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(a) Conventional RGB Stripe arrangement, (b) PenTile RGB subpixel arrangement utilizing 33% fewer subpixels, (c) PenTile RGBW subpixel arrangement utilizing 33% fewer subpixels. 

(a) Conventional RGB Stripe arrangement, (b) PenTile RGB subpixel arrangement utilizing 33% fewer subpixels, (c) PenTile RGBW subpixel arrangement utilizing 33% fewer subpixels. 

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Many of portable devices such as smart phones, portable multimedia players (PMP), and digital single-lens reflex (DSLR) cameras are capable of capturing high-resolution images (e.g. 10 mega-pixel in DSLR) or even video. The limited battery power supply in the portable devices often prevents these systems to use high-power large liquid crystal displ...

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... single pixel on a color liquid crystal display (LCD) contains several individual color primaries, typically three color elements ordered (on various displays) either as blue, green, and red (BGR), or as red, green, and blue (RGB). 1 Some displays may have more than three primaries, often called multi-primary, such as the combination of red, green, blue, and yellow (RGBY), or red, green, blue, and white (RGBW), or even red, green, blue, yellow, and cyan (RGBYC) [1]. These color primaries, sometimes called subpixels, are fused together to appear as a single color to human due to the blurring by the optics and spatial integration by nerve cells in the human eyes. Methods that take the interaction between display technology and human visual system (HVS) into account are called subpixel rendering algorithms [2, 3]. Subpixel rendering technology is well suited to LCDs, where each (logical) pixel corresponds directly to three or more independent color subpixels, but less so for cathode ray tube (CRT). This is because in a CRT the light from the pixel components often spreads across pixels, and the outputs of adjacent pixels are not perfectly independent (see footnote 1). About 20 years ago, the Apple II personal computer Introduced a proprietary high-resolution LCD graphics display, in which each pixel has two vertical stripe subpixels with green and magenta colors, respectively. Without subpixel technology, a diagonal white line on Apple II display could only be drawn using “whole” white pixels composed of a paired green and purple subpixels, as shown in Fig. 1(a) [3]. Thanks to Apple’s built-in subpixel technology, white pixels are often composed of adjacent subpixels to yield a much smoother result, as shown in Fig. 1(d). Similar situation exists for modern-day RGB vertical stripe LCD panels. Figure 2 shows a common problem when a sloping edge is displayed by pixel rendering, and how it can be suppressed by subpixel rendering. Simple pixel-based rendering causes sawtooth in the sloping edge in Fig. 2(a). Thanks to the fact that a pixel is composed of three separable subpixels, we can “borrow” subpixels from adjacent whole pixels. Figure 2(b) depicts that using subpixel rendering, the apparent position of the sloping edge is micro-shifted by a one or two subpixel width, giving a much smoother result compared to Fig. 2(a). However, subpixel rendering may cause local color imbalance called “color fringing artifact” [3–5], because for some pixels, only one or two subpixels are turned on/off, as shown in Fig. 2(c). Although the components of the pixels (primary colors: RGB) in an image sensor or display can be ordered in different patterns or pixel geometry, the geometrical arrangement of the primary colors within a pixel can be varied depending on the usage. In computer monitors such as LCDs that are mostly used to display edges or rectangles, the companies would typically arrange the subpixel components in vertical stripes. However, in displays for motion pictures, companies would tend to arrange the components to have delta (or tri- angular) or other two dimensional (2D) patterns so that the image variation is perceived better by the viewer. In 2000, Clairvoyante developed the “PenTile” matrix as a new approach to build and drive color flat panel displays [6, 7]. The PenTile design takes advantage of the way the human eye and brain process visual information and optimizes the pixel layout to match this process. Various subpixel layouts have been proposed by Clairvoy- ante/Nouvoyance (and demonstrated by Samsung) as members of the PenTile matrix family [6, 7]. Illustrated in Fig. 3 are a conventional RGB vertical stripe subpixel arrangement and higher-efficiency PenTile RGB TM (RGBG), PenTile RGBW TM subpixel arrangements. PenTile RGBG layout uses green pixels interleaved with alternating red and blue pixels, due to the fact that the human eye is most sensitive to green, especially for high- resolution luminance information. As a result, the RGBG scheme creates a color display with one third fewer subpixels than the traditional RGB–RGB scheme but with the same measured luminance display resolution. The PenTile RGBG offers improvements in cost performance and power efficiency compared to conventional RGB stripe display, due to the combined effect of increased aperture ratio in LCD devices or decreased current density in Organic light- emitting (OLED) devices. And it has been widely used in various phones, such as the Google/HTC Nexus One Android phone, Samsung i9000 Galaxy S phone, Samsung Wave S8500 series phones as well as the newly released Galaxy Nexus phone. In PenTile RGBW layout, one pixel contains two subpixels only and every two consecutive pixels would have these four subpixels: red, green, blue, and white. For any two consecutive rows, the color pattern of the second row is shifted to the right by 1 pixel location. Thus, all the subpixels in PenTile RGBW appear to have delta configuration, which should be good for displaying edges in many orientations. Displays made using the PenTile RGBW TM pattern offer improvements in cost performance and power efficiency compared to conventional RGB stripe displays, due to the combined effect of increased aperture ratio and improved light transmission through the white (clear) subpixel. Note that Motorola Atrix 4G phone uses PenTile RGBW TM pixel geometry display. VP (visual perception) dynamics is another company working on displays with special dedicated subpixel rendering technologies. They have two major products: VPX and VPW [8, 9]. In their VPX LCD panel, they modify the regular RGB stripe pixel geometry by shifting every other line to the right by one subpixel location, as shown in Fig. 4(c), making it similar to the delta configuration. With this modification, the VPX LCD panel combined with a subpixel rendering driver can achieve three times ( 3 × ) higher horizontal resolution than the regular RGB stripe LCD panel. As they only change the arrangement of the color filter for the subpixels, the VPX panel can be manufactured with essentially the same process as regular LCD. In their VPW panel, they modified the LCD panel such that a regular RGB stripe pixel with three subpixels (RGB) is replaced by a VPW pixel with 4 square-shaped subpixels corresponding to red, green, blue and white color (RGBW), as shown in Fig. 4(d). The main advantage of VPW (RGBW) technology is four times ( 4 × ) higher resolution ( 2 × horizontal resolution and 2 × vertical resolution) and lower power consumption. As the shapes of the VPW subpixels are different from the regular RGB stripe LCD, VPW manufacturing probably requires more modification than VPX. Subpixel rendering techniques originate from the problem of monochrome font rendering on LCDs. Previously, simple pixel-based font display was used and the smallest level of detail that a computer could display on an LCD was a single pixel. However, researchers found that, by control- ling the subpixel values of neighboring pixels, the number of points that may be independently addressed to reconstruct the image is increased, and it is possible to micro-shift the apparent position or orientation of a line (such as the edge of a font), by one or two subpixel width, to achieve better edge reconstruction [10, 11]. In 1998, Microsoft announced a subpixel-based font display technology called “ClearType” [2]. Note that Microsoft ClearType is software-only subpixel technique capable of improving the readability of text on regular LCD with three vertical stripe subpixels (red, green, and blue), which requires no change of display hardware. With ClearType running on an LCD monitor, features of text as small as a fraction of a pixel in width can be displayed. Figure 5 illustrates an example of displaying the letter “m” with traditional pixel rendering and ClearType [2]. It is obvious that ClearType can reduce staircase artifacts effectively and reconstruct the shape information more faithfully. Microsoft ClearType is especially suitable when rendering relatively small-size font, and the width of consecutive font size probably differs by subpixel only. While subpixel rendering may cause local color imbalance (color fringing artifacts), Microsoft ClearType suppresses color artifacts via “energy sharing”, where each subpixel’s “energy” spreads across it and its two neighboring subpixels by turning on such subpixel and its two immedi- ately adjacent neighbors each with 1/3. Hence, the energy of a single subpixel is shared with its two neighbors instead of putting all the energy entirely within it [10, 11]. ...
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... flat panel dis- plays [6,7]. The PenTile design takes advantage of the way the human eye and brain process visual informa- tion and optimizes the pixel layout to match this process. Various subpixel layouts have been proposed by Clairvoy- ante/Nouvoyance (and demonstrated by Samsung) as mem- bers of the PenTile matrix family [6,7]. Illustrated in Fig. 3 are a conventional RGB vertical stripe subpixel arrange- ment and higher-efficiency PenTile RGB TM (RGBG), Pen- Tile RGBW TM subpixel arrangements. PenTile RGBG layout uses green pixels interleaved with alternating red and blue pixels, due to the fact that the human eye is most sensitive to green, especially for high- resolution ...

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