FreeType 2 Tutorial
Step 2 — managing glyphs
© 2003 David Turner
(
mailto:david@freetype.org
david@freetype.org
)
© 2003 The FreeType Development Team
(
http://www.freetype.org
www.freetype.org
)
Introduction
This is the second section of the FreeType 2 tutorial. It describes
how to
retrieve glyph metrics
easily manage glyph images
retrieve global metrics (including kerning)
render a simple string of text, with kerning
render a centered string of text (with kerning)
render a transformed string of text (with centering)
access metrics in design font units when needed,
and how to scale them to device space
1. Glyph metrics
Glyph metrics are, as their name suggests, certain distances
associated with each glyph in order to describe how to use it to layout
text.
There are usually two sets of metrics for a single glyph: Those used
to layout the glyph in horizontal text layouts (Latin, Cyrillic, Arabic,
Hebrew, etc.), and those used to layout the glyph in vertical text
layouts (Chinese, Japanese, Korean, 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 is true when 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
../reference/ft2-base_interface.html#FT_Glyph_Metrics
FT_Glyph_Metrics
.  We will discuss this in more detail
below; for now, we only note that it contains the following fields:
width
This is the width of the glyph image's bounding box.  It is
independent of the layout direction.
height
This is the height of the glyph image's bounding box.  It is
independent of the layout direction.
horiBearingX
For
horizontal text layouts
, this is the horizontal
distance from the current cursor position to the leftmost border of
the glyph image's bounding box.
horiBearingY
For
horizontal text layouts
, this is the vertical
distance from the current cursor position (on the baseline) to the
topmost border of the glyph image's bounding box.
horiAdvance
For
horizontal text layouts
, this is the horizontal
distance used to increment the pen position when the glyph is drawn
as part of a string of text.
vertBearingX
For
vertical text layouts
, this is the horizontal
distance from the current cursor position to the leftmost border of
the glyph image's bounding box.
vertBearingY
For
vertical text layouts
, this is the vertical distance
from the current cursor position (on the baseline) to the topmost
border of the glyph image's bounding box.
vertAdvance
For
vertical text layouts
, this is the vertical distance
used to increment the pen position when the glyph is drawn as part
of a string of text.
NOTE: As not all fonts do contain vertical
metrics, the values of
vertBearingX
,
vertBearingY
and
vertAdvance
should not be considered reliable when
FT_HAS_VERTICAL
is false.
The following graphics illustrate the metrics more clearly.  First,
for horizontal metrics, where the baseline is the horizontal axis:
horizontal layout
For vertical text layouts, the baseline is vertical, identical to the
vertical axis:
vertical layout
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 will be expressed
in original font units.
The glyph slot object has also a few other interesting fields that
will ease a developer's work.  You can access them through
face->glyph->xxx
, where
xxx
is one of the
following fields:
advance
This field is a
FT_Vector
which holds the transformed
advance for the glyph.  That is useful when you are using a transform
through
FT_Set_Transform
, as shown in the rotated text
example of section I.  Other than that, its value is
by default (metrics.horiAdvance,0), unless you specify
FT_LOAD_VERTICAL
when loading the glyph image;
it will then be (0,metrics.vertAdvance)
linearHoriAdvance
This field contains the linearly scaled value of the glyph's
horizontal advance width.  Indeed, the value of
metrics.horiAdvance
that is returned in the glyph slot is
normally rounded to integer pixel coordinates (i.e., it will be a
multiple of 64) by the font driver used to load the glyph
image.
linearHoriAdvance
is a 16.16 fixed float 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.
linearVertAdvance
This is the similar to
linearHoriAdvance
but for the
glyph's vertical advance height.  Its value is only reliable if the
font face contains vertical metrics.
2. Managing glyph images
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 which is capable of
dealing with glyph images in a flexible and generic way.  To use it, you
first need to include the
../reference/ft2-header_file_macros.html#FT_GLYPH_H
FT_GLYPH_H
header file, as in:
#include FT_GLYPH_H
We will now explain how to use the functions defined in this
file:
a. Extracting the glyph image:
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 ) { ... }
As you see, we have:
Created a variable, named
glyph
, of type
../reference/ft2-glyph_management.html#FT_Glyph
FT_Glyph
.  This is a handle (pointer) to an
individual glyph image.
Loaded the glyph image normally in the face's glyph slot.  We
did not use
FT_LOAD_RENDER
because we want to grab a
scalable glyph image, in order to later transform it.
Copy the glyph image from the slot into a new
FT_Glyph
object, by calling
../reference/ft2-glyph_management.html#FT_Get_Glyph
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 than 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
will really be 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
../reference/ft2-glyph_management.html#FT_Done_Glyph
FT_Done_Glyph
.
The glyph object contains exactly one glyph image and a 2D vector
representing the glyph's advance in 16.16 fixed float 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.
b. Transforming & copying the glyph image
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
../reference/ft2-glyph_management.html#FT_Glyph_Transform
FT_Glyph_Transform
.
You can also copy a single glyph image with
../reference/ft2-glyph_management.html#FT_Glyph_Copy
FT_Glyph_Copy
.  Here is some example code:
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 transform matrix is always applied to the
16.16 advance vector in the glyph; you thus don't need to recompute
it.
c. Measuring the glyph image
You can also retrieve the control (bounding) box of any glyph image
(scalable or not) through the
../reference/ft2-glyph_management.html#FT_Glyph_Get_CBox
FT_Glyph_Get_CBox
function, as in:
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,
be in in integer or 26.6 pixels, with:
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 will also be grid-fitted, which corresponds to
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
.
d. Converting the glyph image to a bitmap
You may need to convert the glyph object to a bitmap once you have
convienently cached or transformed it.  This can be done easily with
the
../reference/ft2-glyph_management.html
FT_Glyph_To_Bitmap
function. It is in charge of
converting any glyph object into a bitmap, as in:
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:
The first parameter is the address of the source glyph's
handle.  When the function is called, it reads its to access the
source glyph object.  After the call, the handle will point to a
new
glyph object that contains the rendered bitmap.
The second parameter is a standard render mode, that is used to
specify what kind of bitmap we want.  It can be
FT_RENDER_MODE_DEFAULT
for an 8-bit anti-aliased pixmap,
or
FT_RENDER_MODE_MONO
for a 1-bit monochrome bitmap.
The third parameter is a pointer to a two-dimensional vector
that is used to translate the source glyph image before the
conversion.  Note that the source image will be translated back to
its original position (and will thus be left unchanged) after the
call.  If you do not need to translate the source glyph before
rendering, set this pointer to 0.
The last parameter is a boolean that indicates whether the
source glyph object should be destroyed by the function.  If
false, the original glyph object is never destroyed, even if its
handle is lost (it is up to client applications to keep it).
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
../reference/ft2-glyph_management.html#FT_BitmapGlyphRec
FT_BitmapGlyphRec
):
left
Just like the
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.
top
Just like the
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
This is a bitmap descriptor for the glyph object, just like the
bitmap
field in a glyph slot.
3. Global glyph metrics
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 design ‘font units’ for scalable
formats.
a. Design global metrics
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 chapter of this section of the
tutorial.  You can access them directly as simple fields of a
FT_Face
handle.
However, you need to check that the font face's format is scalable
before using them.  One can do it by using the macro
FT_IS_SCALABLE
which returns true when appropriate.
In this case, you can access the global design metrics as:
units_per_EM
This is the size of the EM square for the font face.  It is
used by scalable formats to scale design coordinates to device
pixels, as described in the last chapter of this section.  Its
value usually is 2048 (for TrueType) or 1000 (for Type 1),
but others are possible too.  It is set to 1 for fixed-size
formats like FNT/FON/PCF/BDF.
global_bbox
The global bounding box is defined as the largest rectangle
that can enclose all the glyphs in a font face.  It is defined for
horizontal layouts only.
ascender
The ascender is the vertical distance from the horizontal
baseline to the highest ‘character’ coordinate in a
font face.  Unfortunately, font formats define the ascender
differently.  For some, it represents the ascent of all capital
latin characters (without accents), for others it is the ascent of
the highest accented character, and finally, other formats define
it as being equal to
global_bbox.yMax
.
descender
The descender is the vertical distance from the horizontal
baseline to the lowest ‘character’ coordinate in a
font face.  Unfortunately, font formats define the descender
differently.  For some, it represents the descent of all capital
latin characters (without accents), for others it is the ascent of
the lowest accented character, and finally, other formats define
it as being equal to
global_bbox.yMin
.  This field is
negative for values below the baseline.
text_height
This field is simply used to compute a default line spacing
(i.e., the baseline-to-baseline distance) when writing text with
this font.  Note that it usually is larger than the sum of the
ascender and descender taken as absolute values.  There is also no
guarantee that no glyphs extend above or below subsequent
baselines when using this distance.
max_advance_width
This field gives the maximum horizontal cursor advance for all
glyphs in the font.  It can be used to quickly compute the maximum
advance width of a string of text.
It doesn't correspond to
the maximum glyph image width!
max_advance_height
Same as
max_advance_width
but for vertical text
layout.  It is only available in fonts providing vertical glyph
metrics.
underline_position
When displaying or rendering underlined text, this value
corresponds to the vertical position, relative to the baseline, of
the underline bar.  It is negative if it is below the
baseline.
underline_thickness
When displaying or rendering underlined text, this value
corresponds to the vertical thickness of the underline.
Notice how, unfortunately, the values of the ascender and the
descender are not reliable (due to various discrepancies in font
formats).
b. Scaled global metrics
Each size object also contains a scaled versions of some of the
global metrics described above.  They can be accessed directly through
the
face->size->metrics
structure.
Note that these values correspond to scaled versions of the design
global metrics,
with no rounding/grid-fitting performed
.
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.
ascender
The scaled version of the original design ascender.
descender
The scaled version of the original design descender.
height
The scaled version of the original design text height.  This
is probably the only field you should really use in this
structure.
max_advance
The scaled version of the original design max advance.
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 chapter.
c. Kerning
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.  Basically, it means that when the glyph for an
‘A’ is followed by the glyph for a ‘V’, the
space between them 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 the
horizontal direction in nearly all cases except really extreme
ones.
Not all font formats contain kerning information.  Instead, they
sometimes rely on 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 a file with
extension
.pfa
or
.pfb
, and where kerning metrics
can be found in an additional file with extension
.afm
or
.pfm
.
FreeType 2 allows you to deal with this, by providing the
../reference/ft2-base_interface.html#FT_Attach_File
FT_Attach_File
and
../reference/ft2-base_interface.html#FT_Attach_Stream
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.  For example, you could open a
Type 1 font by doing the following:
error = FT_New_Face( library, "/usr/shared/fonts/cour.pfb",
0, &face );
if ( error ) { ... }
error = FT_Attach_File( face, "/usr/shared/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 a
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 for
two glyphs through the
FT_Get_Kerning
function, whose
interface looks like:
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         */
As you see, the function takes a handle to a face object, the
indices of the left and right glyph for which the kerning value is
desired, as well as 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 chapter.  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
is used 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 chapter of this section.
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 text, or simply when
writing right-to-left text.
4. Simple text rendering: kerning + centering
In order to show off what we just learned, we will now demonstrate
how to modify the example code that was provided in section I to
render a string of text, and enhance it to support kerning and delayed
rendering.
a. Kerning support
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.  The code
looks like:
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_mode_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:
As kerning is determined from glyph indices, we need to
explicitely convert our character codes into a glyph indices, then
later call
FT_Load_Glyph
instead of
FT_Load_Char
.
We use a boolean named
use_kerning
which is set with
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.
We move the position of the pen
before
a new glyph is
drawn.
We initialize the variable
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.
We do not check the error code returned by
FT_Get_Kerning
.  This is because the function always sets
the content of
delta
to (0,0) when an error occurs.
b. Centering
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
layout 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 will position individual glyph
images on the baseline, while the second one will render the glyphs.
As we will see, this has many advantages.
We will thus start by storing individual glyph images, as well as
their position on the baseline.  This can be done with code like:
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 where we
extract each glyph image from the slot, and 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 like:
void  compute_string_bbox( FT_BBox  *abbox )
{
FT_BBox  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_BBox  glyph_bbox;
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 as in:
/* 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:
The pen position is expressed in the cartesian space (i.e.,
y upwards).
We call
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 typecasted to
FT_BitmapGlyph
.
We use translation when calling
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.
Of course, we still need to convert pixel coordinates from
cartesian to device space before rendering, hence the
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 could also decide to implement word wrapping, and only
draw
5. Advanced text rendering: transformation + centering + kerning
We are now going to modify our code in order to be able to easily
transform the rendered string, for example to rotate it.  We will start
by performing a few minor improvements:
a. packing & translating glyphs
We start by packing the information related to a single glyph image
into a single structure instead of parallel arrays.  We thus define
the following structure type:
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 as
several advantages.  Our glyph sequence loader thus becomes:
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.  The code becomes:
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], &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;
}
Now take a closer look: The
compute_string_bbox
function
can now compute the bounding box of a transformed glyph string.  For
example, we can do something like:
FT_BBox    bbox;
FT_Matrix  matrix;
FT_Vector  delta;
... load glyph sequence ...
... setup "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 );
b. Rendering a transformed glyph sequence
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, as in the
following code:
FT_Vector  start;
FT_Matrix  transform;
/* 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 );
for ( n = 0; n < num_glyphs; n++ )
{
FT_Glyph  image;
FT_Vector pen;
FT_BBox   bbox;
/* 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, &start );
/* 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( bitmap->bitmap,
bitmap->left,
my_target_height - bitmap->top );
FT_Done_Glyph( image );
}
}
There are a few changes compared to the original version of this
code:
We keep the original glyph images untouched; instead, we
transform a copy.
We perform clipping computations in order to avoid rendering
& drawing glyphs that are not within our target surface
We always destroy the copy when calling
FT_Glyph_To_Bitmap
in order to get rid of the transformed
scalable image.  Note that the image is destroyed even when the
function returns an error code (which is why
FT_Done_Glyph
is only called within the compound
statement.
The translation of the glyph sequence to the start pen position
is integrated in the call to
FT_Glyph_Transform
instead
of
FT_Glyph_To_Bitmap
.
It is possible to call this function several times to render the
string width 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&rsquo'.  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).
6. Accessing metrics in design font units, and scaling them
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 nominal
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 a
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 chapter 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:
‘true’ WYSIWYG text layout
accessing font content for conversion or analysis purposes
a. Scaling distances to device space
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 when
appropriate.
When you call the function
FT_Set_Pixel_Sizes
, you are
specifying the value 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.
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.  These fields
are:
x_ppem
The field name stands for ‘x pixels per EM’;
this is the horizontal size in integer pixels of the EM square,
which also is the
horizontal character pixel size
, called
pixel_size_x
in the above example.
y_ppem
The field name stands for ‘y pixels per EM’;
this is the vertical size in integer pixels of the EM square,
which also is the
vertical character pixel size
, called
pixel_size_y
in the above example.
x_scale
This is a 16.16 fixed float scale that is used to directly
scale horizontal distances from design space to 1/64th of device
pixels.
y_scale
This is a 16.16 fixed float scale that is used to directly
scale vertical distances from design space to 1/64th of device
pixels.
You can scale a distance expressed in font units to 26.6 pixel
format directly with the help of the
FT_MulFix
function, as
in:
/* 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 );
However, you can also scale the value directly with more accuracy
by using doubles:
FT_Size_Metrics*  metrics = &face->size->metrics;
/* shortcut */
double            pixels_x, pixels_y;
double            em_size, x_scale, y_scale;
/* compute floating point scale factors */
em_size = 1.0 * face->units_per_EM;
x_scale = metrics->x_ppem / em_size;
y_scale = metrics->y_ppem / em_size;
/* convert design distances to floating point pixels */
pixels_x = design_x * x_scale;
pixels_y = design_y * y_scale;
b. Accessing design metrics (glyph & global)
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
chapter 3 of this section.
Conclusion
This is the end of the second section of the FreeType 2
tutorial.  You are now able to access glyph metrics, manage glyph
images, and render text much more intelligently (kerning, measuring,
transforming & caching).
You have now sufficient knowledge to build a pretty decent text
service on top of FreeType 2, and you could possibly stop here if
you want.
The next section will deal with FreeType 2 internals (like
modules, vector outlines, font drivers, renderers), as well as a few
font format specific issues (mainly, how to access certain TrueType or
Type 1 tables).  [This section has not been written yet.]
step1.html
FreeType 2 Tutorial Step 1
