[Freetype-wiki-commit] [A freetype Wiki] Update of "FreeType 2
tutorial/part 2"
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The following page has been changed by WernerLemberg:
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#pragma section-numbers on
= Managing glyphs =
----
[[TableOfContents(3)]]
== Introduction ==
This is the second section of the Free''''''Type[[nbsp]]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
----
== 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
[http://freetype.freedesktop.org/freetype2/docs/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:
http://freetype.freedesktop.org/wiki-img/metrics.png
For vertical text layouts, the baseline is vertical, identical to the
vertical axis:
http://freetype.freedesktop.org/wiki-img/metrics2.png
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 part[[nbsp]]1. 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[[nbsp]]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.
----
== 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 Free''''''Type[[nbsp]]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
[http://freetype.freedesktop.org/freetype2/docs/reference/ft2-header_file_macros.html#FT_GLYPH_H FT_GLYPH_H]
header file, as in:
{{{#!cplusplus
#include FT_GLYPH_H
}}}
We now explain how to use the functions defined in this file.
=== Extracting the glyph image ===
You can extract a single glyph image very easily. Here some code that
shows how to do it.
{{{#!cplusplus
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
[http://freetype.freedesktop.org/freetype2/docs/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.
* copied the glyph image from the slot into a new `FT_Glyph` object,
by calling
[http://freetype.freedesktop.org/freetype2/docs/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 True''''''Type 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
[http://freetype.freedesktop.org/freetype2/docs/reference/ft2-glyph_management.html#FT_Done_Glyph FT_Done_Glyph].
The glyph object contains exactly one glyph image and a
two-dimensional 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 Free''''''Type 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.''
=== 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
[http://freetype.freedesktop.org/freetype2/docs/reference/ft2-glyph_management.html#FT_Glyph_Transform FT_Glyph_Transform].
You can also copy a single glyph image with
[http://freetype.freedesktop.org/freetype2/docs/reference/ft2-glyph_management.html#FT_Glyph_Copy FT_Glyph_Copy].
Here is some example code:
{{{#!cplusplus
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.
=== Measuring the glyph image ===
You can also retrieve the control (bounding) box of any glyph image
(scalable or not) through the
[http://freetype.freedesktop.org/freetype2/docs/reference/ft2-glyph_management.html#FT_Glyph_Get_CBox FT_Glyph_Get_CBox]
function, as in
{{{#!cplusplus
FT_BBox bbox;
...
FT_Glyph_Get_CBox( glyph, bbox_mode, &bbox );
}}}
Coordinates are relative to the glyph origin (0,0), using the
y[[nbsp]]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 it in integer or 26.6 pixels, with:
{{{#!cplusplus
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
{{{#!cplusplus
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`.
=== Converting the glyph image to a bitmap ===
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
[http://freetype.freedesktop.org/freetype2/docs/reference/ft2-glyph_management.html FT_Glyph_To_Bitmap]
function. It is in charge of converting any glyph object into a
bitmap, as in
{{{#!cplusplus
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[[nbsp]]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 contents. This type is a
sort of ‘subclass’ of `FT_Glyph` that contains additional fields (see
[http://freetype.freedesktop.org/freetype2/docs/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[[nbsp]]y.
bitmap:: This is a bitmap descriptor for the glyph object, just like the `bitmap` field in a glyph slot.
----
== 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.
=== 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 True''''''Type) or 1000 (for Type[[nbsp]]1), but others are possible too. It is set to[[nbsp]]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).
=== 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 which is really useful in practice.
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.
=== 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[[nbsp]]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`.
Free''''''Type[[nbsp]]2 allows you to deal with this, by providing the
[http://freetype.freedesktop.org/freetype2/docs/reference/ft2-base_interface.html#FT_Attach_File FT_Attach_File] and
[http://freetype.freedesktop.org/freetype2/docs/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[[nbsp]]1 font by doing the following.
{{{#!cplusplus
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[[nbsp]]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.
Free''''''Type[[nbsp]]2 allows you to retrieve the kerning information
for two glyphs through the
[http://freetype.freedesktop.org/freetype2/docs/reference/ft2-base_interface.html#FT_Get_Kerning FT_Get_Kerning]
function, whose interface looks like
{{{#!cplusplus
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 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[[nbsp]]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.
----
== Simple text rendering: kerning + centering ==
In order to show off what we have just learned, we will now
demonstrate how to modify the example code that was provided in
part[[nbsp]]I to render a string of text, and enhance it to support
kerning and delayed rendering.
=== 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:
{{{#!cplusplus
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[[nbsp]]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.
=== 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 this.
{{{#!cplusplus
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:
{{{#!cplusplus
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
{{{#!cplusplus
/* 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[[nbsp]]upwards).
* We call `FT_Glyph_To_Bitmap` with the `destroy` parameter set
to[[nbsp]]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.
----
== 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.
=== 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:
{{{#!cplusplus
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
{{{#!cplusplus
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
{{{#!cplusplus
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:
{{{#!cplusplus
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 );
}}}
=== 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:
{{{#!cplusplus
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 transformation (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 and
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 if 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 Free''''''Type[[nbsp]]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 ‘Free''''''Type’. 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).
----
== 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
=== 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'':
{{{#!cplusplus
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` Free''''''Type 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
{{{#!cplusplus
/* 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:
{{{#!cplusplus
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;
}}}
=== 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 above.
----
== Conclusion ==
This is the end of the second section of the Free''''''Type[[nbsp]]2
tutorial. You are now able to access glyph metrics, manage glyph
images, and render text much more intelligently (kerning, measuring,
transforming, and caching).
You have now sufficient knowledge to build a pretty decent text
service on top of Free''''''Type[[nbsp]]2, and you could possibly stop
here if you want.
The next section will deal with Free''''''Type[[nbsp]]2 internals
(like modules, vector outlines, font drivers, renderers), as well as a
few font format specific issues (mainly, how to access certain
True''''''Type or Type[[nbsp]]1 tables). [This section has not been
written yet.]
["../part 1"]
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