Patent Grant
9208730
This invention relates generally to a display system, and more particularly to a display system having a backlight.
Some display devices, such as liquid crystal displays (LCDs), often use a backlight as a light source to display an image. Typically, the backlight is on at a constant, preselected brightness level to illuminate the display panel. The brightness level of the backlight can be high or low. While a brighter backlight usually results in a higher image quality, it results in high power consumption. High power consumption is disadvantageous, especially for a portable display system that draws its power from a battery. Adjusting the backlight level to a lower brightness decreases power consumption but yields lower image quality, for example with noticeable visual artifacts in parts of the image that show bright, saturated colors.
Different techniques have been developed to optimize the backlight level by balancing the image quality concerns with the power conservation goals. One such technique involves dynamically adjusting the global backlight level as an image is displayed. However, the dynamic global backlight adjustment technique has room for improvement. For example, dynamic adjustment often results in a backlight level that is higher than what is needed to show a high-quality image, using more power than necessary.
A method of adjusting the backlight level in a power-efficient manner without compromising image quality is desired.
In one aspect, the invention is a method of optimizing the luminance level of a backlight system. The method entails dividing the backlight system into zones, each of the zones having a light source; determining a target illumination level for each of the zones based on data values of a display panel that is to be illuminated by the backlight system; and setting a backlight drive value of the light source in a particular zone based on target illumination level of the particular zone and a backlight drive value assigned to its neighboring zone.
The invention makes high-quality image display achievable with reduced power consumption by taking advantage of the “communal” aspect of the zones in a local dimming backlight system. In determining the backlight drive value of a zone, the invention takes into account the illumination contributions from neighboring zones.
One way to improve power usage efficiency is to adjust the backlight level section-by-section instead of controlling the entire backlight with a single drive value. Sometimes referred to as “local dimming,” this method allows parts of the image that does not need a bright backlight to operate with a lower backlight (thereby saving power) while the parts of the image that need bright backlight will still get bright backlight to display a high-quality image. Local dimming method entails dividing a backlight into multiple zones and controlling the brightness level of each zone with its own backlight drive value. The brightness level of each zone is controlled independently of other zones although, as will be described below, the brightness of neighboring zones will be taken into account in determining the brightness of a particular zone.
The backlight drive value of each of the zones may be selected on a frame-by-frame basis depending on the image that is being displayed. For example, if a dark cave scene is being displayed where an explorer in the cave discovers a bright yellow object, the zones that display the bright yellow object will be assigned a high backlight level while the other zones may be assigned a relatively low backlight level. With this type of localized backlight adjustment, it is possible to achieve optimum display quality at low power consumption.
Although the particular embodiment of
Although an embodiment where each zone has one pixel or sub-pixel is contemplated, the invention will be described in the context of there being multiple pixels in a zone. The zones do not necessarily have to be equal in size, the number of pixels, or the number and configuration of the assigned light sources 32.
The reason for the backlight level ending up higher than needed is because light from a light source 32 spreads, illuminating not just its own display zone but also contributing to the illumination of the neighboring display zones.
The invention stems from a recognition that there is a “communal” aspect to local dimming stemming from the fact that light in each zone is not completely contained in that zone. Each zone receives light from its own light source 32 and also from neighboring zones that contribute to the luminance of the given zone. A “neighboring” zone includes but is not limited to an immediately adjacent zone. For example, depending on the size of the zones and the point spread function of the light sources 32, a zone that is multiple zones away can contribute to the illumination of a zone.
The invention includes a recursive technique for minimizing light source drive values in a display system that implements local dimming. The local backlight optimization process is based on a number of concurrent processes, the number equaling the total number of zones whose drive values are to be optimized. Each process keeps track of the total current backlight of all the spreading functions at one or more points in a given backlight zone. Each process also keeps track of the difference (A) between the current backlight illumination level and the target backlight illumination level.
The current illumination level at each zone is surveyed as well, and set to have[i] (step 64). The LEDs are characterized, and each LED drive value has a PSF associated with it. Hence, have[i] value may be determined through mathematical modeling based on the drive values.
The difference (A) between need[i] and have[i] is calculated (step 66) and checked for convergence (step 67). Depending on the implementation, the condition of “convergence,” as used herein, may deemed to be satisfied if a preselected number of iterations have completed, if Δ is less than a preset amount, or if Δ keeps bouncing back between two values (e.g., +1 and −1). If the convergence condition is not satisfied, have[i] is incremented again (step 68) and the iterative process continues until the processes converge to optimum backlight drive values that meet or exceed the desired effective backlight targets for all the zones i.
An “incrementation,” as used herein, may be done by a positive amount or a negative amount (i.e., it includes decrementation). Typically, if need[i]>have[i], the backlight drive value is incremented by a positive amount to get have[i] closer to need[i]. On the other hand, if need[i]<have[i], the backlight drive value is incremented by a negative amount. In addition, in making the decision in step 68, each zone takes into account the state that their neighboring zones are in. For example, a zone increments the backlight drive value if its neighboring zones have reached 100% of their drive values and have still not reached their target luminance (i.e., the neighbors are “exhausted”). Hence, even if the Δ value for that zone is already smaller than the preset value and its own needs are satisfied, its backlight drive value may still get incremented to help its neighbor(s). Each process for a zone “i” can consider the data values of all the other processes and independently make a decision as to whether to increment its associated backlight driving value up or down. In one embodiment, the values need[i] and have[i] are represented in nits or candelas.
Each incrementing of the value is usually done by a small preset magnitude. However, a multiplicative gain factor may also be used to increment the drive values up or down. Alternatively, the magnitude of the increment amount can be dynamically adjusted (instead of being a constant present amount) to speed up the rate of steady state convergence. Small increments are preferred when trying to minimize oscillatory behavior. Such small increment magnitudes can be thought of as high dampening factors on the recursive processes.
As shown, the effective backlight values (i.e., the illumination levels) that are determined in the LED Drive Value Decision block 100 are fed back into the Digital Processing block. The Digital Processing block uses the backlight information to control the valve/shutter of the pixels in the display panel 20. The degree of brightness displayed by a pixel depends on both the illumination level of the backlight and how much of the light is transmitted through the valve/shutter of the pixels. For example, in the case of an LCD, the orientation of the liquid crystal molecules acts as the valve/shutter and determines the amount of light transmission. Hence, to achieve the same level of brightness, the display system 10 can use a high backlight illumination level and use the shutter/valve to let through a portion of it, or use a lower backlight illumination level and open the shutter/valve wider to let more of the backlight through. The Digital Processing block sets the shutter/valve to achieve the desired level of brightness in the ultimate image that is shown, taking into account the effective backlight values.
Using the relative luminance requirements, the Target Backlight Data Structure 96 for need[i] is populated. During this process, each zone may be further suvidivided into subdivisions to enhance the accuracy of the illumination adjustment. For example, with six zones, each zone can be divided into two subdivisions such that there will be twelve subdivisions (e.g., [i] would have [i1] and [i2]). The statistical data extracted from every subdivision constitutes a “survey.” “Effective backlight,” as used herein, is the illumination level.
A sample LUA Code for One-dimensional local dimming system is provided below. The survey block 90 divides the input image into subdivisions, the number of which is controlled by SUBDIV. The statistical data extracted from every subdivision is referred to as a “survey.”
Although the Example above is for a one-dimensional local dimming system, the optimization method provided herein may be adapted to two-dimensional systems. The optimization method may be used for backlight systems with white light sources or backlight systems having colored light sources (e.g., R, G, B or other primary).
After clamping, the in-gamut values may enter a Sub-Pixel Rendering (SPR) block 86. If the display panel 20 contains a novel subpixel repeating group, such as what is described in U.S. Pat. Nos. 7,876,341, 7,701,476, or 7,583,279, subpixel rendering would be applied to convert the data to the particular subpixel layout. The data may then be processed in an optional output gamma block 87 before being output as backlight drive values 84.
Embodiments of the invention and all of the functional operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the invention can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a computer-readable medium for execution by, or to control the operation of, data processing apparatus.
A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.
The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit).
Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio player, a Global Positioning System (GPS) receiver, to name just a few. Computer-readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.
To display images, embodiments of the invention can be implemented on a computer that is connected to a display device, e.g., a CRT (cathode ray tube), OLED (organic light emitting diode), or LCD (liquid crystal display) monitor and an input device (e.g., keyboard and a pointing device, e.g., a mouse or a trackball). Other kinds of devices can be used to provide for interaction with a player as well; for example, feedback provided to the player can be any form of sensory feedback, e.g. visual feedback, auditory feedback, or tactile feedback; and input from the player can be received in any form, including acoustic, speech, brain waves, other physiological input, eye movements, gestures, body movements, or tactile input.
While this specification contains many specifics, these should not be construed as limitations on the scope of the invention or of what can be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features can be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination can be directed to a subcombination or variation of a subcombination.
Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing can be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.
While the foregoing has been with reference to particular embodiments of the invention, it will be appreciated by those skilled in the art that changes in this embodiment may be made without departing from the principles and spirit of the invention.
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