CN101290754A - Method for LCD backlight dimming, LCD/image signal compensation and control of LCD display - Google Patents

Abstract

A method for reducing energy consumption in a liquid crystal display illuminated by a backlight device includes dimming the backlight and adjusting image brightness to compensate for the dimmed backlight. A backlight dimming factor is determined based on a clipping point, the clipping point being determined based on a pixel brightness distribution of an image signal. The image brightness is adjusted based on the dimming factor, wherein a first image signal mapping function is used to adjust the brightness of pixels below a brightness threshold, and a second image signal mapping function is used to adjust the brightness of pixels above the brightness threshold.

Description

LCD Backlight Dimming, LCD/Image Signal Compensation and Method of Controlling LCD Displays

technical field

The present invention relates to liquid crystal display (LCD) devices that use a backlight to illuminate a displayed image. In particular, the present invention relates to dimming the backlight to reduce power consumption while simultaneously adjusting the LCD transmittance value or image intensity value to compensate for dimming the backlight.

Background of the invention

Various liquid crystal display (LCD) devices use a light source to project an image onto a display surface, such as a screen, or place the light source behind the LCD panel to directly view the resulting image on the LCD panel. The desire to reduce power consumption and increase the contrast of image displayed by LCDs has given rise to various techniques for dynamically changing the light intensity of the light source and/or the backlight of such LCD devices. Often this variation in the brightness of the light source results in an overall dimming of the displayed image below the level at which it can be viewed.

To overcome this problem, many compensation techniques have been proposed for backlight dimming. Examples of various techniques are disclosed in US Patent 5,717,422 to Fergasson, US Patent 7,053,881 to Itoh, US Patent 7,176,878 to Lew et al., and US Patent Application 2007/0092139. US Patent 5,717,422 proposes a backlight dimming factor based on the average luminance of the image signal and simply scales the LCD transmittance, which results in clipping of high luminance values. US Patent 7,053,881 proposes a backlight dimming factor based on the average and maximum luminance values of the image signal without LCD compensation, which results in a perceived dimming of the displayed image. US Patent 7,176,878 proposes a linear amplitude increase of the LCD signal to compensate for backlight dimming, but does not discuss the dimming method. Finally, US Patent Application 2007/0092139 proposes a backlight dimming factor based on the highest luminance value of the image signal and simply augments the LCD transmittance, which results in truncation of high luminance values.

Summary of the invention

Therefore, it is an object of the present invention to provide a method of dimming the backlight of an LCD display to reduce power consumption. Another object of the present invention is to provide a method of adjusting LCD transmittance values or image brightness values to compensate for backlight dimming.

A method of maintaining the perceived brightness of a visual image on a dimmed backlit liquid crystal display is disclosed herein, the method comprising the steps of:

dims the LCD backlight by a dimming factor, and

Adjusting the pixel brightness distribution of image pixels based on the dimming factor, wherein a first tone mapping function is used to adjust the brightness of pixels below the brightness threshold, and a second tone mapping function is used to adjust the brightness above the brightness threshold pixel brightness.

Preferably, the method for dimming the backlight of a liquid crystal display comprises the following steps:

Determine a clipping point based on the pixel brightness distribution of the image signal,

determine a backlight dimming factor based on the clipping point, and

Illuminates the backlight at a determined dimming factor.

Preferably, the clipping point is between the 60% and 99% percentile of the pixel intensity distribution.

Preferably, the pixel intensity distribution is based on the achromatic intensity of the pixels within the image signal.

Preferably, the pixel brightness distribution is based on the maximum value of the red, green and blue channel brightness of the pixels within the image signal.

Preferably, the dimming factor is determined by the following equation:

$\mathrm{<span\; class="notranslate">\; BLdim</span>}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; CP</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; P</span>}}$

Among them, CP is the clipping point value, Imax is the maximum value of pixel brightness, and p is a constant not less than 1. The p value is determined according to the LCD input and output characteristics, and is usually close to the gamma value of the display.

Preferably, the brightness threshold is smaller than the clipping point.

Preferably, the threshold is determined by the following equation: |FP-CP|=|CP-Imax|. Among them, FP is the threshold value, CP is the clipping point, and Imax is the maximum value of pixel brightness.

Preferably, one of the first and second image signal mapping functions is non-linear.

Preferably, the second mapping function is a non-linear mapping function.

Preferably, the nonlinear second image signal mapping function is generated according to pixel brightness distribution.

Description of drawings

An exemplary embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a block diagram illustrating backlight dimming and LCD/image signal compensation in accordance with the present invention;

Figure 2 is a histogram of the brightness of an LCD or image signal, used to describe the clipping point used to determine backlight dimming.

Figure 3 is a signal brightness histogram used to describe the expansion of the signal brightness range to compensate for backlight dimming.

Figure 4 illustrates the image signal mapping between the original signal luminance range of Figure 2 and the extended luminance range of Figure 3, and

Figure 5 illustrates image signal mapping to improve clipping of higher luminance values in accordance with the present invention.

Detailed Description of Typical Embodiments

Reference will now be made in detail to an exemplary embodiment of the invention, examples of which are illustrated in the accompanying drawings.

In accordance with the present invention, a method of controlling the brightness of an LCD display device, such as an LCD television, includes dimming a backlight while increasing light transmittance of pixels on an LCD panel, thereby maintaining or improving the original visual brightness characteristics of a displayed image. The invention includes two aspects. The first aspect is an adaptive method for determining a backlight dimming factor based on image brightness characteristics, and in particular analyzing the brightness distribution of the image signal. A second aspect of the present invention is a method of adaptively adjusting the LCD light transmittance, or also adjusting the brightness value of the image signal to compensate for the decrease in backlight brightness so that the brightness of the seen image does not change due to dimming of the backlight . In the second aspect, a compensation range is determined based on the backlight dimming factor. Find a threshold (Fidelity Point) within the compensation range, and then use different image signal mapping algorithms to map the compensation range above and below the threshold to enhance the brightness characteristics of the displayed image. When these two aspects of the invention are used in a backlit LCD display device, the invention results in lower power consumption without compromising the visible brightness of the displayed image.

The method of the present invention will now be described in detail. When describing, the terms brightness, luminance, and intensity are used interchangeably and refer to the relative amount of visible light emanating from an image, or each image pixel, that is necessary for a human to view an image can feel it. The brightness value of each pixel in the image is provided by the luma channel of the image signal. In an LCD display, the luma channel value determines the light transmittance of the LCD pixel. A second aspect of the invention is illustrated by LCD light transmission compensation, but this is not intended to limit the scope or functionality of the invention. Those skilled in the art will understand that the present invention can be practiced by directly compensating the LCD drive signal or by compensating the luma value within the image signal. The relationship between perceived brightness (B) of an image, backlight brightness (L) and LCD transmittance (T) is B=L×T.

The main steps of a preferred embodiment of the present invention are shown in FIG. 1 . An input LCD signal is input to the luminance distribution analysis 2 to find the signal luminance histogram 10 . In the backlight control aspect of the present invention, a clipping point (Clipping Point) is determined in the brightness histogram, and then a dimming factor 4 is determined according to the clipping point. In terms of LCD control (such as image compensation) of the present invention, the first step 5 is to determine a compensation range and a threshold of an LCD (or image) signal. Then steps 6 and 7 determine respective image signal mapping curves, which are used to map the original LCD signal range to the compensation signal range above and below the threshold. These two image signal mapping curves are combined in step 8 to form one final image signal mapping curve, which produces the output LCD signal 9 .

Figure 2 depicts a preferred method of using the brightness histogram 10 of the input LCD signal 1 to find the clipping point and backlight dimming factor. In the image histogram shown in Figure 2, the X or horizontal axis represents the original signal brightness value (such as the white gray level of the image-Y channel code value) from the minimum value (Imin) to the maximum value (Imax). The Y or vertical axis represents the number of pixels within signal 1 that have that luminance value. The peak value of pixel intensity in an image, Ip, is equal to or less than Imax; in this figure Ip is less than Imax. In a typical 8-bit signal, the minimum intensity value Imin is 0 and the maximum intensity value Imax is 255. For the present invention, these specific values are not required, and can be arbitrary values representing minimum and maximum brightness levels.

In order to facilitate the explanation of the present invention, the inventor introduces a concept of clipping point 11 (Clipping Point). The clipping point 11 is located at the 95% percentile of the luminance distribution range, above which only 5% of the pixels fall within the signal histogram 11 . This value of 5% was found by the inventors to provide optimum results, but is not essential to the invention. Typically, the clipping point will lie between the 60% and 99% percentile of the brightness distribution range, but depending on the particular image type, higher or lower values may yield better or equally acceptable results. An important feature of clipping point 11 is: for a darker image, clipping point 11 will be a low value (low value), because most pixels will have a low brightness value, but for a bright image, clipping point 11 will be a low value (low value), because most pixels will have a low brightness value, but for a bright image, the clipping point A magnitude of 11 would be a high value because more pixels will have higher brightness values. Those skilled in the art should understand that the position of the slice point 11 in the video image will dynamically change frame by frame.

The backlight dimming factor (BLdim) is a function of the dimmed backlight brightness with respect to a clipping point value, namely:

$\mathrm{<span\; class="notranslate">\; BLdim</span>}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; CP</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; P</span>}}$

Among them, CP is the clipping point value, Imax is the maximum pixel brightness value, and p is a constant not less than 1. The p value is determined according to the LCD input and output characteristics, and is usually close to the gamma (gamma) value of the display.

In a preferred embodiment, the backlight dimming factor (BLdim) is limited between an upper limit and a lower limit. The maximum dimming factor, ie the minimum amount of dimming, is 85%, and the minimum dimming factor, ie the maximum amount of dimming, is 55%. If the backlight dimming factor is higher than the upper limit, then the backlight dimming factor becomes the upper limit or 85%. If the dimming factor is below the lower limit, the dimming factor becomes the lower limit or 55%. This limit can be expressed by the following equation.

If BLdim > 85%, then BLdim = 85%

If BLdim<55%, then BLdim=55%

The upper and lower limits of the backlight dimming factor are not intended to limit the scope or functionality of the invention. The inventors believe that the 55% and 85% margins give the best results in the method according to the invention. However, other bounds can also be found to provide equally acceptable or possibly even better results. In a particular embodiment of the invention, this limit can be adjusted by the viewer to satisfy the value that yields the best results for personal subjective viewing.

To avoid an overall dimming in the perceived brightness of the image, the LCD transmittance value, or image brightness value, must be adjusted before displaying the image. However, while this is not critical to the first aspect of the invention, the method of determining a backlight dimming factor can be used in LCD displays without compensating the LCD or image signal to allow dimming of the backlight. The method of determining the backlight dimming level is dynamic and takes into account the brightness characteristics of the image signal, so this is an improvement over the backlight dimming method used here.

Compensation for LCD signal input 1 is shown by histogram 12 of Fig. 3, where the image luminance range extends from Ip to Imax/(BLdim) ^1/p . Another method for range expansion is to lengthen the image histogram 12 to move the clipping point 11 to Imax, so that the brightness of 5% of the pixels in the image is greater than Imax.

FIG. 4 is used for illustration only, showing the compensation image signal mapping described in FIG. 3 . The horizontal X-axis represents the original brightness range of the LCD signal, while the vertical Y-axis represents the latest or target brightness range of the signal. Dashed line 15 represents a 1 to 1 mapping where the original range is unchanged. The dotted line 16 represents a linear mapping from the original luminance range to the latest luminance range, while the solid line 17 represents a non-linear mapping from the original luminance range to the latest luminance range. The type of mapping used within the present invention is not critical and any linear or non-linear image signal mapping algorithm well known in the art may be used. Those skilled in the art will attest that it is practically impossible to extend the image brightness range above the Imax value, since brightness values above Imax will be clipped by the display hardware. In order to overcome this problem, the present invention introduces a threshold concept, below the threshold, the brightness range of the signal is expanded through the first image signal mapping function, thereby increasing the light transmittance of the LCD crystal to compensate for backlight dimming. Above the threshold, the luminance range of the signal is compressed using a second image signal mapping function in order to limit the image luminance values to the actual maximum luminance value Imax.

Figure 5 depicts a method of limiting image signal mapping above a threshold 18 to minimize clipping of higher luminance values. Threshold 18 is selected at a suitable luminance value below clipping point 11 . In the preferred embodiment, the threshold is chosen according to the equation |FP-CP|=|CP-Imax|, however, this is not essential to the invention, and those skilled in the art will recognize that the various parameters of choosing an appropriate threshold This method will produce suitable results. Below the threshold, the signal is mapped to the latest luminance range using a first linear or non-linear image signal mapping function. The first image signal mapping function is represented by the solid line 19 . This ensures that below the threshold, the LCD transmittance or image signal brightness increases to compensate for backlight dimming. However, above the threshold, a different image signal mapping function is used to limit the luminance value so that the maximum luminance value does not exceed Imax, the actual maximum value of luminance. The second image signal mapping function is shown by line 20 . In effect, the luminance range of the image signal is extended below the threshold to compensate for backlight dimming, and limited above the threshold to avoid clipping.

An example and a typical embodiment of the present invention have been described above. This is not intended to limit the scope of use or functionality of the invention. It should be understood that modifications and changes apparent to those skilled in the art are not to be considered outside the scope of the present invention.

Claims (26)

1. method that on the LCD that illuminates by back light apparatus, cuts down the consumption of energy, this method may further comprise the steps:

Determine to be displayed on the pixel brightness contribution of the picture signal on the LCD,

Determine a clipping point based on the pixel brightness contribution of this picture signal,

Determine the factor that dims backlight based on this clipping point, and dim factor adjustment backlight illumination according to this,

Dim the pixel brightness contribution that the factor is adjusted image pixel based on this, wherein use the first picture signal mapping function to be adjusted at the following pixel intensity of luminance threshold, and use the second picture signal mapping function to be adjusted at the above pixel intensity of luminance threshold.

2. method according to claim 1, wherein clipping point is between 60% and 99% percentage point of pixel brightness contribution.

3. method according to claim 1, wherein pixel brightness contribution is based on the white gray brightness value of picture signal interior pixel.

4. method according to claim 1, wherein pixel brightness contribution is based on the maximal value of redness, green and the blue channel brightness of picture signal interior pixel.

5. method according to claim 1, wherein dimming the factor is to be determined by following equation:

$\mathrm{BLdim}={\left(\frac{\mathrm{CP}}{I\max }\right)}^P$

Wherein CP is the amplitude limit point value, and Imax is the maximal value of pixel intensity, and p is one and is not less than 1 constant that the p value is to determine according to LCD input and output feature, common gamma value near display.

6. method according to claim 1, wherein luminance threshold is less than clipping point.

7. method according to claim 6, wherein threshold value is to be determined by following equation:

|FP-CP|＝|CP-Imax|

Wherein FP is a threshold value, and CP is a clipping point, and Imax is the maximal value of pixel intensity.

8. method according to claim 1, wherein a function in the first and second picture signal mapping functions is non-linear.

9. method according to claim 1, wherein second mapping function is a Nonlinear Mapping function.

10. method according to claim 9, the wherein non-linear second picture signal mapping function produces according to pixel brightness contribution.

11. a method of controlling the LCD display back lighting, this method may further comprise the steps:

Determine a clipping point based on the pixel brightness contribution of picture signal,

Based on clipping point determine one backlight dim the factor and

Illuminate backlight on the factor in definite dimming.

12. method according to claim 11, wherein clipping point is between 60% and 99% percentage point of pixel brightness contribution.

13. method according to claim 11, wherein pixel brightness contribution is based on the no colour brightness of picture signal interior pixel.

14. method according to claim 11, wherein pixel brightness contribution is based on the maximal value of redness, green and the blue channel brightness of picture signal interior pixel.

15. method according to claim 11, wherein dimming the factor is to be determined by following equation:

$\mathrm{BLdim}={\left(\frac{\mathrm{CP}}{I\max }\right)}^P$

Wherein CP is the amplitude limit point value, and Imax is the maximal value of pixel intensity, and p is one and is not less than 1 constant that the p value is to determine according to LCD input and output feature, common gamma value near display.

16. one kind is dimming the method that keeps the image visual perceived brightness on the LCD backlight, this method may further comprise the steps:

By one dim the factor dim backlight liquid crystal display and

Adjust the pixel brightness contribution of image pixel based on dimming the factor, wherein the first picture signal mapping function is used to be adjusted at the following pixel intensity of luminance threshold, and the second picture signal mapping function is used to be adjusted at the above pixel intensity of luminance threshold.

17. method according to claim 16 wherein dims backlight liquid crystal display and may further comprise the steps:

Determine a clipping point based on the pixel brightness contribution of picture signal,

Based on clipping point determine one backlight dim the factor and

Illuminate backlight on the factor in definite dimming.

18. method according to claim 17, wherein clipping point is between 60% and 99% percentage point of pixel brightness contribution.

19. method according to claim 17, wherein pixel brightness contribution is based on the no colour brightness of picture signal interior pixel.

20. method according to claim 17, wherein pixel brightness contribution is based on the maximal value of redness, green and the blue channel brightness of picture signal interior pixel.

21. method according to claim 17, wherein dimming the factor is to be determined by following equation:

$\mathrm{BLdim}={\left(\frac{\mathrm{CP}}{I\max }\right)}^P$

Wherein CP is the amplitude limit point value, and Imax is the maximal value of pixel intensity, and p is one and is not less than 1 constant that the p value is to determine according to LCD input and output feature, common gamma value near display.

22. method according to claim 18, wherein luminance threshold is less than clipping point.

23. method according to claim 18, wherein threshold value is to be determined by following equation:

|FP-CP|＝|CP-Imax|

Wherein FP is a threshold value, and CP is a clipping point, and Imax is the maximal value of pixel intensity.

24. method according to claim 18, wherein a function in the first and second picture signal mapping functions is non-linear.

25. method according to claim 18, wherein second mapping function is a nonlinear mapping function.

26. method according to claim 25, the wherein non-linear second picture signal mapping function produces according to pixel brightness contribution.

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