WO2014191049A1

WO2014191049A1 - Method for controllling the backlight of a lcd panel

Info

Publication number: WO2014191049A1
Application number: PCT/EP2013/061277
Authority: WO
Inventors: Jacob Lerenius
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2013-05-31
Filing date: 2013-05-31
Publication date: 2014-12-04

Abstract

The present invention relates to a method for controlling a backlight of a LCD panel for a display device, said LCD panel being arranged to use a RGB backlight system, the method being comprising the steps of: applying a Content Adaptation Backlight Control, CABC, algorithm separately on each of the colors red, green, and blue of the RGB backlight system; and adjusting the backlight of the LCD panel based on the three separate outputs from the CABC algorithm for each color red, green, and blue. Furthermore, the invention also relates to a display device, a computer program, and a computer program product thereof.

Description

METHOD FOR CONTROLLLING THE BACKLIGHT OF A LCD PANEL

Technical Field

The present invention relates to a method for controlling the backlight of a LCD panel. Furthermore, the invention also relates to a display device, a computer program, and a computer program product thereof.

Background of the Invention

LCD panels are common in handheld devices, such as mobile phones and tablet devices. When converting electricity to light seen by a user looking onto an LCD panel, there are mainly three steps required, i.e.: converting the light from electricity, filtering the colors, and adjusting the intensity of the light in each sub-pixel in the LCD panel.

In a white LED based system, a white light with a broad spectrum is generated from a blueish LED covered with a phosphor layer. This light is then evenly distributed using a light guide and filtered into different intensity blue, red and green sub-pixels on the screen that makes up a pixel. The three sub-pixels are small enough to make the human eyes to see it as one dot with a particular color. In an RGB (red, green, blue) backlight system, the light consists of three narrow peaks in the wavelength spectrum. Each of the three colors is generated by a single LED and no phosphor layer is used. By using narrow peaks, these can be adjusted to match that of the color filters. The difference in efficiency between mentioned two systems is that in the latter system the colors can match the filters better, the conversion efficiency loss in the phosphor layer is not present, and a lower voltage can be used to drive the red LED.

There is also another white backlight technology used in LCD backlight applications using quantum dots. However, the same applies for this technology where the conversion efficiency between the blue LED and the white light has to be added along with color filter matching and a higher driving voltage for the red light.

Most of the power consumed in LCD panels comes from the screen backlight and the process of converting the electricity to light and keeping losses as low as possible when leading the light to and through the display. A problem with a LCD panel is that the full backlight intensity is used to illuminate the full LCD even when only one pixel is lit. There are however a few technologies that address this problem. For example, using many sections of backlighting is possible on large screens to reduce the energy consumption but is not convenient on small screens due to the small size.

Another technique to reduce backlight intensity is denoted Content Adaptive Backlight Control (CABC). This means that the image content to be displayed is analyzed and the backlight intensity is changed according to the analyzed content. For example, if a dark image is shown, the backlight intensity may be adjusted to a lower value and when the image is bright, the backlight intensity may be increased. In current mobile handsets, normally white LEDs are used to illuminate the LCD panel. By detecting the image content and adjusting the backlight together with the full image according to the brightest pixel, power can be saved. Smart algorithms are used to reduce the dynamic range in the image to get the higher possible power saving, i.e. dynamic compression.

According to a prior art solution, by performing filtering, a highest value of the backlight pixel data is found. This highest value is then translated together with all the other pixels to the maximum brightness level. At this stage, a dynamic compression in the peak values or for the full range may also take place along with other image processing such as enhancement of the contrast. To account for this pixel level translation, the backlight intensity is translated down equally to compensate for the pixel translation. As pixels in a LCD panel consumes near equal amount of power independently of what is shown, the power saved when lowering the backlight level is significant.

To be able to save power in mobile devices is one of the highest priorities. However, saving power in the user interface according to prior art is almost impossible with the white LED CABC as the contrast ratio is most often 100 % on one or more colors, resulting in no power saved unless quality (contrast) is affected. Moreover, mentioned prior art was developed for white LED backlight systems only. Summary of the Invention

An object of the present invention is to provide a solution which mitigates or solves the drawbacks and problems of prior art solutions. Another object of the invention is to provide a backlight solution for LCD panels which reduces power consumption compared to prior art.

According to a first aspect of the invention, the above mentioned objects are achieved by a method for controlling a backlight of a LCD panel for a display device, said LCD panel being arranged to use a RGB backlight system, the method comprising the steps of:

- applying a Content Adaptation Backlight Control, CABC, algorithm separately on each of the colors red, green, and blue of the RGB backlight system; and

- adjusting the backlight of the LCD panel based on the three separate outputs from the CABC algorithm for each color red, green, and blue.

Different preferred embodiments of the above method are defined in the appended dependent claims.

The method according to the present invention may be comprised in computer program having code means which when run by processing means executes the present method.

According to a second aspect of the invention, the above mentioned objects are achieved with a display device comprising at least one LCD panel and at least one image processing unit, said LCD panel comprising at least one display section and being arranged to use a RGB backlight system, wherein the RGB backlight system is controlled by said image processing unit, wherein said image processing unit is arranged to

- apply a Content Adaptation Backlight Control, CABC, algorithm separately on each of the colors red, green, and blue of the RGB backlight system; and is further arranged to

- adjust the backlight of the LCD panel based on the three separate outputs from the CABC algorithm for each color red, green, and blue. Such a display device may be arranged in a mobile unit, such as a mobile phone or a tablet device or any other suitable mobile unit. The device may also be modified according to the different embodiments of the present method. Further applications and advantages of the present invention will be apparent from the following detailed description.

Brief Description of the Drawings

The appended drawings are intended to clarify and explain different embodiments of the present invention in which:

Fig. 1 illustrates the present invention;

Fig. 2 illustrates how the user interface can be adjusted with the use of the present invention;

Fig. 3 shows a first flow chart according to an embodiment of the invention; and - Fig. 4 shows a second flow chart according to an embodiment of the invention.

Detailed Description of the Invention

To achieve the aforementioned and other objects, the present invention relates to a method for controlling a backlight of a LCD panel for a display device. In the present invention it is assumed that the LCD panel is arranged to use a RGB backlight system which is well known in the art.

The present method comprises the steps of applying the CABC algorithm separately on each of the colors red, green, and blue of the RGB backlight system, and thereafter adjusting the backlight of the LCD panel based on the three separate outputs from the CABC algorithm for each color red, green, and blue. The CABC algorithm is applied simultaneously and in parallel for the three different colors for each frame.

A solution that uses RGB backlight and a CABC algorithm for each color channel is therefore proposed. As the LCD panel is one of the main consumers of energy in a mobile device, by using RGB backlight as well as implementing the present invention, the total power efficiency will be improved which e.g. means longer user times for the end consumer. In RGB backlight systems the potential power saving is large as the three channels can be optimized independently compared to prior art with the use of the present invention.

The image (frame) will be analyzed and adjusted together with the corresponding backlight for each color individually according to the invention. By adjusting each color's data in the image by adjusting the dynamic range, peak intensities and increasing the "brightness" of the image to the highest possible, the backlight of the corresponding color backlight contribution may be reduced accordingly. Instead of using the full pixel brightness data to calculate the backlight level, the data from each sub-pixel is used together with the RGB backlight level control. Each frame will analyzed based on the intensity data for each color and adjusted together with the corresponding backlight for each color individually. Fig. 1 illustrates the present invention. By combining each sub-color (red, green and blue) image consisting of the image data and the corresponding color's backlight brightness data the final image is shown and should show the same information as if the RGB-CABC was not used. However, depending on the level of filtering and processing used in the algorithm, there may be small artifacts compared with a system where the RGB-CABC is not used. Hence, according to an embodiment of the invention the steps of applying the CABC algorithm for each color and each image frame involves the steps of deriving a maximum pixel brightness of each color (red, green, and blue). Increasing a brightness of each color and decreasing the backlight of the LCD panel by the same ratio for each color. This embodiment is illustrated in the flow chart of fig. 3.

The step of increasing the brightness may according to another embodiment of the invention involve increasing the brightness of all pixels of each image frame until the brightest pixels has reached a maximum value. The maximum value can be the maximum available brightness level for the used number of bits per color. This means that the LCD sub-pixel is open 100 %.

According to yet another embodiment of the invention the step of deriving the maximum pixel brightness of each color is preceded by image preprocessing so as making the CABC algorithm as efficient as possible and also to further improving the performance of the present invention. These preprocessing algorithms may in one alternative include dynamic compression. After the preprocessing, the data is analyzed and the peak value is detected. By using the peak value as input, the whole matrix of pixels for this color is translated to the maximum brightness and the backlight is than adjusted to a lower value accordingly. The result is a pixel brightness that is the same as original frame but possibly with some dynamic range losses.

The preprocessing may however affect the image quality and may therefore have parameters to control the balance between power saving and quality. It is however possible to save power even without changing the quality. It is possible to increase the brightness of the image and lowering the backlight as long as the brightest pixel on the screen is not fully white.

The maximum pixel brightness for each color can be derived either directly or by applying a low-pass filter. When finding the maximum brightness for a color in the image, the actual maximum value can be used, or a value obtained by low pass filtering where for example the maximum value of the average between two or more adjacent pixels is used. This can be an option so that not only one single bright pixel will impact alone on the algorithm.

The above embodiment is illustrated in the flow chart of fig. 4. The individual filtering and processing of the image data for the different colors of each frame is used to adopt the data to be able to save more energy in the system on the expense of dynamic range or contrast. The amount of adjustment done will depend on what information is being shown and a general trade-off between power and quality. According to yet another embodiment of the invention anti-flicker compensation for each color is applied between each image frame. This will prevent flickering of the image due to fast changes of the backlight. If this is an optically visible issue when the backlight changes in large steps, an anti flickering function may be used to reduce the change between each frame. By taking advantage of the present invention also in the User Interface (UI) design, significant power can be saved. For example, a red clock on the screen will only use the red LED backlight, powered at 2 V and matching the red filter exactly resulting in much lower power consumption compared with a solution using a white LED backlight and thus in this case keeping the LEDs at the maximum level set by the system. Saving power in the UI is almost impossible with the while LED content adaptive backlight control as the contrast ratio is most often 100 % on one or more colors. See fig. 2 for an illustration of a UI which uses the present invention. In fig 2 only the green color of the three colors is used in the UI so as to reduce the power consumption. It is therefore realized that the design of the UI can be adjusted so as to take the full advantage of the present invention thereby increasing the power saving even more.

The invention may also be used in a system (display) using two or more separate sections where the backlight may be controlled independently for each section. This means that the present method is applied on each display section separately. If only one section displays bright data, the other sections do not have to adjust to the first section but can adjust according to the brightest pixel in its own section which saves power. The present invention also relates to a display device comprising at least one LCD panel and at least one image processing unit. The device comprises the means and is arranged to perform the present method. The device may e.g. have at least one image processing unit arranged to control the RGB backlight system of the LCD panel. The image processing unit is arranged to apply the CABC algorithm separately on each of the colors red, green, and blue of the RGB backlight system. The unit is further arranged to adjust the backlight of the LCD panel based on the three separate outputs from the CABC algorithm for each color. The present device has the same advantages as the present method. It should also be realized that the device can be modified, mutatis mutandis, according to the different embodiments of the present method.

Moreover, the present device is preferably arranged in a mobile unit such as a mobile phone or a tablet device. However, the skilled person realizes that the method and device of the invention can also be used in stationary devices such as television and computer screens. Furthermore, as understood by the person skilled in the art, any method according to the present invention may also be implemented in a computer program, having code means, which when run by processing means causes the processing means to execute the steps of the method. The computer program is included in a computer readable medium of a computer program product. The computer readable medium may comprises of essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive. It is however noted that the present invention may also be implemented in dedicated hardware of suitable devices.

Finally, it should be understood that the present invention is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claims.

Claims

1. Method for controlling a backlight of a LCD panel for a display device, said LCD panel being arranged to use a RGB backlight system, the method being characterized by the steps of:

2. Method according to claim 1, wherein for each image frame the applying step involves the steps of:

- deriving a maximum pixel brightness of each color red, green, and blue;

- increasing a brightness of each color red, green, and blue; and

- decreasing the backlight of the LCD panel by the same ratio for each color red, green, and blue.

3. Method according to claim 2, wherein the step of deriving said maximum pixel brightness is preceded by the steps of preprocessing, namely:

- analyzing an image for each color component red, green, and blue; and

- dynamically compressing each color red, green, and blue based on the output from the analyzing step.

4. Method according to claim 2, wherein said maximum pixel brightness of each color red, green, and blue is derived directly or by applying a low-pass filter for each color red, green, and blue.

5. Method according to claim 2, wherein the step of increasing said brightness of each color red, green, and blue involves:

- increasing a brightness of all pixels of each image frame until the brightest pixels has reached a maximum value.

6. Method according to claim 2, further comprising the step of: - applying anti-flicker compensation for each color red, green, and blue between each image frame.

7. Method according claim 1, wherein said LCD panel comprises a plurality of separate display sections and the method steps are performed separately for each display section.

8. Method according to claim 1, wherein the applying steps of the CABC algorithm for each color red, green, and blue are performed in parallel.

9. Computer program, characterized in code means, which when run by processing means causes said processing means to execute said method according to any of claims 1-8.

10. Computer program product comprising a computer readable medium and a computer program according to claim 8, wherein said computer program is included in the computer readable medium, and comprises of one or more from the group: ROM (Read-Only Memory), PROM (Programmable ROM), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically EPROM) and hard disk drive.

11. Display device comprising at least one LCD panel and at least one image processing unit, said LCD panel comprising at least one display section and being arranged to use a RGB backlight system, wherein the RGB backlight system is controlled by said image processing unit, characterized in that said image processing unit is arranged to

- adjust the backlight of the LCD panel based on the three separate outputs from the CABC algorithm for each color red, green, and blue.

12. Mobile unit, such as a mobile phone or a pad, comprising at least one display device according to claim 11.