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|
The FreeType Engine
Extension Library Reference
-----------------------------------
Table of Contents:
I. Engine extensions
1. kerning (ftxkern)
2. PostScript names (ftxpost)
3. TrueType Open (ftxopen)
a. The `BASE' table (baseline data)
b. The `GDEF' table (glyph definitions)
c. The `GPOS' table (glyph positions)
d. The `GSUB' table (glyph substitions)
e. The `JSTF' table (justification data)
f. auxiliary functions
4. embedded bitmaps (ftxsbit)
II. API extensions
1. cmap iteration (ftxcmap)
2. internationalized error messages (ftxerr18)
3. access to the `gasp' table (ftxgasp)
4. fast retrieval of glyph widths and heights (ftxwidth)
III. Error codes
--------------------
Please read the file `user.txt' for an introduction into FreeType's
extension mechanism and the difference between engine and API
extensions.
I. Engine extensions
====================
To use engine extensions you have to compile the library with the
macro TT_CONFIG_OPTION_EXTEND_ENGINE. By default, this macro is set
and all engine extensions are linked to the FreeType library.
1. kerning (ftxkern)
--------------------
TT_Init_Kerning_Extension( TT_Engine engine )
Initializes the kerning extension for a given engine. This must
be called just after the engine creation, and before any face
object allocation. Example:
TT_Init_FreeType( &engine );
TT_Init_Kerning_Extension( engine );
..................................................................
TT_Get_Kerning_Directory( TT_Face face,
TT_Kerning* directory )
Queries the kerning directory found in a face object. If no
kerning table is found in the TrueType file, the error
TT_Err_Table_Is_Missing will be returned.
You can access the subtables through the pointers of the
directory. However, by default, the directory is only loaded if
a face object is created. You must load the subtables that
interest you with a call to TT_Load_Kerning_Table().
The layout of all kerning structures is defined in the file
`lib/extend/ftxkern.h'. Formats 0 and 2 (as defined in the
Microsoft TrueType specification) are exposed by this API.
NOTE:
This function must be called after the kerning extension has
been initialized.
..................................................................
TT_Load_Kerning_Table( TT_Face face,
TT_UShort kernIndex )
Loads the kerning subtable number `kern_index' into memory. The
subtable can be accessed through the pointers provided by the
kerning directory, obtained from a call to the function
TT_Get_Kerning_Directory().
Note that the interpretation of the kerning data is left to the
client application. Read the TrueType specification for more
information on kerning encoding.
NOTE 1:
This function must be called after the kerning extension were
initialized.
NOTE 2:
An example can be found in the file `test/ftstrtto.c'.
====================================================================
2. PostScript names (ftxpost)
-----------------------------
TT_Init_Post_Extension( TT_Engine engine )
Initializes the PostScript name extension to load the PostScript
glyph names given in the `post' table. This must be called just
after creation of the engine, and before any face object
allocation. See description of TT_Get_PS_Name() for an example.
..................................................................
TT_Load_PS_Names( TT_Face face,
TT_Post* post )
Loads the PostScript glyph names into memory. This must be done
before TT_Get_PS_Name() is called. In case of error, either
TT_Err_Invalid_Post_Table or TT_Err_Invalid_Post_Table_Format is
returned. See description of TT_Get_PS_Name() for an example.
..................................................................
TT_Get_PS_Name( TT_Face face,
TT_UShort index,
TT_String** PSname )
Get the PostScript glyph name for a given glyph index. A
pointer to the name is returned in `PSname'. Example:
...
TT_Post post;
char* PSname;
...
TT_Init_Post_Extension( engine );
TT_Load_PS_Names( face, &post );
...
TT_Get_PS_Name( face, index, &PSname );
NOTE:
You must not alter the PostScript glyph name string returned
in `PSname'.
====================================================================
3. TrueType Open (ftxopen)
--------------------------
Please note that TrueType Open specific error codes have the
prefix `TTO_Err_' instead of `TT_Err_'.
a. The `BASE' table (baseline data)
Not yet supported.
b. The `GDEF' table (glyph definitions)
TT_Init_GDEF_Extension( TT_Engine engine )
Initializes the GDEF extension to load the glyph definition
data given in the `GDEF' table. This must be called just
after creation of the engine, and before any face object
allocation. See the file `ftstrtto.c' for an example.
................................................................
TT_Load_GDEF_Table( TT_Face face,
TTO_GDEFHeader* gdef )
Loads the GDEF table into `gdef' for a given face `face'.
This must be done before any queries about glyph properties
with TT_GSUB_Get_Property(). Returns TT_Err_Table_Missing if
there is no GDEF table in the font.
................................................................
TT_GDEF_Get_Glyph_Property( TTO_GDEFHeader* gdef,
TT_UShort glyphID,
TT_UShort* property )
Use this function to get glyph properties in the variable
`property' for the glyph with index `glyphID' stored in the
`gdef' table. This data is essential for many fonts to
correctly apply contextual substitution (both GSUB and GPOS).
As a special case, zero is assigned to `property' if `gdef' is
NULL or the glyph has no special glyph property assigned to
it. The other return values of `property' are TTO_BASE,
TTO_LIGATURE, TTO_MARK, and TTO_COMPONENT; you can directly
apply the `LookupFlag' mask to check for certain properties:
TTO_GDEFHeader* gdef;
TTO_Lookup* lo;
TT_UShort glyph_ID;
TT_UShort p;
TT_GDEF_Get_Property( gdef, glyph_ID, &p );
if ( p & lo->LookupFlag )
return TTO_Err_Not_Covered;
Usually, you don't need to take care of glyph properties by
yourself since TT_GSUB_Apply_String() will do this for you by
calling TT_GDEF_Get_Property().
Some fonts need GDEF-like data even if no GDEF table is
provided (for example, the Arabic script needs information
which glyphs are base glyphs and which are mark glyphs). In
such cases, you should use TT_GDEF_Build_ClassDefinition() to
build the necessary structures so that TT_GDEF_Get_Property()
returns meaningful values.
See also TT_Load_GSUB_Table() and TT_Load_GPOS_Table().
................................................................
TT_GDEF_Build_ClassDefinition( TTO_GDEFHeader* gdef,
TT_UShort num_glyphs,
TT_UShort glyph_count,
TT_UShort* glyph_array,
TT_UShort* class_array )
Fills a `gdef' structure with data to make
TT_GDEF_Get_Property() work in case no GDEF table is
available. `num_glyphs' is the number of glyphs in the font.
`glyph_count' is the number of glyphs resp. class values (as
specified in the GDEF specification) given in the arrays
`glyph_array' and `class_array', respectively. The glyph
indices in `glyph_array' must be all different and sorted in
ascending order; the function expects that glyph index
`glyph_array[n]' has its class value in `class_array[n]'.
Note that mark attach classes as defined in OpenType 1.2 are
not supported for constructed GDEF tables.
See `arabic.c' for an example.
c. The `GPOS' table (glyph positions)
Not yet supported.
d. The `GSUB' table (glyph substitions)
For glyph substitution, a string object of type TTO_GSUB_String
is used. The field `length' holds the length of the string,
`pos' points to the actual position in the glyph string `string'
(for input strings) resp. the position where the substituted
glyphs should be placed at (for output strings).
Within the `properties' array (which must always have the same
length as `string' if set), you can define properties for glyphs
-- `string[n]' is associated with `properties[n]'. The idea is
that some features apply to specific glyphs only. As an
example, the `fina' feature for Arabic applies only to glyphs
which appear at the end of a word (strictly speaking, this
feature is used only for glyphs which have a `final' property;
such glyphs can occur in the middle of a word also under certain
circumstances which is dependent on Arabic script rules). The
relationship between properties and features can be set up with
the function TT_GSUB_Add_Feature(). If `properties' is set to
NULL, all selected features apply to all glyphs in the given
string object. If `properties' is non-NULL, the elements of the
array are treated as bit fields. A set flag means that the
corresponding feature (as set with the TT_GSUB_Add_Feature()
function) should not be applied to the particular glyph. As a
consequence, a zero value for these 16 bits means that all
features should be applied, and a value of 0xFFFF implies that
no feature at all should be used for the glyph.
The field `allocated' is for internal use only and must not be
modified. If its value is larger than zero, you should use
free() to deallocate the memory used by `string' (and
`properties' if set) in case you want to destroy a
TTO_GSUB_String object (memory for `string' and `properties' is
allocated automatically e.g. by TT_GSUB_Apply_String() for
output string objects).
struct TTO_GSUB_String_
{
TT_ULong length;
TT_ULong pos;
TT_ULong allocated;
TT_UShort* string;
TT_UShort* properties;
};
typedef struct TTO_GSUB_String_ TTO_GSUB_String;
Note that the `string' field must contain glyph indices, not
character codes.
For initializing an input string object, you should set the
`length', `string', and `properties' fields (the last one only
if necessary) to appropriate values. `pos' has to be set to the
position where the substitution lookups should start (usually
zero). The `allocated' field will be ignored.
For initializing an output string object, all fields (except
`length' which will be ignored) must be set to zero. If you
want to reuse the object, set `pos' to the position where the
substituted glyphs should be placed at (usually zero), but don't
touch the `allocated', `string', and `properties' fields.
................................................................
TT_Init_GSUB_Extension( TT_Engine engine )
Initializes the GSUB extension to load the glyph substitution
data given in the `GSUB' table. This must be called just
after creation of the engine, and before any face object
allocation. See the files `ftstrtto.c' and `ftdump.c' for
detailed examples.
................................................................
TT_Load_GSUB_Table( TT_Face face,
TTO_GSUBHeader* gsub,
TTO_GDEFHeader* gdef )
Loads the GSUB table into `gsub' for a given face `face'.
This must be done before any queries about or selection of
scripts, languages, and features. Returns
TT_Err_Table_Missing if there is no GSUB table in the font.
`gdef' should contain a pointer to an associated GDEF table,
either a native one loaded with TT_Load_GDEF_Table() or
emulated with TT_GDEF_Build_ClassDefinition(). If `gdef' is
set to NULL, no GDEF data will be used.
................................................................
TT_GSUB_Select_Script( TTO_GSUBHeader* gsub,
TT_ULong script_tag,
TT_UShort* script_index )
This function sets the script index of a given script tag
`script_tag' in the variable `script_index' for the GSUB table
`gsub'. Returns TTO_Err_Not_Covered if the script tag cannot
be found. The available script tags can be queried with the
function TT_GSUB_Query_Scripts().
................................................................
TT_GSUB_Select_Language( TTO_GSUBHeader* gsub,
TT_ULong language_tag,
TT_UShort script_index,
TT_UShort* language_index,
TT_UShort* req_feature_index )
This function sets the language index of a given language tag
`language_tag' and script index `script_index' in the variable
`language_index' for the GSUB table `gsub'. Returns
TTO_Err_Not_Covered if the language tag cannot be found. The
available language tags can be queried with the function
TT_GSUB_Query_Languages().
Additionally, the index of the required feature for this
language system is returned in `req_feature_index'; it must be
later registered for use with TT_GSUB_Add_Feature().
................................................................
TT_GSUB_Select_Feature( TTO_GSUBHeader* gsub,
TT_ULong feature_tag,
TT_UShort script_index,
TT_UShort language_index,
TT_UShort* feature_index )
This function sets the feature index of a feature tag
`feature_tag', script index `script_index', and language index
`language_index' in the variable `feature_index' for the GSUB
table `gsub'. Returns TTO_Err_Not_Covered if the feature tag
cannot be found. The available feature tags can be queried
with the function TT_GSUB_Query_Features(). Setting
language_index to 0xFFFF selects the default language system.
................................................................
TT_GSUB_Query_Scripts( TTO_GSUBHeader* gsub,
TT_ULong** script_tag_list )
Returns the available script tags for a given GSUB table
`gsub' in the array `script_tag_list'. The array can be later
deallocated with free().
................................................................
TT_GSUB_Query_Languages( TTO_GSUBHeader* gsub,
TT_UShort script_index,
TT_ULong** language_tag_list )
Returns the available language tags for a given GSUB table
`gsub' and script index `script_index' in the array
`language_tag_list'. The array can be later deallocated with
free().
................................................................
TT_GSUB_Query_Features( TTO_GSUBHeader* gsub,
TT_UShort script_index,
TT_UShort language_index,
TT_ULong** feature_tag_list )
Returns the available feature tags for a given GSUB table
`gsub', script index `script_index', and language index
`language_index' in the array `feature_tag_list'. If language
index is set to 0xFFFF, the values for the default language
system are returned. The array can be later deallocated with
free().
................................................................
TT_GSUB_Add_Feature( TTO_GSUBHeader* gsub,
TT_UShort feature_index,
TT_UShort property )
To prepare a call to TT_GSUB_Apply_String() which should
process a given feature with index `feature_index' and the
property `property', you must use this function. Its task is
to mark the lookup tables used by the feature for further
processing.
`property' defines a relationship between the input string
object and the specific feature. The client must handle this
variable as a bit field, i.e., up to 16 user properties can be
defined. If `property' is set to ALL_GLYPHS (0xFFFF, the only
predefined value), or if the input string object has no set
bits in the `properties' field, the feature applies to all
glyphs. If bit `x' is set in `property', the feature applies
only to glyphs which have bit `x' not set in glyph's
`properties' field in the input string object. See
TT_GSUB_Apply_String() for an example.
Returns TT_Err_Invalid_Argument in case of error.
................................................................
TT_GSUB_Clear_Features( TTO_GSUBHeader* gsub )
Clears the list of used features and properties. No lookup
tables will be processed.
................................................................
TT_GSUB_Register_Alternate_Function( TTO_GSUBHeader* gsub,
TTO_AltFunction alt,
void* data )
Installs a callback function to handle alternate
substitutions. `data' is a generic pointer to a user-defined
structure which will be completely ignored by the ftxopen
extension. `alt' is a function pointer defined as
typedef TT_UShort
(*TTO_AltFunction)(TT_ULong pos,
TT_UShort glyphID,
TT_UShort num_alternates,
TT_UShort* alternates,
void* data );
If TT_GSUB_Apply_String() encounters an alternate lookup while
processing the input string, it will call the function `alt'
to find out which alternate glyph it should use. `pos' gives
the position in the string, `glyphID' the glyph ID of the
glyph to be replaced with an alternate glyph, and
`num_alternates' the number of alternate glyphs in the
`alternates' array. A pointer to the user-defined `data'
structure is also available. `alt' must return an index into
`alternates'.
If `alt' is NULL or if this function isn't called at all,
TT_GSUB_Apply_String() will always use the first alternate
glyph.
Please note that theoretically an alternate lookup can happen
during any other lookup! For example, a lookup chain like
single subst -> alternate subst -> ligature subst
*is* possible (albeit rather unlikely). Thus be warned that
`pos' does not necessarily reflect the position of a glyph to
be displayed at all, nor does `glyphID' specifies a glyph
which will be in the final output string.
................................................................
TT_GSUB_Apply_String( TTO_GSUBHeader* gsub,
TTO_GSUB_String* in,
TTO_GSUB_String* out )
This is the very function which handles glyph substitution
according to the features set up with TT_GSUB_Add_Feature(),
using both GSUB and GDEF data (if defined). Two
TTO_GSUB_String objects `in' and `out' (as described above)
are taken for input and output; after successful excecution,
the converted glyph string can been found in `out', and
`out.length' gives the actual length of the output string
(counted from the begin of the array).
Example:
/* We assume that the feature `xxxx' has index 5, and */
/* feature `yyyy' has index 8, applying only to glyphs */
/* with the property `FINAL_GLYPHS' set (0x1000 is an */
/* ad-hoc value just for this example). */
/* The order of calls to TT_GSUB_Add_Feature() is not */
/* significant. */
#define FINAL_GLYPHS 0x1000
TT_GSUB_Clear_Features( &gsub );
TT_GSUB_Add_Feature( &gsub, 8, FINAL_GLYPHS );
TT_GSUB_Add_Feature( &gsub, 5, ALL_GLYPHS );
TT_GSUB_Apply_String( &gsub, &in, &out );
You must initialize both `in' and `out' structures; allocation
necessary for the `out' object will be handled automatically.
In case you don't register a function to handle alternate
substitutions (GSUB lookup 3), always the first alternate
glyph will be used. See TT_GSUB_Register_Alternate_Function()
above for more details.
e. The `JSTF' table (justification data)
Not yet supported.
f. auxiliary functions
TT_GSUB_Add_String( TTO_GSUB_String* in,
TT_UShort num_in,
TTO_GSUB_String* out,
TT_UShort num_out,
TT_UShort* data )
This function copies `num_out' elements from `data' to `out',
advancing the array pointer `in.pos' by `num_in' elements and
`out.pos' by `num_out' elements. If the string (resp. the
properties) array in `out' is empty or too small, it allocates
resp. reallocates the string (and properties) array.
Finally, it sets `out.length' equal to `out.pos'.
The properties for all replaced glyphs are taken from the
glyph at position `in->pos'.
TT_GSUB_Add_String() is normally used in
TT_GSUB_Apply_String(); you will need it for the special case
to skip some glyphs (i.e., copy glyphs directly from the `in'
to the `out' object), bypassing a possible GSUB substitution.
Here an example which copies one glyph:
TT_GSUB_Add_String( in, 1,
out, 1,
&in->string[in->pos] );
====================================================================
4. embedded bitmaps (ftxsbit)
-----------------------------
TT_Init_SBit_Extension( TT_Engine engine )
Initializes the embedded bitmap extension to load the bitmap
glyphs given in the various sbit tables: `EBLC', `bloc', `EBDT',
and `bdat' (`bloc' and `bdat' tables are only in Apple fonts;
the former is equivalent to `EBLC', the latter equivalent to
`EBDT'). This must be called just after creation of the engine,
and before any face object allocation. See description of
TT_Load_Glyph_Bitmap() for an example.
..................................................................
TT_Get_Face_Bitmaps( TT_Face face,
TT_EBLC* eblc_table )
Loads the `EBLC' (resp. `bloc') table from a font file into the
`eblc_table' structure. Use this function for testing whether
embedded bitmaps are available or not.
This function returns TT_Err_Table_Missing if the font contains
no embedded bitmaps. All fields in `eblc_table' will then be
set to 0.
..................................................................
TT_New_SBit_Image( TT_SBit_Image** image )
Allocates a new embedded bitmap container, pointed to by
`image'.
..................................................................
void TT_Done_SBit_Image( TT_SBit_Image* image )
^^^^
Releases the embedded bitmap container `image'.
..................................................................
TT_Get_SBit_Strike( TT_Face face,
TT_Instance instance,
TT_SBit_Strike* strike )
Loads a suitable strike (i.e. bitmap sizetable) for the given
instance. Will return TT_Err_Invalid_PPem if there is no strike
for the current x_ppem and y_ppem values.
..................................................................
TT_Load_Glyph_Bitmap( TT_Face face,
TT_Instance instance,
TT_UShort glyph_index,
TT_SBit_Image* bitmap );
Loads a glyph bitmap for a given glyph index `glyph_index' (in
face `face' and instance `instance') into `bitmap'. It calls
TT_Get_SBit_Strike() internally for checking the current x_ppem
and y_ppem values.
This function returns an error if there is no embedded bitmap
for the glyph at the given instance.
Example (omitting the error handling):
...
TT_SBit_Image* bitmap;
...
TT_Init_SBit_Extension( engine );
TT_New_SBit_Image( &bitmap );
...
TT_Load_Glyph_Bitmap( face, instance, glyph_index, bitmap );
--------------------------------------------------------------------
--------------------------------------------------------------------
II. API extensions
==================
To use API extensions, simply compile the specific extension file
and link it to the library or your program. By default, all
extensions described below are linked to the library.
1. cmap iteration (ftxcmap)
---------------------------
TT_Long TT_CharMap_First( TT_CharMap charMap,
^^^^^^^ TT_UShort* glyph_index )
Returns the first valid character code in a given character map
`charMap'. Also returns the corresponding glyph index (in the
parameter `*glyph_index'). In case of failure, -1 is returned,
and `*glyph_index' is undefined.
..................................................................
TT_Long TT_CharMap_Next( TT_CharMap charMap,
^^^^^^^ TT_UShort last_char,
TT_UShort* glyph_index )
Returns the next valid character code in a given character map
`charMap' which follows `last_char'. Also returns the
corresponding glyph index (in the parameter `*glyph_index'). In
case of failure, -1 is returned, and `glyph_index' is undefined.
..................................................................
TT_Long TT_CharMap_Last( TT_CharMap charMap,
^^^^^^^ TT_UShort* glyph_index )
Returns the last valid character code in a given character map
`charMap'. Also returns the corresponding glyph index (in the
parameter `*glyph_index'). In case of failure, -1 is returned,
and `*glyph_index' is undefined.
====================================================================
2. internationalized error messages (ftxerr18)
----------------------------------------------
This extension provides internationalized error strings from the
various error messages. It uses the `gettext' package if
available or returns English/American message strings if not.
Currently, the gettext package is only available on UNIX-like
systems like Linux; this means that for other platforms only
English error strings are returned.
If the gettext package is found on your system, the configure
script automatically includes it by default. In case you don't
want to use it, or if you encounter some problems compiling this
file, try to disable nls support by configuring FreeType with
`./configure --disable-nls'.
Please read the gettext info files for more information how to set
up your system for internationalized messages. A short
introduction is also given in the file `i18n.txt'.
TT_String* TT_ErrToString18( TT_Error i )
^^^^^^^^^^
This function returns an error string for a given error code
`i'. The type `TT_String' usually defaults to `char'; see
apiref.txt for more details.
An example how to use this function (in connection with the
gettext interface) is given e.g. in test/ftdump.c.
====================================================================
3. access to the `gasp' table (ftxgasp)
---------------------------------------
The `gasp' table is currently loaded by the core engine, but the
standard API doesn't give access to it.
TT_Get_Face_Gasp_Flags( TT_Face face,
TT_UShort point_size,
TT_Bool* grid_fit,
TT_Bool* smooth_font )
This function returns for a given `point_size' the values of the
gasp flags `grid_fit' and `smooth_font'. The returned values
are booleans (where 0 = NO, and 1 = YES).
Note that this function will return TT_Err_Table_Missing if the
font file doesn't contain any gasp table.
====================================================================
4. fast retrieval of glyph widths and heights (ftxwidth)
--------------------------------------------------------
This extension is used to parse the `glyf' table of a TrueType
file in order to extract the bounding box of a given range of
glyphs.
The bounding box is then used to build font unit widths and
heights that are returned in two parallel arrays.
This extension is needed by the FreeType/2 OS/2 Font Driver.
TT_Get_Face_Widths( TT_Face face,
TT_UShort first_glyph,
TT_UShort last_glyph,
TT_UShort* widths,
TT_UShort* heights )
Returns the widths (in array `widths') and heights (in array
`heights') of a glyph range which starts at `first_glyph' and
ends at `last_glyph'. All returned values are returned in font
units. If either `widths' or `heights' is set to a NULL
pointer, no data will be returned for that particular array.
Note: TT_Get_Face_Widths() does *not* allocate the two arrays!
--------------------------------------------------------------------
--------------------------------------------------------------------
III. Error Messages
===================
Most functions return an error code, typed to TT_Error. A return
value of zero indicates no error. The error values are defined in
the various extension header files (e.g. ftxkern.h). In the
following table, the prefix `TT_Err_' is omitted, e.g. `Ok' ->
`TT_Err_Ok'.
Error Unprefixed Error
Code Macro Name Description
------------------------------------------------------------------
0x0A00 Invalid_Kerning_Table_Format
An invalid kerning subtable
format was found in this font.
0x0A01 Invalid_Kerning_Table A kerning table contains illegal
glyph indices.
0x0B00 Invalid_Post_Table_Format
The post table format specified
in the font is invalid.
0x0B01 Invalid_Post_Table The post table contains illegal
entries.
Here the TrueType Open error codes. In the following table, the
prefix `TTO_Err_' is omitted.
Error Unprefixed Error
Code Macro Name Description
------------------------------------------------------------------
0x1000 Invalid_SubTable_Format The TrueType Open subtable format
specified in the font is invalid.
0x1001 Invalid_SubTable A TrueType Open subtable contains
illegal entries.
0x1002 Not_Covered The requested feature, glyph,
etc. isn't covered by the actual
function.
0x1010 Invalid_GSUB_SubTable_Format
The GSUB subtable format
specified in the font is invalid.
0x1011 Invalid_GSUB_SubTable The GSUB subtable contains
illegal entries.
0x1020 Invalid_GPOS_SubTable_Format
The GPOS subtable format
specified in the font is invalid.
0x1021 Invalid_GPOS_SubTable The GPOS subtable contains
illegal entries.
--- end of apirefx.txt ---
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