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Font hinting, also known asinstructing, is the use of mathematical instructions to adjust the display of anoutline font so that it lines up with arasterized grid. At lowscreen resolutions, hinting is critical for producing clear, legible text. It can be accompanied byantialiasing and (onliquid crystal displays)subpixel rendering for further clarity.
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For the purpose of on-screen text display, font hinting designates which primary pixels areinterpolated to more clearly render afont.[citation needed] Hints are usually created in afont editor during the typeface design process and embedded in the font. A font can be hinted either automatically (through processed algorithms based on the character outlines) or set manually. Most font editors are able to do automatic hinting, and this approach is suitable for many fonts. However, high-quality commercial fonts are often manually hinted to provide the sharpest appearance on computer displays.Verdana is one example of a font that contains a large amount of hinting data, much of which was accomplished manually by type engineerTom Rickner.[1]
In theTrueType font format, released in 1991 byApple Inc, hinting invokes tables of font data used to render fonts properly on screen. One aspect of TrueType hinting isgrid-fitting, which modifies the height and width of font characters to line up to the set pixel grid of screen display. The open-sourceFreeType 2 font rendering engine uses an auto-hinter when such hinting data are not present or their use is restricted by asoftware patent.[2] As of 2011, the FreeType website states that the relevant font hinting patents have now all expired, and hinting is now enabled in FreeType by default.[2]
According to the TrueType Reference Manual,[3] font instructors (those performing font hinting) must balance the following two constraints when hinting a font:
The reference manual suggests that, for screen viewing, fonts should be readable at 9 pixels perem at 72pixels per inch. Particular attention should be paid to thecap height,x-height, andbaseline, so that the font retains its normal character while not producing exaggerated effects at small sizes.
With the advent ofhigh-DPI displays (generally considered to be displays with more than 300 pixels per inch), font hinting has become less relevant, as aliasing effects become un-noticeable to the human eye. As a result Apple'sQuartz text renderer, which is targeted for Apple'sRetina displays, now ignores font hint information completely.[4][5]
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