Apple Type Services for Unicode Imaging

Apple Type Services for Unicode Imaging (ATSUI) was a framework for developed by Apple for rendering Unicode-encoded text in Mac OS 8.5 through early versions of Mac OS X.

History
In 1998, Mac OS 8.5 introduced support for Unicode through ATSUI 1.0, which initially operated alongside the older WorldScript text engine. In 1999, Mac OS 8.6 updated ATSUI to version 1.1.

Deprecation
As ATSUI itself was based on the transitional Carbon API, it also started being phased out in favor of Core Text with the release of Mac OS X 10.5 "Leopard" in October 2007. The framework was officially deprecated with the release of Xcode 4.6 in December 2012: "Source code using ATS APIs will generate warnings while being compiled. For 10.8, there will be no loss of functionality but there could be areas where performance will suffer. Programmers are instructed to replace all their ATS code (including ATSUI) with CoreText as ATS functionality will be completely removed in future releases of OS X."

Articles

 * A look at OS X’s font capabilities at Macworld (2001-10-11)