HarfBuzz includes a copy of the UCDN (Unicode Database and Normalization) library, which provides functions for accessing basic Unicode character properties, performing canonical composition, and performing both canonical and compatibility decomposition.

Currently, UCDN supports direct queries for several more character properties than HarfBuzz's built-in set of Unicode functions does, such as the BiDirectional Class, East Asian Width, Paired Bracket and Resolved Linebreak properties. If you need to access more properties than HarfBuzz's internal implementation provides, using the built-in UCDN functions may be a useful solution.

The built-in UCDN functions are compiled by default when building HarfBuzz from source, but this can be disabled with a compile-time switch.