This section describes the FreeType 2 cache sub-system, which is used to limit the number of concurrently opened FT_Face and FT_Size objects, as well as caching information like character maps and glyph images while limiting their maximum memory usage.
Note that all types and functions begin with the ‘FTC_’ prefix.
The cache is highly portable and thus doesn't know anything about the fonts installed on your system, or how to access them. This implies the following scheme:
First, available or installed font faces are uniquely identified by FTC_FaceID values, provided to the cache by the client. Note that the cache only stores and compares these values, and doesn't try to interpret them in any way.
Second, the cache calls, only when needed, a client-provided function to convert an FTC_FaceID into a new FT_Face object. The latter is then completely managed by the cache, including its termination through FT_Done_Face. To monitor termination of face objects, the finalizer callback in the ‘generic’ field of the FT_Face object can be used, which might also be used to store the FTC_FaceID of the face.
Clients are free to map face IDs to anything else. The most simple usage is to associate them to a (pathname,face_index) pair that is used to call FT_New_Face. However, more complex schemes are also possible.
Note that for the cache to work correctly, the face ID values must be persistent, which means that the contents they point to should not change at runtime, or that their value should not become invalid.
If this is unavoidable (e.g., when a font is uninstalled at runtime), you should call FTC_Manager_RemoveFaceID as soon as possible, to let the cache get rid of any references to the old FTC_FaceID it may keep internally. Failure to do so will lead to incorrect behaviour or even crashes.
To use the cache, start with calling FTC_Manager_New to create a new FTC_Manager object, which models a single cache instance. You can then look up FT_Face and FT_Size objects with FTC_Manager_LookupFace and FTC_Manager_LookupSize, respectively.
If you want to use the charmap caching, call FTC_CMapCache_New, then later use FTC_CMapCache_Lookup to perform the equivalent of FT_Get_Char_Index, only much faster.
If you want to use the FT_Glyph caching, call FTC_ImageCache, then later use FTC_ImageCache_Lookup to retrieve the corresponding FT_Glyph objects from the cache.
If you need lots of small bitmaps, it is much more memory efficient to call FTC_SBitCache_New followed by FTC_SBitCache_Lookup. This returns FTC_SBitRec structures, which are used to store small bitmaps directly. (A small bitmap is one whose metrics and dimensions all fit into 8-bit integers).
We hope to also provide a kerning cache in the near future.