Basically, what we have done at our company, Clairvoyante Inc., in Cupertino, Calif., is to rectify these shortcomings of the Bayer pattern. First, our pattern of subpixels, called the PenTile Matrix, addresses the Bayer’s color imbalance by going easy on the green. In addition, our pattern is rotated with respect to that of the Bayer by 45 degrees; this rotation has the effect of mapping the conventionally square orientation of the incoming pixel data to the high-luminance green subpixels on a one-to-one basis. We also resize the subpixels with respect to each other, making the green ones a lot smaller, a process that renders information with higher resolution.
Another seemingly slight tweak enhances the display’s performance immensely. By adding a white (or clear) subpixel to form a red-green-blue-white pixel, we dispense with one out of four color filters. We thereby boost efficiency: color filters absorb wavelengths from the backlight, so they sap energy from a display. We call this scheme the PenTile RGBW pattern.
We weren’t the first to come up with an RGBW display; past experimenters did it by simply swapping one of the two green subpixels for a white one in every pixel of a Bayer display. What we did differently was to squeeze the subpixels into long rectangles.
Even so, there is still an inherent inefficiency in using four subpixels instead of three. To get around the problem, we found ways to render a pixel with an average of just two subpixels--two-thirds as many as in the conventional RGB pattern. We do it by using software algorithms to create, in effect, virtual pixels. Basically, the algorithm fools the eye. It defines an edge of an object in an image with the red, green, and white subpixels, and adds the requisite dash of blue off to the side, on the ground that the eye cannot discern the exact location of the blue bits, anyway. Such tricks provide very crisp images with good color.
Another trick enhances the color on a single pixel indirectly. The goal is to minimize the number of color subpixels needed to display an image by getting each one to work as hard as possible in resolving the image. And there is a lot of freedom in doing so. For instance, to enhance a red pixel on a gray background, you can add a dash of white--increasing the luminance of the red--and also turn down the surrounding blue and green. The eye perceives this reduction of the blue and green as an enhancement of the red.
The bottom line is that brightness and color can be conveyed in more than one combination of red, green, blue, and white. Orange, for instance, will look the same to a human eye whether it comes from a single, pure wavelength at 600 nanometers or from the combination of two or more wavelengths from the red and yellow bands of the spectrum.
In tests, our PenTile Matrix pattern did well on many kinds of image files, including video, computer-generated graphics, and still and moving images [see illustration, ” ”]. The PenTile technology, however, works well only in high-resolution formats. If the resolution is too low--below about 185 dots per inch for near-range devices like cellphones--it can produce artifacts, such as texture in the background. The matrix did especially well displaying the higher-performance versions of the Joint Photographic Experts Group (JPEG) and MPEG-2 compression formats. A version of MPEGâ''2 is what is used to compress video data so that a whole movie and more can fit on a DVD. Briefly, MPEG-2 has several versions; the kind used on DVDs is known as 4:2:0, which indicates the ratio of the samples used to convey the moving image’s brightness (the 4) and the color (the 2 and the 0). A superior form of MPEGâ''2 is 4:2:2, because it allows for more detailed color sampling.
Our PenTile Matrix did particularly well with this superior form of MPEG-2. That was encouraging to us, because MPEG 4:2:2 is the compression format used for high-definition video, which is growing in popularity with the proliferation of big-screen TVs and the imminent arrival of high-definition video players, such as those offering the competing Blu-ray and HD DVD formats.
Vertical black-and-white lines show off the advantages of the PenTile RGBW [see illustration, ”Two Do the Job of Three”]. While the conventional pattern must turn three adjacent columns on and three other columns off, our display renders the same detail by turning just two on and two off. Because this scheme requires just two-thirds as many subpixels as the conventional one, fewer transistors and drive lines are needed to control the subpixels, and less of the display’s area needs to be obscured by opaque elements. In other words, the aperture ratio increases, letting more light through. This improvement, together with the white subpixel, provides about twice the brightness for a given draw of power. The savings in manufacturing costs more than balances any increase occasioned by the addition of a fourth, clear, color filter.
Engineers can use the gains to save power or to rev up the brightness. In a cellphone with a 2.8-inch display, you can get a luxurious 350-candela-per-square-meter level of brightness for 475 milliwatts. That’s a power level that would give you a meager 175 cd/m2 if you used a conventional pattern. If, however, you are content with the meager brightness, you can make do with 233 mW--and with a backlight having half as many light-emitting diodes.
In anticipation of the expected demand for optimized displays, several manufacturers have already begun incorporating these power-efficient subpixel patterns and rendering algorithms. So far, the following LCD companies have publicly demonstrated PenTile display technology: AU Optronics, BOE Hydis, CPT, LG Innotek, Samsung, and Wintek. Silicon Works and Tomato LSI have also developed chips.
Clairvoyante hopes to see a PenTile display in a commercial product by the end of this year.
The art of designing products to conform to the needs of the body has been dignified by a name, ergonomics. The expansion of information technology into new domains means that engineers must now learn to make products that conform to the needs of the mind and the senses, as well.
About the Author
Joel Pollack is president and chief executive officer of Clairvoyante Inc., a display-architecture company in Cupertino, Calif. He has also worked in a variety of research and management jobs in the display businesses of Sharp, Tektronix, and Xerox.
To Probe Further
The advantages of biomimetic displays are discussed in ”Reducing Pixel Count Without Reducing Image Quality,” by C.H.B. Elliott, Information Display , December 1999, Vol. 15, pp. 22–25.
Check out the blog by Greg Hitchcock and Bert Keely, inventors of Microsoft’s ClearType, at http://blogs.msdn.com/fontblog/default.aspx?p=2.
For a discussion on the total list of processing operations required to view a pixel, see ”What Is a Pixel?” by J.F. Blinn, in IEEE Computer Graphics and Applications , September-October 2005, Vol. 25, no. 5, pp. 82–87.
More information on subpixel patterns is available in ”Subpixel Rendering on Nonstriped Colour Matrix Displays,” by D.S. Messing et al. in Proceedings of the 2003 International Conference on Image Processing , Vol. 2, pp. 949–52.