Embodiments of the present invention comprise methods and systems for generating, modifying and applying backlight driving values for an LED backlight array.
Some displays, such as LCD displays, have backlight arrays with individual elements that can be individually addressed and modulated. The displayed image characteristics can be improved by systematically addressing backlight array elements.
Some embodiments of the present invention comprise methods and systems for generating, modifying and applying backlight driving values for an LED backlight array.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.
Embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The figures listed above are expressly incorporated as part of this detailed description.
It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the methods and systems of the present invention is not intended to limit the scope of the invention but it is merely representative of the presently preferred embodiments of the invention.
Elements of embodiments of the present invention may be embodied in hardware, firmware and/or software. While exemplary embodiments revealed herein may only describe one of these forms, it is to be understood that one skilled in the art would be able to effectuate these elements in any of these forms while resting within the scope of the present invention.
In a high dynamic range (HDR) display, comprising an LCD using an LED backlight, an algorithm may be used to convert the input image into a low resolution LED image, for modulating the backlight LED, and a high resolution LCD image. To achieve high contrast and save power, the backlight should contain as much contrast as possible. The higher contrast backlight image combined with the high resolution LCD image can produce much higher dynamic range image than a display using prior art methods. However, one issue with a high contrast backlight is motion-induced flickering. As a moving object crosses the LED boundaries, there is an abrupt change in the backlight: In this process, some LEDs reduce their light output and some increase their output; which causes the corresponding LCD to change rapidly to compensate for this abrupt change in the backlight. Due to the timing difference between the LED driving and LCD driving, or an error in compensation, fluctuation in the display output may occur causing noticeable flickering along the moving objects. The current solution is to use infinite impulse response (IIR) filtering to smooth the temporal transition, however, this is not accurate and also may cause highlight clipping.
An LCD has limited dynamic range due the extinction ratio of polarizers and imperfections in the LC material. In order to display high-dynamic-range images, a low resolution LED backlight system may be used to modulate the light that feeds into the LCD. By the combination of modulated LED backlight and LCD, a very high dynamic range (HDR) display can be achieved. For cost reasons, the LED typically has a much lower spatial resolution than the LCD. Due to the lower resolution LED, the HDR display, based on this technology, can not display high dynamic pattern of high spatial resolution. But, it can display an image with both very bright areas (>2000 cd/m2) and very dark areas (<0.5 cd/m2) simultaneously. Because the human eye has limited dynamic range in a local area, this is not a significant problem in normal use. And, with visual masking, the eye can hardly perceive the limited dynamic range of high spatial frequency content.
Another problem with modulated-LED-backlight LCDs is flickering along the motion trajectory, i.e. the fluctuation of display output. This can be due to the mismatch in LCD and LED temporal response as well as errors in the LED point spread function (PSF). Some embodiments may comprise temporal low-pass filtering to reduce the flickering artifact, but this is not accurate and may also cause highlight clipping. In embodiments of the present invention, a motion adaptive LED driving algorithm may be used. A motion map may be derived from motion detection. In some embodiments, the LED driving value may also be dependent on the motion status. In a motion region, an LED driving value may be derived such that the contrast of the resulting backlight is reduced. The reduced contrast also reduces a perceived flickering effect in the motion trajectory.
Some embodiments of the present invention may be described with reference to
In some embodiments, the backlight image is given by
bl(x,y)=LED(i,j)*psf(x,y) (1)
where LED(i,j) is the LED output level of each individual LED in the backlight array, psf(x,y) is the point spread function of the diffusion layer and * denotes a convolution operation. The backlight image may be further modulated by the LCD.
The displayed image is the product of the LED backlight and the transmittance of the LCD: TLCD(x,y).
img(x,y)=bl(x,y)TLCD(x,y)=(led(i,j)*psf(x,y))TLCD(x,y) (2)
By combining the LED and LCD, the dynamic range of the display is the product of the dynamic range of LED and LCD. For simplicity, in some embodiments, we use a normalized LCD and LED output between 0 and 1.
Some exemplary embodiments of the present invention may be described with reference to
In these embodiments, the image may be low-pass filtered and sub-sampled 12 to an intermediate resolution. In some embodiments, the intermediate resolution will be a multiple of the LED array size (aM×aN). In an exemplary embodiment, the intermediate resolution may be 8 times the LED resolution (8M×8N). The extra resolution may be used to detect motion and to preserve the specular highlight. The maximum of the intermediate resolution image forms the Blockmax image (M×N) 14. This Blockmax image may be formed by taking the maximum value in the intermediate resolution image (aM×sN) corresponding to each block to form an M×N image. A Blockmean image 16 may also be created by taking the mean of each block used for the Blockmax image.
In some embodiments, the Blockmean image 16 may then be tone mapped 20. In some embodiments, tone mapping may be accomplished with a 1D LUT, such as is shown in
Flickering in the form of intensity fluctuation can be observed when an object moves cross LED boundaries. This object movement can cause an abrupt change in LED driving values. Theoretically, the change in backlight can be compensated by the LCD. But due to timing differences between the LED and the LCD, and mismatch in the PSF used in calculating the compensation and the actual PSF of the LED, there is typically some small intensity variation. This intensity variation might not be noticeable when the eye is not tracking the object motion, but when the eye is tracking the object motion, this small intensity change can become a periodic fluctuation. The frequency of the fluctuation is the product of video frame rate and object motion speed in terms of LED blocks per frame. If an object moves across an LED block in 8 video frames and the video frame rate is 60 Hz, the flickering frequency is 60 hz*0.125=7.5 Hz. This is about the peak of human visual sensitivity to flickering and it can result in a very annoying artifact.
To reduce this motion flickering, a motion adaptive algorithm may be used to reduce the sudden LED change when an object moves across the LED grids. Motion detection may be used to divide a video image into two classes: a motion region and a still region. In the motion region, the backlight contrast is reduced so that there is no sudden change in LED driving value. In the still region, the backlight contrast is preserved to improve the contrast ratio and reduce power consumption.
Motion detection may be performed on the subsampled image at aM×aN resolution. The value at a current frame may be compared to the corresponding block in the previous frame. If the difference is greater than a threshold, then the backlight block that contains this block may be classified as a motion block. In an exemplary embodiment, each backlight block contains 8×8 sub-elements. In some exemplary embodiments, the process of motion detection may be performed as follows:
For each frame,
The LED driving value is given by
where LEDmax is the local max of LEDs in a window that centers on the current LED. One example is a 3×3 window. Another example is a 5×5 window.
In some embodiments, motion estimation may be used. In these embodiments, the window may be aligned with a motion vector. In some embodiments, the window may be one-dimensional and aligned with the direction of the motion vector. This approach reduces the window size and preserves the contrast in the non-motion direction, but the computation of a motion vector is much more complex than simple motion detection. In some embodiments, the motion vector values may be used to create the enlarged motion map. In some embodiments, the motion vector values may be normalized to a value between 0 and 1. In some embodiments, any motion vector value above 0 may be assigned a value of 1. The motion status map may then be created as described above and the LED driving values may be calculated according to equation 4, however, LEDmax would be determined with a 1D window aligned with the motion vector.
Since the PSF of the LED is larger than the LED spacing to provide a more uniform backlight image, there is considerable crosstalk between the LED elements that are located close together.
Because of the PSF of the LEDs, any LED has contribution from each of its neighboring LEDs. Although Equation 2 can be used to calculate the backlight, given an LED driving signal, deriving the LED driving signal to achieve a target backlight image is an inverse problem. This is an ill-posed de-convolution problem. In one approach, a convolution kernel is used to derive the LED driving signal as shown in Equation 3. The crosstalk correction kernel coefficients (c1 and c2) are negative to compensate for the crosstalk from neighboring LEDs.
The crosstalk correction matrix does reduce the crosstalk effect from its immediate neighbors, but the resulting backlight image is still inaccurate with a too-low contrast. Another problem is that it produces many out of range driving values that have to be truncated and can result in more errors.
Since the LCD output can not be more than 1, the LED driving value must be derived so that backlight is larger than target luminance, e.g.,
led(i,j):{led(i,j)*psf(x,y)≧I(x,y)} (5)
In Equation 5, “:” is used to denote the constraint to achieve the desired LED values of the function in the curly bracket. Because of the limited contrast ratio (CR), due to leakage, LCD(x,y) can no longer reach 0. The solution is that when a target value is smaller than LCD leakage, the led value may be reduced to reproduce the dark luminance.
led(i,j):{led(i,j){circle around (x)}psf(x,y)<I(x,y)·CR} (6)
In some embodiments, another goal may be a reduction in power consumption so that the total LED output is reduced or minimized.
Flickering may be due to the non-stationary response of the LED combined with the mismatch between the LCD and LED. The mismatch can be either spatial or temporal. Flickering can be reduced or minimized by reducing the total led output fluctuation between frames.
where vx and vy are the motion speed in term of LED blocks. Combining Equations 5 and 8 yields Equation 9 below.
In some embodiments, the algorithm to derive the backlight values that satisfy Eq. 8 comprises the following steps:
Finding an LED driving value from a target value is an ill-posed problem that requires an iterative algorithm, which is difficult to implement in hardware. The method, of some embodiments of the present invention, can be implemented as a single pass method. These embodiments may be described with reference to
In some embodiments, the derived LED value 67 from the single pass algorithm can be less than 0 and greater than 1. Since the LED can only be driven between 0 (minimum) and 1 (maximum), these values may be truncated to 0 or 1. Truncation to 0 still satisfies Eq. 4, but truncation to 1 does not. This truncation causes a shortfall in backlight illumination. In some embodiments, this shortfall may be compensated by increasing the driving value of neighboring LEDs. In some embodiments, this may be performed by error diffusion methods. An exemplary error diffusion method is illustrated in
In some embodiments, a post processing algorithm may be used to diffuse this error as follows:
1. For these ledi,j>1
2. tmpVal=ledi,j−1;
3. set ledi,j=1;
4. Sort the 4 neighboring LEDs to ascending order
5. If (max−min<min(diffThd, tmpVal/2)
else
In some situations, the LED output may be non-linear with respect to the driving value, and, if the driving value is an integer, inverse gamma correction and quantization may be performed to determine the LED driving value.
LED driving is commonly done with pulse width modulation (PWM), where the LED driving current is fixed and its duration or “on” time determines the light output. This pulse width driving at a 60 Hz frame rate can cause flickering. Therefore, two PWM pulses are typically used in prior art methods. This doubles the backlight refresh rate so that flickering is reduced or eliminated. However, the use of two PWM pulses may cause motion blur at higher duty-cycles or ghosting (double edges) at lower duty-cycles. To reduce both flickering and motion blur, motion adaptive LED driving may be used.
To compensate for the time difference between LCD driving from top to bottom, a BLANK signal is used to synchronize PWM driving with the LCD driving. These embodiments may be further illustrated with reference to
The use of two PWM pulses in one LCD enables motion adaptive backlight flashing. If there is no detected motion, the two PWM pulses may have the same width, but may be offset in time by half of an LCD frame time. If the LCD frame rate is 60 Hz, the perceived image is actually 120 Hz, thereby eliminating the perception of flickering. If motion is detected, PWM pulse 192 may be reduced or eliminated, while the width of PWM pulse 293 is increased to maintain the overall brightness. Elimination of PWM pulse 192 may significantly reduce the temporal aperture thereby reducing motion blur.
In some embodiments, the next step is to predict the backlight image from the LED. The LED image may be upsampled to the LCD resolution (m×n) and convolved with the PSF of the LED.
The LCD transmittance may be determined using Equation 10.
TLCD(x,y)=img(x,y)/bl(x,y) (10)
In some embodiments, inverse gamma correction may also be performed to correct the nonlinear response of the LCD. In these embodiments, a normalized LCD transmittance value 100 may be mapped with a tonescale curve 102 to an LCD driving value 104.
The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalence of the features shown and described or portions thereof.
This application claims the benefit of U.S. Provisional Patent Application No. 60/940,378, entitled “Methods and Systems for Motion Adaptive Backlight Driving for LCD Displays with Area Adaptive Backlight,” filed on May 25, 2007; this application is also a continuation-in-part of U.S. patent application Ser. No. 10/966,258, entitled “Adaptive Flicker and Motion Blur Control,” filed on Oct. 15, 2004; this application is also a continuation-in-part of U.S. patent application Ser. No. 11/219,888, entitled “Black Point Insertion,” filed on Sep. 6, 2005; and this application is also a continuation-in-part of U.S. patent application Ser. No. 11/157,231, entitled “Image Display Device with Reduced Flickering and Blur,” filed on Jun. 20, 2005. All applications listed in this section are hereby incorporated herein by reference.
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Child | 11843529 | US | |
Parent | 11157231 | Jun 2005 | US |
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Parent | 10966258 | Oct 2004 | US |
Child | 11157231 | US |