Font hinting is the use of mathematical instructions to adjust the display of an outline font so that it lines up with a rasterized grid. At small screen sizes, with or without antialiasing, hinting is critical for producing a clear, legible text for human readers. It is also known as instructing.

Overview

For the purpose of on-screen text display, font hinting instructs which primary pixels are interpolated to more clearly render a font.

One popular and recognizable form of hinting is found in the TrueType font format, released in 1991 by Apple Computer. Hinting in TrueType invokes tables of font data used to render fonts properly on screen. One aspect of TrueType hinting is grid-fitting, which modifies the height and width of font characters to line up to the set pixel grid of screen display. The open-source FreeType font rendering engine uses an auto-hinter when such hinting data is not present or its use is restricted by a software patent.

Hints are usually created in a font 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, commercial fonts of the highest quality are often manually hinted to provide the sharpest appearance on computer displays.

Guidelines

According to the TrueType Reference Guidehttp://developer.apple.com/textfonts/TTRefMan/RM03/Chap3.html#features TrueType Reference Guide, font instructors (those performing font hinting) must balance the following two constraints when hinting a font:

• At small sizes, chance effects should not be allowed to magnify small differences in the original outline design of a glyph.
• At large sizes, the subtlety of the original design should emerge.

The Guide suggests that, for screen viewing, fonts should be readable at 9 points per em at 72 dpi. Particular attention should be paid to the cap height, x-height, and baseline, so that the font retains its normal character while not producing exaggerated effects at small sizes.

See also

• Antialiasing
• Subpixel rendering
• Kell factor
• TrueType

External links

• The raster tragedy at low resolution
• FreeType and Patents
• Tutorial on the DejaVu font wiki
• Texts Rasterization Exposures Article from the Anti-Grain Geometry Project.