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Engineers at Clairvoyante (Cupertino, CA) are using a red/green/blue/white (RGBW) pixel architecture, dubbed “PenTile,” and subpixel rendering to display scenes with more-natural luminance and chrominance than traditional RGB stripe techniques (see Fig. 1). While RGBW has been used for some time-to illuminate imagery on signboards, for example-according to the company’s president and CEO, Joel Pollack, viewing artifacts such as washed-out colors have plagued the technique. To address these artifacts, Clairvoyante has developed subpixel rendering algorithms to match pixel patterning to the receptive characteristics of the human eye.

Saturated colors in natural scenery are almost never bright, and bright colors are almost never saturated. Reproducing the natural palette based on subtractive coloration in additive electronic display systems (that add red, green, and blue subpixels to create each white pixel) almost always calls for compromises in brightness and color saturation, as well as tradeoffs between image quality and power consumption. Adding a white subpixel to the conventional RGB mix, however, can significantly boost the visual range from unsaturated brightness to deep, dark, richly saturated hues in a single display.1 It can also boost contrast for reading black and white text.

Matching to human vision

For Clairvoyante, the keys to achieving this practically and economically enough for the highly competitive mobile-display market lie in matching the architecture and function of RGBW displays to the approximations, assumptions, and interpolations of the human vision system. Unlike a video camera, which faithfully transmits whatever it “sees,” the human eye, which is actually part of the brain, tends to pass on visual information that varies temporally or spatially. Simply stated, the visual cortex receives the derivative of a field of view that it must then integrate to reconstruct an actual scene.

The RGBW display process focuses on working in tandem with the processes of visual physiology. For instance, red and green photoreceptors are an order of magnitude more plentiful in the human eye than blue, which thus plays a much smaller role than the other two in visual resolution. Therefore, the Clairvoyante displays tend to have fewer blue subpixels than do conventional RGB stripe architectures. In addition, their white subpixels, while brightening the display, also improve resolution. The architecture and algorithms behind the Clairvoyante displays are also designed around what cofounder, CTO, and board chairman Candice Brown Elliott has described as the three perceptual channels of retinal processing: luminance, red-versus-green chrominance, and yellow-versus-blue chrominance.2

The competitive economics show up in the fact that the RGBW PenTile approach requires one-third fewer pixels to display a scene than RGB stripe, thereby achieving the same modulation-transfer function (MTF) with one-third fewer transistors, and cutting cost and power consumption for similar displays (see Fig. 2). Alternatively, it can enable up to 50% brighter displays than with RGB stripes at the same level of cost and power consumption. The former is particularly important for Clairvoyante because a primary market focus is the mobile-display market, in which low power consumption becomes a key enabler for adding functionality to visual displays. Pollack notes that on cell phones, for instance, display backlighting is a major consumer of power (about one-third), and that adding video content would boost that percentage to about one-half.

Hassaun A. Jones-Bey

REFERENCES

1. C.H.B. Elliott et al., Information Display 5(05) 26.

2. C. Johnson, EE Times, www.eetimes.com/news/latest/showArticle.jhtml?articleID=175004265

Tue Aug 01 00:00:00 CDT 2006

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