Currently, two versions are available, representing the bytecode interpreter with and without subpixel hinting support, respectively. The default is subpixel support if TT_CONFIG_OPTION_SUBPIXEL_HINTING is defined, and no subpixel support otherwise (since it isn't available then).

If subpixel hinting is on, many TrueType bytecode instructions behave differently compared to B/W or grayscale rendering. The main idea is to render at a much increased horizontal resolution, then sampling down the created output to subpixel precision. However, many older fonts are not suited to this and must be specially taken care of by applying (hardcoded) font-specific tweaks.

Details on subpixel hinting and some of the necessary tweaks can be found in Greg Hitchcock's whitepaper at ‘http://www.microsoft.com/typography/cleartype/truetypecleartype.aspx’.

The following example code demonstrates how to activate subpixel hinting (omitting the error handling).

  FT_Library  library;
  FT_Face     face;
  FT_UInt     interpreter_version = TT_INTERPRETER_VERSION_38;


  FT_Init_FreeType( &library );

  FT_Property_Set( library, "truetype",
                            "interpreter-version",
                            &interpreter_version );

This property can be used with FT_Property_Get also.

#define TT_INTERPRETER_VERSION_35  35
#define TT_INTERPRETER_VERSION_38  38

A list of constants used for the interpreter-version property to select the hinting engine for Truetype fonts.

The numeric value in the constant names represents the version number as returned by the ‘GETINFO’ bytecode instruction.

This property controls the behaviour of the bytecode interpreter and thus how outlines get hinted. It does not control how glyph get rasterized! In particular, it does not control subpixel color filtering.

If FreeType has not been compiled with configuration option FT_CONFIG_OPTION_SUBPIXEL_HINTING, selecting version 38 causes an ‘FT_Err_Unimplemented_Feature’ error.

Depending on the graphics framework, Microsoft uses different bytecode engines. As a consequence, the version numbers returned by a call to the ‘GETINFO[1]’ bytecode instruction are more convoluted than desired.

   framework   Windows version   result of GETINFO[1]
  ----------------------------------------------------
    GDI         before XP         35
    GDI         XP and later      37
    GDI+ old    before Vista      37
    GDI+ old    Vista, 7          38
    GDI+        after 7           40
    DWrite      before 8          39
    DWrite      8 and later       40

Since FreeType doesn't provide all capabilities of DWrite ClearType, using version 38 seems justified.