Currently Used Rasterization Systems
Microsoft Windows has supported subpixel rendering since Windows XP. The Windows rasterizer is an example of one that prioritizes clarity; by forcing text into integral coordinate positions (and not even antialiasing certain fonts at certain sizes), it becomes easier to read on the screen, but may appear somewhat different when printed.
Mac OS X's Quartz is distinguished by the use of floating-point positioning; it does not force glyphs into exact pixel locations, instead using various antialiasing techniques, including subpixel rendering, to position characters and lines more accurately. The result is that the on-screen display looks extremely similar to printed output, but can occasionally be difficult to read at smaller point sizes. Contrary to other rasterizers, Quartz ignores any Postscript or TrueType hints in the font and solely relies on its own algorithm. A simpler type of font antialiasing was introduced in Mac OS 8.5, in 1998. Apple's technique can be seen on Windows in older versions of Safari for Windows. In more recent versions, however, Apple has switched to using system settings by default.
RISC OS includes font anti-aliasing, first introduced before January 1989. It uses its own font rendering system which favours accurate shapes over readability, with features such as scaffolding and hinting, sub-pixel positioning and background blending.
PDF documents are usually rendered with Adobe CoolType.
Most other systems use the FreeType library, which falls somewhere between Microsoft's and Apple's implementations; it supports hinting and anti-aliasing, and optionally performs subpixel rendering. The Free fonts included with most Linux distributions look better with FreeType's "auto-hinting" mode, which is high-quality and not encumbered by patents.
D-Type Font Engine is an independent, proprietary and portable font rasterization library. It provides anti-aliasing, subpixel precision, automatic hinting, bitmap filtering and other techniques that can improve the appearance and legibility of text on screen. According to the authors, the display quality of D-Type Font Engine can be configured to match or exceed the quality of Windows and Mac OS X font rasterizers while using only non-hinted TrueType, OpenType or Type 1 fonts.
Read more about this topic: Font Rasterization
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