Patent Application
20140340429
1. Field of the Invention
The present invention relates to an image display apparatus and a control method for the image display apparatus.
2. Description of the Related Art
In an image display apparatus including a backlight including a plurality of light-emitting blocks, light emission brightness of which can be individually controlled, there is a technique for reducing the light emission brightness of the backlight for each of the light-emitting blocks according to an input image signal. There is also a technique for reducing the light emission brightness of the backlight for each of the light-emitting blocks according to an input image signal and correcting an image signal of an image region corresponding to the light-emitting block according to the reduction of the light emission brightness. These techniques are called local dimming. By using the local dimming, it is possible to improve the contrast of a display image and suppress a misadjusted black level (see, for example, WO2009/054223).
In the conventional local dimming, the light emission brightness is controlled according to an input image for each of the light-emitting blocks to, when a maximum gradation value of an image region corresponding to each of the light-emitting blocks of the backlight is low, reduce the light emission brightness of the light-emitting block and, when the maximum gradation value is high, increase the light emission brightness. A pixel value (a gradation value) of the image region corresponding to the light-emitting block, the light emission brightness of which is reduced, is extended to prevent the display brightness of pixels in the image region from changing with respect to the brightness of the input signal. In this way, the light emission brightness of the light-emitting block is determined according to the maximum gradation value of one image region corresponding to the light-emitting block. Therefore, when the maximum gradation value of the image region greatly changes, the light emission brightness of the light-emitting block also greatly changes. This sometimes causes a flicker.
In WO2009/054223, by determining the light emission brightness of a light-emitting block on the basis of a weighted average of a maximum and an average of the brightness of pixels in an image region corresponding to the light-emitting block, a flicker is further suppressed compared with determining the light emission brightness on the basis of only the maximum. However, in the method of WO2009/054223, when the weight of the average in calculating the weighted average is larger than the weight of the maximum, the display brightness of the pixels having the maximum sometimes falls below the brightness of an input signal.
Therefore, the present invention provides a technique capable of suppressing a flicker in an image display apparatus that performs local dimming and suppressing deterioration in reproducibility of brightness.
According to a first aspect of the present invention, there is provided an image display apparatus including: a light-emitting unit, a light emission amount of which is variably controllable; a display unit configured to display an image by modulating, according to imago data, light emitted from the light-emitting unit; and a control unit configured to control the light emission amount of the light-emitting unit according to a maximum and a minimum of a pixel value in a frame. The control unit maximizes the light emission amount of the light-emitting unit irrespective of the maximum when the minimum is larger than a first threshold.
According to a second aspect of the present invention, there is provided a control method for an image display apparatus including: a light-emitting unit, a light emission amount of which is variably controllable; and a display unit configured to display an image by modulating, according to image data, light emitted from the light-emitting unit, the control method including: acquiring a maximum and a minimum of a pixel value in a frame; and controlling the light emission amount of the light-emitting unit according to the maximum and the minimum. In the controlling the light emission amount of the light-emitting unit, the light emission amount of the light-emitting unit is maximized irrespective of the maximum when the minimum is larger than a first threshold.
According to the present invention, it is possible to suppress a flicker in an image display apparatus that performs local dimming and suppressing deterioration in reproducibility of brightness.
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
A liquid crystal display apparatus and a control method for the liquid crystal display apparatus according to a first embodiment of the present invention are explained below. The liquid crystal display apparatus according to this embodiment includes a backlight including a plurality of light-emitting blocks, light emission brightness (a light emission amount) of which is variably controllable individually. The blocks are regions obtained by dividing a screen. For example, the blocks are regions obtained by dividing the screen into N (N is an integer equal to or larger than 2) in the horizontal direction and M (M is an integer equal to or larger than 2) in the vertical direction. The blocks may be regions obtained by dividing the screen only in the horizontal direction or may be regions obtained by dividing the screen only in the vertical direction.
Input image data (image signal) is sent to a maximum detecting unit 1 and a minimum detecting unit 2. The maximum detecting unit 1 acquires, for each of image regions corresponding to the blocks, a maximum of pixel values (gradation values) of pixels in the image region. The minimum detecting unit 2 acquires, for each of the image regions corresponding to the blocks, a minimum of the pixel values of the pixels in the image region. The maximum and the minimum of the pixel values for each of the blocks are sent to a backlight control value determining unit 3.
The backlight control value determining unit 3 determines a backlight control value for each of the blocks from the maximum and the minimum of the pixel values of each of the blocks. The backlight control value corresponds to the light emission brightness of the backlight. The backlight control value determining unit 3 determines the backlight control value according to a two-dimensional table shown in
The backlight 4 is a lighting device including a plurality of blocks. Each of the blocks includes one or a plurality of light sources. The backlight 4 irradiates a liquid crystal panel 6 from the back. The light emission brightness of each of the blocks can be independently controlled. The light emission brightness of each of the blocks is controlled according to a signal (a backlight control value) output by the backlight control value determining unit 3. The liquid crystal panel 6 transmits light emitted from the backlight 4 at transmittance determined for each of pixels, whereby an observable image is displayed on the screen.
The liquid crystal panel 6 displays an image by modulating, according to image data, the light emitted from the backlight 4. Image data is input to the liquid crystal panel 6 via a correction processing unit 7. A correction coefficient calculated by a correction coefficient calculating unit 5 is input to the correction processing unit 7. The correction processing unit 7 corrects the image data on the basis of the correction coefficient. The correction coefficient calculating unit 5 calculates a brightness distribution of the backlight on the basis of a signal (a backlight control value) output by the backlight control value determining unit 3 and corrects, on the basis of the brightness distribution, the image data such that the brightness of transmitted light is brightness corresponding to a pixel value of the input image data.
The two-dimensional table shown in
In the image display apparatus in this embodiment, a relation (a gradation brightness characteristic) between an output brightness value (a display brightness value) L of the liquid crystal panel 6 and an input signal (a pixel value) S is indicated by the following expression when it is assumed that the gradation of the pixel value is 8 bits (0 to 255 gradations):
L=K1+(W5−K1)×(S/255)γ (Expression 1)
When control gains Gx at the respective backlight control value levels x are represented as G1=0.067, G2=0.2, G3=0.33, G4=0.67, and G5=1, an input signal S′ and a display brightness value L at each of the levels are indicated by the following expression:
L=Gx×{K5+(W5−K5)×(S′/255)γ}
(Expression 2)
A brightness range displayable by the image display apparatus when the input signal is changed between 0 and 255 at each of the backlight control value levels is calculated from Expression 2. A gradation range corresponding to the displayable brightness range is calculated from Expression 1.
For example, when it is assumed that W5=1 and K5=0.005, the displayable brightness range in the case of the backlight control value level 1 is calculated as 0.000335 to 0.067 from Expression 2. A gradation range corresponding to the brightness range is calculated as 0 to 74 from Expression 1. Similarly, a gradation range corresponding to the brightness range displayable at each of the backlight control value levels (hereinafter referred to as displayable gradation range) is 10 to 122 at the level 2, 13 to 154 at the level 3, 19 to 212 at the level 4, and 23 to 255 at the level 5.
If the block is caused to emit light at the backlight control value level at which both of the minimum and the maximum of the pixel values of the pixels in the image region corresponding to the block are included in the displayable gradation range, it is possible to display all the pixels in the image region corresponding to the block at correct brightness.
In the case of this embodiment, displayable gradation ranges respectively corresponding to the five backlight control value levels overlap one another. Therefore, depending on a combination of the maximum and the minimum, in some case, there are a plurality of backlight control value levels at which the brightness of all the pixels in the image region corresponding to the block can be correctly displayed. On the other hand, depending on a combination of the maximum and the minimum, in some case, there could be no backlight control value level at which both of the minimum and the maximum are included in the displayable gradation range.
Therefore, in this embodiment, a backlight control value level of each of the blocks is determined as explained below on the basis of a maximum and a minimum of pixel values of pixels in an image region corresponding to the block.
(A) A backlight control value level is selected at which the maximum is included in a displayable gradation range and a portion common to a displayable gradation range and a range from the minimum to the maximum is the widest.
(B) When there are a plurality of backlight control value levels that satisfy the condition (A), a maximum backlight control value level among the backlight control value levels is selected.
By determining the backlight control value level as explained above, it is possible to suppress the display brightness of a pixel, a pixel value of which is the maximum, in the image region corresponding to each of the blocks from becoming low with respect to the pixel value. Therefore, it is possible to suppress deterioration in brightness reproducibility.
The two-dimensional table shown in
As shown in
A specific example of the determination method for a backlight control value in this embodiment is explained with reference to
In a second block from the left in a first row in
On the other hand, when a backlight control value level is determined according to only a maximum as in the related art, the backlight control value level greatly changes from 2 to 5 according to a change of the maximum. Therefore, as indicated by the broken line in
When
In this embodiment, the backlight control value levels are five levels. However, the backlight control value levels are not limited to this and may be set finer. When the backlight control value levels are finer, a change in light emission brightness at the time when the backlight control value level changes decreases. Therefore, it is possible to more effectively suppress a flicker. Even when the backlight control value levels are set finer, a flicker involved in a change of the maximum and the minimum of the pixel values is suppressed by increasing a ratio of the highest backlight control value level on the table as shown in
As explained above, according to this embodiment, the light emission brightness is controlled according to the minimum and the maximum of the pixel values of each of the blocks such that a change in the backlight control value level involved in a change in the maximum and the minimum decreases. Therefore, it is possible to realize local dimming with a flicker suppressed. Since a backlight control value level at which brightness of a pixel, a pixel value of which is the maximum, is displayable is selected, it is possible to suppress deterioration in brightness reproducibility.
In a second embodiment, an example is explained in which a flicker is further suppressed with respect to a change in a minimum and a maximum of pixel values of each of blocks.
A minimum output from the minimum detecting unit 2, a maximum output from the maximum detecting unit 1, and a backlight control value output from the backlight control value determining unit 3 are input to the backlight control value LPF unit 8. The backlight control value LPF unit 8 outputs the backlight control value subjected to time LPF processing.
The backlight control value LPF unit 8 is explained below.
A backlight control value is determined by the backlight control value determining unit 3 on the basis of the two-dimensional table shown in
The backlight control value LPF unit 8 suppresses a sudden change in the backlight control value. Specifically, the backlight control value LPF unit 8 stores a backlight control value of the preceding frame and compares a backlight control value determined anew in the present frame and the backlight control value of the preceding frame. When an amount of change in the backlight control value level is larger than a threshold Th, the backlight control value LPF unit 8 reduces the change amount to be equal to or smaller than the threshold Th.
For example, when it is assumed that the threshold Th is 1, the backlight control value of the preceding frame is 1, and the backlight control value determined anew in she present frame is 5, a difference between the backlight control values is 4, which is larger than the threshold Th (=1). In this case, the change amount is reduced to 1. The backlight control value level applied to the present frame is changed to 2. A threshold for determining whether processing for reducing the amount of change in the backlight control value in this way is performed is hereinafter referred to as change amount threshold (second threshold). In this embodiment, when the amount of change in the backlight control value is larger than the change amount threshold, the backlight control value applied to the present frame is changed (corrected) such that the amount of change in the backlight control value is equal to or smaller than the change amount threshold (equal to or smaller than the second threshold).
In this embodiment, further, the backlight control value LPF unit 8 varies the threshold according to whether the change in the backlight control value level is involved in the change in the maximum or the change in the minimum and according to whether the change is an increasing change or a decreasing change. Specifically, when the maximum increases or the minimum decreases, the backlight control value LPF unit 8 sets the backlight control value to quickly change. When the minimum increases or the maximum decreases, the backlight control value LPF unit 8 sets the backlight control value to slowly change.
This is because, concerning pixel values of each of the blocks, even if the minimum of the present frame increases with respect to the minimum of the preceding frame, if the maximum does not increase, pixels of the present frame can be displayed at the backlight control value level of the preceding frame. That is, an influence on brightness reproducibility is small even if an increase in the backlight control value level is delayed with respect to the increase in the minimum. Similarly, concerning the pixel values of each of the blocks, even if the maximum of the present frame decreases with respect to the maximum of the preceding frame, if the minimum does not decrease, pixels of the present frame can be displayed at the backlight control value level of the preceding frame. That is, an influence on brightness reproducibility is small even if a decrease in the backlight control value level is delayed with respect to the decrease in the minimum.
Specifically, the backlight control value LPF unit 8 stores the minimum and the maximum of the preceding frame concerning the pixel values of each of the blocks. When an increase amount of the maximum is larger than a threshold (a third threshold) or a decrease amount of the minimum is larger than the threshold (the third threshold), the backlight control value LPF unit 8 sets the change amount threshold Th to Th_high. When the increase amount of the maximum is equal to or smaller than the threshold (equal to or smaller than the third threshold) or the decrease amount of the minimum is equal to or smaller than the threshold (equal to or smaller than the third threshold), the backlight control value LPF unit 8 sets the change amount threshold Th to Th_low (<Th_high). When both of the increase amount of the maximum and the decrease amount of the minimum are equal to or smaller than the threshold (equal to or smaller than the third threshold), the backlight control value LPF unit 8 uses the change amount threshold (the second threshold) of the preceding frame as it is. When the amount of change in the backlight control value of the present frame with respect to the preceding frame is larger than the change amount threshold, the backlight control value LPF unit 8 sets, as a backlight control value applied to the present frame, a value obtained by adding the change amount threshold to the backlight control value of the preceding frame. Consequently, when the amount of change in the backlight control value is large, change speed of the backlight control value is reduced. When the change in the backlight control value is involved in a large increase in the maximum or a large decrease in the minimum, the backlight control value changes at high speed while being reduced.
The change amount thresholds Th_high and Th_low are not limited to integer values and may be fractional values. The backlight control value LPF unit 8 calculates the backlight control value as a fractional value and retains a value of the preceding frame. A backlight control value obtained by converting the fractional value into an integer value (by, for example, rounding-off) is sent to the backlight 4 and the correction coefficient calculating unit 5.
A specific example is explained with reference to
In this embodiment, the threshold of the maximum change is set to 30, the threshold of the minimum change is set to 5, the change amount threshold Th_high of the backlight control value is set to 1, and the change amount threshold Th_low of the backlight control value is set to 0.4.
In an upper left block surrounded by a circle in
Both of the maximum and the minimum or the block do not change in T2 to T3. Therefore, the change amount threshold Th remains at Th_low =0.4. Since the maximum is 160 and the minimum is 120 in the block, the backlight control value determined by the backlight control value determining unit 3 on the basis of the table shown in
On the other hand, in a lower left block surrounded by a square in
A backlight control value of T2 determined on the basis of the table shown in
According to this embodiment, when a slow change in a backlight control value causes a brightness fall (insufficiency of brightness) and gradation collapse, for example, when the maximum increases or the minimum decreases, changing speed of the backlight control value is less easily reduced. Consequently, the brightness fall and the gradation collapse are suppressed. Otherwise, a sudden change in the backlight control value is further suppressed than in the first embodiment. Therefore, it is possible to more effectively suppress a flicker.
As explained above, according to this embodiment, light emission brightness is controlled according to the minimum and the maximum of the pixel values of each of the blocks such that a change in a backlight control value level involved in a change of the maximum and the minimum decreases. Therefore, it is possible to realize local dimming with a flicker suppressed. Since a backlight control value level at which brightness of a pixel, a pixel value of which is the maximum, is displayable is selected, it is possible to suppress deterioration in brightness reproducibility.
The embodiments are examples in which the present invention is applied to the image display apparatus in which the backlight includes the plurality of light-emitting blocks and a light emission amount can be variably controlled independently for each of the light-emitting blocks. However, the present invention can also be applied to an image display apparatus having a configuration in which the backlight is not divided by light-emitting blocks. In this case, the control in the embodiments can be applied without any modification, on the premise that the number of divisions by the light-emitting blocks of the backlight in the embodiments is 1. In this way, in the case of the image display apparatus that performs brightness control of the backlight uniformly over the entire screen, the maximum detecting unit 1 and the minimum detecting unit 2 detect a maximum and a minimum of pixel values in a frame of image data. By applying the present invention to such an image display apparatus, the effects of a reduction in a flicker and suppression of deterioration in brightness reproducibility are obtained. The embodiments are examples in which the present invention is applied to the image display apparatus including the liquid crystal panel. However, the liquid crystal panel is an example of a display panel. In the image display apparatus of the present invention, the display panel is not limited to the liquid crystal panel.
Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory Computer-readable storage medium) to perform the functions of one or more of the above-described embodiment(s) of the present invention, and by a method performed by the computer of the system or apparatus by, tor example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2013-102286, filed on May 14, 2013, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-102286 | May 2013 | JP | national |
