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Incremental Loading¶

Synopsis¶

This section contains various functions used to perform so-called ‘incremental’ glyph loading. This is a mode where all glyphs loaded from a given FT_Face are provided by the client application.

Apart from that, all other tables are loaded normally from the font file. This mode is useful when FreeType is used within another engine, e.g., a PostScript Imaging Processor.

To enable this mode, you must use FT_Open_Face, passing an FT_Parameter with the FT_PARAM_TAG_INCREMENTAL tag and an FT_Incremental_Interface value. See the comments for FT_Incremental_InterfaceRec for an example.

FT_Incremental¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

  typedef struct FT_IncrementalRec_*  FT_Incremental;

An opaque type describing a user-provided object used to implement ‘incremental’ glyph loading within FreeType. This is used to support embedded fonts in certain environments (e.g., PostScript interpreters), where the glyph data isn't in the font file, or must be overridden by different values.

note

It is up to client applications to create and implement FT_Incremental objects, as long as they provide implementations for the methods FT_Incremental_GetGlyphDataFunc, FT_Incremental_FreeGlyphDataFunc and FT_Incremental_GetGlyphMetricsFunc.

See the description of FT_Incremental_InterfaceRec to understand how to use incremental objects with FreeType.

FT_Incremental_MetricsRec¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

  typedef struct  FT_Incremental_MetricsRec_
  {
    FT_Long  bearing_x;
    FT_Long  bearing_y;
    FT_Long  advance;
    FT_Long  advance_v;     /* since 2.3.12 */

  } FT_Incremental_MetricsRec;

A small structure used to contain the basic glyph metrics returned by the FT_Incremental_GetGlyphMetricsFunc method.

fields

bearing_x	Left bearing, in font units.
bearing_y	Top bearing, in font units.
advance	Horizontal component of glyph advance, in font units.
advance_v	Vertical component of glyph advance, in font units.

note

These correspond to horizontal or vertical metrics depending on the value of the ‘vertical’ argument to the function FT_Incremental_GetGlyphMetricsFunc.

FT_Incremental_Metrics¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

   typedef struct FT_Incremental_MetricsRec_*  FT_Incremental_Metrics;

A handle to an FT_Incremental_MetricsRec structure.

FT_Incremental_GetGlyphDataFunc¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

  typedef FT_Error
  (*FT_Incremental_GetGlyphDataFunc)( FT_Incremental  incremental,
                                      FT_UInt         glyph_index,
                                      FT_Data*        adata );

A function called by FreeType to access a given glyph's data bytes during FT_Load_Glyph or FT_Load_Char if incremental loading is enabled.

Note that the format of the glyph's data bytes depends on the font file format. For TrueType, it must correspond to the raw bytes within the ‘glyf’ table. For PostScript formats, it must correspond to the unencrypted charstring bytes, without any lenIV header. It is undefined for any other format.

input

incremental	Handle to an opaque `FT_Incremental` handle provided by the client application.
glyph_index	Index of relevant glyph.

output

adata

A structure describing the returned glyph data bytes (which will be accessed as a read-only byte block).

return

FreeType error code. 0 means success.

note

If this function returns successfully the method FT_Incremental_FreeGlyphDataFunc will be called later to release the data bytes.

Nested calls to FT_Incremental_GetGlyphDataFunc can happen for compound glyphs.

FT_Incremental_FreeGlyphDataFunc¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

  typedef void
  (*FT_Incremental_FreeGlyphDataFunc)( FT_Incremental  incremental,
                                       FT_Data*        data );

A function used to release the glyph data bytes returned by a successful call to FT_Incremental_GetGlyphDataFunc.

input

incremental	A handle to an opaque `FT_Incremental` handle provided by the client application.
data	A structure describing the glyph data bytes (which will be accessed as a read-only byte block).

FT_Incremental_GetGlyphMetricsFunc¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

  typedef FT_Error
  (*FT_Incremental_GetGlyphMetricsFunc)
                      ( FT_Incremental              incremental,
                        FT_UInt                     glyph_index,
                        FT_Bool                     vertical,
                        FT_Incremental_MetricsRec  *ametrics );

A function used to retrieve the basic metrics of a given glyph index before accessing its data. This is necessary because, in certain formats like TrueType, the metrics are stored in a different place from the glyph images proper.

input

incremental	A handle to an opaque `FT_Incremental` handle provided by the client application.
glyph_index	Index of relevant glyph.
vertical	If true, return vertical metrics.
ametrics	This parameter is used for both input and output. The original glyph metrics, if any, in font units. If metrics are not available all the values must be set to zero.

output

ametrics

The replacement glyph metrics in font units.

FT_Incremental_FuncsRec¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

  typedef struct  FT_Incremental_FuncsRec_
  {
    FT_Incremental_GetGlyphDataFunc     get_glyph_data;
    FT_Incremental_FreeGlyphDataFunc    free_glyph_data;
    FT_Incremental_GetGlyphMetricsFunc  get_glyph_metrics;

  } FT_Incremental_FuncsRec;

A table of functions for accessing fonts that load data incrementally. Used in FT_Incremental_InterfaceRec.

fields

get_glyph_data	The function to get glyph data. Must not be null.
free_glyph_data	The function to release glyph data. Must not be null.
get_glyph_metrics	The function to get glyph metrics. May be null if the font does not provide overriding glyph metrics.

FT_Incremental_InterfaceRec¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

  typedef struct  FT_Incremental_InterfaceRec_
  {
    const FT_Incremental_FuncsRec*  funcs;
    FT_Incremental                  object;

  } FT_Incremental_InterfaceRec;

A structure to be used with FT_Open_Face to indicate that the user wants to support incremental glyph loading. You should use it with FT_PARAM_TAG_INCREMENTAL as in the following example:

  FT_Incremental_InterfaceRec  inc_int;
  FT_Parameter                 parameter;
  FT_Open_Args                 open_args;


  // set up incremental descriptor
  inc_int.funcs  = my_funcs;
  inc_int.object = my_object;

  // set up optional parameter
  parameter.tag  = FT_PARAM_TAG_INCREMENTAL;
  parameter.data = &inc_int;

  // set up FT_Open_Args structure
  open_args.flags      = FT_OPEN_PATHNAME | FT_OPEN_PARAMS;
  open_args.pathname   = my_font_pathname;
  open_args.num_params = 1;
  open_args.params     = &parameter; // we use one optional argument

  // open the font
  error = FT_Open_Face( library, &open_args, index, &face );
  ...

FT_Incremental_Interface¶

Defined in FT_INCREMENTAL_H (freetype/ftincrem.h).

  typedef FT_Incremental_InterfaceRec*   FT_Incremental_Interface;

A pointer to an FT_Incremental_InterfaceRec structure.