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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 upward y coordinate. |
count |
The number of spans to draw on this scanline. |
spans |
A table of |
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 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 |
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 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 |
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 |
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 |
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.