Subpixel rendering is the process of using computer software to break apart a single pixel on a computer screen into the component colors that, in various intensities, combine to create the perceived color. The result of using subpixels to render an image is that angles and curves will appear smoother because units of display smaller than a standard pixel are used to fill areas where there might otherwise be only a solid pixel or no pixel at all. The process of subpixel rendering is most often used to improve the appearance of fonts on computer displays. Other applications, such as sharpening an image or improving the appearance of vector-based graphics, also have been developed but do not enjoy the widespread use that fonts do.
The basic premise behind subpixel rendering has to do with how some monitors display information, specifically liquid crystal displays (LCDs). When an image is drawn on the screen, it is made up of many small dots known as pixels. Each of these pixels can be one of millions of colors, depending on the display technology. The individual pixel that most users see only as a single dot is actually made from three or more smaller dots, each of which can only illuminate in a single color, usually red, green or blue. The combination of these colors determines the final color that is seen as a single pixel.
By manipulating the subpixels of a larger pixel, more gradual gradations are able to be achieved when rendering an image. The end result is that the space that would normally be unoccupied by a whole pixel can be only partially filled in with a subpixel, creating the illusion of a smooth line. Subpixel rendering is often compared to another image processing technique known as anti-aliasing, which can sometimes achieve the same effect, though usually on a larger scale.
One complication that can make subpixel rendering ineffective is the specifics of the display technology being used. While many LCD monitors arrange their pixels in a red, green and blue sequence, not all do. Some instead arrange the colors blue, red and green. This could cause a subpixel rendering algorithm to incorrectly activate certain pixels. The result of the rendering text in this type of environment is a faint, but visible, area of color around the primary pixels in a letter that actually degrades the quality of the text.
The inconsistencies in display hardware also can make any standardization of subpixel rendering in the display of full-color images difficult to implement. Some algorithms exist, especially for situations in which images are being resized or edited for quality, although these improvements are not guaranteed to translate to another computer using a different monitor. Several operating systems do, however, attempt to use subpixel rendering for the display of text by default, although the eyesight of the viewer also can play a role in whether this is a desirable effect.