interpreter-version

Currently, three versions are available, two representing the bytecode interpreter with subpixel hinting support (old ‘Infinality’ code and new stripped-down and higher performance ‘minimal’ code) and one without, 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 (except if ‘native ClearType’ is selected by the font). Microsoft's 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) tweaks in Microsoft's interpreter.

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’. Note that FreeType currently doesn't really ‘subpixel hint’ (6x1, 6x2, or 6x5 supersampling) like discussed in the paper. Depending on the chosen interpreter, it simply ignores instructions on vertical stems to arrive at very similar results.

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

  FT_Library  library;
  FT_Face     face;
  FT_UInt     interpreter_version = TT_INTERPRETER_VERSION_35;


  FT_Init_FreeType( &library );

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

note

This property can be used with FT_Property_Get also.

This property can be set via the ‘FREETYPE_PROPERTIES’ environment variable (using values ‘35’, ‘38’, or ‘40’).

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TT_INTERPRETER_VERSION_XXX

Defined in FT_TRUETYPE_DRIVER_H (freetype/ftttdrv.h).

#define TT_INTERPRETER_VERSION_35  35
#define TT_INTERPRETER_VERSION_38  38
#define TT_INTERPRETER_VERSION_40  40

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.

values

TT_INTERPRETER_VERSION_35	Version 35 corresponds to MS rasterizer v.1.7 as used e.g. in Windows 98; only grayscale and B/W rasterizing is supported.
TT_INTERPRETER_VERSION_38	Version 38 corresponds to MS rasterizer v.1.9; it is roughly equivalent to the hinting provided by DirectWrite ClearType (as can be found, for example, in the Internet Explorer 9 running on Windows 7). It is used in FreeType to select the ‘Infinality’ subpixel hinting code. The code may be removed in a future version.
TT_INTERPRETER_VERSION_40	Version 40 corresponds to MS rasterizer v.2.1; it is roughly equivalent to the hinting provided by DirectWrite ClearType (as can be found, for example, in Microsoft's Edge Browser on Windows 10). It is used in FreeType to select the ‘minimal’ subpixel hinting code, a stripped-down and higher performance version of the ‘Infinality’ code.

note

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 the configuration option TT_CONFIG_OPTION_SUBPIXEL_HINTING, selecting version 38 or 40 causes an ‘FT_Err_Unimplemented_Feature’ error.

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

Here are two tables that try to shed some light on the possible values for the MS rasterizer engine, together with the additional features introduced by it.

  GETINFO framework               version feature
  -------------------------------------------------------------------
      3   GDI (Win 3.1),            v1.0  16-bit, first version
          TrueImage
     33   GDI (Win NT 3.1),         v1.5  32-bit
          HP Laserjet
     34   GDI (Win 95)              v1.6  font smoothing,
                                          new SCANTYPE opcode
     35   GDI (Win 98/2000)         v1.7  (UN)SCALED_COMPONENT_OFFSET
                                            bits in composite glyphs
     36   MGDI (Win CE 2)           v1.6+ classic ClearType
     37   GDI (XP and later),       v1.8  ClearType
          GDI+ old (before Vista)
     38   GDI+ old (Vista, Win 7),  v1.9  subpixel ClearType,
          WPF                             Y-direction ClearType,
                                          additional error checking
     39   DWrite (before Win 8)     v2.0  subpixel ClearType flags
                                            in GETINFO opcode,
                                          bug fixes
     40   GDI+ (after Win 7),       v2.1  Y-direction ClearType flag
          DWrite (Win 8)                    in GETINFO opcode,
                                          Gray ClearType

The ‘version’ field gives a rough orientation only, since some applications provided certain features much earlier (as an example, Microsoft Reader used subpixel and Y-direction ClearType already in Windows 2000). Similarly, updates to a given framework might include improved hinting support.

   version   sampling          rendering        comment
            x        y       x           y
  --------------------------------------------------------------
    v1.0   normal  normal  B/W           B/W    bi-level
    v1.6   high    high    gray          gray   grayscale
    v1.8   high    normal  color-filter  B/W    (GDI) ClearType
    v1.9   high    high    color-filter  gray   Color ClearType
    v2.1   high    normal  gray          B/W    Gray ClearType
    v2.1   high    high    gray          gray   Gray ClearType

Color and Gray ClearType are the two available variants of ‘Y-direction ClearType’, meaning grayscale rasterization along the Y-direction; the name used in the TrueType specification for this feature is ‘symmetric smoothing’. ‘Classic ClearType’ is the original algorithm used before introducing a modified version in Win XP. Another name for v1.6's grayscale rendering is ‘font smoothing’, and ‘Color ClearType’ is sometimes also called ‘DWrite ClearType’. To differentiate between today's Color ClearType and the earlier ClearType variant with B/W rendering along the vertical axis, the latter is sometimes called ‘GDI ClearType’.

‘Normal’ and ‘high’ sampling describe the (virtual) resolution to access the rasterized outline after the hinting process. ‘Normal’ means 1 sample per grid line (i.e., B/W). In the current Microsoft implementation, ‘high’ means an extra virtual resolution of 16x16 (or 16x1) grid lines per pixel for bytecode instructions like ‘MIRP’. After hinting, these 16 grid lines are mapped to 6x5 (or 6x1) grid lines for color filtering if Color ClearType is activated.

Note that ‘Gray ClearType’ is essentially the same as v1.6's grayscale rendering. However, the GETINFO instruction handles it differently: v1.6 returns bit 12 (hinting for grayscale), while v2.1 returns bits 13 (hinting for ClearType), 18 (symmetrical smoothing), and 19 (Gray ClearType). Also, this mode respects bits 2 and 3 for the version 1 gasp table exclusively (like Color ClearType), while v1.6 only respects the values of version 0 (bits 0 and 1).

Keep in mind that the features of the above interpreter versions might not map exactly to FreeType features or behavior because it is a fundamentally different library with different internals.

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