FreeType » Docs » Support API » Scanline Converter

Scanline Converter¶

Synopsis¶

A raster or a rasterizer is a scan converter in charge of producing a pixel coverage bitmap that can be used as an alpha channel when compositing a glyph with a background. FreeType comes with two rasterizers: bilevel raster1 and anti-aliased smooth are two separate modules. They are usually called from the high-level FT_Load_Glyph or FT_Render_Glyph functions and produce the entire coverage bitmap at once, while staying largely invisible to users.

Instead of working with complete coverage bitmaps, it is also possible to intercept consecutive pixel runs on the same scanline with the same coverage, called spans, and process them individually. Only the smooth rasterizer permits this when calling FT_Outline_Render with FT_Raster_Params as described below.

Working with either complete bitmaps or spans it is important to think of them as colorless coverage objects suitable as alpha channels to blend arbitrary colors with a background. For best results, it is recommended to use gamma correction, too.

This section also describes the public API needed to set up alternative FT_Renderer modules.

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 to model a single span of consecutive 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 anti-aliased rasterizer produces coverage values from 0 to 255, this is, from completely transparent to completely opaque.

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 pixel spans on each scan line.

input

y	The scanline's upward 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 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 using alpha compositing. It can also be used for oversampling and averaging.

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 parameters used by a raster's render function, passed as an argument to FT_Outline_Render.

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 span clipping box expressed in integer pixels (not in 26.6 fixed-point units).

note

The FT_RASTER_FLAG_AA bit flag must be set in the flags to generate an anti-aliased glyph bitmap, otherwise a monochrome bitmap is generated. The target should have appropriate pixel mode and its dimensions define the clipping region.

If both FT_RASTER_FLAG_AA and FT_RASTER_FLAG_DIRECT bit flags are set in flags, the raster calls an FT_SpanFunc callback gray_spans with user data as an argument ignoring target. This allows direct composition over a pre-existing user surface to perform the span drawing and composition. To optionally clip the spans, set the FT_RASTER_FLAG_CLIP flag and clip_box. The monochrome raster does not support the direct mode.

The gray-level rasterizer always uses 256 gray levels. If you want fewer gray levels, you have to use FT_RASTER_FLAG_DIRECT and reduce the levels in the callback function.

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
#define FT_RASTER_FLAG_SDF      0x8

  /* 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 and the output will be clipped to its size. 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. Otherwise, the `clip_box` is effectively set to the bounding box and all spans are generated.
FT_RASTER_FLAG_SDF	This flag is set to indicate that a signed distance field glyph image should be generated. This is only used while rendering with the `FT_RENDER_MODE_SDF` render mode.

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.

note

In FreeType 2, all rasters are now encapsulated within specific FT_Renderer modules and only used in their context.

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.

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.