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# FreeType-2.4.4 API Reference

# Scanline Converter

## Synopsis

| FT_Raster |  | FT_RASTER_FLAG_XXX |  | FT_Raster_SetModeFunc
| FT_Span |  | FT_Raster_Params |  | FT_Raster_RenderFunc
| FT_SpanFunc |  | FT_Raster_NewFunc |  | FT_Raster_Funcs
| FT_Raster_BitTest_Func |  | FT_Raster_DoneFunc |  |
| FT_Raster_BitSet_Func |  | FT_Raster_ResetFunc |  |

This section contains technical definitions.

#### FT_Raster

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef struct FT_RasterRec_*  FT_Raster;

A handle (pointer) to a raster object. Each object can be used independently to convert an outline into a bitmap or pixmap.

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#### FT_Span

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef struct  FT_Span_
      {
        short           x;
        unsigned short  len;
        unsigned char   coverage;

      } FT_Span;

A structure used to model a single span of gray (or black) pixels when rendering a monochrome or anti-aliased bitmap.

fields

x | The span's horizontal start position.
len | The span's length in pixels.
coverage | The span color/coverage, ranging from 0 (background) to 255 (foreground). Only used for anti-aliased rendering.

note
This structure is used by the span drawing callback type named FT_SpanFunc which takes the y coordinate of the span as a a parameter. The coverage value is always between 0 and 255. If you want less gray values, the callback function has to reduce them.

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#### FT_SpanFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef void
      (*FT_SpanFunc)( int             y,
                      int             count,
                      const FT_Span*  spans,
                      void*           user );

    #define FT_Raster_Span_Func  FT_SpanFunc

A function used as a call-back by the anti-aliased renderer in order to let client applications draw themselves the gray pixel spans on each scan line.

input

y | The scanline's y coordinate.
count | The number of spans to draw on this scanline.
spans | A table of ‘count’ spans to draw on the scanline.
user | User-supplied data that is passed to the callback.

note

This callback allows client applications to directly render the gray spans of the anti-aliased bitmap to any kind of surfaces.

This can be used to write anti-aliased outlines directly to a given background bitmap, and even perform translucency.

Note that the ‘count’ field cannot be greater than a fixed value defined by the ‘FT_MAX_GRAY_SPANS’ configuration macro in ‘ftoption.h’. By default, this value is set to 32, which means that if there are more than 32 spans on a given scanline, the callback is called several times with the same ‘y’ parameter in order to draw all callbacks.

Otherwise, the callback is only called once per scan-line, and only for those scanlines that do have ‘gray’ pixels on them.

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#### FT_Raster_BitTest_Func

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef int
      (*FT_Raster_BitTest_Func)( int    y,
                                 int    x,
                                 void*  user );

THIS TYPE IS DEPRECATED. DO NOT USE IT. A function used as a call-back by the monochrome scan-converter to test whether a given target pixel is already set to the drawing ‘color’. These tests are crucial to implement drop-out control per-se the TrueType spec.

input

y | The pixel's y coordinate.
x | The pixel's x coordinate.
user | User-supplied data that is passed to the callback.

return
1 if the pixel is ‘set’, 0 otherwise.

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#### FT_Raster_BitSet_Func

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef void
      (*FT_Raster_BitSet_Func)( int    y,
                                int    x,
                                void*  user );

THIS TYPE IS DEPRECATED. DO NOT USE IT. A function used as a call-back by the monochrome scan-converter to set an individual target pixel. This is crucial to implement drop-out control according to the TrueType specification.

input

y | The pixel's y coordinate.
x | The pixel's x coordinate.
user | User-supplied data that is passed to the callback.

return
1 if the pixel is ‘set’, 0 otherwise.

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#### FT_RASTER_FLAG_XXX

Defined in FT_IMAGE_H (freetype/ftimage.h).

    #define FT_RASTER_FLAG_DEFAULT  0x0
    #define FT_RASTER_FLAG_AA       0x1
    #define FT_RASTER_FLAG_DIRECT   0x2
    #define FT_RASTER_FLAG_CLIP     0x4

      /* deprecated */
    #define ft_raster_flag_default  FT_RASTER_FLAG_DEFAULT
    #define ft_raster_flag_aa       FT_RASTER_FLAG_AA
    #define ft_raster_flag_direct   FT_RASTER_FLAG_DIRECT
    #define ft_raster_flag_clip     FT_RASTER_FLAG_CLIP

A list of bit flag constants as used in the ‘flags’ field of a FT_Raster_Params structure.

values

FT_RASTER_FLAG_DEFAULT | This value is 0.
FT_RASTER_FLAG_AA | This flag is set to indicate that an anti-aliased glyph image should be generated. Otherwise, it will be monochrome (1-bit).
FT_RASTER_FLAG_DIRECT | This flag is set to indicate direct rendering. In this mode, client applications must provide their own span callback. This lets them directly draw or compose over an existing bitmap. If this bit is not set, the target pixmap's buffer must be zeroed before rendering. Note that for now, direct rendering is only possible with anti-aliased glyphs.
FT_RASTER_FLAG_CLIP | This flag is only used in direct rendering mode. If set, the output will be clipped to a box specified in the ‘clip_box’ field of the FT_Raster_Params structure. Note that by default, the glyph bitmap is clipped to the target pixmap, except in direct rendering mode where all spans are generated if no clipping box is set.

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#### FT_Raster_Params

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef struct  FT_Raster_Params_
      {
        const FT_Bitmap*        target;
        const void*             source;
        int                     flags;
        FT_SpanFunc             gray_spans;
        FT_SpanFunc             black_spans;  /* doesn't work! */
        FT_Raster_BitTest_Func  bit_test;     /* doesn't work! */
        FT_Raster_BitSet_Func   bit_set;      /* doesn't work! */
        void*                   user;
        FT_BBox                 clip_box;

      } FT_Raster_Params;

A structure to hold the arguments used by a raster's render function.

fields

target | The target bitmap.
source | A pointer to the source glyph image (e.g., an FT_Outline).
flags | The rendering flags.
gray_spans | The gray span drawing callback.
black_spans | The black span drawing callback. UNIMPLEMENTED!
bit_test | The bit test callback. UNIMPLEMENTED!
bit_set | The bit set callback. UNIMPLEMENTED!
user | User-supplied data that is passed to each drawing callback.
clip_box | An optional clipping box. It is only used in direct rendering mode. Note that coordinates here should be expressed in integer pixels (and not in 26.6 fixed-point units).

note

An anti-aliased glyph bitmap is drawn if the FT_RASTER_FLAG_AA bit flag is set in the ‘flags’ field, otherwise a monochrome bitmap is generated.

If the FT_RASTER_FLAG_DIRECT bit flag is set in ‘flags’, the raster will call the ‘gray_spans’ callback to draw gray pixel spans, in the case of an aa glyph bitmap, it will call ‘black_spans’, and ‘bit_test’ and ‘bit_set’ in the case of a monochrome bitmap. This allows direct composition over a pre-existing bitmap through user-provided callbacks to perform the span drawing/composition.

Note that the ‘bit_test’ and ‘bit_set’ callbacks are required when rendering a monochrome bitmap, as they are crucial to implement correct drop-out control as defined in the TrueType specification.

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#### FT_Raster_NewFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef int
      (*FT_Raster_NewFunc)( void*       memory,
                            FT_Raster*  raster );

    #define FT_Raster_New_Func  FT_Raster_NewFunc

A function used to create a new raster object.

input

memory | A handle to the memory allocator.

output

raster | A handle to the new raster object.

return
Error code. 0 means success.

note
The ‘memory’ parameter is a typeless pointer in order to avoid un-wanted dependencies on the rest of the FreeType code. In practice, it is an FT_Memory object, i.e., a handle to the standard FreeType memory allocator. However, this field can be completely ignored by a given raster implementation.

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#### FT_Raster_DoneFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef void
      (*FT_Raster_DoneFunc)( FT_Raster  raster );

    #define FT_Raster_Done_Func  FT_Raster_DoneFunc

A function used to destroy a given raster object.

input

raster | A handle to the raster object.

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#### FT_Raster_ResetFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef void
      (*FT_Raster_ResetFunc)( FT_Raster       raster,
                              unsigned char*  pool_base,
                              unsigned long   pool_size );

    #define FT_Raster_Reset_Func  FT_Raster_ResetFunc

FreeType provides an area of memory called the ‘render pool’, available to all registered rasters. This pool can be freely used during a given scan-conversion but is shared by all rasters. Its content is thus transient. This function is called each time the render pool changes, or just after a new raster object is created.

input

raster | A handle to the new raster object.
pool_base | The address in memory of the render pool.
pool_size | The size in bytes of the render pool.

note
Rasters can ignore the render pool and rely on dynamic memory allocation if they want to (a handle to the memory allocator is passed to the raster constructor). However, this is not recommended for efficiency purposes.

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#### FT_Raster_SetModeFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef int
      (*FT_Raster_SetModeFunc)( FT_Raster      raster,
                                unsigned long  mode,
                                void*          args );

    #define FT_Raster_Set_Mode_Func  FT_Raster_SetModeFunc

This function is a generic facility to change modes or attributes in a given raster. This can be used for debugging purposes, or simply to allow implementation-specific ‘features’ in a given raster module.

input

raster | A handle to the new raster object.
mode | A 4-byte tag used to name the mode or property.
args | A pointer to the new mode/property to use.

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#### FT_Raster_RenderFunc

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef int
      (*FT_Raster_RenderFunc)( FT_Raster                raster,
                               const FT_Raster_Params*  params );

    #define FT_Raster_Render_Func  FT_Raster_RenderFunc

Invoke a given raster to scan-convert a given glyph image into a target bitmap.

input

raster | A handle to the raster object.
params | A pointer to an FT_Raster_Params structure used to store the rendering parameters.

return
Error code. 0 means success.

note

The exact format of the source image depends on the raster's glyph format defined in its FT_Raster_Funcs structure. It can be an FT_Outline or anything else in order to support a large array of glyph formats.

Note also that the render function can fail and return a ‘FT_Err_Unimplemented_Feature’ error code if the raster used does not support direct composition.

XXX: For now, the standard raster doesn't support direct composition but this should change for the final release (see the files ‘demos/src/ftgrays.c’ and ‘demos/src/ftgrays2.c’ for examples of distinct implementations which support direct composition).

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#### FT_Raster_Funcs

Defined in FT_IMAGE_H (freetype/ftimage.h).

      typedef struct  FT_Raster_Funcs_
      {
        FT_Glyph_Format        glyph_format;
        FT_Raster_NewFunc      raster_new;
        FT_Raster_ResetFunc    raster_reset;
        FT_Raster_SetModeFunc  raster_set_mode;
        FT_Raster_RenderFunc   raster_render;
        FT_Raster_DoneFunc     raster_done;

      } FT_Raster_Funcs;

A structure used to describe a given raster class to the library.

fields

glyph_format | The supported glyph format for this raster.
raster_new | The raster constructor.
raster_reset | Used to reset the render pool within the raster.
raster_render | A function to render a glyph into a given bitmap.
raster_done | The raster destructor.

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