Patent Application
20120327303
The present invention is directed to backlit video display devices and more particularly to dynamic luminance scaling of a video signal for a backlit video display device to reduce power consumption of the backlit video display device.
In a backlit video display, a backlight panel illuminates an electronically controlled image panel. The backlight panel is of basically uniform brightness, although it may be partitioned so that parts of it can have their brightness controlled separately. The image panel has a matrix of pixel elements whose light transmission is modulated by electronic image input signals to form the image displayed. An example of a backlit display is a backlit liquid crystal display (LCD) in which the pixel matrix comprises liquid crystal material that modifies the polarization of light passing through it as a function of an electric field applied to the pixel. The image panel also includes orthogonal polarizing filters so that the intensity of the light transmitted through the pixel and the polarizing filters is a function of the electric field at each pixel. Examples of LCD image panels are thin-film transistor and active matrix image panels.
The power consumption of the backlight of a display may represent a significant proportion of the total power consumption of a mobile device, up to one third or even half of the total power consumption, for example. It is possible to reduce the backlight power consumption by dimming the backlight (that is to say reducing its luminance) during periods when the image to be displayed is less bright and compensating the backlight dimming by increasing the light transmission of the image panel, a technique known as dynamic luminance scaling (DLS). The adjustment of the luminance of the backlight may be obtained by pulse-width modulation (PWM), for example, in which the instantaneous luminance is maintained, but the proportion of the frame period during which the backlight is illuminated is modulated so as to modulate the average, perceived luminance during the frame. The adjustment of the luminance of the backlight may alternatively be obtained by control of a power management application specific integrated circuit (ASIC), for example.
The range of linearity of light transmission of the image panel is limited, due to saturation of the pixels. Accordingly, for example, if the backlight luminance is insufficient, too many of the pixels of the image panel in the brightest parts of the image may already be at their maximum light transmission, resulting in washout of the image displayed, that is to say lack of contrast in the brighter parts of the image. This limits the acceptable degree of backlight dimming.
It is undesirable if DLS changes of backlight luminance and compensating changes of image panel light transmission to achieve the power saving goal are perceptible as screen flickering, especially of the background parts of the picture, or as non-smooth visual quality.
The present invention is illustrated by way of example and is not limited by embodiments thereof shown in the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
In more detail, the input video signal VIDEOIN is provided by an input source 208 and may comprise luminance and chrominance data for individual pixel elements of the matrix 104. The dynamic luminance scaling module 204 includes a luminance adjustment module 210 which defines the target adjustments αcalc to luminance of the backlight 102 and to light transmission of the pixel matrix 104 so as to tend to reduce power consumption of the backlight to a value at which a compensating increase in transmission of light by the image panel maintains saturated the light transmission of a defined number of the pixel elements. The actual adjustments αm which the adaptive filter 206 generates are applied to vary the voltage VL and current IL for the backlight by a backlight luminance module 212, and are applied as a coefficient by an LCD light transmission module 214 to modify the luminance data of the input video signal VIDEOIN and provide the output video signal VIDEOOUT for the image pixel matrix.
The method 300 comprises detecting changes of luminance distribution between a plurality of frames of the input video signal VIDEOIN, the luminance distribution being a function of a distribution of pixel luminance values in the frames. Target adjustments αcalc to luminance of the backlight 102 and to light transmission of the image panel 104, 106, 108 for a current frame of the input video signal are defined to compensate luminance of the displayed image for the adjustment to luminance of the backlight. Actual adjustments αm to luminance of the backlight 102 and to light transmission of the image panel 104, 106, 108 for the current frame are functions of the target adjustments αcalc for the current frame and of the actual adjustments αm for a previous frame in proportions which are a function of the changes detected of luminance distribution.
In the processor 200 and the method 300, the changes of luminance distribution are detected between successive frames of the input video signal VIDEOIN. However, it is also possible to detect changes of luminance distribution between a plurality of frames of the input video signal VIDEOIN which are not consecutive. In this example, the actual adjustments αm to luminance of the backlight and to light transmission of the image panel for the current frame are substantially equal to the target adjustments αcalc for changes of luminance distribution between successive frames which correspond to rapid and/or large changes of scene in the input video signal, known as shot boundaries. However, the actual adjustments αm for the current frame are a greater proportion of the actual adjustments αm for a previous frame for changes of luminance distribution between successive frames which correspond to slower and/or smaller changes of scene in the input video signal.
In more detail, the method 300 starts for a frame N at 302. At 304, the input video signal VIDEOIN is down sampled if necessary to reduce the quantity of data to be processed by the data processor 200 in calculating the luminance adjustments αm, knowing that the full input data of the input video signal VIDEOIN is transferred to the backlit display device 100 in the output video signals VIDEOOUT after scaling by the luminance adjustments αm.
At 306, a distribution of pixel luminance values in the current frame N is calculated. Graphic representations of examples of such distributions of pixel luminance values in different frames are shown in
In general, the perceived luminance Li of the pixel i is given by:
L
i
=T(xi)×B
where B is the actual backlight luminance and T(xi) is the light transmission of the pixel i, that is to say the ratio of transmitted luminance to incident luminance. The unadjusted input video signal VIDEOIN defines a perceived luminance (Li)IN of the pixel i given by:
(Li)IN=T(xi)IN×B0
where B0 is the unadjusted backlight luminance implied by the input video signal VIDEOIN, and T(x)IN is the light transmission that the input video signal VIDEOIN defines for the pixel i. The compensating target adjustments to be made to the luminance of the backlight 102 and to the light transmission of the pixel matrix 104 can be represented as follows:
B
CALC
=B
0
÷f(αCALC)
T(xi)CALC=T(xi)IN×f(αCALC), so that
(Li)CALC=T(xi)CALC×BCALC=(Li)IN,
where B0 is the unadjusted backlight luminance implied by the input video signal VIDEOIN, BCALC is the adjusted target backlight luminance, T(xi)CALC is the adjusted target luminance data for the pixel i, and (Li)CALC is the target perceived luminance of the displayed pixel i. The target adjustments f(αcalc) for the current frame are chosen in this example to reduce power consumption of the backlight 102 to a value at which a compensating increase in transmission of light by the image panel maintains saturated the light transmission of a defined number of pixels of the matrix 104 in the brightest part of the image to be displayed, causing a degree of wash-out of the contrast of the brightest parts of the image. In this example, the target adjustment factor αcalc is chosen to saturate a selected percentage of the pixels, the backlight 102 being dimmed by the corresponding target adjustment 1/f(αcalc). In one example, where the range of possible luminance values contains 128 levels, the target adjustment factor αcalc is selected so that the pixels in the five brightest levels of the picture saturated.
In this example of an embodiment of the invention, the target adjustments f(αcalc) to the light transmission of the pixel matrix 104 are applied by multiplying the luminance component xi of the video signal data for each pixel i of the frame by the chosen factor αcalc., so that:
T(xi)CALC=T(xi×αCALC).
Often the light transmission of the pixels of the matrix 104 can be represented approximately as:
T(xi)=M×xiN,
where M and N are constant parameters. It then follows that:
T(xi)CALC=T(xi)IN×f(αCALC), therefore
f(αCALC)=αCALCN,
and 1/αCALCN is the target adjustment to be applied to the luminance of the backlight 102.
In this example, the actual adjustments αm to luminance of the backlight 102 and to light transmission of the image panel 104, 106, 108 for the current frame are smoothed at 312 as a function of the changes detected of luminance distribution between a processor of frames, representing scene changes in the video, which are detected at 310, before repeating the calculations for the next frame at 313. In this example, the actual adjustments αm applied to luminance of the backlight 102 and to light transmission of the image panel 104, 106, 108 for the current frame at 312 are a function represented by:
αm=w(x)αprev+(1−w(x))αcalc,
where w(x) is a weighting coefficient whose value is between 0 and 1 and which defines the proportions of the target adjustments for the current frame F and of the actual adjustments for a previous frame (F−1). The value of the weighting coefficient w(x) in this example is a function of the nature of the scene changes in the video, as indicated by the rapidity and/or magnitude of the changes of luminance distribution between a processor of frames detected at 310.
A video shot is defined as a series of interrelated consecutive frames taken contiguously by a single camera and representing a continuous action in time and space. A shot boundary is an abrupt change of scene. When a shot boundary occurs, the viewer's attention will be attracted by the rapid and/or large scene change and he will pay less attention to possible visual quality degradation caused by imperfect compensation of backlight scaling by the image panel transmission. In this example, at 314 the correlation C of histograms of the distribution of pixel luminance values between a current frame F and a previous frame (F−1) is calculated. In this example, C has a value between 0 and 1 and is calculated as C=(2×X×Y)÷(X2+Y2), where X and Y are vectors representing the numbers of pixels in each histogram bin of the luminance distribution of the frames F and (F−1) respectively. A shot boundary is considered to be detected if C is less than a threshold value CTH and the value of the weighting coefficient w(x) is then set to 0 at 316, so that the actual adjustments αm are set immediately to the same value as the target adjustments αcalc at 312. A typical value of the threshold value CTH in this example is 0.8.
Referring again to
In this example, change of the range of the distribution of pixel luminance values between the current frame and previous frames is measured after discarding a defined percentage of pixels having extreme pixel luminance values. In this example, thresholds are set LLOW and LHIGH and pixels having luminance values less than the lower threshold LLOW and pixels having luminance values greater than the upper threshold LHIGH are ignored in calculating the range of the distribution of pixel luminance values in the frame. In this example, the lower threshold LLOW and the upper threshold LHIGH are set so as to eliminate a set proportion of the total number of pixels in the frame, such as 2% for example. The range of the distribution of pixel luminance values is then measured as LHIGH−LLOW−. The pixels eliminated typically result from noise and could have a disproportionate distortional effect on the minimum and maximum values of pixel luminance, and hence on measurement of change of the range of the distribution of pixel luminance values. Noise may be introduced by the image capture process, such as the camera, or by the conversion to electronic signals and encoding/decoding, for example.
Referring again to
The saving of backlight power consumption obtained with the data processor 200 and the method 300 are a function of the video sequence being displayed. However, in typical video sequences, a power saving between 20% and 45% has been obtained with satisfactory viewing quality. Since the same adjustments to the transmission of light by the image panel can be applied to all the pixels of the matrix for a whole frame, the dynamic luminance scaling can be performed by modifying color space conversion (‘CSC’) parameters and can be accelerated by hardware such as an image processor unit.
However, the invention may also be implemented at least partially in a computer program for running on a computer system, at least including code portions for performing steps of a method according to the invention when run on a programmable apparatus, such as a computer system or enabling a programmable apparatus to perform functions of a device or system according to the invention.
A computer program is a list of instructions such as a particular application program and/or an operating system. The computer program may for instance include one or more of: a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.
The computer program may be stored internally on computer readable storage medium or transmitted to the computer system via a computer readable transmission medium. All or some of the computer program may be provided on computer readable media permanently, removably or remotely coupled to an information processing system. A computer process typically includes an executing (running) program or portion of a program, current program values and state information, and the resources used by the operating system to manage the execution of the process. An operating system (OS) is the software that manages the sharing of the resources of a computer and provides programmers with an interface used to access those resources. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the system.
In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims.
The terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.
The connections as discussed herein may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise, the connections may for example be direct connections or indirect connections. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice versa. Also, plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed manner. Likewise, single connections carrying multiple signals may be separated out into various different connections carrying subsets of these signals. Therefore, many options exist for transferring signals.
Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. Any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermediate components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality.
Those skilled in the art will recognize that boundaries between the above described operations are merely illustrative. Multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.
Also for example, the examples, or portions thereof, may be implemented as software or code representations of physical circuitry or of logical representations convertible into physical circuitry, such as in a hardware description language.
Also, the invention is not limited to physical devices or units implemented in non-programmable hardware but can also be applied in programmable devices or units able to perform the desired device functions by operating in accordance with suitable program code, such as mainframes, minicomputers, servers, workstations, personal computers, notepads, personal digital assistants, electronic games, automotive and other embedded systems, cell phones and various other wireless devices, commonly denoted in this application as ‘computer systems’.
However, other modifications, variations and alternatives are also possible. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive.
In the claims, the word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. The terms “a” or “an,” as used herein, are defined as one or more than one. The use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements The fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201110189595.6 | Jun 2011 | CN | national |
