dark pixel ratio (S36). Next, the backlight control unit 13 calculates a correction coefficient for correcting the lower limit value in accordance with the bright pixel ratio input from the frequency distribution processing unit 14 (S37). The backlight control unit 13 stores the relationship between the bright pixel ratio and the correction coefficient in the form of a function, a numerical value table, or the like. In step S37, the correction coefficient corresponding to the bright pixel ratio is calculated according to the stored content.

  FIG. 11 is a characteristic diagram showing the relationship between the bright pixel ratio and the correction coefficient. In the figure, the horizontal axis indicates the bright pixel ratio, and the vertical axis indicates the correction coefficient corresponding to the bright pixel ratio. When the bright pixel ratio is small, the correction coefficient has a minimum value. The correction coefficient is a minimum value until the bright pixel ratio increases to a predetermined threshold value e. Until the bright pixel ratio increases from the threshold e to the predetermined threshold f, the correction coefficient changes linearly, and the correction coefficient increases as the bright pixel ratio increases. In the state where the bright pixel ratio is the threshold value f, the correction coefficient is a maximum value of 1. In the state until the bright pixel ratio increases from the threshold value f to the maximum value of 1, the correction coefficient is the maximum value of 1. . Overall, the relationship between the bright pixel ratio and the correction coefficient is such that the correction coefficient increases monotonously according to the magnitude of the bright pixel ratio, and the correction coefficient increases as the bright pixel ratio increases. .

  The relationship between the bright pixel ratio and the correction coefficient shown in FIG. 11 is an example. If the correction coefficient monotonically increases with respect to the bright pixel ratio, the relationship is other than the relationship shown in FIG. Also good. For example, the relationship between the bright pixel ratio and the correction coefficient may be a smooth function. Further, for example, the correction coefficient may take a jump value with a predetermined bright pixel ratio as a boundary.

Next, the backlight control unit 13 performs a process of correcting the lower limit value of the luminance calculated in step S36 using the calculated correction coefficient (S38). Specifically, the backlight control unit 13 calculates the corrected lower limit value by multiplying the lower limit value before correction by the correction coefficient as in the following equation.
(Lower limit value after correction) = (Lower limit value before correction) x Correction coefficient

  Next, the backlight control unit 13 adjusts the luminance in each area of the backlight 12 according to the gradation value at each pixel in the video indicated by the input video signal, and the luminance becomes less than the lower limit value. A process of adjusting the luminance in the area to the lower limit value is performed (S39). Each area of the backlight 12 emits light with brightness controlled by the backlight control unit 13, and the liquid crystal panel 11 adjusts the amount of light transmission in the part corresponding to each pixel in the video according to the video signal. The video display device displays video. The video display device ends the process of displaying one frame of video. The video display device executes the processes of steps S31 to S39 every time one frame of video is displayed.

  As described above in detail, in the present embodiment, the lower limit value of the luminance in the plurality of areas of the backlight 12 is corrected by multiplying the lower limit value of the luminance by a correction coefficient that increases as the bright pixel ratio increases. Since the correction coefficient increases as the bright pixel ratio increases, when an image with a large bright pixel ratio and a large number of bright pixels is displayed, the minimum luminance of the area of the backlight 12 is increased, and the occurrence of luminance blur is effectively generated. It is suppressed. Since an image with a large bright pixel ratio and a large number of bright pixels tends to have a noticeable luminance blur, the quality of the image displayed by the image display device is improved by effectively suppressing the luminance blur for an image with a large number of bright pixels.

(Embodiment 4)
In the fourth embodiment, the lower limit value of the luminance of the backlight 12 determined according to the dark pixel ratio is corrected according to the brightness of the entire video and the bright pixel ratio. The internal configuration of the video display apparatus according to Embodiment 4 is the same as that of Embodiment 1, and the description thereof is omitted.

  FIG. 12 is a flowchart illustrating a procedure of processing performed by the video display apparatus according to the fourth embodiment. The video processing unit 15 generates a video signal and inputs the video signal to the frequency distribution processing unit 14. The frequency distribution processing unit 14 uses each pixel included in one frame of video based on the video signal from the video signal. The frequency distribution of the gradation values is acquired (S401). Next, the frequency distribution processing unit 14 calculates an average value of gradation values in one frame of video represented by the video signal from the acquired frequency distribution (S402). Next, the frequency distribution processing unit 14 calculates the number of dark pixels included in the video from the acquired frequency distribution (S403), and calculates a dark pixel ratio indicating the ratio of dark pixels included in the video (S404). Next, the frequency distribution processing unit 14 calculates the number of bright pixels included in the video from the acquired frequency distribution (S405), and calculates a bright pixel ratio indicating the ratio of bright pixels included in the video (S406). After step S406 is completed, the frequency distribution processing unit 14 inputs the video signal, the average value of the calculated gradation values, the dark pixel ratio, and the bright pixel ratio to the backlight control unit 13.

  The backlight control unit 13 performs a process of calculating lower limit values of luminance in a plurality of areas included in the backlight 12 according to the input dark pixel ratio (S407). Next, the backlight control unit 13 uses a method similar to that of the second embodiment to change the first correction coefficient for correcting the lower limit value according to the average value of the gradation values input from the frequency distribution processing unit 14. Is calculated (S408). Next, the backlight control unit 13 calculates a second correction coefficient for correcting the lower limit value according to the bright pixel ratio input from the frequency distribution processing unit 14 by the same method as in the third embodiment. (S409).

Next, the backlight control unit 13 performs a process of correcting the lower limit value calculated in step S407 using the calculated first correction coefficient and second correction coefficient (S410). Specifically, the backlight control unit 13 calculates the corrected lower limit value by multiplying the lower limit value before correction by the first correction coefficient and the second correction coefficient as in the following equation.
(Lower limit value after correction) = (Lower limit value before correction) × (first correction coefficient) × (second correction coefficient)

  Next, the backlight control unit 13 adjusts the luminance in each area of the backlight 12 according to the gradation value at each pixel in the video indicated by the input video signal, and the luminance becomes less than the lower limit value. A process of adjusting the luminance in the area to the lower limit value is performed (S411). Each area of the backlight 12 emits light with brightness controlled by the backlight control unit 13, and the liquid crystal panel 11 adjusts the amount of light transmission in the part corresponding to each pixel in the video according to the video signal. The video display device displays video. The video display device ends the process of displaying one frame of video. The video display device executes the processes of steps S401 to S411 each time one frame of video is displayed.

  As described above in detail, in the present embodiment, the lower limit value of luminance is multiplied by the first correction coefficient that decreases as the average value of gradation values decreases and the second correction coefficient that increases as the bright pixel ratio increases. The lower limit value of the luminance in the plurality of areas of the backlight 12 is corrected. FIG. 13 is a characteristic diagram showing a histogram of an image with a small average value of gradation values but many bright pixels. By correcting the lower limit value according to the average value of the gradation value and the bright pixel ratio of the image, the occurrence of black floating is suppressed in the entire dark image, and the average value of the gradation value as shown in FIG. For an image with a small number of bright pixels and luminance blur that is noticeable, the minimum luminance of the backlight 12 can be increased to effectively suppress the occurrence of luminance blur. Therefore, the quality of the video displayed by the video display device is improved.

(Embodiment 5)
When the minimum luminance of the backlight 12 is increased in order to suppress the occurrence of luminance blur, the contrast ratio of the video is lowered. On the other hand, the appearance of the video displayed by the video display device is affected by ambient light, and when the viewer views the video in a somewhat bright environment, the viewer hardly perceives a change in the brightness of black and the contrast ratio decreases. Even if it is difficult to detect. Therefore, in a somewhat bright environment, even if the minimum luminance of the backlight 12 is increased, a decrease in contrast ratio is hardly perceived. In the fifth embodiment, a lower limit value of luminance determined according to the dark pixel ratio is corrected according to ambient light.

  FIG. 14 is a block diagram showing the internal configuration of the video display apparatus of the present invention according to Embodiment 5. In FIG. The video display device includes an illuminance sensor 16 that measures the brightness around the video display device, and the illuminance sensor 16 is connected to the backlight control unit 13. The illuminance sensor 16 is disposed in the vicinity of the liquid crystal panel 11, measures the illuminance due to external light, and inputs the measurement result to the backlight control unit 13. The other internal configuration of the video display apparatus according to Embodiment 5 is the same as that of Embodiment 1, and the corresponding parts are denoted by the same reference numerals and description thereof is omitted.

  FIG. 15 is a flowchart illustrating a procedure of processing performed by the video display apparatus according to the fifth embodiment. The video processing unit 15 generates a video signal and inputs the video signal to the frequency distribution processing unit 14. The frequency distribution processing unit 14 uses each pixel included in one frame of video based on the video signal from the video signal. The frequency distribution of the gradation values is acquired (S51). Next, the frequency distribution processing unit 14 calculates the number of dark pixels included in the video from the acquired frequency distribution (S52), and calculates a dark pixel ratio indicating the ratio of dark pixels included in the video (S53). After step S53 ends, the frequency distribution processing unit 14 inputs the video signal and the calculated dark pixel ratio to the backlight control unit 13. The illuminance sensor 16 inputs the measured illuminance to the backlight control unit 13.

  The backlight control unit 13 performs a process of calculating lower limit values of luminance in a plurality of areas included in the backlight 12 according to the dark pixel ratio input from the frequency distribution processing unit 14 (S54). Next, the backlight control unit 13 calculates a correction coefficient for correcting the lower limit value according to the illuminance input from the illuminance sensor 16 (S55). The backlight control unit 13 stores the relationship between the illuminance and the correction coefficient in the form of a function, a numerical table, or the like. In step S55, the correction coefficient corresponding to the illuminance is calculated according to the stored content.

  FIG. 16 is a characteristic diagram showing the relationship between the illuminance and the correction coefficient. In the figure, the horizontal axis represents illuminance, and the vertical axis represents a correction coefficient corresponding to the illuminance. When the illuminance is small, the correction coefficient is 1 which is the minimum value. The correction coefficient remains at the minimum value of 1 until the illuminance increases to the predetermined threshold value g. Until the illuminance increases from the threshold value g to the predetermined threshold value h, the correction coefficient changes linearly, and the correction coefficient increases as the illuminance increases. When the illuminance is the threshold value h, the correction coefficient is the maximum value, and when the illuminance is the threshold value h or more, the correction coefficient is a constant maximum value. As a whole, the relationship between the illuminance and the correction coefficient is such that the correction coefficient increases monotonously according to the magnitude of the illuminance, and the correction coefficient decreases as the bright pixel ratio decreases.

  The relationship between the illuminance and the correction coefficient illustrated in FIG. 16 is an example, and may be a relationship other than the relationship illustrated in FIG. 16 as long as the correction coefficient monotonously increases with respect to the illuminance. For example, the relationship between the illuminance and the correction coefficient may be a smooth function. Further, for example, the correction coefficient may take a jump value with a predetermined illuminance as a boundary.

Next, the backlight control unit 13 performs a process of correcting the lower limit value calculated in step S54 using the calculated correction coefficient (S56). Specifically, the backlight control unit 13 calculates the lower limit value after correction by multiplying the lower limit value before correction by the correction coefficient calculated in step S55 as in the following equation.
(Lower limit value after correction) = (Lower limit value before correction) x Correction coefficient

  Next, the backlight control unit 13 adjusts the luminance in each area of the backlight 12 according to the gradation value at each pixel in the video indicated by the input video signal, and the luminance becomes less than the lower limit value. A process of adjusting the luminance in the area to the lower limit value is performed (S57). Each area of the backlight 12 emits light with brightness controlled by the backlight control unit 13, and the liquid crystal panel 11 adjusts the amount of light transmission in the part corresponding to each pixel in the video according to the video signal. The video display device displays video. The video display device ends the process of displaying one frame of video. The video display device executes the processes of steps S51 to S57 each time a video for one frame is displayed.

  As described above in detail, in the present embodiment, the lower limit value of the luminance in the plurality of areas of the backlight 12 is obtained by multiplying the lower limit value of the luminance by a correction coefficient that increases as the illuminance indicating the ambient brightness increases. To correct. Since the correction factor increases as the illuminance increases, when the image is displayed in an environment where the outside light is bright, the lower limit value increases, the minimum luminance of the backlight 12 increases, and the occurrence of luminance blur is suppressed. The In the state where the minimum brightness of the backlight 12 is increased, the contrast ratio of the image is decreased, but the decrease in the contrast ratio is difficult to be recognized in a bright environment. While preventing it from standing up, the occurrence of luminance blur can be effectively suppressed. Therefore, the quality of the video displayed by the video display device is improved.

(Embodiment 6)
In the sixth embodiment, a mode in which the range in which the minimum luminance is increased in a plurality of areas of the backlight 12 is limited according to the video is shown. The internal configuration of the video display apparatus according to the sixth embodiment is the same as that of the first embodiment, and a description thereof is omitted.

  FIG. 17 is a flowchart illustrating a procedure of processing performed by the video display device according to the sixth embodiment. The video processing unit 15 generates a video signal and inputs the video signal to the frequency distribution processing unit 14. The frequency distribution processing unit 14 uses each pixel included in one frame of video based on the video signal from the video signal. The frequency distribution of the gradation values is acquired (S61). Next, the frequency distribution processing unit 14 calculates the number of dark pixels included in the video from the acquired frequency distribution (S62), and calculates a dark pixel ratio indicating the ratio of dark pixels included in the video (S63). After step S63 ends, the frequency distribution processing unit 14 inputs the video signal and the calculated dark pixel ratio to the backlight control unit 13.

  The backlight control unit 13 performs a process of calculating lower limit values of luminance in a plurality of areas included in the backlight 12 according to the dark pixel ratio input from the frequency distribution processing unit 14 (S64). Next, the backlight control unit 13 obtains the luminance in each area according to the gradation value at each pixel in the video indicated by the video signal, and obtained from the plurality of areas included in the backlight 12. An area whose luminance value is equal to or greater than the lower limit is specified (S65). Next, the backlight control unit 13 performs a process of limiting the range of the area where the luminance less than the lower limit value should be adjusted to the lower limit value (S66). Specifically, the backlight control unit 13 sets the range of the area adjacent to the area specified in step S65 to a limited range that is the range of the area where the luminance less than the lower limit value should be adjusted to the lower limit value. The area included in the limited range is not limited to the area adjacent to the area specified in step S65, and an area within a predetermined distance determined in advance from the area specified in step S65 is included in the limited range. You can do it.

  Next, the backlight control unit 13 adjusts the luminance in each area of the backlight 12 according to the gradation value at each pixel in the video indicated by the input video signal, and the luminance becomes less than the lower limit value. A process of adjusting the luminance in the area within the limited range to the lower limit value is performed (S67). In step S67, the backlight control unit 13 adjusts the luminance in the area to the calculated luminance for the area where the luminance value obtained in step S65 is equal to or higher than the lower limit value. Further, the backlight control unit 13 adjusts the luminance in the area to the lower limit value for the area included in the limited range set in step S66 among the areas where the luminance value obtained in step S65 is less than the lower limit value. For areas not included in the limited range, the brightness in the area is adjusted to the calculated brightness.

  Each area of the backlight 12 emits light with brightness controlled by the backlight control unit 13, and the liquid crystal panel 11 adjusts the amount of light transmission in the part corresponding to each pixel in the video according to the video signal. The video display device displays video. The video display device ends the process of displaying one frame of video. The video display device executes the processes of steps S61 to S67 each time one frame of video is displayed.

  FIG. 18 is a schematic diagram illustrating a screen example of the video display device according to the sixth embodiment. In the figure, the boundary of the area of the backlight 12 is indicated by a broken line. Among the areas of the backlight 12, an area adjacent to an area that illuminates a bright portion in the video is included in the limited range. Within the limited range, the brightness in the area where the luminance corresponding to the video is less than the lower limit is adjusted to the lower limit, and outside the limited range, the luminance in each area even if the luminance corresponding to the video is less than the lower limit Remains below the lower limit. Since the luminance of each area of the backlight 12 within the limited range is equal to or higher than the lower limit value, the difference in luminance between the area that illuminates both the bright and dark parts of the video and the area adjacent to the area is reduced. In addition, the occurrence of brightness bleeding is suppressed. Outside the limited range, the luminance in the area that illuminates the dark part of the video is less than the lower limit value, and an increase in power consumption can be suppressed.

  In the above first to sixth embodiments, the dark pixel ratio and the bright pixel ratio are used as the dark pixel amount and the bright pixel amount in the present invention. However, the present invention is not limited to this. A form using the number of dark pixels as the dark pixel quantity or a form using the bright pixel quantity as the bright pixel quantity may be used. In the first to sixth embodiments, the processing to be performed by the video display device is executed by each hardware. However, the video display device of the present invention is configured to execute part or all of the processing by software. It may be.

  In the first to sixth embodiments, the backlight 12 using a plurality of LEDs as the light source is shown. However, the backlight according to the present invention is not limited to this, and the brightness in each area is individually set. If it is the structure which can be adjusted to, the backlight which used other light emitting elements, such as an EL (electroluminescence) element, as the light source may be sufficient. In the first to sixth embodiments, the liquid crystal panel 11 is used as the video display panel in the present invention. However, the video display panel in the present invention is not limited to this, and the light from the backlight is used. Other video display panels may be used as long as they are transmissive video display panels that use them to display video.

1 is a block diagram showing an internal configuration of a video display device according to the present invention according to Embodiment 1. FIG. It is a schematic diagram which shows the structure of a backlight. 3 is a flowchart illustrating a procedure of processing performed by the video display apparatus according to Embodiment 1. It is a characteristic view which shows the example of the histogram which represented frequency distribution by the figure. It is a characteristic view which shows the relationship between a dark pixel ratio and the minimum value of a brightness | luminance. It is a schematic diagram which shows a part of screen of the video display apparatus of this invention, and distribution of the brightness of a screen. 10 is a flowchart illustrating a procedure of processing performed by the video display device according to the second embodiment. FIG. 5 is a characteristic diagram showing a relationship between an average value of gradation values of video and a correction coefficient. 10 is a flowchart illustrating a procedure of processing performed by the video display device according to Embodiment 3. It is a characteristic view which shows the histogram which added the bright reference value. It is a characteristic view which shows the relationship between a bright pixel ratio and a correction coefficient. 14 is a flowchart illustrating a procedure of processing performed by the video display device according to the fourth embodiment. FIG. 11 is a characteristic diagram showing a histogram of an image with a small average value of gradation values but many bright pixels. It is a block diagram which shows the internal structure of the video display apparatus of this invention which concerns on Embodiment 5. FIG. 14 is a flowchart illustrating a procedure of processing performed by the video display device according to the fifth embodiment. It is a characteristic view which shows the relationship between illumination intensity and a correction coefficient. 18 is a flowchart illustrating a procedure of processing performed by the video display device according to Embodiment 6. FIG. 10 is a schematic diagram illustrating a screen example of a video display device according to a sixth embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Brightness control apparatus 11 Liquid crystal panel 12 Backlight 121,122,123 Area 13 Backlight control part 14 Frequency distribution process part 16 Illuminance sensor

Claims (6)

The brightness of a plurality of areas into which a planar backlight that illuminates a video display panel that displays video based on a video signal is divided, and the brightness of the video displayed in the portion illuminated by each area in the video display panel. In the brightness control device that individually controls the brightness according to the tone value,
Means for acquiring a frequency distribution of gradation values of pixels included in the video based on the video signal;
Means for obtaining a dark pixel amount indicating an amount of pixels whose gradation value is equal to or less than a predetermined first reference value from the frequency distribution acquired by the means;
Means for calculating a lower limit value of luminance in the area to a value that monotonously decreases in accordance with the magnitude of the dark pixel amount;
A backlight luminance control device, characterized in that, in a plurality of areas of the backlight, means for adjusting the luminance in an area where the luminance according to the gradation value of the video is less than the lower limit value to the lower limit value. An image display panel that displays an image based on an image signal, a backlight that emits light in a planar shape and illuminates the image display panel, and brightness in a plurality of areas into which the backlight is divided are displayed in the image display panel. In the video display device comprising: means for individually controlling the brightness according to the gradation value of the video displayed in the portion illuminated by each area
Means for acquiring a frequency distribution of gradation values of pixels included in the video based on the video signal;
Means for obtaining a dark pixel amount indicating an amount of pixels whose gradation value is equal to or less than a predetermined first reference value from the frequency distribution acquired by the means;
Means for calculating a lower limit value of luminance in the area to a value that monotonously decreases in accordance with the magnitude of the dark pixel amount;
A video display apparatus comprising: means for adjusting a luminance in an area where a luminance corresponding to a gradation value of an image is less than the lower limit value among the plurality of areas of the backlight to the lower limit value. Means for calculating an average value of gradation values from the frequency distribution;
3. The means for correcting the lower limit value by further multiplying the calculated lower limit value by a coefficient that monotonously increases according to the magnitude of the average value calculated by the means. Video display device. Means for obtaining from the frequency distribution a bright pixel amount indicating an amount of pixels having a gradation value equal to or greater than a predetermined second reference value greater than the first reference value;
A means for correcting the lower limit value by multiplying the calculated lower limit value by a coefficient that monotonously increases in accordance with the magnitude of the bright pixel amount obtained by the means is further provided. 4. The video display device according to 3. Means for measuring ambient brightness;
The apparatus further comprises means for correcting the lower limit value by multiplying the calculated lower limit value by a coefficient that monotonously increases in accordance with the ambient brightness measured by the means. 5. The video display device according to any one of 4 to 4. A specifying means for specifying an area in which the luminance according to the gradation value of the video is equal to or higher than the lower limit value from the plurality of areas of the backlight;
Means for limiting an area whose luminance is adjusted to the lower limit value among areas where the luminance according to the gradation value of the video is less than the lower limit value to an area within a predetermined distance from the area specified by the specifying means; The video display device according to claim 2, further comprising:

Images