FreeType » Docs » Core API » Glyph Retrieval
Glyph Retrieval¶
Synopsis¶
The functions and structures collected in this section operate on single glyphs, of which FT_Load_Glyph is most important.
FT_GlyphSlot¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
typedef struct FT_GlyphSlotRec_* FT_GlyphSlot;
A handle to a given ‘glyph slot’. A slot is a container that can hold any of the glyphs contained in its parent face.
In other words, each time you call FT_Load_Glyph or FT_Load_Char, the slot's content is erased by the new glyph data, i.e., the glyph's metrics, its image (bitmap or outline), and other control information.
also
See FT_GlyphSlotRec for the publicly accessible glyph fields.
FT_GlyphSlotRec¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
typedef struct FT_GlyphSlotRec_
{
FT_Library library;
FT_Face face;
FT_GlyphSlot next;
FT_UInt glyph_index; /* new in 2.10; was reserved previously */
FT_Generic generic;
FT_Glyph_Metrics metrics;
FT_Fixed linearHoriAdvance;
FT_Fixed linearVertAdvance;
FT_Vector advance;
FT_Glyph_Format format;
FT_Bitmap bitmap;
FT_Int bitmap_left;
FT_Int bitmap_top;
FT_Outline outline;
FT_UInt num_subglyphs;
FT_SubGlyph subglyphs;
void* control_data;
long control_len;
FT_Pos lsb_delta;
FT_Pos rsb_delta;
void* other;
FT_Slot_Internal internal;
} FT_GlyphSlotRec;
FreeType root glyph slot class structure. A glyph slot is a container where individual glyphs can be loaded, be they in outline or bitmap format.
fields
| library |
A handle to the FreeType library instance this slot belongs to. |
| face |
A handle to the parent face object. |
| next |
In some cases (like some font tools), several glyph slots per face object can be a good thing. As this is rare, the glyph slots are listed through a direct, single-linked list using its |
| glyph_index |
[Since 2.10] The glyph index passed as an argument to |
| generic |
A typeless pointer unused by the FreeType library or any of its drivers. It can be used by client applications to link their own data to each glyph slot object. |
| metrics |
The metrics of the last loaded glyph in the slot. The returned values depend on the last load flags (see the Note that even when the glyph image is transformed, the metrics are not. |
| linearHoriAdvance |
The advance width of the unhinted glyph. Its value is expressed in 16.16 fractional pixels, unless |
| linearVertAdvance |
The advance height of the unhinted glyph. Its value is expressed in 16.16 fractional pixels, unless |
| advance |
This shorthand is, depending on |
| format |
This field indicates the format of the image contained in the glyph slot. Typically |
| bitmap |
This field is used as a bitmap descriptor. Note that the address and content of the bitmap buffer can change between calls of |
| bitmap_left |
The bitmap's left bearing expressed in integer pixels. |
| bitmap_top |
The bitmap's top bearing expressed in integer pixels. This is the distance from the baseline to the top-most glyph scanline, upwards y coordinates being positive. |
| outline |
The outline descriptor for the current glyph image if its format is [Since 2.10.1] If |
| num_subglyphs |
The number of subglyphs in a composite glyph. This field is only valid for the composite glyph format that should normally only be loaded with the |
| subglyphs |
An array of subglyph descriptors for composite glyphs. There are |
| control_data |
Certain font drivers can also return the control data for a given glyph image (e.g. TrueType bytecode, Type 1 charstrings, etc.). This field is a pointer to such data; it is currently internal to FreeType. |
| control_len |
This is the length in bytes of the control data. Currently internal to FreeType. |
| other |
Reserved. |
| lsb_delta |
The difference between hinted and unhinted left side bearing while auto-hinting is active. Zero otherwise. |
| rsb_delta |
The difference between hinted and unhinted right side bearing while auto-hinting is active. Zero otherwise. |
note
If FT_Load_Glyph is called with default flags (see FT_LOAD_DEFAULT) the glyph image is loaded in the glyph slot in its native format (e.g., an outline glyph for TrueType and Type 1 formats). [Since 2.9] The prospective bitmap metrics are calculated according to FT_LOAD_TARGET_XXX and other flags even for the outline glyph, even if FT_LOAD_RENDER is not set.
This image can later be converted into a bitmap by calling FT_Render_Glyph. This function searches the current renderer for the native image's format, then invokes it.
The renderer is in charge of transforming the native image through the slot's face transformation fields, then converting it into a bitmap that is returned in slot->bitmap.
Note that slot->bitmap_left and slot->bitmap_top are also used to specify the position of the bitmap relative to the current pen position (e.g., coordinates (0,0) on the baseline). Of course, slot->format is also changed to FT_GLYPH_FORMAT_BITMAP.
Here is a small pseudo code fragment that shows how to use lsb_delta and rsb_delta to do fractional positioning of glyphs:
FT_GlyphSlot slot = face->glyph;
FT_Pos origin_x = 0;
for all glyphs do
<load glyph with `FT_Load_Glyph'>
FT_Outline_Translate( slot->outline, origin_x & 63, 0 );
<save glyph image, or render glyph, or ...>
<compute kern between current and next glyph
and add it to `origin_x'>
origin_x += slot->advance.x;
origin_x += slot->lsb_delta - slot->rsb_delta;
endfor
Here is another small pseudo code fragment that shows how to use lsb_delta and rsb_delta to improve integer positioning of glyphs:
FT_GlyphSlot slot = face->glyph;
FT_Pos origin_x = 0;
FT_Pos prev_rsb_delta = 0;
for all glyphs do
<compute kern between current and previous glyph
and add it to `origin_x'>
<load glyph with `FT_Load_Glyph'>
if ( prev_rsb_delta - slot->lsb_delta > 32 )
origin_x -= 64;
else if ( prev_rsb_delta - slot->lsb_delta < -31 )
origin_x += 64;
prev_rsb_delta = slot->rsb_delta;
<save glyph image, or render glyph, or ...>
origin_x += slot->advance.x;
endfor
If you use strong auto-hinting, you must apply these delta values! Otherwise you will experience far too large inter-glyph spacing at small rendering sizes in most cases. Note that it doesn't harm to use the above code for other hinting modes also, since the delta values are zero then.
FT_Glyph_Metrics¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
typedef struct FT_Glyph_Metrics_
{
FT_Pos width;
FT_Pos height;
FT_Pos horiBearingX;
FT_Pos horiBearingY;
FT_Pos horiAdvance;
FT_Pos vertBearingX;
FT_Pos vertBearingY;
FT_Pos vertAdvance;
} FT_Glyph_Metrics;
A structure to model the metrics of a single glyph. The values are expressed in 26.6 fractional pixel format; if the flag FT_LOAD_NO_SCALE has been used while loading the glyph, values are expressed in font units instead.
fields
| width |
The glyph's width. |
| height |
The glyph's height. |
| horiBearingX |
Left side bearing for horizontal layout. |
| horiBearingY |
Top side bearing for horizontal layout. |
| horiAdvance |
Advance width for horizontal layout. |
| vertBearingX |
Left side bearing for vertical layout. |
| vertBearingY |
Top side bearing for vertical layout. Larger positive values mean further below the vertical glyph origin. |
| vertAdvance |
Advance height for vertical layout. Positive values mean the glyph has a positive advance downward. |
note
If not disabled with FT_LOAD_NO_HINTING, the values represent dimensions of the hinted glyph (in case hinting is applicable).
Stroking a glyph with an outside border does not increase horiAdvance or vertAdvance; you have to manually adjust these values to account for the added width and height.
FreeType doesn't use the ‘VORG’ table data for CFF fonts because it doesn't have an interface to quickly retrieve the glyph height. The y coordinate of the vertical origin can be simply computed as vertBearingY + height after loading a glyph.
FT_Load_Glyph¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
Load a glyph into the glyph slot of a face object.
inout
| face |
A handle to the target face object where the glyph is loaded. |
input
| glyph_index |
The index of the glyph in the font file. For CID-keyed fonts (either in PS or in CFF format) this argument specifies the CID value. |
| load_flags |
A flag indicating what to load for this glyph. The |
return
FreeType error code. 0 means success.
note
For proper scaling and hinting, the active FT_Size object owned by the face has to be meaningfully initialized by calling FT_Set_Char_Size before this function, for example. The loaded glyph may be transformed. See FT_Set_Transform for the details.
For subsetted CID-keyed fonts, FT_Err_Invalid_Argument is returned for invalid CID values (that is, for CID values that don't have a corresponding glyph in the font). See the discussion of the FT_FACE_FLAG_CID_KEYED flag for more details.
If you receive FT_Err_Glyph_Too_Big, try getting the glyph outline at EM size, then scale it manually and fill it as a graphics operation.
FT_LOAD_XXX¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
#define FT_LOAD_DEFAULT 0x0
#define FT_LOAD_NO_SCALE ( 1L << 0 )
#define FT_LOAD_NO_HINTING ( 1L << 1 )
#define FT_LOAD_RENDER ( 1L << 2 )
#define FT_LOAD_NO_BITMAP ( 1L << 3 )
#define FT_LOAD_VERTICAL_LAYOUT ( 1L << 4 )
#define FT_LOAD_FORCE_AUTOHINT ( 1L << 5 )
#define FT_LOAD_CROP_BITMAP ( 1L << 6 )
#define FT_LOAD_PEDANTIC ( 1L << 7 )
#define FT_LOAD_IGNORE_GLOBAL_ADVANCE_WIDTH ( 1L << 9 )
#define FT_LOAD_NO_RECURSE ( 1L << 10 )
#define FT_LOAD_IGNORE_TRANSFORM ( 1L << 11 )
#define FT_LOAD_MONOCHROME ( 1L << 12 )
#define FT_LOAD_LINEAR_DESIGN ( 1L << 13 )
#define FT_LOAD_SBITS_ONLY ( 1L << 14 )
#define FT_LOAD_NO_AUTOHINT ( 1L << 15 )
/* Bits 16-19 are used by `FT_LOAD_TARGET_` */
#define FT_LOAD_COLOR ( 1L << 20 )
#define FT_LOAD_COMPUTE_METRICS ( 1L << 21 )
#define FT_LOAD_BITMAP_METRICS_ONLY ( 1L << 22 )
#define FT_LOAD_NO_SVG ( 1L << 24 )
A list of bit field constants for FT_Load_Glyph to indicate what kind of operations to perform during glyph loading.
values
| FT_LOAD_DEFAULT |
Corresponding to 0, this value is used as the default glyph load operation. In this case, the following happens:
Note that by default the glyph loader doesn't render outlines into bitmaps. The following flags are used to modify this default behaviour to more specific and useful cases. |
| FT_LOAD_NO_SCALE |
Don't scale the loaded outline glyph but keep it in font units. This flag is also assumed if This flag implies If the font is ‘tricky’ (see |
| FT_LOAD_NO_HINTING |
Disable hinting. This generally generates ‘blurrier’ bitmap glyphs when the glyphs are rendered in any of the anti-aliased modes. See also the note below. This flag is implied by |
| FT_LOAD_RENDER |
Call This flag is unset by |
| FT_LOAD_NO_BITMAP |
Ignore bitmap strikes when loading. Bitmap-only fonts ignore this flag.
|
| FT_LOAD_SBITS_ONLY |
[Since 2.12] This is the opposite of Note that this load flag was part of FreeType since version 2.0.6 but previously tagged as internal. |
| FT_LOAD_VERTICAL_LAYOUT |
Load the glyph for vertical text layout. In particular, the In case |
| FT_LOAD_FORCE_AUTOHINT |
Prefer the auto-hinter over the font's native hinter. See also the note below. |
| FT_LOAD_PEDANTIC |
Make the font driver perform pedantic verifications during glyph loading and hinting. This is mostly used to detect broken glyphs in fonts. By default, FreeType tries to handle broken fonts also. In particular, errors from the TrueType bytecode engine are not passed to the application if this flag is not set; this might result in partially hinted or distorted glyphs in case a glyph's bytecode is buggy. |
| FT_LOAD_NO_RECURSE |
Don't load composite glyphs recursively. Instead, the font driver fills the Don't use this flag for retrieving metrics information since some font drivers only return rudimentary data. This flag implies |
| FT_LOAD_IGNORE_TRANSFORM |
Ignore the transform matrix set by |
| FT_LOAD_MONOCHROME |
This flag is used with Note that this has no effect on the hinting algorithm used. You should rather use |
| FT_LOAD_LINEAR_DESIGN |
Keep |
| FT_LOAD_NO_AUTOHINT |
Disable the auto-hinter. See also the note below. |
| FT_LOAD_COLOR |
Load colored glyphs. FreeType searches in the following order; there are slight differences depending on the font format. [Since 2.5] Load embedded color bitmap images (provided [Since 2.12] If the glyph index maps to an entry in the face's ‘SVG ’ table, load the associated SVG document from this table and set the [Since 2.10, experimental] If the glyph index maps to an entry in the face's ‘COLR’ table with a ‘CPAL’ palette table (as defined in the OpenType specification), make |
| FT_LOAD_NO_SVG |
[Since 2.13.1] Ignore SVG glyph data when loading. |
| FT_LOAD_COMPUTE_METRICS |
[Since 2.6.1] Compute glyph metrics from the glyph data, without the use of bundled metrics tables (for example, the ‘hdmx’ table in TrueType fonts). This flag is mainly used by font validating or font editing applications, which need to ignore, verify, or edit those tables. Currently, this flag is only implemented for TrueType fonts. |
| FT_LOAD_BITMAP_METRICS_ONLY |
[Since 2.7.1] Request loading of the metrics and bitmap image information of a (possibly embedded) bitmap glyph without allocating or copying the bitmap image data itself. No effect if the target glyph is not a bitmap image. This flag unsets |
| FT_LOAD_CROP_BITMAP |
Ignored. Deprecated. |
| FT_LOAD_IGNORE_GLOBAL_ADVANCE_WIDTH |
Ignored. Deprecated. |
note
By default, hinting is enabled and the font's native hinter (see FT_FACE_FLAG_HINTER) is preferred over the auto-hinter. You can disable hinting by setting FT_LOAD_NO_HINTING or change the precedence by setting FT_LOAD_FORCE_AUTOHINT. You can also set FT_LOAD_NO_AUTOHINT in case you don't want the auto-hinter to be used at all.
See the description of FT_FACE_FLAG_TRICKY for a special exception (affecting only a handful of Asian fonts).
Besides deciding which hinter to use, you can also decide which hinting algorithm to use. See FT_LOAD_TARGET_XXX for details.
Note that the auto-hinter needs a valid Unicode cmap (either a native one or synthesized by FreeType) for producing correct results. If a font provides an incorrect mapping (for example, assigning the character code U+005A, LATIN CAPITAL LETTER Z, to a glyph depicting a mathematical integral sign), the auto-hinter might produce useless results.
FT_LOAD_TARGET_MODE¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
#define FT_LOAD_TARGET_MODE( x ) \
FT_STATIC_CAST( FT_Render_Mode, ( (x) >> 16 ) & 15 )
Return the FT_Render_Mode corresponding to a given FT_LOAD_TARGET_XXX value.
FT_LOAD_TARGET_XXX¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
#define FT_LOAD_TARGET_( x ) ( FT_STATIC_CAST( FT_Int32, (x) & 15 ) << 16 )
#define FT_LOAD_TARGET_NORMAL FT_LOAD_TARGET_( FT_RENDER_MODE_NORMAL )
#define FT_LOAD_TARGET_LIGHT FT_LOAD_TARGET_( FT_RENDER_MODE_LIGHT )
#define FT_LOAD_TARGET_MONO FT_LOAD_TARGET_( FT_RENDER_MODE_MONO )
#define FT_LOAD_TARGET_LCD FT_LOAD_TARGET_( FT_RENDER_MODE_LCD )
#define FT_LOAD_TARGET_LCD_V FT_LOAD_TARGET_( FT_RENDER_MODE_LCD_V )
A list of values to select a specific hinting algorithm for the hinter. You should OR one of these values to your load_flags when calling FT_Load_Glyph.
Note that a font's native hinters may ignore the hinting algorithm you have specified (e.g., the TrueType bytecode interpreter). You can set FT_LOAD_FORCE_AUTOHINT to ensure that the auto-hinter is used.
values
| FT_LOAD_TARGET_NORMAL |
The default hinting algorithm, optimized for standard gray-level rendering. For monochrome output, use |
| FT_LOAD_TARGET_LIGHT |
A lighter hinting algorithm for gray-level modes. Many generated glyphs are fuzzier but better resemble their original shape. This is achieved by snapping glyphs to the pixel grid only vertically (Y-axis), as is done by FreeType's new CFF engine or Microsoft's ClearType font renderer. This preserves inter-glyph spacing in horizontal text. The snapping is done either by the native font driver, if the driver itself and the font support it, or by the auto-hinter. Advance widths are rounded to integer values; however, using the If configuration option |
| FT_LOAD_TARGET_MONO |
Strong hinting algorithm that should only be used for monochrome output. The result is probably unpleasant if the glyph is rendered in non-monochrome modes. Note that for outline fonts only the TrueType font driver has proper monochrome hinting support, provided the TTFs contain hints for B/W rendering (which most fonts no longer provide). If these conditions are not met it is very likely that you get ugly results at smaller sizes. |
| FT_LOAD_TARGET_LCD |
A variant of |
| FT_LOAD_TARGET_LCD_V |
A variant of |
note
You should use only one of the FT_LOAD_TARGET_XXX values in your load_flags. They can't be ORed.
If FT_LOAD_RENDER is also set, the glyph is rendered in the corresponding mode (i.e., the mode that matches the used algorithm best). An exception is FT_LOAD_TARGET_MONO since it implies FT_LOAD_MONOCHROME.
You can use a hinting algorithm that doesn't correspond to the same rendering mode. As an example, it is possible to use the ‘light’ hinting algorithm and have the results rendered in horizontal LCD pixel mode, with code like
FT_Load_Glyph( face, glyph_index,
load_flags | FT_LOAD_TARGET_LIGHT );
FT_Render_Glyph( face->glyph, FT_RENDER_MODE_LCD );
In general, you should stick with one rendering mode. For example, switching between FT_LOAD_TARGET_NORMAL and FT_LOAD_TARGET_MONO enforces a lot of recomputation for TrueType fonts, which is slow. Another reason is caching: Selecting a different mode usually causes changes in both the outlines and the rasterized bitmaps; it is thus necessary to empty the cache after a mode switch to avoid false hits.
FT_Render_Glyph¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
FT_EXPORT( FT_Error )
FT_Render_Glyph( FT_GlyphSlot slot,
FT_Render_Mode render_mode );
Convert a given glyph image to a bitmap. It does so by inspecting the glyph image format, finding the relevant renderer, and invoking it.
inout
| slot |
A handle to the glyph slot containing the image to convert. |
input
| render_mode |
The render mode used to render the glyph image into a bitmap. See If |
return
FreeType error code. 0 means success.
note
When FreeType outputs a bitmap of a glyph, it really outputs an alpha coverage map. If a pixel is completely covered by a filled-in outline, the bitmap contains 0xFF at that pixel, meaning that 0xFF/0xFF fraction of that pixel is covered, meaning the pixel is 100% black (or 0% bright). If a pixel is only 50% covered (value 0x80), the pixel is made 50% black (50% bright or a middle shade of grey). 0% covered means 0% black (100% bright or white).
On high-DPI screens like on smartphones and tablets, the pixels are so small that their chance of being completely covered and therefore completely black are fairly good. On the low-DPI screens, however, the situation is different. The pixels are too large for most of the details of a glyph and shades of gray are the norm rather than the exception.
This is relevant because all our screens have a second problem: they are not linear. 1 + 1 is not 2. Twice the value does not result in twice the brightness. When a pixel is only 50% covered, the coverage map says 50% black, and this translates to a pixel value of 128 when you use 8 bits per channel (0-255). However, this does not translate to 50% brightness for that pixel on our sRGB and gamma 2.2 screens. Due to their non-linearity, they dwell longer in the darks and only a pixel value of about 186 results in 50% brightness – 128 ends up too dark on both bright and dark backgrounds. The net result is that dark text looks burnt-out, pixely and blotchy on bright background, bright text too frail on dark backgrounds, and colored text on colored background (for example, red on green) seems to have dark halos or ‘dirt’ around it. The situation is especially ugly for diagonal stems like in ‘w’ glyph shapes where the quality of FreeType's anti-aliasing depends on the correct display of grays. On high-DPI screens where smaller, fully black pixels reign supreme, this doesn't matter, but on our low-DPI screens with all the gray shades, it does. 0% and 100% brightness are the same things in linear and non-linear space, just all the shades in-between aren't.
The blending function for placing text over a background is
dst = alpha * src + (1 - alpha) * dst ,
which is known as the OVER operator.
To correctly composite an anti-aliased pixel of a glyph onto a surface,
-
take the foreground and background colors (e.g., in sRGB space) and apply gamma to get them in a linear space,
-
use OVER to blend the two linear colors using the glyph pixel as the alpha value (remember, the glyph bitmap is an alpha coverage bitmap), and
-
apply inverse gamma to the blended pixel and write it back to the image.
Internal testing at Adobe found that a target inverse gamma of 1.8 for step 3 gives good results across a wide range of displays with an sRGB gamma curve or a similar one.
This process can cost performance. There is an approximation that does not need to know about the background color; see https://bel.fi/alankila/lcd/ and https://bel.fi/alankila/lcd/alpcor.html for details.
ATTENTION: Linear blending is even more important when dealing with subpixel-rendered glyphs to prevent color-fringing! A subpixel-rendered glyph must first be filtered with a filter that gives equal weight to the three color primaries and does not exceed a sum of 0x100, see section ‘Subpixel Rendering’. Then the only difference to gray linear blending is that subpixel-rendered linear blending is done 3 times per pixel: red foreground subpixel to red background subpixel and so on for green and blue.
FT_Render_Mode¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
typedef enum FT_Render_Mode_
{
FT_RENDER_MODE_NORMAL = 0,
FT_RENDER_MODE_LIGHT,
FT_RENDER_MODE_MONO,
FT_RENDER_MODE_LCD,
FT_RENDER_MODE_LCD_V,
FT_RENDER_MODE_SDF,
FT_RENDER_MODE_MAX
} FT_Render_Mode;
/* these constants are deprecated; use the corresponding */
/* `FT_Render_Mode` values instead */
#define ft_render_mode_normal FT_RENDER_MODE_NORMAL
#define ft_render_mode_mono FT_RENDER_MODE_MONO
Render modes supported by FreeType 2. Each mode corresponds to a specific type of scanline conversion performed on the outline.
For bitmap fonts and embedded bitmaps the bitmap->pixel_mode field in the FT_GlyphSlotRec structure gives the format of the returned bitmap.
All modes except FT_RENDER_MODE_MONO use 256 levels of opacity, indicating pixel coverage. Use linear alpha blending and gamma correction to correctly render non-monochrome glyph bitmaps onto a surface; see FT_Render_Glyph.
The FT_RENDER_MODE_SDF is a special render mode that uses up to 256 distance values, indicating the signed distance from the grid position to the nearest outline.
values
| FT_RENDER_MODE_NORMAL |
Default render mode; it corresponds to 8-bit anti-aliased bitmaps. |
| FT_RENDER_MODE_LIGHT |
This is equivalent to |
| FT_RENDER_MODE_MONO |
This mode corresponds to 1-bit bitmaps (with 2 levels of opacity). |
| FT_RENDER_MODE_LCD |
This mode corresponds to horizontal RGB and BGR subpixel displays like LCD screens. It produces 8-bit bitmaps that are 3 times the width of the original glyph outline in pixels, and which use the |
| FT_RENDER_MODE_LCD_V |
This mode corresponds to vertical RGB and BGR subpixel displays (like PDA screens, rotated LCD displays, etc.). It produces 8-bit bitmaps that are 3 times the height of the original glyph outline in pixels and use the |
| FT_RENDER_MODE_SDF |
The positive (unsigned) 8-bit bitmap values can be converted to the single-channel signed distance field (SDF) by subtracting 128, with the positive and negative results corresponding to the inside and the outside of a glyph contour, respectively. The distance units are arbitrarily determined by an adjustable |
note
The selected render mode only affects scalable vector glyphs of a font. Embedded bitmaps often have a different pixel mode like FT_PIXEL_MODE_MONO. You can use FT_Bitmap_Convert to transform them into 8-bit pixmaps.
FT_Get_Kerning¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
FT_EXPORT( FT_Error )
FT_Get_Kerning( FT_Face face,
FT_UInt left_glyph,
FT_UInt right_glyph,
FT_UInt kern_mode,
FT_Vector *akerning );
Return the kerning vector between two glyphs of the same face.
input
| face |
A handle to a source face object. |
| left_glyph |
The index of the left glyph in the kern pair. |
| right_glyph |
The index of the right glyph in the kern pair. |
| kern_mode |
See |
output
| akerning |
The kerning vector. This is either in font units, fractional pixels (26.6 format), or pixels for scalable formats, and in pixels for fixed-sizes formats. |
return
FreeType error code. 0 means success.
note
Only horizontal layouts (left-to-right & right-to-left) are supported by this method. Other layouts, or more sophisticated kernings, are out of the scope of this API function – they can be implemented through format-specific interfaces.
Note that, for TrueType fonts only, this can extract data from both the ‘kern’ table and the basic, pair-wise kerning feature from the GPOS table (with TT_CONFIG_OPTION_GPOS_KERNING enabled), though FreeType does not support the more advanced GPOS layout features; use a library like HarfBuzz for those instead. If a font has both a ‘kern’ table and kern features of a GPOS table, the ‘kern’ table will be used.
Also note for right-to-left scripts, the functionality may differ for fonts with GPOS tables vs. ‘kern’ tables. For GPOS, right-to-left fonts typically use both a placement offset and an advance for pair positioning, which this API does not support, so it would output kerning values of zero; though if the right-to-left font used only advances in GPOS pair positioning, then this API could output kerning values for it, but it would use left_glyph to mean the first glyph for that case. Whereas ‘kern’ tables are always advance-only and always store the left glyph first.
Use FT_HAS_KERNING to find out whether a font has data that can be extracted with FT_Get_Kerning.
FT_Kerning_Mode¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
typedef enum FT_Kerning_Mode_
{
FT_KERNING_DEFAULT = 0,
FT_KERNING_UNFITTED,
FT_KERNING_UNSCALED
} FT_Kerning_Mode;
/* these constants are deprecated; use the corresponding */
/* `FT_Kerning_Mode` values instead */
#define ft_kerning_default FT_KERNING_DEFAULT
#define ft_kerning_unfitted FT_KERNING_UNFITTED
#define ft_kerning_unscaled FT_KERNING_UNSCALED
An enumeration to specify the format of kerning values returned by FT_Get_Kerning.
values
| FT_KERNING_DEFAULT |
Return grid-fitted kerning distances in 26.6 fractional pixels. |
| FT_KERNING_UNFITTED |
Return un-grid-fitted kerning distances in 26.6 fractional pixels. |
| FT_KERNING_UNSCALED |
Return the kerning vector in original font units. |
note
FT_KERNING_DEFAULT returns full pixel values; it also makes FreeType heuristically scale down kerning distances at small ppem values so that they don't become too big.
Both FT_KERNING_DEFAULT and FT_KERNING_UNFITTED use the current horizontal scaling factor (as set e.g. with FT_Set_Char_Size) to convert font units to pixels.
FT_Get_Track_Kerning¶
Defined in FT_FREETYPE_H (freetype/freetype.h).
FT_EXPORT( FT_Error )
FT_Get_Track_Kerning( FT_Face face,
FT_Fixed point_size,
FT_Int degree,
FT_Fixed* akerning );
Return the track kerning for a given face object at a given size.
input
| face |
A handle to a source face object. |
| point_size |
The point size in 16.16 fractional points. |
| degree |
The degree of tightness. Increasingly negative values represent tighter track kerning, while increasingly positive values represent looser track kerning. Value zero means no track kerning. |
output
| akerning |
The kerning in 16.16 fractional points, to be uniformly applied between all glyphs. |
return
FreeType error code. 0 means success.
note
Currently, only the Type 1 font driver supports track kerning, using data from AFM files (if attached with FT_Attach_File or FT_Attach_Stream).
Only very few AFM files come with track kerning data; please refer to Adobe's AFM specification for more details.