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Scanline Converter¶

Synopsis¶

This section contains technical definitions.

FT_Raster¶

Defined in FT_IMAGE_H (freetype/ftimage.h).

  typedef struct FT_RasterRec_*  FT_Raster;

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

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 pixels when rendering an 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).

note

This structure is used by the span drawing callback type named FT_SpanFunc that takes the y coordinate of the span as a parameter.

The coverage value is always between 0 and 255. If you want less gray values, the callback function has to reduce them.

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.

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;  /* unused */
    FT_Raster_BitTest_Func  bit_test;     /* unused */
    FT_Raster_BitSet_Func   bit_set;      /* unused */
    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	Unused.
bit_test	Unused.
bit_set	Unused.
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. This allows direct composition over a pre-existing bitmap through user-provided callbacks to perform the span drawing and composition. Not supported by the monochrome rasterizer.

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

  /* these constants are deprecated; use the corresponding */
  /* `FT_RASTER_FLAG_XXX' values instead                   */
#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. 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.

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.

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.

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 used to provide an area of memory called the ‘render pool’ available to all registered rasterizers. This was not thread safe, however, and now FreeType never allocates this pool.

This function is called after a new raster object is created.

input

raster	A handle to the new raster object.
pool_base	Previously, the address in memory of the render pool. Set this to NULL.
pool_size	Previously, the size in bytes of the render pool. Set this to 0.

note

Rasterizers should rely on dynamic or stack allocation if they want to (a handle to the memory allocator is passed to the rasterizer constructor).

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.

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 that support direct composition).

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.

FT_Raster_BitTest_Func¶

Defined in FT_IMAGE_H (freetype/ftimage.h).

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

Deprecated, unimplemented.

FT_Raster_BitSet_Func¶

Defined in FT_IMAGE_H (freetype/ftimage.h).

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

Deprecated, unimplemented.