A DISPLAY DEVICE AND THE CONTROL METHOD THEREOF

[0001] Technical Field

[0002] The present invention relates to a display device, the backlight of which is automatically adjusted, and the control method thereof.

[0003] The Previous Technique

[0004] In LCD panels that are widely used in devices such as television, cell phone and laptop computer, an external light source is needed in order to generate the image. In televisions wherein 'transmissive' LCD panels that operate by the pixels, which generate the image, being illuminated from behind are used, a backlight is used as the light source. The generation of the image by being illuminated from behind shows generally positive results in terms of contrast and color quality. However, in the situation that the ambient light is excessive or depending on the change of the brightness values of the image reflected on the screen, the audience sometimes has difficulty in distinguishing the image details. In images, the brightness value of which is too low (for example in images that are completely black), the backlight leaking from the LCD panel adversely affects the contrast ratio. In the state of the art, various embodiments wherein the backlight level is changed according to the image or the ambient light are available.

[0005] In the state of the art International Patent Document No WO2005029459, a display device, wherein the image brightness is adjusted by changing the backlight intensity according to the ambient light, and the control method thereof are described.

[0006] In the state of the art United States of America Patent Document No

US2005088401 , an LCD device that realizes the automatic adjustment of the backlight by using the data about the image reflected on the screen is described.

[0007] Brief Description of the Present Invention

[0008] The aim of the present invention is the realization of a display device, the backlight of which is automatically adjusted according to the image reflected on the screen and to the ambient light, and the control method thereof.

[0009] According to the display device and the control method thereof, realized in order to attain the aim of the present invention and explicated in the first claim and the respective claims thereof, measurements about the image reflected on the screen are performed by a control unit and data are provided, and backlight control is realized in compliance with these data.

[0010] The display device, which comprises a screen, an illumination element that provides the generation of an image by illuminating the screen from behind, and a control unit that receives the data regarding the image and controls the illumination element in compliance with these data, first determines the brightness histogram (Histogramp]) and the current average brightness value (CurrentAvg) of the image displayed on the screen. Then, by comparing the current (present) image and the previous image, it is detected whether a scene change between the broadcasted images has occurred or not. If it is detected that no scene change has occurred between two images, the new average brightness value indicating the new value to be obtained by the current average brightness value is calculated as the weighted average of CurrentAvg and the previous average brightness value (PreAvg). After the scene change control, it is controlled whether CurrentAvg is between the maximum threshold value (MaxThr) and the minimum threshold value (MinThr) or not. In the situation that CurrentAvg is smaller than MinThr, it is equalized with MinThr and in the situation that CurrentAvg is greater than MaxThr, it is equalized with MaxThr. In the situation wherein it is determined that CurrentAvg is within the desired interval (MinThr-MaxThr), the target backlight value (TargetBL) is calculated such that it will be between the minimum backlight value (MinBL) and the maximum backlight value (MaxBL), and in proportion to the point between MinThr and MaxThr wherein CurrentAvg is situated.

[0011] Afterwards, TargetBL is compared with a predetermined middle backlight value (MidBL) for a second calibration. If Histogramp] is greater than HistThr, TargetBL is increased in direct proportion to i and if Histogramp] is not greater than HistThr, the histogram information of the image (Histogramp]) is controlled in N stages.

[0012] Finally, it is determined whether the difference of CurrentAvg and PreAvg is above AvDiffThr or not, and the scene change control is performed one more time. In the situation that no scene change has occurred, TargetBL is applied to the illumination element slowly, and in the situation that a scene change has occurred, TargetBL is applied to the illumination element at a time.

[0013] Accordingly, with a two-stage calibration, the display device precisely calculates the target backlight value (TargetBL) that will be applied to the illumination element. For the calculation of the backlight value, as well as the current average brightness values, the maximum and minimum threshold brightness values (MaxThr and MinThr) are also taken as references and the maximum and minimum backlight values (MaxBL and MinBL) are also used in the calculation.

[0014] In an embodiment of the present invention, by using the light sensor on the device, MaxBL and MinBL are determined automatically according to the intensity of the ambient light. In another embodiment of the present invention, the said values are determined by the user by hand (manually).

[0015] Detailed Description of the Present Invention

[0016] A display device and the control method thereof, realized in order to attain the aim of the present invention, is illustrated in the attached figures, where:

[0017] Figure 1 - is the schematic view of the display device of the present invention.

[0018] Figure 2 - is another schematic view of the display device of the present invention.

[0019] Figure 3 - is the data flow diagram that belongs to the control method of the present invention.

[0020] Figure 4 - is the continuation of the diagram illustrated in Figure 3.

[0021] Figure 5 - is the continuation of the diagram illustrated in Figure 4.

[0022] Figure 6 - is the continuation of the diagrams illustrated in Figure 4 and Figure 5.

[0023] The elements illustrated in the figures are numbered as follows: 1. Display device

2. Screen

3. Illumination element

4. Control unit

[0024] The display device (1) of the present invention comprises a screen (2), an illumination element (3) that provides the generation of an image by illuminating the screen (2) from behind, and a control unit (4) that receives the data regarding the image and controls the illumination element (3) in compliance with these data (Figure 1 , Figure 2).

[0025] In the display device (1), the method, in which the intensity of the backlight provided by the illumination element (3) is controlled, comprises the following steps:

- Calculating the current average brightness value (CurrentAvg) and the brightness histogram (Histogramp]) of the image (101),

- Determining whether the difference of CurrentAvg and the previous average brightness value (PreAvg) is below a threshold value (AvDiffThr) predetermined by the producer (102),

- Calculating the new average brightness value as the weighted average of CurrentAvg and PreAvg if the difference of CurrentAvg and the previous average brightness value (PreAvg) is below a certain threshold value (AvDiffThr) (103),

- Decreasing the Delay parameter by 1 (104),

- Controlling whether Delay is 0 or not (105),

- Equalizing PreAvg with CurrentAvg if it is detected that Delay is not 0 (106), and returning to step 102,

- Equalizing the Delay value with the predetermined Delay (DelayPreset) value if it is detected that Delay is 0 (107),

- Equalizing the Delay value with the predetermined Delay (DelayPreset) value if the difference of CurrentAvg and the previous average brightness value (PreAvg) is above the threshold value (AvDiffThr) (107),

- Detecting whether CurrentAvg is greater than the maximum threshold value (MaxThr) or not (201), - Equalizing the target backlight value (TargetBL) with the maximum backlight value (MaxBL) if CurrentAvg is greater than the maximum threshold value (MaxThr) (202),

- Detecting whether CurrentAvg is smaller than the minimum threshold value (MinThr) or not if CurrentAvg is not greater than the maximum threshold value (MaxThr) (203),

- Equalizing TargetBL with the minimum backlight value (MinBL) if CurrentAvg is smaller than the minimum threshold value (MinThr) (204),

- Calculating TargetBL according to CurrentAvg using minimum and maximum backlight values (MinBL, MaxBL), minimum and maximum threshold values (MinThr, MaxThr), with the formula TargetBL=MinBL+[(CurrentAvg-MinThr)*(MaxBL-MinBL)/(MaxThr-Mi nT hr)], if CurrentAvg is not smaller than the minimum threshold value (MinThr) (205),

- Comparing the target backlight value (TargetBL) with the middle backlight value (MidBL) (300),

- Equalizing the variable i, which shows a certain brightness value in the image brightness histogram (histogramp]), with N if the target backlight value (TargetBL) is smaller than the middle backlight value (MidBL) (301),

- Comparing the image histogram (Histogramp]), which shows the amount of pixels in the brightness value i, with the threshold histogram value (HistThr) (302),

- Increasing TargetBL as much as the product of the backlight increase amount (BLInc), predetermined by the producer, and i according to the formula TargetBL=TargetBL + i*BLInc (303) and proceeding with the step wherein it is determined whether the difference of CurrentAvg and the previous average brightness value (PreAvg) is above a certain threshold value (AvDiffThr) (401), if the image histogram no. i is greater than the threshold histogram value (HistThr),

- Decreasing i by 1 if the image histogram no. i is not greater than the threshold histogram value (HistThr) (304),

- Detecting whether i is lower than the minimum i (i_min) or not (305), - Returning to step 302 if i is greater than the minimum i (i_min),

- Determining whether the difference of CurrentAvg and the previous average brightness value (PreAvg) is above a certain threshold value (AvDiffThr) or not if i is not greater than the minimum i (Lrnin) or if the target backlight value (TargetBL) is not smaller than the middle backlight value (MidBL) (401),

- Equalizing the backlight value (BL) with the target backlight value (TargetBL) and PreAvg with CurrentAvg if the difference of CurrentAvg and the previous average brightness value (PreAvg) is above a certain threshold value (AvDiffThr) (402),

- Detecting whether the backlight value (BL) is lower than the target backlight value (TargetBL) or not if the difference of CurrentAvg and the previous average brightness value (PreAvg) is not above a certain threshold value (AvDiffThr) (403),

- Increasing the backlight value (BL) by one level and equalizing PreAvg with CurrentAvg if the backlight value (BL) is lower than the target backlight value (TargetBL) (404),

- Detecting whether the backlight value (BL) is greater than the target backlight value (TargetBL) or not if the backlight value (BL) is not lower than the target backlight value (TargetBL) (405),

- Decreasing the backlight value (BL) by one level and equalizing PreAvg with CurrentAvg if the backlight value (BL) is greater than the target backlight value (TargetBL) (406),

- Keeping the backlight value (BL) unchanged and equalizing PreAvg with CurrentAvg if the backlight value (BL) is not greater than the target backlight value (TargetBL) (407).

[0026] According to the method of the present invention, the control unit (4) first generates the brightness histogram (Histogramp]) for each frame reflected on the screen (2) and calculates the current average brightness value (CurrentAvg) (101).

[0027] The image histogram shows the amount of pixels at a certain brightness value (i) in an image. For example, Histogram[0] shows the amount of pixels, the brightness value of which is 0, and Histogram[1] shows the amount of pixels, the brightness value of which is 1. Using histograms with fewer levels instead of keeping separate data for each brightness value is also possible. For example, in a system wherein the image is processed as 8 bits and the brightness value for each image can be between 0-255, an 8-level histogram wherein Histogram[0] shows the amount of pixels, the brightness value of which is between 0 and 31 , Histogram[1] shows the amount of pixels, the brightness value of which is between 32 and 63 etc., can also be used. The current average brightness value (CurrentAvg) is calculated by calculating the average of the brightness values of all pixels in an image, and is a parameter that indicates the average brightness of the image.

[0028] After the calculation of CurrentAvg and Histogramp] values (101), by comparing the current (present) image and the previous image, it is detected whether a scene change in the broadcast has occurred or not. This process is realized by controlling whether the difference between CurrentAvg and the previous average brightness value (PreAvg) is below a certain threshold value (AvDiffThr) or not (102). It is assumed that no scene change has occurred if the difference is below the threshold value (AvDiffThr), and that a scene change has occurred if the difference is above thereof.

[0029] If it is detected that no scene change has occurred between two images, the new average brightness value is calculated as the weighted average of CurrentAvg and PreAvg (103). Thus, the average brightness (CurrentAvg) that will be used in the adjustment of the back light (BL) intensity is adjusted according to the small changes in image transitions. This calculation is performed according to the following formula:

[0030] CurrentAvg=α ^* CurrentAvg + (1-α) ^* PreAvg

[0031] In the applied formula, α is a constant smaller than 1 and predetermined by the producer, and can receive different values by being adjusted beforehand according to the desired suppression ratio for the small changes in the brightness. In the situation that no scene change has occurred, the Delay parameter, the value of which is predetermined as DelayPreset, is used to indicate the number of frames the said calculation will be performed for. Accordingly, the Delay parameter shows how many times the cycle (102-106-102), which comprises calculating the weighted average (103) and wherein the step (102) in which the scene change control is performed again is started over, will be repeated as long as no scene change occurs. After calculating the new average brightness value (CurrentAvg) with the weighted average, the value of the Delay parameter is decreased by 1 (104) and then, it is controlled whether the Delay value has reached zero or not (105). In the situation that Delay is not zero, the previous average brightness value (PreAvg) is equalized with the current average brightness value (CurrentAvg) (106) and the step of scene change control (102) is started over for repeating the cycle one more time.

[0032] In the situation that the change in the average brightness is greater than the threshold value (AvDiffThr), that is, a scene change is detected, the weighted average is not calculated. Thus, the backlight intensity, which will be applied for the present image, is aimed to be provided by using only the brightness of the new scene. When it is detected that no scene change has occurred between the present image and the previous image, or when calculating the average brightness values of the images as much as the Delay value is finished (Delay = 0), that current Delay value is equalized with the predetermined Delay value (DelayPreset) (107) (Sekil-3).

[0033] In the next phases of the method, the target backlight intensity (TargetBL), that is, the backlight intensity (TargetBL) that will be applied to the illumination element (3) is calculated. For the calculation of TargetBL, the current average brightness value (CurrentAvg) of the image, the maximum and minimum threshold brightness values (MaxThr, MinThr) indicating the desired limits of CurrentAvg, and the maximum and minimum backlight values (MaxBL, MinBL) indicating the desired limits of the backlight value are taken into consideration. The MaxBL and MinBL values are selected according to the light in the ambient wherein the display device (1) is situated as the lowest and highest backlight values wherein the image on the screen (2) will be perceived by the audience most easily.

[0034] In an embodiment of the present invention, MaxBL and MinBL are adjusted by the user by hand (manually) according to the light conditions of the ambient wherein the display device (1) is situated and to the preference of the user.

[0035] In another embodiment of the present invention, MaxBL and MinBL are adjusted automatically according to the ambient light intensity that is measured by a sensor situated in the display device (1).

[0036] After equalizing the Delay value with DelayPreset (107), it is detected whether CurrentAvg is greater than MaxThr or not (201). If CurrentAvg is greater than MaxThr, the target backlight value (TargetBL) is equalized with the maximum backlight value (MaxBL) (202). If CurrentAvg is not greater than MaxThr, it is detected whether CurrentAvg is smaller than the minimum threshold value (MinThr) or not (203). If CurrentAvg is smaller than MinThr, TargetBL is equalized with the minimum backlight value (MinBL) (204). Thus, in the situation that CurrentAvg is outside the MinThr-MaxThr interval, CurrentAvg is brought within these limits.

[0037] In the situation that CurrentAvg is determined to be in the desired interval (MinThr-MaxThr), TargetBL is calculated according to the value of CurrentAvg (205). The following formula is used for calculating TargetBL:

[0038] TargetBL=MinBL+[(CurrentAvg-MinThr)*(MaxBL-MinBL)/(MaxThr-Mi nThr)

]

[0039] According to the formula, TargetBL is calculated such that it will be between MinBL and MaxBL, and in proportion to the point between MinThr and MaxThr wherein CurrentAvg is situated (Figure 4).

[0040] In the next phase of the method, TargetBL is put through a second calibration in order to serve the audience the image in a comprehensible way without being adversely affected by the ambient light and the brightness values it owns. Accordingly, TargetBL is compared with a predetermined middle backlight value (MidBL) (for example with the average value of MinBL and MaxBL) (300). If TargetBL is smaller than MidBL according to the result of the comparison, the content of the image is controlled one more time. In the first step of the control process, in order to determine how many levels the control will have, the variable i is equalized with N that indicates the number of control levels (301). The whole number N can be a constant value determined during the production of the device as well as changeable from the menus of the display device (1) by the user in order to adjust the degree of precision of the backlight control. Afterwards, the value of the image histogram no. i (Histogramp]) is compared with the threshold histogram value (HistThr) (302). If Histogramp] is greater than HistThr, TargetBL is increased in direct proportion to i (303). Increasing TargetBL is realized according to the following formula:

[0041] TargetBL = TargetBL + i*BLInc

[0042] In the formula, BLInc is the target backlight increase amount and is predetermined during the production of the display device (1). In order that the new TargetBL, calculated by increasing TargetBL, does not exceed the maximum backlight value (MaxBL), the new TargetBL is required not to be greater than MaxBL even in the situation that the whole number N is used as the greatest possible value for i. Since the step of increasing TargetBL in direct proportion to i (303) is applied under the condition that TargetBL is not greater than MidBL (300), the greatest value BLInc can have should provide the inequality

[0043] MaxBL>MidB+i ^* BLInc,

[0044] therefore

[0045] BLInc<(MaxBL-MidBL)/N.

[0046] Accordingly, the greatest value BLInc can have is calculated as (MaxBL-MidBL)/N.

[0047] Thus, even if TargetBL during this step (303) of the method is low, if it is detected that there are more bright pixels in the image than those in HistThr, TargetBL is adjusted in proportion to the values of MidBL and of the bright pixels.

[0048] If Histogramp] is not greater than HistThr, the value i is decreased by 1

(304) and it is controlled whether i is smaller than the minimum i (Lrnin) or not (305) and if i is not smaller than i_min, the step 302 is started over. Thus, by the cycle consisting of steps 302, 304 and 305, the N stage of the histogram data (Histogramp]) of the image is controlled from the end to the start (such that i will gradually decrease) (Figure 5). In this phase (301-305), before applying a low backlight intensity, it is detected whether there are bright portions in the image or not (302). In the situation that bright portions are detected in the image, TargetBL is increased (303). Because, when TargetBL is decreased considering the image is dark-intensive, the image portions that may even be less bright appear pale (for example channel logos, clocks, stock market data etc. that constantly remain fixed), the problem of some portions, which are desired to be seen, becoming meaningless occurs. In this phase, these problems are prevented by putting TargetBL through a second calibration.

[0049] After determining that TargetBL is not smaller than MidBL as a result of comparing TargetBL with MidBL (300), or i is not smaller than i_min as a result of comparing i and i_min (305), or after increasing TargetBL in direct proportion to i (303), the last step of the method is started. The scene change control is performed one more time by determining whether the difference of CurrentAvg and PreAvg is above AvDiffThr or not (401). If the difference of CurrentAvg and PreAvg is above AvDiffThr, that is, if a scene change has occurred, the backlight intensity (BL) is equalized with TargetBL and PreAvg is equalized with CurrentAvg (402). If the difference of CurrentAvg and PreAvg is not above AvDiffThr, it is detected whether the backlight value (BL) is lower than TargetBL or not (403). If the backlight value (BL) is lower than TargetBL, BL is increased by one level and PreAvg is equalized with CurrentAvg (404). If the backlight value (BL) is not lower than TargetBL, it is detected whether BL is greater than TargetBL or not (405). If the backlight value (BL) is greater than TargetBL, BL is decreased by one level and PreAvg is equalized with CurrentAvg (406). If the backlight value (BL) is not greater than TargetBL, BL is not changed and PreAvg is equalized with CurrentAvg (407). By means of the steps in this phase, TargetBL is provided to be applied slowly in the situation that no scene change has occurred and it is provided to be applied at a time in the situation that a scene change has occurred.

[0050] By means of the display device (1) and the backlight adjustment method explained above, the intensity of the backlight (BL) emitted by the illumination element (3) that provides images to occur is adjusted automatically. The brightness histogram (Histogramp]) and the brightness values of each image are used for adjusting the backlight. The backlight intensity of the illumination element (3) is determined and applied by the control unit (4) by taking into consideration whether a scene change has occurred between sequential images or not, and by taking the desired brightness value interval (MaxThr-MinThr) and the backlight intensity interval (MaxBL-MinBL) into consideration. Alongside controlling whether a scene change has occurred between the images or not, a predetermined average backlight value (MidBL) is also taken as reference for the final determination of the backlight value (TargetBL) to be applied. After the scene change control that is repeated one more time in the last phase of the control method, TargetBL is applied to the illumination element (3) gradually or at a time.