Glyph metrics are, as the name suggests, certain distances associated with each glyph that describe how to position this glyph while creating a text layout.
There are usually two sets of metrics for a single glyph: Those used to represent glyphs in horizontal text layouts (Latin, Cyrillic, Arabic, Hebrew, etc.), and those used to represent glyphs in vertical text layouts (Chinese, Japanese, Korean, Mongolian, etc.).
Note that only a few font formats provide vertical
metrics. You can test whether a given face object
contains them by using the
macro FT_HAS_VERTICAL
,
which returns true if appropriate.
Individual glyph metrics can be accessed by first loading
the glyph in a face's glyph slot, then accessing them
through the face->glyph->metrics
structure, whose type
is FT_Glyph_Metrics
.
We will discuss this in more detail below; for now, we
only note that it contains the following fields.
FT_Size_Metrics
structure.As not all fonts do contain vertical
metrics, the values of vertBearingX
,
vertBearingY
and vertAdvance
should not be considered reliable
if FT_HAS_VERTICAL
returns false.
The following graphics illustrate the metrics more clearly. In case a distance is directed, it is marked with a single arrow, indicating a positive value. The first image displays horizontal metrics, where the baseline is the horizontal axis.
For vertical text layouts, the baseline is vertical,
identical to the vertical axis. Contrary to all other
arrows, bearingX
shows a negative value in
this image.
The metrics found
in face->glyph->metrics
are normally
expressed in 26.6 pixel format (i.e., 1/64th of pixels),
unless you use the FT_LOAD_NO_SCALE
flag when
calling FT_Load_Glyph
or FT_Load_Char
. In this case, the metrics
are expressed in original font units.
The glyph slot object has also a few other interesting
fields that eases a developer's work. You can access them
through face->glyph->xxx
,
where xxx
is one of the following fields.
FT_Vector
that holds the
transformed advance for the glyph. That is useful when
you are using a transformation
through FT_Set_Transform
, as shown in the
rotated text
example of part I. Other than that, its value
is by default (metrics.horiAdvance,0), unless you
specify FT_LOAD_VERTICAL
when loading the
glyph image; it is then (0,metrics.vertAdvance).metrics.horiAdvance
that is returned in the
glyph slot is normally rounded to integer pixel
coordinates (i.e., being a multiple of 64) by the
font driver that actually loads the glyph
image. linearHoriAdvance
is a 16.16
fixed-point number that gives the value of the original
glyph advance width in 1/65536th of pixels. It can be
use to perform pseudo device-independent text
layouts.linearHoriAdvance
but for the glyph's vertical advance height. Its value
is only reliable if the font face contains vertical
metrics.The glyph image that is loaded in a glyph slot can be
converted into a bitmap, either by
using FT_LOAD_RENDER
when loading it, or by
calling FT_Render_Glyph
.
Each time you load a new glyph image, the previous one is
erased from the glyph slot.
There are situations, however, where you may need to extract this image from the glyph slot in order to cache it within your application, and even perform additional transformations and measures on it before converting it to a bitmap.
The FreeType 2 API has a specific extension that is
capable of dealing with glyph images in a flexible and
generic way. To use it, you first need to include
the FT_GLYPH_H
header file.
#include FT_GLYPH_H
You can extract a single glyph image very easily. Here some code that shows how to do it.
FT_Glyph glyph; /* a handle to the glyph image */
...
error = FT_Load_Glyph( face, glyph_index, FT_LOAD_NORMAL );
if ( error ) { ... }
error = FT_Get_Glyph( face->glyph, &glyph );
if ( error ) { ... }
The following steps are performed.
glyph
, of
type FT_Glyph
.
This is a handle (pointer) to an individual glyph
image.FT_LOAD_RENDER
because we want to grab a scalable glyph image that we
can transform later on.FT_Glyph
object by
calling FT_Get_Glyph
.
This function returns an error code and
sets glyph
.It is important to note that the extracted glyph is in
the same format as the original one that is still in the
slot. For example, if we are loading a glyph from a
TrueType font file, the glyph image is really a scalable
vector outline. You can access the
field glyph->format
if you want to know
exactly how the glyph is modeled and stored.
A new glyph object can be destroyed with a call
to FT_Done_Glyph
.
The glyph object contains exactly one glyph image and a
2D vector representing the glyph's advance in 16.16
fixed-point coordinates. The latter can be accessed
directly as glyph->advance
Note that unlike other FreeType objects,
the library doesn't keep a list of all allocated glyph
objects. This means you have to destroy them yourself
instead of relying on FT_Done_FreeType
doing
all the clean-up.
If the glyph image is scalable (i.e.,
if glyph->format
is not equal
to FT_GLYPH_FORMAT_BITMAP
), it is possible to
transform the image anytime by a call
to FT_Glyph_Transform
.
You can also copy a single glyph image
with FT_Glyph_Copy
.
FT_Glyph glyph, glyph2; FT_Matrix matrix; FT_Vector delta; ... load glyph image in `glyph' ... /* copy glyph to glyph2 */ error = FT_Glyph_Copy( glyph, &glyph2 ); if ( error ) { ... could not copy (out of memory) ... } /* translate `glyph' */ delta.x = -100 * 64; /* coordinates are in 26.6 pixel format */ delta.y = 50 * 64; FT_Glyph_Transform( glyph, 0, &delta ); /* transform glyph2 (horizontal shear) */ matrix.xx = 0x10000L; matrix.xy = 0.12 * 0x10000L; matrix.yx = 0; matrix.yy = 0x10000L; FT_Glyph_Transform( glyph2, &matrix, 0 );
Note that the 2×2 transformation matrix is always applied to the 16.16 advance vector in the glyph; you thus don't need to recompute it.
You can also retrieve the control (bounding) box of any
glyph image (scalable or not) through
the FT_Glyph_Get_CBox
function.
FT_BBox bbox; ... FT_Glyph_Get_CBox( glyph, bbox_mode, &bbox );
Coordinates are relative to the glyph origin (0,0), using the y upwards convention. This function takes a special argument, the bbox mode, to indicate how box coordinates are expressed.
If the glyph has been loaded
with FT_LOAD_NO_SCALE
, bbox_mode
must be set to FT_GLYPH_BBOX_UNSCALED
to get
unscaled font units in 26.6 pixel format. The
value FT_GLYPH_BBOX_SUBPIXELS
is another name
for this constant.
Note that the box's maximum coordinates are exclusive, which means that you can always compute the width and height of the glyph image (regardless of using integer or 26.6 coordinates) with a simple subtraction.
width = bbox.xMax - bbox.xMin; height = bbox.yMax - bbox.yMin;
Note also that for 26.6 coordinates, if
FT_GLYPH_BBOX_GRIDFIT
is used as the bbox
mode, the coordinates are also grid-fitted, which
corresponds to the following four lines of
pseudo-code.
bbox.xMin = FLOOR( bbox.xMin ) bbox.yMin = FLOOR( bbox.yMin ) bbox.xMax = CEILING( bbox.xMax ) bbox.yMax = CEILING( bbox.yMax )
To get the bbox in integer pixel coordinates,
set bbox_mode
to FT_GLYPH_BBOX_TRUNCATE
.
Finally, to get the bounding box in grid-fitted pixel
coordinates, set bbox_mode
to FT_GLYPH_BBOX_PIXELS
.
[Computing exact bounding boxes can be done with
function FT_Outline_Get_BBox
,
at the cost of slower execution. You probably don't need
it with the possible exception of rotated glyphs.]
You may need to convert the glyph object to a bitmap once
you have conveniently cached or transformed it. This can
be done easily with
the FT_Glyph_To_Bitmap
function, which handles any glyph object.
FT_Vector origin; origin.x = 32; /* 1/2 pixel in 26.6 format */ origin.y = 0; error = FT_Glyph_To_Bitmap( &glyph, render_mode, &origin, 1 ); /* destroy original image == true */
Some notes.
FT_RENDER_MODE_DEFAULT
for an 8-bit
anti-aliased pixmap, or FT_RENDER_MODE_MONO
for a 1-bit monochrome bitmap.The new glyph object always contains a bitmap (if no
error is returned), and you must typecast its
handle to the FT_BitmapGlyph
type in order to
access its content. This type is a sort of
‘subclass’ of FT_Glyph
that
contains additional fields
(see FT_BitmapGlyphRec
).
bitmap_left
field of a
glyph slot, this is the horizontal distance from the
glyph origin (0,0) to the leftmost pixel of the glyph
bitmap. It is expressed in integer pixels.bitmap_top
field of a glyph
slot, this is the vertical distance from the glyph
origin (0,0) to the topmost pixel of the glyph bitmap
(more precise, to the pixel just above the bitmap).
This distance is expressed in integer pixels, and is
positive for upwards y.bitmap
field in a glyph slot.Unlike glyph metrics, global metrics are used to describe distances and features of a whole font face. They can be expressed either in 26.6 pixel format or in (unscaled) font units for scalable formats.
For scalable formats, all global metrics are expressed in
font units in order to be later scaled to the device
space, according to the rules described in the last
section of this tutorial part. You can access them
directly as fields of an FT_Face
handle.
However, you need to check that the font face's format is
scalable before using them. One can do it with
macro FT_IS_SCALABLE
, which returns true when
appropriate.
Here a table of the global design metrics for scalable faces.
bbox.yMax
.bbox.yMin
. This field is
negative for values below the baseline.max_advance_width
but for
vertical text layout.Notice that the values of the ascender and the descender are not reliable (due to various discrepancies in font formats), unfortunately.
Each size object also contains a scaled version of some
of the global metrics described above, to be directly
accessed through
the face->size->metrics
structure (of
type
FT_Size_Metrics
). No grid-fitting
is performed for those values. They are also
completely independent of any hinting process. In other
words, don't rely on them to get exact metrics at the
pixel level. They are expressed in 26.6 pixel format but
rounded for historical reasons.
The scaled version of the original design text height (the vertical distance from one baseline to the next). This is probably the only field you should really use in this structure. It is rounded to an integer value.
Be careful not to confuse it with the
‘height’ field in
the FT_Glyph_Metrics
structure.
Note that the face->size->metrics
structure contains other fields that are used to scale
design coordinates to device space. They are described in
the last section.
Kerning is the process of adjusting the position of two subsequent glyph images in a string of text in order to improve the general appearance of text. For example, if a glyph for an uppercase ‘A’ is followed by a glyph for an uppercase ‘V’, the space between the two glyphs can be slightly reduced to avoid extra ‘diagonal whitespace’.
Note that in theory kerning can happen both in the horizontal and vertical direction between two glyphs; however, it only happens in a single direction in nearly all cases.
Not all font formats contain kerning information, and not all kerning formats are supported by FreeType; in particular, for TrueType fonts, the API can only access kerning via the ‘kern’ table. OpenType kerning via the ‘GPOS’ table is not supported! You need a higher-level library like HarfBuzz, Pango, or ICU, since GPOS kerning requires contextual string handling.
Sometimes, the font file is associated with an additional
file that contains various glyph metrics, including
kerning, but no glyph images. A good example is the
Type 1 format where glyph images are stored in files
with extension .pfa
or .pfb
,
while kerning metrics can be found in files with extension
.afm
or .pfm
.
FreeType 2 allows you to deal with this, by
providing
the FT_Attach_File
and FT_Attach_Stream
APIs. Both functions are used to load additional metrics
into a face object by reading them from an additional
format-specific file. Here an example, opening a
Type 1 font.
error = FT_New_Face( library, "/usr/share/fonts/cour.pfb", 0, &face ); if ( error ) { ... } error = FT_Attach_File( face, "/usr/share/fonts/cour.afm" ); if ( error ) { ... could not read kerning and additional metrics ... }
Note that FT_Attach_Stream
is similar to
FT_Attach_File
except that it doesn't take a
C string to name the extra file but
an FT_Stream
handle. Also, reading a metrics file is in no way
mandatory.
Finally, the file attachment APIs are very generic and can be used to load any kind of extra information for a given face. The nature of the additional content is entirely font format specific.
FreeType 2 allows you to retrieve the kerning
information between two glyphs through
the FT_Get_Kerning
function.
FT_Vector kerning; ... error = FT_Get_Kerning( face, /* handle to face object */ left, /* left glyph index */ right, /* right glyph index */ kerning_mode, /* kerning mode */ &kerning ); /* target vector */
This function takes a handle to a face object, the indices of the left and right glyph for which the kerning value is desired, an integer, called the kerning mode, and a pointer to a destination vector that receives the corresponding distances.
The kerning mode is very similar to the bbox mode described in a previous section. It is a enumeration that indicates how the kerning distances are expressed in the target vector.
The default value is FT_KERNING_DEFAULT
,
which has value 0. It corresponds to kerning
distances expressed in 26.6 grid-fitted pixels (which
means that the values are multiples of 64). For scalable
formats, this means that the design kerning distance is
scaled, then rounded.
The value FT_KERNING_UNFITTED
corresponds to
kerning distances expressed in 26.6 unfitted pixels (i.e.,
that do not correspond to integer coordinates). It is the
design kerning distance that is scaled without
rounding.
Finally, the value FT_KERNING_UNSCALED
indicates to return the design kerning distance, expressed
in font units. You can later scale it to the device space
using the computations explained in the last section of
this part.
Note that the ‘left’ and ‘right’ positions correspond to the visual order of the glyphs in the string of text. This is important for bidirectional or right-to-left text.
In order to show off what we have just learned, we now demonstrate how to modify the example code that was provided in part I to render a string of text, and enhance it to support kerning and delayed rendering.
Adding support for kerning to our code is trivial, as long as we consider that we are still dealing with a left-to-right script like Latin. We simply need to retrieve the kerning distance between two glyphs in order to alter the pen position appropriately.
FT_GlyphSlot slot = face->glyph; /* a small shortcut */ FT_UInt glyph_index; FT_Bool use_kerning; FT_UInt previous; int pen_x, pen_y, n; ... initialize library ... ... create face object ... ... set character size ... pen_x = 300; pen_y = 200; use_kerning = FT_HAS_KERNING( face ); previous = 0; for ( n = 0; n < num_chars; n++ ) { /* convert character code to glyph index */ glyph_index = FT_Get_Char_Index( face, text[n] ); /* retrieve kerning distance and move pen position */ if ( use_kerning && previous && glyph_index ) { FT_Vector delta; FT_Get_Kerning( face, previous, glyph_index, FT_KERNING_DEFAULT, &delta ); pen_x += delta.x >> 6; } /* load glyph image into the slot (erase previous one) */ error = FT_Load_Glyph( face, glyph_index, FT_LOAD_RENDER ); if ( error ) continue; /* ignore errors */ /* now draw to our target surface */ my_draw_bitmap( &slot->bitmap, pen_x + slot->bitmap_left, pen_y - slot->bitmap_top ); /* increment pen position */ pen_x += slot->advance.x >> 6; /* record current glyph index */ previous = glyph_index; }
We are done. Some notes.
FT_Load_Glyph
instead of FT_Load_Char
.use_kerning
, which
is set to the result of the
macro FT_HAS_KERNING
. It is certainly
faster not to call FT_Get_Kerning
when we
know that the font face does not contain kerning
information.previous
with
the value 0, which always corresponds to the
‘missing glyph’ (also
called .notdef
in the PostScript world).
There is never any kerning distance associated with this
glyph.FT_Get_Kerning
. This is because the
function always sets the content of delta
to (0,0) if an error occurs.Our code begins to become interesting but it is still a bit too simple for normal use. For example, the position of the pen is determined before we do the rendering; normally, you would rather determine the layout of the text and measure it before computing its final position (centering, etc.), or perform things like word-wrapping.
Let us now decompose our text rendering function into two distinct but successive parts: The first one positions individual glyph images on the baseline, while the second one renders the glyphs. As we will see, this has many advantages.
We thus start by storing individual glyph images, as well as their position on the baseline.
FT_GlyphSlot slot = face->glyph; /* a small shortcut */ FT_UInt glyph_index; FT_Bool use_kerning; FT_UInt previous; int pen_x, pen_y, n; FT_Glyph glyphs[MAX_GLYPHS]; /* glyph image */ FT_Vector pos [MAX_GLYPHS]; /* glyph position */ FT_UInt num_glyphs; ... initialize library ... ... create face object ... ... set character size ... pen_x = 0; /* start at (0,0) */ pen_y = 0; num_glyphs = 0; use_kerning = FT_HAS_KERNING( face ); previous = 0; for ( n = 0; n < num_chars; n++ ) { /* convert character code to glyph index */ glyph_index = FT_Get_Char_Index( face, text[n] ); /* retrieve kerning distance and move pen position */ if ( use_kerning && previous && glyph_index ) { FT_Vector delta; FT_Get_Kerning( face, previous, glyph_index, FT_KERNING_DEFAULT, &delta ); pen_x += delta.x >> 6; } /* store current pen position */ pos[num_glyphs].x = pen_x; pos[num_glyphs].y = pen_y; /* load glyph image into the slot without rendering */ error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT ); if ( error ) continue; /* ignore errors, jump to next glyph */ /* extract glyph image and store it in our table */ error = FT_Get_Glyph( face->glyph, &glyphs[num_glyphs] ); if ( error ) continue; /* ignore errors, jump to next glyph */ /* increment pen position */ pen_x += slot->advance.x >> 6; /* record current glyph index */ previous = glyph_index; /* increment number of glyphs */ num_glyphs++; }
This is a very slight variation of our previous code; we extract each glyph image from the slot, then store it, along with the corresponding position, in our tables.
Note also that pen_x
contains the total
advance for the string of text. We can now compute the
bounding box of the text string with a simple
function.
void compute_string_bbox( FT_BBox *abbox ) { FT_BBox bbox; FT_BBox glyph_bbox; /* initialize string bbox to "empty" values */ bbox.xMin = bbox.yMin = 32000; bbox.xMax = bbox.yMax = -32000; /* for each glyph image, compute its bounding box, */ /* translate it, and grow the string bbox */ for ( n = 0; n < num_glyphs; n++ ) { FT_Glyph_Get_CBox( glyphs[n], ft_glyph_bbox_pixels, &glyph_bbox ); glyph_bbox.xMin += pos[n].x; glyph_bbox.xMax += pos[n].x; glyph_bbox.yMin += pos[n].y; glyph_bbox.yMax += pos[n].y; if ( glyph_bbox.xMin < bbox.xMin ) bbox.xMin = glyph_bbox.xMin; if ( glyph_bbox.yMin < bbox.yMin ) bbox.yMin = glyph_bbox.yMin; if ( glyph_bbox.xMax > bbox.xMax ) bbox.xMax = glyph_bbox.xMax; if ( glyph_bbox.yMax > bbox.yMax ) bbox.yMax = glyph_bbox.yMax; } /* check that we really grew the string bbox */ if ( bbox.xMin > bbox.xMax ) { bbox.xMin = 0; bbox.yMin = 0; bbox.xMax = 0; bbox.yMax = 0; } /* return string bbox */ *abbox = bbox; }
The resulting bounding box dimensions are expressed in integer pixels and can then be used to compute the final pen position before rendering the string.
In general, the above function
does not compute an exact bounding box of a
string! As soon as hinting is involved, glyph
dimensions must be derived from the resulting
outlines. For anti-aliased pixmaps,
FT_Outline_Get_BBox
then yields proper
results. In case you need 1-bit monochrome bitmaps, it is
even necessary to actually render the glyphs because the
rules for the conversion from outline to bitmap can also
be controlled by hinting instructions
(cf. dropout
control).
/* compute string dimensions in integer pixels */ string_width = string_bbox.xMax - string_bbox.xMin; string_height = string_bbox.yMax - string_bbox.yMin; /* compute start pen position in 26.6 Cartesian pixels */ start_x = ( ( my_target_width - string_width ) / 2 ) * 64; start_y = ( ( my_target_height - string_height ) / 2 ) * 64; for ( n = 0; n < num_glyphs; n++ ) { FT_Glyph image; FT_Vector pen; image = glyphs[n]; pen.x = start_x + pos[n].x; pen.y = start_y + pos[n].y; error = FT_Glyph_To_Bitmap( &image, FT_RENDER_MODE_NORMAL, &pen, 0 ); if ( !error ) { FT_BitmapGlyph bit = (FT_BitmapGlyph)image; my_draw_bitmap( bit->bitmap, bit->left, my_target_height - bit->top ); FT_Done_Glyph( image ); } }
Some remarks.
FT_Glyph_To_Bitmap
with
the destroy
parameter set to 0
(false), in order to avoid destroying the original glyph
image. The new glyph bitmap is accessed through
image
after the call and is typecast to
FT_BitmapGlyph
.FT_Glyph_To_Bitmap
. This ensures
that the left
and top
fields
of the bitmap glyph object are already set to the
correct pixel coordinates in the Cartesian space.my_target_height - bitmap->top
in the
call to my_draw_bitmap
.The same loop can be used to render the string anywhere on our display surface, without the need to reload our glyph images each time.
We are now going to modify our code in order to be able to easily transform the rendered string, for example, to rotate it. First, some minor improvements.
We start by packing the information related to a single glyph image into a single structure instead of parallel arrays.
typedef struct TGlyph_ { FT_UInt index; /* glyph index */ FT_Vector pos; /* glyph origin on the baseline */ FT_Glyph image; /* glyph image */ } TGlyph, *PGlyph;
We also translate each glyph image directly after it is loaded to its position on the baseline at load time. As we will see, this has several advantages. Here is our new glyph sequence loader.
FT_GlyphSlot slot = face->glyph; /* a small shortcut */ FT_UInt glyph_index; FT_Bool use_kerning; FT_UInt previous; int pen_x, pen_y, n; TGlyph glyphs[MAX_GLYPHS]; /* glyphs table */ PGlyph glyph; /* current glyph in table */ FT_UInt num_glyphs; ... initialize library ... ... create face object ... ... set character size ... pen_x = 0; /* start at (0,0) */ pen_y = 0; num_glyphs = 0; use_kerning = FT_HAS_KERNING( face ); previous = 0; glyph = glyphs; for ( n = 0; n < num_chars; n++ ) { glyph->index = FT_Get_Char_Index( face, text[n] ); if ( use_kerning && previous && glyph->index ) { FT_Vector delta; FT_Get_Kerning( face, previous, glyph->index, FT_KERNING_MODE_DEFAULT, &delta ); pen_x += delta.x >> 6; } /* store current pen position */ glyph->pos.x = pen_x; glyph->pos.y = pen_y; error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT ); if ( error ) continue; error = FT_Get_Glyph( face->glyph, &glyph->image ); if ( error ) continue; /* translate the glyph image now */ FT_Glyph_Transform( glyph->image, 0, &glyph->pos ); pen_x += slot->advance.x >> 6; previous = glyph->index; /* increment number of glyphs */ glyph++; } /* count number of glyphs loaded */ num_glyphs = glyph - glyphs;
Note that translating glyphs now has several advantages. The first one is that we don't need to translate the glyph bbox when we compute the string's bounding box.
void compute_string_bbox( FT_BBox *abbox ) { FT_BBox bbox; bbox.xMin = bbox.yMin = 32000; bbox.xMax = bbox.yMax = -32000; for ( n = 0; n < num_glyphs; n++ ) { FT_BBox glyph_bbox; FT_Glyph_Get_CBox( glyphs[n], ft_glyph_bbox_pixels, &glyph_bbox ); if (glyph_bbox.xMin < bbox.xMin) bbox.xMin = glyph_bbox.xMin; if (glyph_bbox.yMin < bbox.yMin) bbox.yMin = glyph_bbox.yMin; if (glyph_bbox.xMax > bbox.xMax) bbox.xMax = glyph_bbox.xMax; if (glyph_bbox.yMax > bbox.yMax) bbox.yMax = glyph_bbox.yMax; } if ( bbox.xMin > bbox.xMax ) { bbox.xMin = 0; bbox.yMin = 0; bbox.xMax = 0; bbox.yMax = 0; } *abbox = bbox; }
With the above modifications,
the compute_string_bbox
function can now
compute the bounding box of a transformed glyph string,
which allows further code simplications.
FT_BBox bbox; FT_Matrix matrix; FT_Vector delta; ... load glyph sequence ... ... set up `matrix' and `delta' ... /* transform glyphs */ for ( n = 0; n < num_glyphs; n++ ) FT_Glyph_Transform( glyphs[n].image, &matrix, &delta ); /* compute bounding box of transformed glyphs */ compute_string_bbox( &bbox );
However, directly transforming the glyphs in our sequence is not a good idea if we want to reuse them in order to draw the text string with various angles or transformations. It is better to perform the affine transformation just before the glyph is rendered.
FT_Vector start; FT_Matrix matrix; FT_Glyph image; FT_Vector pen; FT_BBox bbox; /* get bbox of original glyph sequence */ compute_string_bbox( &string_bbox ); /* compute string dimensions in integer pixels */ string_width = (string_bbox.xMax - string_bbox.xMin) / 64; string_height = (string_bbox.yMax - string_bbox.yMin) / 64; /* set up start position in 26.6 Cartesian space */ start.x = ( ( my_target_width - string_width ) / 2 ) * 64; start.y = ( ( my_target_height - string_height ) / 2 ) * 64; /* set up transform (a rotation here) */ matrix.xx = (FT_Fixed)( cos( angle ) * 0x10000L ); matrix.xy = (FT_Fixed)(-sin( angle ) * 0x10000L ); matrix.yx = (FT_Fixed)( sin( angle ) * 0x10000L ); matrix.yy = (FT_Fixed)( cos( angle ) * 0x10000L ); pen = start; for ( n = 0; n < num_glyphs; n++ ) { /* create a copy of the original glyph */ error = FT_Glyph_Copy( glyphs[n].image, &image ); if ( error ) continue; /* transform copy (this will also translate it to the */ /* correct position */ FT_Glyph_Transform( image, &matrix, &pen ); /* check bounding box; if the transformed glyph image */ /* is not in our target surface, we can avoid rendering it */ FT_Glyph_Get_CBox( image, ft_glyph_bbox_pixels, &bbox ); if ( bbox.xMax <= 0 || bbox.xMin >= my_target_width || bbox.yMax <= 0 || bbox.yMin >= my_target_height ) continue; /* convert glyph image to bitmap (destroy the glyph copy!) */ error = FT_Glyph_To_Bitmap( &image, FT_RENDER_MODE_NORMAL, 0, /* no additional translation */ 1 ); /* destroy copy in "image" */ if ( !error ) { FT_BitmapGlyph bit = (FT_BitmapGlyph)image; my_draw_bitmap( bit->bitmap, bit->left, my_target_height - bit->top ); /* increment pen position -- */ /* we don't have access to a slot structure, */ /* so we have to use advances from glyph structure */ /* (which are in 16.16 fixed float format) */ pen.x += image.advance.x >> 10; pen.y += image.advance.y >> 10; FT_Done_Glyph( image ); } }
There are a few changes compared to the original version of this code.
FT_Glyph_To_Bitmap
in order to get rid of
the transformed scalable image. Note that the image is
not destroyed if the function returns an error code
(which is why FT_Done_Glyph
is only called
within the compound statement).FT_Glyph_Transform
instead
of FT_Glyph_To_Bitmap
.It is possible to call this function several times to
render the string with different angles, or even change
the way start
is computed in order to move it
to different place.
This code is the basis of the FreeType 2
demonstration program
named ftstring.c
.
It could be easily extended to perform advanced text
layout or word-wrapping in the first part, without
changing the second one.
Note, however, that a normal implementation would use a glyph cache in order to reduce memory needs. For example, let us assume that our text string is ‘FreeType’. We would store three identical glyph images in our table for the letter ‘e’, which isn't optimal (especially when you consider longer lines of text, or even whole pages).
A FreeType demo program that shows how glyph caching can
be implemented
is ftview.c
.
In general, ‘ftview’ is the main program used
by the FreeType developer team to check the validity of
loading, parsing, and rendering fonts.
Another very useful demo program
is ftdiff.c
,
demonstrating the use and the optical results of the
various rendering and hinting modes provided by FreeType.
In particular, it also demonstrates how to do sub-pixel
positioning (for unhinted glyphs and ‘light’
hinting mode) – all code in this tutorial assumes
integer coordinates.
Scalable font formats usually store a single vectorial
image, called an outline, for each glyph in a
face. Each outline is defined in an abstract grid called
the design space, with coordinates expressed in
font units. When a glyph image is loaded, the
font driver usually scales the outline to device space
according to the current character pixel size found in
an FT_Size
object. The driver may also modify the scaled outline in
order to significantly improve its appearance on a
pixel-based surface (a process known as hinting
or grid-fitting).
This section describes how design coordinates are scaled to the device space, and how to read glyph outlines and metrics in font units. This is important for a number of things.
Design coordinates are scaled to the device space using a simple scaling transformation whose coefficients are computed with the help of the character pixel size.
device_x = design_x * x_scale device_y = design_y * y_scale x_scale = pixel_size_x / EM_size y_scale = pixel_size_y / EM_size
Here, the value EM_size
is font-specific and
corresponds to the size of an abstract square of the
design space (called the EM), which is used by
font designers to create glyph images. It is thus
expressed in font units. It is also accessible directly
for scalable font formats
as face->units_per_EM
. You should check
that a font face contains scalable glyph images by using
the FT_IS_SCALABLE
macro, which returns true
if appropriate.
When you call the
function FT_Set_Pixel_Sizes
,
you are specifying integer values
of pixel_size_x
and
pixel_size_y
FreeType shall use. The library
will immediately compute the values
of x_scale
and
y_scale
.
When you call the
function FT_Set_Char_Size
,
you are specifying the character size in
physical points, which is used, along with the
device's resolutions, to compute the character pixel size
and the corresponding scaling factors. Here, the scaling
factors can correspond to fractional ppem values.
Note that after calling any of these two functions, you
can access the values of the character pixel size and
scaling factors as fields of
the face->size->metrics
structure.
pixel_size_x
in the above
example.pixel_size_y
in the above
example.You can scale a distance expressed in font units to 26.6
pixel format directly with the help of
the FT_MulFix
function.
/* convert design distances to 1/64th of pixels */
pixels_x = FT_MulFix( design_x, face->size->metrics.x_scale );
pixels_y = FT_MulFix( design_y, face->size->metrics.y_scale );
Alternatively, you can also scale the value directly by using doubles.
FT_Size_Metrics* metrics = &face->size->metrics; /* shortcut */ double pixels_x, pixels_y; double x_scale, y_scale; /* compute floating point scale factors */ x_scale = face->size->metrics.x_scale / 65536.0; y_scale = face->size->metrics.y_scale / 65536.0; /* convert design distances to floating point pixels */ pixels_x = design_x * x_scale; pixels_y = design_y * y_scale;
You can access glyph metrics in font units simply by
specifying the FT_LOAD_NO_SCALE
bit flag
in FT_Load_Glyph
or FT_Load_Char
. The metrics returned
in face->glyph->metrics
will all be in
font units.
You can access unscaled kerning data using the
FT_KERNING_MODE_UNSCALED
mode.
Finally, a few global metrics are available directly in
font units as fields of the FT_Face
handle,
as described in section 3 of
this part.
This is the end of the second part of the FreeType tutorial. You are now able to access glyph metrics, manage glyph images, and render text much more intelligently (kerning, measuring, transforming & caching); this is sufficient knowledge to build a pretty decent text service on top of FreeType.
The demo programs in the ‘ft2demos’ bundle (especially ‘ftview’) are a kind of reference implementation, and are a good resource to turn to for answers. They also show how to use additional features, such as the glyph stroker and cache.
Last update: 16-Jan-2021