# FreeType-2.3.4 API Reference

# Incremental Loading

## Synopsis

| FT_Incremental |  | FT_Incremental_FuncsRec
| FT_Incremental_Metrics |  | FT_Incremental_InterfaceRec
| FT_Incremental_GetGlyphDataFunc |  | FT_Incremental_Interface
| FT_Incremental_FreeGlyphDataFunc |  | FT_PARAM_TAG_INCREMENTAL
| FT_Incremental_GetGlyphMetricsFunc |  |

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
      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.

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#### FT_Incremental_Metrics
      typedef struct  FT_Incremental_MetricsRec_
      {
        FT_Long  bearing_x;
        FT_Long  bearing_y;
        FT_Long  advance;

      } FT_Incremental_MetricsRec, *FT_Incremental_Metrics;
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 | 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.

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#### FT_Incremental_GetGlyphDataFunc
      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.

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#### FT_Incremental_FreeGlyphDataFunc
      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).

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#### FT_Incremental_GetGlyphMetricsFunc
      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.

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#### FT_Incremental_FuncsRec
      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.

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#### FT_Incremental_InterfaceRec
      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 );
      ...

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#### FT_Incremental_Interface
      typedef FT_Incremental_InterfaceRec*   FT_Incremental_Interface;
A pointer to an FT_Incremental_InterfaceRec structure.

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#### FT_PARAM_TAG_INCREMENTAL
    #define FT_PARAM_TAG_INCREMENTAL  FT_MAKE_TAG( 'i', 'n', 'c', 'r' )
A constant used as the tag of FT_Parameter structures to indicate an incremental loading object to be used by FreeType.

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