Chapter 2 Color Breakup and Prior Solutions
2.4 Summary
The physiology of eye movements and the mechanism of CBU caused by different eye movements have been introduced. In order to suppress the CBU issue on FSC-LCDs and achieve novel type displays with lower power consumption, higher color saturation, and lower cost, many researchers have proposed several methods for CBU suppression, like increasing field rate, inserting multi-primary color fields, and utilizing motion compensation. However, these methods have their own challenges, such as LC limitation, image distortion, and uncertain eye movement, so they are hard to be applied on the hardware. Therefore, we proposed the “Stencil Field Sequential Color (Stencil-FSC) method to effectively suppress CBU with field rate of 240 Hz, and it is easier to be implemented on hardware.
Chapter 3 Stencil Field Sequential Color Method
The “Stencil Field Sequential Color (Stencil-FSC)” method with 240 Hz field rate was proposed for suppressing CBU. The concept and algorithm will be introduced at first, and the two processes for optimization, backlight colorful method and Fourier transformation process in will be presented in the following. Finally, the performance and the feasibility of Stencil-FSC method could be obtained.
3.1 Stencil Field Sequential Color Method
3.1.1 Concept
The conventional RGB color sequence utilized to display full color image on the FSC-LCD causes serious CBU because the field images are colorful and bright, like Fig. 3-2(b). Therefore, the “Stencil” concept was utilized to suppress CBU. The meaning of “stencil” is a unique technique of painting. Take Fig. 3-1 for example, if a boy will be painted on the wall by the stencil technique, a base color, white (Fig. 3-1 (b)), is drawn firstly, and then blocks of detail parts with different colors are put on the base color in order to add each detail color (Fig. 3-1(c)). Finally, the painting can be completed by the stencil technique as shown in Fig. 3-1(d). By the concept of stencil, the “Stencil Field Sequential (Stencil-FSC) Method” was proposed to suppress CBU. The Stencil-FSC method is a color sequence with four fields. A multi-color image is displayed in the first multi-color field instead conventional single-color fields, and the red, green, and blue field images are displayed to add red, green, and
B-field R-field G-field
R-field G-field B-field Multi-color field
(b)
(c) (a)
Target image
blue image details as shown in Fig. 3-2(c). By the method, the most color and luminance information are shown at the first multi-color field, so the red, green, and blue field images are darker and less colorful compared to those of conventional RGB color sequence, and it will be helpful for suppressing CBU. Additionally, the concept of utilizing multi-color fields instead of conventional single-color fields to display images was first proposed in FSC technique.
Fig. 3-1 The flowchart of stencil method. (a) Put block of base image, (b) paint base color, (c) put blocks of detail parts with different colors and paint color, and (d) complete the painting. (From http://www.wretch.cc/blog/Bbrother&article)
Fig. 3-2 (a) Target image. The field images of (b) RGB color sequence and (c) Stencil-FSC method.
(a) (b) (c) (d)
3.1.2 Display module
In order to get the first multi-color field, the FSC-LCD with Stencil-FSC method can be based on a locally controlled backlight system. The backlight system is divided into several regions, and it can be locally controlled according to the displayed image as shown in Fig. 3-3. Furthermore, the locally controlled backlight can be intensity or color, like Fig. 3-3(b) and Fig. 3-3(c) respectively. The technique is also called High Dynamic Range (HDR) technique [27-28], and the flowchart is given in Fig. 3-4. At first, the LC signal of target image with full-on white backlight (Io) is inputted, and then backlight LED signals (ILED) of each region are gotten by an algorithm of locally controlled backlight, such as maximum, root, or Inverse of a Mapping Function (IMF)[29]. Next, in order to get the compensated LC signal (Compensated LC), the backlight distribution of locally controlled backlight is obtained by convolution of ILED and light spread function (LSF). The compensated LC can be gotten by comparing the backlight distribution to the target image. Finally, the image can be displayed by composing the real backlight distribution of ILED and compensated LC.
The backlight can be locally controlled according to the image, so it can enhance the contrast ratio, save power consumption, and increase color saturation compared to those of conventional full-on backlight system. Therefore, by implementing the technique on the FSC-LCD, not only can the first multi-color field be generated, but
the display can also have the advantages of local controlled backlight technique.
Fig. 3-3 Locally controlled backlight technique. (a) Target image, (b) intensity locally controlled backlight, and (c) color locally controlled backlight.
Fig. 3-4 The flowchart of locally controlled backlight technique
(High Dynamic Range technique)
3.1.3 Algorithm
The Stencil-FSC method displays multi-color image at the first field to reduce the luminance and color of red, green, and blue fields, and it is helpful to suppress CBU. The algorithm can be explained by Fig. 3-5. At first, the locally controlled backlight and the compensated LC signals (LCR, LCG, and LCB) can be gotten by the algorithm of locally controlled backlight process as show in Fig. 3-4. Then, the
(a)
(b) (c)
Io ⊗ LED
I LSF I
ILED
Target image
( 8 × 8 )
Compensated LC Locally controlled B/L
⊗ LED LSF I
minimum compensated LC signal of red, green, and blue is taken as LCmin, like Eq.
3-1, and it will be the LC signal of first multi-color field.
LCmin= min (LCR, LCG, LCB) (3-1) By composing the LCmin with the color backlight from locally controlled backlight, the first multi-color field can be completed. At the second red-field, the LC signal (LCR’) is equal to Eq. 3-2 and the backlight signal is the red component of locally controlled backlight (Red B/L).
LCR’= LCR- LCmin (3-2)
Then, the second red-field can be generated by composing the LC signal of LCR’ and the backlight of Red B/L. Furthermore, the green and blue fields also can be gotten by the algorithm of red-field. Finally, the field images of color, red, green, and blue can be produced. By displaying them in time sequence, a full color image can be perceived on the FSC-LCD.
Fig. 3-5 The algorithm of Stencil-FSC method
Target image
multi-color field red-field green-field blue-field
3.2 Algorithm Optimization
The Stencil-FSC method can be completed by utilizing the algorithm introduced in last section 3.1.2. However, the algorithm has two issues. First, the first multi-color field is not colorful enough, and it will cause the luminance and color of red, green, and blue field are obvious. Thus, it can not suppress CBU effectively. Second, the generating process of backlight distribution is too complicate. In general, convolution method is usually used to get backlight distribution. However, the method has complicate computation complexity, and it is almost impossible to be utilized on hardware. Therefore, backlight colorful method and Fourier transformation process are utilized to solve these two issues and optimize the algorithm of Stencil-FSC method.
3.2.1 Backlight colorful method
The field images generated from the algorithm mentioned in section 3.1.2 are shown in Fig. 3-6. Unfortunately, the multi-color field is not colorful enough.
Therefore, the most color and luminance information are displayed at red, green, and blue field, and the CBU suppression is limited. Therefore, the backlight colorful method is proposed to overcome this issue. In order to make the first multi-color field be more colorful, it is important to reduce the difference between compensated LC signal of red, green, and blue, which the backlight colorful method is based on the concept. The original locally controlled backlight signal is gotten by the backlight process as show in Fig. 3-7(a). In order to make the backlight more colorful, the minimum backlight is dimmed by a ratio (ex. 10%), and new locally controlled backlight signal is gotten, like Fig. 3-7(b). By a fundamental equation of displaying, Eq. 3-3, Iimage and IBL denote the emitting intensity of image and backlight
respectively; GLLC denotes the gray level of LC signal.
Iimage = (GLLC/255)γ × IBL (3-3) If the backlight intensity is reduced, the LC signal will be enhanced. Therefore, a larger minimum LC signal can be generated, and the multi-color field will be more colorful by the method, as shown in Fig. 3-8. Thus, the most color information is displayed in the first multi-color field, so the field images of red, green, and blue will be less colorful, and the CBU suppression will be more effective.
Furthermore, by utilizing the backlight colorful method to get the more colorful multi-color field, the determination of the dimming ratio is a critical work. In Table.
3-1, a comparison of different dimming ratio is made to discuss the effect on the CBU and the distortion ratio (D) of image, and the distortion ratio is defined by Eq. 3-4.
Distortion ratio (D) = number of distorted pixels/ number of total pixels (3-4) There are three dimming ratios are presented, and some summaries can be made. The smaller dimming ratio can get more colorful multi-color field, and it is more helpful for suppressing CBU. The green components of CBU with different dimming ratios are shown in the brackets of second row. However, the smaller dimming ratio may cause more image distortion because the LC and backlight cannot display enough luminance. Thus, the dimming ratio of backlight dimming method is needed to be optimized, and the optimization will be discussed in the Chapter 5.
Fig. 3-6 The field images of Stencil-FSC algorithm mentioned in last section
Fig. 3-7 The locally controlled backlight signal of (a) original algorithm and (b) algorithm with the backlight colorful method
Fig. 3-8 The field images of Stencil-FSC algorithm with the backlight dimming method
Table. 3-1The CBU and the distortion ratio (D) of image with different dimming ratio
(225,161,65) (225,161,7)
3.2.2 Backlight distribution generation by Fourier transformation process
In order to get the compensated LC signal, the backlight intensity distribution of the locally controlled backlight must be known. The usual method is utilizing convolution process. By measuring the light intensity distribution of an individual region, a light spread function can be gathered, like Fig. 3-9(a). Then, a backlight distribution can be generated by making a convolution of the light spread function and locally controlled backlight signal, like Fig. 3-9. The convolution method is a straight way to get the backlight distribution. However, the computation complexity is shown in Eq.3-5, and the X and Y mean the size of light spread function; the X1 and Y1 stand for the size of image size, and No. means the number of backlight regions.
Computation complexity = (X*Y)*(No.)+ (No.)*(X1*Y1) (3-5) It is an elaborate computation, and it is extremely hard to be realized on hardware.
Therefore, an alternative method, “Fourier transformation process,” is utilized to get backlight distribution.
A Fourier Transformation with low-pass filter is used to simplify the backlight distribution generation as shown in Fig. 3-10. At first, the locally controlled backlight signal is extended into the image size, and then the signal in frequency domain is gotten by making a Fourier Transformation to the extended locally controlled backlight signal. Next, a low-pass filter covers the locally controlled backlight signal in frequency domain to block the information of high frequency which is the edge part in spatial domain. In the step, a Gaussian low-pass filter is chosen because it can prevent from the ringing phenomenon in the image as shown in Fig. 3-11[30]. Finally, a blur image can be gotten by making an inverse Fourier Transformation to the locally controlled backlight signal passed filter in frequency domain. The computation
complexity is equal to Eq. 3-6, and the X and Y stand for image size.
Computation complexity =4*[(X*Y)+(X*Y)+ (X*Y)] (3-6) In order to prove the computation simplification, a comparison of computation complexity is made between the convolution process and the Fourier Transformation process. The image and light spread function size are set as 1366*768, and the number of backlight regions is divided into 16*12. By inserting the parameters into Eq. (3) and Eq. (4), Fourier transformation process has only 3% computation compared to convolution process. Therefore, generating the backlight distribution by Fourier Transformation process can simplify the computation effectively, and it can enhance the feasibility of the Stencil-FSC method on the hardware.
Fig. 3-9 Convolution process. (a) Light spread function,(b)the locally controlled backlight signal, and (c) the convolution backlight.
Fig. 3-10 Fourier transformation process. (a) The extended locally controlled backlight, (b) the backlight signal in frequency domain, (c) the Gaussian low-pass filter, and (d) The blurred image.
FT Low-pass filter (FT)-1
Locally controlled B/L (Spatial domain)
Locally controlled B/L (Frequency domain)
(a) (b) (c) (d)
12*16
=
Light spread function B/L distribution
X Y
X1
(a) (b) (c)
Y X
B\L distribution
Fig. 3-11 Ringing phenomenon. (a) Target image, (b) blurred image with ringing, and (c) blurred image without ringing.
(From Digital Image Process, Gonzalez and Woods, p.180-185)
3.3 Summary
Base on the concept of “stencil” which is a technique of painting, a Stencil-FSC method was proposed to suppress the CBU on the FSC-LCD. The method was based on locally controlled backlight system, and it displays a multi-color image in the first multi-color field, so it could reduce the color and luminance of red, green, and blue fields to suppress CBU. However, the original algorithm had some issues, so the backlight colorful method was proposed to get more colorful multi-color field, and the Fourier transformation process was utilized to simplify the computation complexity.
By the optimized algorithm, the Stencil-FSC can not only suppress CBU effectively, but also enhance the feasibility on the hardware. Moreover, Stencil-FSC will be verified by simulation and demonstration in the next two chapters.
(a) (b) (c)
Chapter 4 Experimental Demonstration
Stencil-FSC method was proposed to suppress CBU on the FSC-LCD.
Furthermore, two optimize algorithms, the backlight colorful method and the Fourier Transformation process, have been utilized to enhance CBU suppression and reduce the computation complexity. In order to verify the Stencil-FSC method, a demonstration on the real FSC-LCD will be completed.
4.1Approximation of Backlight Intensity Distribution
In the demonstration, the Fourier Transformation (FT) process was utilized instead of the convolution method to generate light distribution of locally controlled backlight and simplify the computation complexity as mentioned in 3.2.2. Therefore, in order to demonstrate correctly, it is important to make the backlight intensity distribution generated form Fourier Transformation process the same as that form convolution process.
In the Fourier transformation process, a Gaussian low-pass filter was used to filter the high frequency component of locally controlled backlight image in frequency domain, and a blur backlight image can be got to be the simulated light distribution. The equation of the Gaussian low-pass filter is shown in Eq. 4-1, and the diagram is shown in Fig. 4-1
2 2( ,)/2 0
) ,
(u v e D uv D
H = − (4-1) H is the magnitude of the low-pass filter, D is a coordinate in frequency domain, and D is the cut-off frequency to determine how blurry the image is. The definition of D
is the ratio of the coordinate where the maximum magnitude deceases to 60.7% and the total width of the Gaussian profile (W). Therefore, the lager D0 denote the Gaussian profile is broader, the more high frequency component of image can pass through, and the clearer image can be generated as shown in Fig. 4-2(b). Conversely, the smaller D0 denote the Gaussian profile is narrower, less high frequency component of image can pass through, and the more blurry image can be generated, as in Fig.
4-2(c). Therefore, the blurry image with different blur level can be generated by adjusting the D0 parameter, and it can be utilized to simulate the backlight intensity distributions with different light spread function.
D0
W
H
60.7%
02 2(, )/2
) ,
(u v e D uv D
H = −
D
Fig. 4-1 Gaussian low-pass filter(a) (b) (c)
Fig. 4-2 (a) Target image. Blurry images by using FT process with (b) D0=0.01 and (c) D0=0.001
Then, an approximation of the real backlight intensity distribution on the 32” FSC-LCD supported by C-Company was made by Fourier Transformation process. Parameter, D0, is adjusted to get different blurry backlight image, and the color difference (ΔE*ab) is
used (Fig. 4-3 (a)-(c)), and the results are shown in Fig. 4-4. When D0 is close to 0.0023, the minimum ΔE*ab can be gotten, that means the blurry backlight is closest to convolution backlight, as in Fig. 4-5. Therefore, when implementing Fourier Transformation process on the 32” FSC-LCD, the D0 will be chosen to be 0.0023 to get the most realistic backlight distribution, and the compensated LC signal can be gotten.
(a) (b)
(c)
Fig. 4-3 Test images. (a) Soccer, (b) color balls, and (c) girl.
0
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 Do
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 Do
Fig. 4-5 (a)Target images, (b) convolution backlights, and (c) FT blurry backlights with D0 =0.0023 of three test images.
(c)FT blurry B/L (a)Target image (b)Convolution B/L
4.2 Experimental Demonstration I-15Hz/Frame
A Conventional LCD was used to simulate a FSC-LCD with Stencil-FSC method, and the Stencil-FSC method can be verified by using the experimental demonstration.
At first, simulated field images of the Stencil-FSC method are generated by a simulation program created by Matlab software, and the simulation parameters were set to match the real 32” FSC-LCD. The number of backlight sub-regions equals 12*16, D0 in Gaussian low-pass filter equals 0.0023, and the dimming ratio was set 50% to prevent image distortion. In the following, the simulate field images were displayed sequentially on the conventional LCD with 60Hz frame rate. However, Stencil-FSC method was a color sequence with four fields so the frame rate of the simulated FSC-LCD was only 15Hz, which was so low that human will perceive flicker phenomenon. Therefore, a High-speed CCD (Charge Coupled Device) camera was utilized to capture images with 15Hz capture frequency, and the images were displayed in 60Hz frame rate. Finally, Stencil-FSC images with 60Hz frame rate (240Hz field rate) can be simulated successfully. Finally, a moving camera was used to simulate eye movement and capture the CBU phenomenon in a conventional RGB color sequence and the Stencil-FSC method on the experimental LCD. Two test images, Lily and Girl, were used to verify the Stencil-FSC method as shown in Fig.
4-6. Lily is a white image which causes serious CBU using conventional RGB color sequence, and Girl is a colorful image utilized to test the CBU and image distortion for each color. The experimental results are presented in Fig. 4-7 and Fig. 4-8. In both test images, the CBU phenomenon is effectively suppressed by utilizing Stencil-FSC method, and there is no image distortion. Therefore, the Stencil-FSC method was
successfully verified by the experimental demonstration.
(a) (b)
Fig. 4-6 The test image. (a) white image: Lily and (b) colorful image : Girl
(a) RGB color sequence (b) Stencil-FSC method Fig. 4-7 The captured CBU image of Lily with (a) conventional RGB color sequence and (b) The Stencil-FSC method
(a) RGB color sequence (b) Stencil-FSC method Fig. 4-8 The captured CBU image of Gril with (a) conventional RGB color sequence and (b) The Stencil-FSC method
4.3 Experimental Demonstration II-60Hz/frame
The second experimental demonstration verified the Stencil-FSC method on a 32” FSC-LCD supported by C-company with local dimming backlight of 12*16 sub-regions and 180Hz field rate. The schemes of the FSC-LCD are presented in Table. 4-1.Because the field rate of the FSC-LCD was only 180Hz, and the frame rate must be set higher than 60Hz to prevent flickering, the panel could only display three fields sequentially in a frame. However, the Stencil-FSC method is a color sequence with four fields, so one of the fields must be rejected. Because human eye is less sensitive to blue compared to red or green, the blue field image can be rejected with less sensitivity to color distortion. Moreover, white and yellowish images were chosen to be the test images since the color contribution from blue field images were not important when utilizing the Stencil-FSC method. Therefore, two test images, Lily and Sunflower, were used in the experimental demonstration, and the field images are shown in Fig. 4-9.
Table. 4-1 Schemes of the 32” FSC-LCD 32-inch FSC-LCD
OCB-mode LC
1366 × 768 16 × 12 Divisions BL
48 × 24 (1152) LEDs Field rate 180 Hz (3-field)
(a)
(b)
Fig. 4-9 Test images of (a) Lily and (b) Sunflower and their field images
In the experimental demonstration, the backlight signal for 12*16 sub-regions was sent to the backlight system to get locally controlled color backlight, as in Fig.
In the experimental demonstration, the backlight signal for 12*16 sub-regions was sent to the backlight system to get locally controlled color backlight, as in Fig.