A fallback font is a reserve typeface containing symbols for as many Unicode characters as possible. When a display system encounters a character which is not part of the repertoire of any of the other available fonts, a symbol from a fallback font is used instead. Typically, a fallback font will contain symbols representative of the various types of Unicode characters. The use of a fallback font is suggested in The Unicode Standard.

Systems that do not offer a fallback font typically display black or white rectangles, question marks, or nothing at all in place of missing characters. Symbols in a fallback font can contain annotations such as the relevant Unicode block and the script system used.

Apple's LastResort font

Sample glyphs from the Apple's LastResort font.

Apple's LastResort font was first included in Mac OS 8.5 in 1998, for the benefit of applications using Apple Type Services for Unicode Imaging (ATSUI). It is also used in Mac OS X. In 2001, for the second release of OS X, the Last Resort font design was revised to include the border text and was re-digitized, and extended by Michael Everson of Evertype, who continues to update it with each new release of Unicode. Apple has now made the Last Resort font available for free download from the Unicode website.

One prototypic glyph per Unicode block is used because the total number of Unicode characters greatly exceeds the address space of an sfnt (TrueType and OpenType) font structure, which has a 16-bit glyph index that can store a maximum of only 65,536 glyphs. Unicode has now over 100,000 defined characters, with a potential address space of over one million characters - over 15 times the sfnt size limit. Using this one-glyph-per-block generalization, the Last Resort font is still capable of showing a glyph for every byte in Unicode. The Last Resort mechanism will therefore not break as Unicode continues to grow and the Basic Multilingual Plane (BMP) and surrogate planes fill up further.

The Unicode BMP Fallback font

The Unicode BMP Fallback font is a Unicode font containing a glyph for every character in the Basic multilingual plane. Each glyph consists of a box containing the four hex digits corresponding to the Unicode value. The example to the left is a mock-up of the glyph for a space character (U+0020).

The font was created for debugging purposes.

See also

• Fonts on Macintosh
• Unicode

References

• Apple Computer (November 2, 2002). Apple's ''LastResort'' font web site. Retrieved 2004-11-02.
• The Unicode Consortium (March 30, 2004). The Unicode Standard, 4.0.1. Retrieved 2004-11-02.
• Description of the Unicode BMP fallback font at SIL International

External links

• Last Resort Font download
• Apple Computer on LastResort
• brief discussion of Last Resort by Michael Everson
• SIL International's Unicode BMP Fallback Font — Free fallback font

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