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Font RasterizationFont rasterization is the process of converting text from a vector description (as found in scalable fonts such as TrueType fonts) to a raster or bitmap desription. This often involves some anti-aliasing on screen text to make it smoother and easier to read. It may also involve "hinting", that is, the use of information pre-computed for a particular font size. Types of rasterization In modern operating systems, rasterization is normally provided by a shared library common to many applications. Such a shared library may be built in to the operating system ot the desktop environment, or it may be added at some later time. In principle, each application may use a different font rasterization library, but in practice most systems attempt to standardize on a single library. The simplest form of rasterization is simple line-drawing with no antialiasing of any sort. This is the fastest method (that is, it requires the least computation to place on screen), and on appropriate displays and with sufficient hinting, may look very clear. However, often the lack of antialiasing leads to undesirable jagged edges and some more sophisticated method may be needed. Most modern systems either use a more sophisticated method or rely on bitmapped fonts - essentially fonts that have been manually rasterized with care, and can therefore be rendered extremely quickly. A more complicated approach is to use standard anti-aliasing techniques from computer graphics. This can be thought of as determining, for each pixel, how much of that pixel is occupied by the letter, and drawing that pixel with that degree of opacity. For example, when drawing a black letter on a white background, if a pixel ideally should be half filled (perhaps by a diagonal line from corner to corner) it would be drawn in 50% gray. Simple application of this procedure can lead to very ugly, blurry letters: for example, if the letter includes a vertical line which should be one pixel wide but falls exactly between two pixels, it will appear on screen a two-pixel-wide gray line. Thus most fonts designed for antialiasing include "hints" on how the renderer should render them. Some of these hinting algorithms are patented. Many modern screens are not well described by the model "one pixel, one color". For example, many laptop flat panel displays have independent red, green, and blue pixels adjacent to one another. Since they are not quite at the same location, rendering can be improved by taking this into account. This is generally known as subpixel rendering. Currently used rasterization systems On most Windows computers, a standard method of anti-aliasing is used that adds relatively weak rasterization to TrueType fonts. With the release of Windows XP, a new type of rasterization, ClearType, was introduced, which does subpixel rendering. In Mac OS X, there are no clear defined types of anti-aliasing. However, on closer examination of text, it is clear that a stronger version of standard rasterization is used. In System Preferences there is an option to change to a type of anti-aliasing similar to the ClearType found on Windows XP machines optimized for LCD displays. The Freetype library (used on most Linux systems) supports hinting, anti-aliasing and does subpixel rendering, thus being on a par if not better than the Windows and Mac OS X font rasterizers.
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