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Sub-pixel rendering can double or triple the resolution when the screen is displaying text.
  technofile
Al Fasoldt's reviews and commentaries, continuously available online since 1983

T e c h n o f i l e
Free feature of Windows XP and Mac OS X coaxes extra sharpness from LCD screens


Dec. 14, 2003


By Al Fasoldt
Copyright © 2003, Al Fasoldt
Copyright © 2003, The Post-Standard

   Thinking about getting a flat-panel LCD monitor this holiday season? A new LCD monitor -- or a new computer that comes standard with one, such as Apple's iMac -- might do more than simply impress the neighbors. It might even improve your vision.
   That's because both Microsoft and Apple have added free software to their latest operating systems that makes LCD screens amazingly sharp when they are displaying text. Microsoft calls its method ClearType; Apple refers to its method generically as sub-pixel rendering. Both methods work the same way, using single-color screen stripes within pixels, or picture elements, to help draw the fine edges of letters and numbers.
   The two methods aren't the same as an older technique of font smoothing first used in Windows and Macs seven or eight years ago. That method uses grayscale shading of nearby pixels to make the edges of letters and numbers seem smoother. The new method works within pixels and is therefore much more effective.
   But the new method is designed for LCD screens, not the picture-tube monitors most users have.
   In LCD monitors, characters are drawn using groups of three tiny color stripes for each blended rectangle on the screen. These three stripes (one long rectangle each of red, green and blue) make up one pixel, or picture element. By varying the intensity of the three colors, the computer's graphic-display circuits can make that pixel any color.
   On normal screens -- the ones that use picture tubes -- the smallest element in a display is the three-stripe (or three-dot) pixel. Getting individual colored stripes or dots to cooperate in sharpening the display is chancy at best. But because LCD screens operate far more precisely, each part of the screen, down to the individual stripes within a pixel, can be mapped on an exact grid in the graphics circuitry.
   And that means the tiniest parts of the characters that make up the words on an LCD screen can be redrawn inside a pixel by making some of the three color stripes brighter or darker. This makes the character's shape much clearer. Instead of limiting the resolution of a typical LCD screen to its rated pixel resolution, sub-pixel rendering can double or triple the resolution when the screen is displaying text.
   To use ClearType or sub-pixel rendering, you must have either a PC running Windows XP or an Apple Macintosh running OS X. (Older versions of Windows or Mac won't work.) You need an LCD flat-panel monitor. (You're free to try using this method if your computer uses a standard monitor, but you probably won't see a benefit.)
   On a Windows XP computer, turn on ClearType this way:
   Click the Start button and then click on the Control Panel. Choose Appearance and Themes, then Display. Click the Appearance tab if it is not already selected, then click Effects. Click the check box labeled "Use the following method to smooth edges of screen fonts," then click ClearType in the list.
   On an Apple Mac OS X computer, turn on sub-pixel rendering this way:
   Click the Apple menu, then click System Preferences. Click the Appearance icon. You'll see "Font smoothing style:" in the window. Choose "Medium - Best for Flat Panel" from the dropdown menu.
   Both methods can benefit from tweaking, mostly because LCD displays differ in their accuracy.
   To make fine adjustments in Windows XP, use the Web-based utility offered by Microsoft at www.microsoft.com/typography/cleartype/cleartypeactivate.htm.
   On an OS X Macintosh, you should first try turning off all font smoothing for very small text sizes. You can also try altering the font smoothing style from "Medium" to "Light" or "Strong."
   For more information on the specific methods of sub-pixel smoothing, use CLEARTYPE as the search term on Microsoft's Web site at www.microsoft.com/typography/cleartype/default.htm and FONT SMOOTHING as the term on Apple's support site at www.info.apple.com. You'll also find Steve Gibson's fascinating examination of sub-pixel rendering at his site, http://grc.com/cleartype.htm.