JP2013015630A - Image display device, image display method, and image processing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To arrest a small gradation level difference of an output image by effectively using the gradation of the output image expressed by light control of a backlight.SOLUTION: An image display section 102 includes an image display element and a backlight and adjusts the luminance of the backlight by a light control value. An image processing section 101 includes: a light control value calculating part 13 for calculating the light control value of the backlight by an input image signal; a correction value calculating part 15 for calculating a correction gain for correcting the input image signal according to the light control value; and an image correcting part 19 for correcting the image signal by multiplying the input image signal by the correction gain and supplying the image signal display part 102 therewith. The correction value calculating part 15 sets a target display image from the input image signal and the light control value, smooths the target display image by a smoothing filter, and calculates the correction gain from the smoothed target display image and the light control value.

Description

  The present invention relates to a video display device that adjusts the luminance of a backlight for illumination and displays a video, a video display method, and a video processing device that corrects a video signal.

  The liquid crystal display is used as a monitor for many video devices such as a television and a personal computer. The LCD display has a backlight for illumination, but in order to reduce the power consumption of the backlight light source, a local dimming method that performs local dimming in synchronization with the distribution of shadows in the video has been proposed. Commercialization has also been made. The dimming of the backlight in the local dimming method is performed in units of areas by dividing the screen into a predetermined number of sections (areas).

  For example, Patent Document 1 describes a method of correcting an image signal in consideration of backlight illumination luminance distribution in the local dimming method. As a result, it is stated that an image quality with a high contrast ratio and little unevenness can be obtained.

JP 2005-258403 A

For example, in the case of a display composed of a backlight of a white light source and liquid crystals of RGB three primary colors, the luminance of the video displayed on the screen of the liquid crystal display is determined by the luminance value (unit: cd / m ² ) of the backlight and the video signal. It is determined by a multiplication value of the transmittance of the liquid crystal controlled in accordance with each value of R, G, B (hereinafter referred to as RGB value).

  As a feature of local dimming, the luminance value of the backlight can be dimmed, so that the contrast ratio of the output video is higher than that of a backlight with a fixed luminance value, and the level of the video signal is adjusted at each dimmed luminance value. Since the tone can be set, the gradation of the output video also increases. Since the light control of the backlight is performed in units of areas by dividing the screen, the gradation of the output video can also be set in units of areas.

  However, in the prior art, the correction of the video signal based on the backlight luminance difference between areas is calculated only by multiplying the RGB value of the video signal of one pixel by the backlight luminance value in area units, so that gradation that can be expressed Cannot be used effectively. For example, the correction of the gradation video according to the prior art is multiplied by one pixel unit by interpolation of the backlight luminance difference between the areas, so that a small gradation step is generated in the output image and a gradation spot pattern remains. Even if the number of gradations of the image is increased, the level difference is not reduced only by increasing the gradation width of the gradation level difference.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems, effectively use the gradation of an output video that can be expressed by dimming a backlight, and eliminate a small gradation step generated in the output video.

  The video display device of the present invention includes a video display unit and a video processing unit, and the video display unit adjusts the transmittance for each pixel by a video signal and a back for illuminating the video display device from the back side. A light control unit for adjusting the luminance of the backlight according to a light control value, wherein the video processing unit calculates a light control value for the backlight from an input video signal; A correction amount calculation unit that calculates a correction gain for correcting the input video signal according to the dimming value, a video signal is corrected by multiplying the input video signal by the correction gain, and the video signal is supplied to the video display unit A video correction unit. The correction amount calculation unit sets a target display video from the input video signal and the dimming value, smoothes the target display video by a smoothing filter, and calculates the target display video and the dimming value from the smoothed target display video. Calculate the correction gain.

  According to the present invention, it is possible to eliminate a small gradation step generated in an output video and improve a gradation feeling of the output video.

1 is a configuration diagram (Example 1) showing an embodiment of a video display device according to the present invention. The figure which shows the case where the memory 23 is provided in the structure of FIG. 1A. Schematic which shows the panel structure of the liquid crystal display 22. FIG. The figure which shows the example of the liquid crystal display 22 which performs local dimming. The figure which shows the example of luminance distribution when one LED light source is lighted. The figure which shows the input-output characteristic of the gamma characteristic of a display. The figure which shows the input-output characteristic of a gamma correction. The figure which shows the relationship between the light control gradation of a backlight, and an input video gradation. The figure which shows the input image gradation at the time of inputting a whole surface white image. The figure which shows the luminance distribution of the backlight at the time of inputting a whole surface white image | video. The figure which shows the input image gradation at the time of inputting a gradation image | video. The figure which shows the luminance distribution of the backlight at the time of inputting a gradation image | video. The figure explaining the detection method of the low frequency area | region by frequency conversion. The figure explaining the smoothing process by an average value filter. The figure which shows the input video gradation at the time of gradation video input. The figure which shows the backlight luminance value at the time of gradation video input. The figure which shows the display image gradation at the time of gradation image input (no gain correction). The figure which shows the target display image gradation at the time of gradation image input (without smoothing). The figure which shows the correction | amendment gain at the time of gradation video input (no smoothing). The figure which shows the correction | amendment image gradation at the time of gradation image input (without smoothing). The figure which shows the target display image gradation at the time of gradation image input (with smoothing). The figure which shows the correction gain at the time of gradation video input (with smoothing). The figure which shows the correction | amendment image gradation at the time of gradation image input (with smoothing). The figure which shows input video gradation related to generation | occurrence | production of a gradation step. The figure which shows correction | amendment gain relevant to generation | occurrence | production of a gradation level | step difference (without smoothing). The figure which shows correction | amendment image gradation related to generation | occurrence | production of a gradation level | step difference (without smoothing). The figure which shows the input image gradation regarding reduction of a gradation level | step difference. The figure which shows correction | amendment gain in connection with reduction of a gradation level difference (with smoothing). The figure which shows correction | amendment image gradation related to reduction of a gradation level difference (with smoothing). The figure explaining the effect of a present Example on a video display screen. The block diagram which shows one Example of the video processing apparatus by this invention (Example 2).

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not necessarily limited to these embodiments. Note that, in the drawings for describing the embodiments, the same members are denoted by the same reference numerals, and repeated description thereof is omitted.

  FIG. 1A is a block diagram showing an embodiment of a video display device according to the present invention. The video display device 100 includes a video processing unit 101 and a video display unit 102. The video processing unit 101 includes an input signal processing unit 11, a dimming value calculation unit 13, a correction gain calculation unit 15, a low frequency detection unit 17, The video correction unit 19 is included, and the video display unit 102 includes a timing control unit 21 and a liquid crystal display 22. The liquid crystal display 22 includes a liquid crystal panel that is a video display element and a backlight that performs local dimming. The video processing unit 101 sends a dimming value 14 for controlling the backlight luminance to the video display unit 102 in accordance with the input video signal 10. Further, the correction gain 16 for the input video signal 10 is determined according to the light control value 14, and the corrected video signal 20 is supplied. The liquid crystal display 22 of the video display unit 102 sets the brightness of the backlight by the dimming value 14, drives the liquid crystal panel by the corrected video signal 20, and displays the video.

  FIG. 2 is a schematic diagram showing a panel configuration of the liquid crystal display 22. 1 is a liquid crystal panel, 2 is a diffusion sheet, and 3 is a backlight. Illumination light from the backlight 3 spreads through the diffusion sheet 2 and irradiates the liquid crystal panel 1 from the back, and changes the transmittance of the liquid crystal panel 1 to adjust the transmission amount of the illumination light to display an image. The liquid crystal panel 1 is provided with three primary color filter layers, thereby displaying a color image. The transmittance of the liquid crystal panel 1 is controlled by the corrected video signal 20, and the brightness (luminance) of the backlight 3 is controlled by the dimming value 14.

  FIG. 3A shows an example of the liquid crystal display 22 that performs local dimming. Here, a case where the screen is divided into 4 × 4 areas with a direct backlight structure is shown. Each area 4 is provided with one light emitting diode (LED) 5 as a light source. FIG. 3B shows an example of the luminance distribution when one LED light source 5 is turned on. The luminance gradually decreases at the area boundary, and the illumination light leaks to the adjacent area.

  In each area of the liquid crystal display 22, if the RGB value of the input video signal is X, the transmittance of the liquid crystal panel 1 is α, and the luminance value by the backlight 3 is BL, the luminance of the displayed video (RGB value) f (X) becomes [Formula 1].

  As described above, the liquid crystal display 22 performs gradation expression by the transmittance α of the liquid crystal panel 1, and the luminance value f (X) of the display image is obtained by multiplying the transmittance α by the luminance value BL of the backlight 3. Determined. The relationship between the RGB value X of the input video signal and the transmittance α is corrected so that the luminance characteristic of the display video is the same as that of a display (CRT) using a cathode ray tube.

Hereinafter, the operation of each unit of the video processing apparatus 101 will be described in detail.
The input signal processing unit 11 converts the format of the input video signal 10 into a signal of a format corresponding to the video display unit 102. For example, in the case of a liquid crystal panel of RGB three primary colors, conversion processing to RGB is performed. Since the input video signal 10 is gamma corrected, a process applied to the gamma characteristic (hereinafter referred to as reverse gamma correction) is performed, and the reverse gamma corrected video signal 12 is output.

  4A and 4B are diagrams for explaining gamma characteristics and gamma correction. The adjustment method of a liquid crystal display is demonstrated using this figure. The input / output characteristics of the CRT are such that the relationship between the RGB value of the input video that is the input value and the luminance value of the display video that is the output value is approximated not by a proportional relationship but by a power of the input value. Call. Here, when the input value is X and the index for the input value is γ (gamma), the output value G (X) is expressed as [Equation 2]. In general, γ is often set to 1.8 or 2.2.

  FIG. 4A shows the gamma characteristic when γ is 2.2. The horizontal axis represents the input video gradation, and the vertical axis represents the output display video gradation. In a region where the video gradation of the input value is small, that is, a dark region, the output gradation is crushed. Gamma correction is performed to solve the problem of gradation collapse. In the gamma correction, the input value X is corrected using the inverse function of [Equation 2], and the output value g (X) becomes [Equation 3].

  FIG. 4B shows gamma correction when γ is 2.2. When a gamma-corrected video signal is input to a display having gamma characteristics, the output value is G (g (X)) = X, and the input value and the output value are in a proportional relationship, so that gradation collapse is eliminated.

  As described above, the liquid crystal display 22 is designed to have a gamma characteristic in order to keep the input data consistent with the CRT. The gamma correction is a correction for displaying the input gradation without losing gradation by making the input image gradation value proportional to the luminance value of the display image. In the following explanation, it is considered that gamma correction is always performed for a display having a gamma characteristic, and the input / output characteristic of the liquid crystal display is γ = 1, that is, the value of the input video gradation and the luminance value of the liquid crystal panel. It will be explained that a proportional relationship holds.

  The dimming value calculation unit 13 receives the inverse gamma correction video signal 12 and outputs a dimming value 14 for setting the luminance of each area of the backlight (LED light source). Here, for dimming the backlight, for example, the gradation of the input video signal is divided equally by the number of gradations of the dimming, and the dimming gradation is determined by the maximum value of the input video signal in one gradation area of dimming. Select the value (dimming value).

  FIG. 5 shows the relationship between the light control gradation of the backlight and the input video gradation. The horizontal axis represents the case where the backlight dimming gradation is 16 gradations, and the vertical axis is the case where the luminance value of the display image is 16 gradations. The straight line represented by the upper dotted line in the figure is the luminance (white luminance level) when the maximum value of the input video is given at each dimming gradation of the backlight, and the straight line represented by the lower dotted line is The luminance (black luminance level) when the minimum value of the input video is given is shown. A solid straight line group between dotted lines indicates that 16 gradations are allocated to the video signal in each dimming gradation. The area dimming value may be selected by selecting a dimming value in which the maximum value of the input video in the area is, for example, about 94% (15/16 of the whole) of the luminance value of the display video. In this way, after setting the maximum luminance area, the luminance of the adjacent area is sequentially obtained.

  The dimming value calculation unit 13 can obtain the dimming value by using a lookup table. In this case, since adjustment is performed over the entire screen area, the dimming value 14 calculated by the dimming value calculation unit 13 is delayed by one frame compared to the corrected video signal 20 calculated by the video correction unit 19. In order to eliminate this one-frame delay, a memory 23 is provided as shown in FIG. 1B, the input of the inverse gamma correction video signal 12 other than the dimming value calculation unit 13 is delayed by one frame, and each process is performed after the timing is adjusted. What is necessary is just to input into a part.

  FIG. 6A and FIG. 6B are diagrams showing the input video gradation (FIG. 6A) and the backlight luminance (FIG. 6B) when an entirely white video is input. The horizontal axis in the figure is the horizontal position across the display in FIG. 3A and is shown divided into areas. The trapezoidal dotted line in FIG. 6B indicates that four LED light sources having the luminance distribution shown in FIG. 3B are arranged. Moreover, since all the white light is emitted from the entire surface, light is emitted at the same luminance peak, and an added value obtained by superimposing them is a constant luminance level indicated by a solid line.

  7A and 7B are diagrams showing the distribution of the input video gradation (FIG. 7A) and the luminance value of the backlight (FIG. 7B) when a gradation video is input. FIG. 7A shows a gradation image in which the image gradation gradually changes in the horizontal direction. The backlight luminance value in this case is obtained from the maximum value of the input video in each area using FIG. The dotted line in FIG. 7B indicates the luminance value of each area, and the four LED light sources emit light with different luminance values, and the added value obtained by superimposing these lights indicates a broken line-like luminance distribution as indicated by a solid line.

Next, the low frequency detection unit 17 and the correction gain calculation unit 15 which are features of the present embodiment will be described.
The low frequency detector 17 receives the inverse gamma corrected video signal 12 to detect a flat low frequency video region, and a gain value (hereinafter referred to as a low frequency gain) 18 indicating the flatness of the video with respect to the pixel of interest. Is calculated and output. The low-frequency gain is calculated by detecting a flat low-frequency image by using frequency conversion such as FFT (First Fourier Transform) or DCT (Discrete Cosine Transform), and reducing the low-frequency gain according to the spectrum in the low-frequency region. 18 is calculated.

FIG. 8 is a diagram for explaining a method of detecting a low frequency region by frequency conversion. A video space (x, y) in the figure indicates an 8 × 8 pixel range including the pixel of interest in the center, and is converted into an 8 × 8 frequency space (u, v) by frequency conversion. Moreover, the frequency region (region a) on the low frequency side and the frequency region (region b) on the high frequency side are divided by the threshold value F _TH in the drawing. That is, the frequency space (u, v) belonging to the low frequency region a is represented by [Expression 4] by the threshold value F _TH .

Next, a method for calculating the low frequency gain will be described. If the spectrum at the frequency (u, v) is s (u, v), and the sum of the absolute value spectra within the threshold _FTH is S, the total spectrum S is obtained from [Equation 5].

The sum S of the absolute value spectra indicates the degree of flatness of the video, and the value of S is normalized by a value of 0 to 1 as a low frequency gain Gain _freq . The low frequency gain Gain _freq is obtained from [Equation 6], where S _{0 is} the threshold value for determining the low frequency region with respect to the obtained S value, and S ₁ is the limit value to which smoothing processing described later is applied.

  As another low frequency gain calculation method, an edge can be calculated using a DoG (Difference of Gaussian) filter, and the gain can be determined according to the size of the edge. For example, a DoG filter in an N × N (N is a positive number) pixel range has a variance value σ, a scale factor k, and a video signal (here, an inverse gamma corrected video signal) of video coordinates (x, y) as X If (x, y), it is expressed by [Formula 7].

In addition, the edge detection may simply use a total value of absolute values for pixel differences adjacent to X (x, y). The edge calculation result is represented by E as an edge, the edge threshold value to be determined as the low frequency region is E ₀ , and the limit value to which smoothing processing described later is applied is E ₁ , and the low frequency gain Gain _freq is obtained from [Equation 8]. .

The correction gain calculation unit 15 receives the inverse gamma correction video signal 12, the dimming value 14, and the low frequency gain 18 and outputs a correction gain signal 16 to the video correction unit 19. First, a display video gradation that is a theoretical display target (hereinafter referred to as a target display video gradation) is created from the inverse gamma corrected video signal 12 and the light control value 14. Next, smoothing processing, which is a feature of this embodiment, is performed on the target display video gradation. Then, the correction gain is obtained from the smoothed target display image gradation and applied according to the magnitude of the low frequency gain 18. When the RGB value of the input video is X and the fixed backlight value is BL ₀ , the target display video gradation f (X) is expressed by [Equation 9].

  Next, in the smoothing process of the target display video gradation f (X), for example, the smoothing filter filter (x, y) is used in an N × N (N is a positive number) pixel range centering on the target pixel. As the smoothing filter, an average value filter represented by [Equation 10] can be used. FIG. 9 is a diagram for explaining the smoothing process by the average value filter. Here, the averaging process is shown in the range of the surrounding 7 × 7 pixels.

Assuming that the video signal of the video coordinates (x, y) is X (x, y) and the target display video gradation is f (X), the target display video gradation f _flt (X (x, y) after smoothing processing is performed. ) Is represented by [Formula 11]. The subscript flt indicates a value after applying the smoothing filter.

  As other smoothing filters, the Gaussian filter Gauss and weighted average filter in [Formula 7] can be used.

Moreover, you may use LPF (low-pass filter) using frequency conversion as a smoothing process. The LPF at this time is a filter that passes only the region a in FIG. If a filter that passes only the region a is a filter _LPF, and DCT and inverse DCT (DCT ⁻¹ ) are used, [Formula 12] is obtained.

Next, from the target display image gradation f _flt (X (x, y)) after the smoothing process and the dimming luminance value BL (X (x, y)) of the _backlight, the correction gain Gain _BLflt at the time of smoothing Is calculated by [Equation 13]. The subscript flt here also indicates a value after the smoothing filter is applied.

Further, the correction gain signal 16 output to the video correction unit 19 applies the smoothing correction gain Gain _BLflt and the non-smoothing correction gain Gain _BL separately according to the low frequency gain 18. That is, when the low frequency gain Gain _freq is close to 1 (when the flatness of the video is large), the smoothing correction gain Gain _BLflt is applied, and when the low frequency gain Gain _freq is close to 0 (the flatness of the video Is small), the correction gain Gain _BL without smoothing is applied. When this is expressed by an equation, a correction gain signal Gain to be finally output with respect to the low-frequency gain Gain _freq is determined as [Equation 14].

  The video correction unit 19 receives the correction gain signal 16 from the correction gain calculation unit 15 and creates a corrected video gradation F (X) by multiplying the inverse gamma correction video signal X and the correction gain signal Gain. Then, the corrected video gradation F (X) is output to the video display unit 102 as the corrected video signal 20.

  The timing control unit 21 of the video display unit 102 generates a display control signal based on the input corrected video signal 20 and the horizontal / vertical synchronization signal, and sends the video display signal and the scanning signal to the liquid crystal display 22. The liquid crystal display 22 receives the display signal and the scanning signal, applies a voltage corresponding to the image gradation to the liquid crystal panel in the corresponding pixel region, and displays the image by controlling the transmittance.

  Next, generation of a gradation step in the local dimming method and a gradation step reduction function according to the present embodiment will be described. FIGS. 10 to 14 show the input video gradation near the boundary between area 1 and area 2, the backlight luminance value, the correction gain, the display video gradation, and the like when the gradation video shown in FIG. 7A is input. ing.

  FIG. 10A, FIG. 10B, and FIG. 10C show cases where gain correction is not performed on the input video signal, and the input video gradation (FIG. 10A), the backlight luminance value (FIG. 10B), and the display video gradation ( FIG. 10C is an enlarged view. The display video gradation is the product of the input video gradation and the backlight luminance value, but there is a difference between the backlight luminance values (dimming luminance values) in area 1 and area 2, and thus the display video gradation is large near the boundary. There are gradation steps. This is a gradation step created to perform dimming for each area, and in order to eliminate this gradation step, a corrected image gradation is created by applying gain correction to the input image signal as described below. .

  11A, 11B, and 11C show a case where gain correction is performed on an input video signal (however, smoothing processing is not performed), and the target display video gradation (FIG. 11A), the correction gain (FIG. 11B), and The corrected video gradation (FIG. 11C) is shown. The corrected video gradation of FIG. 11C is generated by multiplying the input video gradation of FIG. 10A by the correction gain of FIG. 11B. By applying the backlight luminance value of FIG. 10B to this, the display video gradation actually displayed can be obtained.

  The correction gain is obtained by setting a target display image gradation (target display image gradation) and dividing the target display image gradation by the luminance value of the backlight and the input image gradation. FIG. 11A shows the target display video gradation, which is set from the case where the input video gradation is displayed with a backlight having a fixed luminance value. FIG. 11B shows a correction gain for this, and as a result, it is obtained as a ratio (backlight ratio) between the fixed luminance value of the backlight and the dimmed luminance value. If the backlight luminance value of FIG. 10B is applied to the corrected image gradation of FIG. 11C, the display image gradation similar to that of FIG. 11A can be obtained.

These relationships are represented by the following equation. When the RGB value of the input video is X, the target display video gradation is f (X), the fixed backlight value is BL ₀ , and the dimming luminance value of the backlight is BL (X), the correction gain Gain _BL and the correction video The gradation F (X) is as shown in [Formula 15].

  In addition, for example, in the method of calculating the target display video gradation, the white luminance in the dimming value in area 2 is changed from the black luminance value in the dimming value in area 1 in the video signal gradation characteristic between black luminance and white luminance in FIG. The luminance value is obtained as a display video gradation, and the input video gradation is mapped to the luminance value of the display video. However, depending on how the dimming gradation is divided, if the dimming gradation difference between area 1 and area 2 is large, an area where the input video gradation cannot be mapped is created. Cannot be interpolated correctly.

  In order to solve this problem, the following conditions may be added to the dimming value calculation method using the maximum value of the input video in the area using FIG. The condition is selected by the dimming value calculation method for the entire area, that is, the area having the maximum value of the input video in the entire screen. This area is temporarily called a maximum luminance area. Next, the dimming value of the area adjacent to the maximum luminance area is obtained. The dimming gradation of the adjacent area is not less than about 6% (1/16) of the dimming value of the maximum luminance area, and the dimming gradation that minimizes the luminance at the maximum value of the input video in the adjacent area. Should be selected.

The RGB value of the input video is X, the dimming luminance value of the backlight is BL (X), and the display video gradation f (X) is Tbl (Tbl () assuming that the input video gradation is mapped using the lookup table Tbl. X), the correction gain Gain _BL and the correction value F (X) are as shown in [Formula 16].

  In this embodiment, two methods for calculating the correction gain are shown, but the present embodiment is not limited to this. Any other method may be used as long as the dimming luminance value of the backlight and the target display image gradation can be created.

  11A to 11C, a small gradation step is seen in area 1 as shown in FIG. 11C. This gradation step increases in proportion to the luminance difference of the backlight between areas. Hereinafter, the reason why the gradation level difference occurs will be described.

  FIG. 13A, FIG. 13B, and FIG. 13C are diagrams for explaining the problem of the gradation step when the correction gain is applied to the input video gradation (no smoothing process). The horizontal axis is the horizontal position of the central portion of area 1. In this area, the backlight luminance distribution is assumed to be flat as shown in FIG. 10B. FIG. 13A shows the input video gradation, which is a gradation video that gradually changes with the horizontal position. For example, the gradation of the video gradation increases by one gradation every three pixels in the area. On the other hand, it is assumed that a correction gain (Gain = 3) as shown in FIG. 13B is applied by local dimming. As a result, the corrected video gradation changes as shown in FIG. 13C and has a staircase shape that increases three gradations every time three pixels are advanced. Since the brightness of the backlight in this area is constant, the gradation step remains in the displayed gradation video.

  Even if the number of display gradations is increased, the gradation level does not decrease only by increasing the number of gradation levels as shown in FIG. 13C. For example, if the number of gradations in FIG. 13A is doubled, the three gradation steps in FIG. 13C only become six gradation steps.

  On the other hand, FIGS. 12A, 12B, and 12C are diagrams showing a case where the smoothing process of the present embodiment (for example, the average value filter of FIG. 9) is performed, and the target display video gradation (FIG. 12A). And a correction gain (FIG. 12B) and a corrected video gradation (FIG. 12C). In FIG. 12A, smoothing processing is performed on the target display image gradation, and the change in image gradation is smooth compared to the case where the smoothing processing of FIG. 11A is not performed. Next, the correction gain shown in FIG. 12B is obtained by dividing the target display video gradation (with smoothing processing) shown in FIG. 12A by the luminance value of the backlight and the input video gradation, thereby finely changing the correction gain in units of pixels. It becomes like this. When the input video gradation of FIG. 10A is multiplied by the correction gain of FIG. 12B, the smooth corrected video gradation of FIG. 12C is obtained. In the corrected video gradation of FIG. 12C, the gradation step seen in FIG. 11C is eliminated.

  FIG. 14A, FIG. 14B, and FIG. 14C are diagrams for explaining the effect of the smoothing process. FIG. 14A shows the input video gradation and is the same as FIG. 13A. By applying a smoothing process to the target display video gradation for this, a smooth gradient is obtained as shown in FIG. 12A. FIG. 14B is an enlarged view of the correction gain, which is not constant within the area and has different values for each pixel. As a result, as shown in FIG. 14C, the corrected video gradation has a staircase shape that increases by one gradation every time one pixel advances. That is, as compared with 13C, the change width of the gradation step can be reduced from 3 gradations to 1 gradation of the minimum width.

  It should be noted that smoothing the input video in FIG. 14A has no effect because the gradation step is one gradation. When the input video gradation is subjected to dithering or the like, the gradation of the input video itself is increased, but after the correction gain multiplication, the gradation level is three times higher, so that it is visually recognized as noise.

  The technique of this embodiment creates a target display image gradation assuming an image displayed by combining a dimmed backlight and a liquid crystal panel having a transmittance set by a corrected image signal. Since smoothing processing is performed on the target display video gradation that is the visual target, a smooth signal without a gradation step can be created as shown in FIG. 12C or FIG. 14C.

  FIG. 15 is a diagram for explaining the effect of the present embodiment on the video display screen. For example, assume that the display screen is an image of a meadow or grove against the background of the evening sky. The brightness of the backlight is dimmed along a gradation of brightness in the evening sky. At this time, the gradation display according to the prior art has a small gradation step as described with reference to FIG. 11C, and the enlarged image is displayed as a spotted pattern. On the other hand, by introducing the smoothing process of the present embodiment, the gradation step in the gradation portion becomes smooth as described in FIG. 12C, and the spotted pattern generated in the enlarged image is not seen.

  In this embodiment, the input video is frequency-converted and smoothing processing is applied to the low-frequency video. In the frequency domain visualized video in FIG. 15, a white area (background image of the evening sky) is determined as a low frequency area, and a black area (grassland or grove image or sunset outline) is determined as a high frequency area. The degree of the low frequency region is expressed as the low frequency gain in the above [Equation 6] and [Equation 8]. Then, as shown in [Formula 14], smoothing processing is applied in accordance with the degree of the low frequency region (low frequency gain), and control is performed so that correction by the smoothing processing is not performed in the high frequency region. As a result, it is possible to make a smooth correction from the low frequency region to the high frequency region while maintaining a sharp image such as a contour included in the high frequency region.

  In this embodiment, the backlight used for the liquid crystal display of the video display unit adopts a local dimming method in which the screen is divided into a plurality of areas and the luminance is adjusted for each area. However, the gradation step reduction effect according to this embodiment is also effective in the global dimming method in which the luminance of the entire screen is adjusted all at once. That is, there is an effect of smoothing the gradation level difference in the gradation portion when the backlight luminance is lowered in a dark image on the entire screen.

  In the first embodiment, the video display device including the video processing unit and the video display unit (display) has been described. In the second embodiment, a video processing device that connects a video display unit (display) as an external device will be described.

  FIG. 16 shows an embodiment of the video processing apparatus 103 according to the present invention, and shows a system in which a display apparatus 104 is connected to the outside. Here, it is assumed that the display device 104 includes the liquid crystal display 22 and is adjusted to have a gamma characteristic in order to maintain the consistency between the CRT and the input data as described in the first embodiment. Therefore, it is necessary to input a signal subjected to gamma correction to the display device 104.

  The video processing apparatus 103 is configured by adding an output signal processing unit 24 in addition to the input signal processing unit 11, the dimming value calculation unit 13, the correction gain calculation unit 15, the low frequency detection unit 17, and the video correction unit 19. The In the video processing apparatus 103, the dimming value 14 and the corrected video signal 20 are generated by the same method as in the first embodiment. Further, the output signal processing unit 24 performs gamma correction on the corrected video signal 20 and outputs a gamma corrected video signal 25 to the display device 104.

  The dimming value 14 and the gamma correction video signal 25 are input to the display device 104. The input signal processing unit 26 performs reverse gamma correction on the gamma corrected video signal 25 and supplies the reverse gamma corrected video signal 27 to the liquid crystal display 22 via the timing control unit 21. The light control value 14 is supplied to the backlight of the liquid crystal display 22.

  As described above, the video processing apparatus 103 according to the present embodiment can maintain consistency with the display apparatus 104 having the gamma characteristic.

  In each of the embodiments described above, the case where a liquid crystal display is used has been described. However, the video display element may be other than the liquid crystal panel, and may be a display corresponding to a method for dimming a backlight for illumination. Further, the light source of the backlight may be other than the LED, and the structure thereof may be a side light system using a light guide plate.

1 ... LCD panel,
2 ... Diffusion sheet,
3 ... Backlight,
4 ... Area,
5 ... LED light source,
10: Input video signal,
11: Input signal processing unit,
12: Inverse gamma correction video signal,
13: Dimming value calculation unit,
14: Dimming value,
15 ... correction gain calculation unit,
16: Correction gain,
17 ... low frequency detector,
18 ... Low frequency gain,
19 ... Video correction unit,
20 ... corrected video signal,
21 ... Timing control unit,
22 ... Liquid crystal display,
23 ... Memory,
24... Output signal processing unit,
25 ... Gamma corrected video signal,
26: Input signal processing unit,
27: Inverse gamma correction video signal,
100 ... Video display device,
101 ... Video processing unit,
102 ... Video display section,
103: Video processing device,
104: Display device.

Claims (8)

In a video display device having a video display unit and a video processing unit,
The video display unit includes a video display element that adjusts the transmittance for each pixel by a video signal and a backlight that illuminates the video display element from the back, and adjusts the luminance of the backlight according to a dimming value. There,
The video processing unit
A dimming value calculation unit for calculating a dimming value for the backlight from an input video signal;
A correction amount calculation unit that calculates a correction gain for correcting the input video signal according to the dimming value;
A video correction unit that multiplies the input video signal by the correction gain to correct the video signal and supplies the video signal to the video display unit;
The correction amount calculation unit sets a target display video from the input video signal and the dimming value, smoothes the target display video by a smoothing filter, and calculates the target display video and the dimming value from the smoothed target display video. An image display device that calculates a correction gain. The video display device according to claim 1,
The video processing unit includes a low frequency detection unit that obtains a low frequency gain indicating a flatness of the video from the input video signal,
The correction amount calculation unit adjusts an application ratio of the correction gain obtained from the smoothed target display video signal and the correction gain obtained from the target display video signal not smoothed according to the low frequency gain. A characteristic video display device. The video display device according to claim 2,
When the flatness of the image is large due to the low frequency gain, the correction amount calculation unit applies the correction gain obtained from the smoothed target display video signal, and the flatness of the image based on the low frequency gain. A video display device characterized by applying a correction gain obtained from the target display video signal that is not smoothed when the value is small. The video display device according to claim 1,
The image display apparatus according to claim 1, wherein the correction amount calculation unit uses any one of an average value filter, a low-pass filter, and a Gaussian filter as the smoothing filter. The video display device according to claim 2,
In order to obtain the low frequency gain, the low frequency detection unit uses a size of a low frequency region detected by frequency-converting an input video signal or a size of an edge detected by filtering the input video signal. A video display device characterized by the above. The video display device according to claim 1,
The video display unit uses a liquid crystal panel as the video display element, and the backlight corresponds to a display screen divided into a plurality of areas and adjusts luminance for each area. Video display device. In a video processing device that processes a video signal and supplies it to a display device,
The display device has a video display element that adjusts the transmittance for each pixel by a video signal and a backlight that illuminates the video display element from the back, and adjusts the luminance of the backlight according to a dimming value. And
The video processing device includes:
A dimming value calculation unit for calculating a dimming value for the backlight from an input video signal;
A correction amount calculation unit that calculates a correction gain for correcting the input video signal according to the dimming value;
A video correction unit that multiplies the input video signal by the correction gain to correct the video signal and supplies the video signal to the video display unit;
The correction amount calculation unit sets a target display video signal that is a display target from the input video signal and the dimming value, smoothes the target display video signal, and smoothes the target display video signal and the dimming value. An image processing apparatus that calculates the correction gain from a light value. In a video display method for displaying a video signal on a display device having a video display element and a backlight,
Calculating a dimming value for the backlight from an input video signal;
Calculating a correction gain for correcting the input video signal according to the dimming value;
Multiplying the input video signal by the correction gain to correct the video signal;
Providing the dimming value and the corrected video signal to the display device,
The step of calculating the correction gain includes a step of setting a target display video from the input video signal and the dimming value, a step of smoothing the target display video by a smoothing filter, and a smoothed target display video And a step of calculating the correction gain from the dimming value.

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