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<h1><a href="http://freetype.org/index.html">FreeType</a> FAQ</h1>
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<div class="col1wrap">
<div class="col1">
<!-- ************************************************** -->
<div id="contents">
<h3><a href="#general">General questions</a></h3>
<ul>
<li>
<a href="#general-what">What is FreeType?</a>
</li>
<li>
<a href="#general-uses">What can I do with FreeType?</a>
</li>
<li>
<a href="#general-donts">What can I not do with
FreeType?</a>
</li>
<li>
<a href="#general-portability">How portable is
FreeType?</a>
</li>
<li>
<a href="#general-edit">Can I use FreeType to edit fonts
or create new ones?</a>
</li>
<li>
<a href="#general-typesetting">How do I render a line of
text in FreeType?</a>
</li>
</ul>
<h3><a href="#builds">Compilation &
configuration</a></h3>
<ul>
<li>
<a href="#builds-compile">How do I compile the FreeType
library?</a>
</li>
<li>
<a href="#builds-config">How do I configure my build of
the library?</a>
</li>
<li>
<a href="#builds-differences">Why does FreeType render
differently on different platforms?</a>
</li>
</ul>
<h3><a href="#autohint">The FreeType auto-hinter</a></h3>
<ul>
<li>
<a href="#autohint-work">How does the auto-hinter
work?</a>
</li>
<li>
<a href="#autohint-other-scripts">Why doesn't the
auto-hinter work well with my script?</a>
</li>
</ul>
<h3><a href="#other">Other questions</a></h3>
<ul>
<li>
<a href="#other-depth">Can I use FreeType to draw text
on a pixmap with arbitrary depth?</a>
</li>
<li>
<a href="#other-color">How can I set the colour of text
rendered by FreeType?</a>
</li>
<li>
<a href="#other-size">I set the pixel size to 8×8,
but the resulting glyphs are larger (or smaller) than
that. Why?</a>
</li>
<li>
<a href="#other-bbox">How can I compute the bounding box
of a text string without loading its glyphs
before?</a>
</li>
<li>
<a href="#other-antialias">Which anti-aliasing algorithm
is used by FreeType?</a>
</li>
<li>
<a href="#other-opentype">When will FreeType support
OpenType?</a>
</li>
<li>
<a href="#other-vertical">How to implement Asian
vertical writing text with FreeType?</a>
</li>
</ul>
</div>
<!-- ************************************************** -->
<div id="faq">
<a name="general"></a>
<h1>General questions</h1>
<a name="general-what"></a>
<h3>What is FreeType?</h3>
<p>It is a software library that can be used by all kinds of
applications to access the contents of font files. Most
notably, it supports the following features.</p>
<ul>
<li>
<p>It provides a uniform interface to access font files.
It supports both bitmap and scalable formats,
including TrueType, OpenType, Type1, CID, CFF, Windows
FON/FNT, X11 PCF, and others.</p>
</li>
<li>
<p>It supports high-speed, anti-aliased glyph bitmap
generation with 256 gray levels.</p>
</li>
<li>
<p>It is extremely modular, each font format being
supported by a specific module. A build of the
library can be tailored to support only the formats
you need, thus reducing code size. A minimal
anti-aliasing build of FreeType can be less than
30kByte.</p>
</li>
</ul>
<a name="general-uses"></a>
<h3>What can I do with FreeType?</h3>
<p>FreeType is already used in many products. For example,
it serves as a font rendering engine</p>
<ul>
<li>
in graphics subsystem and libraries to display text
(Android OS, Apple's iOS, and others),
</li>
<li>
in text layout and pagination services to measure and
eventually render text,
</li>
<li>
in font inspection and conversion tools.
</li>
</ul>
<p>Generally speaking, the library allows you to access and
manage the contents of font files in a very easy way.</p>
<a name="general-donts"></a>
<h3>What can I not do with FreeType?</h3>
<p>FreeType doesn't try to perform a number of sophisticated
things, because it focuses on being an excellent <em>font
service</em>.</p>
<p>This means that the following features are not supported
directly by the library, even though they can be more or
less implemented on top of it, or by using it.</p>
<ul>
<li>
<p><b>rendering glyphs to arbitrary surfaces</b><br>
FreeType doesn't try to be a graphics library and thus
only supports a very small number of pixel formats
when rendering glyphs, in particular monochrome 1-bit
bitmaps and 8-bit gray-level pixmaps.</p>
<p>If you need to draw glyphs to other kinds of surfaces
(for example, a 24-bit RGB pixmap), you need to use
your favorite graphics library to do just that.</p>
<p><em>Note, however, that in the case of rendering
scalable glyph outlines to anti-aliased pixmaps, an
application can also provide its own rendering
callback in order to draw or compose directly the
anti-aliased glyph on any target surface.</em></p>
</li>
<li>
<p><b>glyph caching</b><br>
Each time you request a glyph image from a font,
FreeType does it by parsing the relevant portion of
the font file or font stream and interpreting it
according to its font format. This can be very slow
for certain formats, including scalable ones like
TrueType or CFF.</p>
<p>Any decent text-rendering sub-system must thus be
capable of caching glyph data in order to reach
appropriate rendering speed.</p>
<p><em>Note that we provide a caching sub-system with
FreeType. However, it might not suit your
needs.</em></p>
</li>
<li>
<p><b>text layout</b><br>
The library doesn't support text layout operations.
Sophisticated features like glyph substitution,
positioning (kerning), justification, bi-directional
ordering, etc., are not part of a
<em>font service</em> in itself. They must be handled
(at least) one level higher.</p>
</li>
</ul>
<a name="general-portability"></a>
<h3>How portable is FreeType?</h3>
<p>FreeType's source code is <em>extremely</em> portable for
the following reasons.</p>
<ul>
<li>
<p>Everything is written in standard ANSI C.</p>
</li>
<li>
<p>We are very pedantic to avoid any kinds of compiler
warnings. The current source code has been compiled
with many compilers without producing a single
warning.</p>
</li>
<li>
<p>The library doesn't use any static writable data at
all, making it an ideal choice on various embedded
systems (e.g., it can be run from ROM directly). It
is completely thread-safe too.</p>
</li>
</ul>
<p>We have made great efforts to ensure that the library is
efficient, compact, and customizable.</p>
<a name="general-edit"></a>
<h3>Can I use FreeType to edit fonts or create new
ones?</h3>
<p>No. The library is specifically designed
to <em>read</em> font files with small code size and very
low memory usage.</p>
<p>A good, freely available font editor
is <a href="https://fontforge.github.io">FontForge</a>.</p>
<a name="general-typesetting"></a>
<h3>How do I render a line of text in FreeType?</h3>
<p>FreeType is a very low-level rendering engine. All it
knows how to do is render individual glyphs and return
metrics for them.</p>
<p>Arranging glyphs into words and lines is best left to a
2D graphics rendering library
like <a href="https://www.cairographics.org">Cairo</a>.
Cairo is able to do some primitive typesetting, including
very basic translation of Unicode character codes to
glyphs; for a more general solution, it
uses <a href="https://pango.gnome.org">Pango</a> for text
layout, which in turn relies
on <a href="https://harfbuzz.github.io/">HarfBuzz</a> to
implement the full OpenType rules for glyph substitution
and placement (Cairo still handles the actual text
drawing). To deal with right-to-left (Arabic, Hebrew)
text layout, you will also need
<a href="https://github.com/fribidi/fribidi">FriBidi</a>,
which handles the details of the Unicode bidirectional
layout algorithm.</p>
<p>And finally, to cope with additional niceties like
dealing with writing-system-specific rules for word breaks
and line breaks, you might like to look at
<a href="http://www.icu-project.org">ICU</a>, which
includes a whole range of internationalization and
localization facilities.</p>
<a name="builds"></a>
<h1>Compilation & configuration</h1>
<a name="builds-compile"></a>
<h3>How do I compile the FreeType library?</h3>
<p>The library can be compiled in various ways, and detailed
documentation is available in documentation directory of
FreeType's source tree.</p>
<p>For compilation on the command
line, <a href="https://www.gnu.org/software/make">GNU
make</a> is the standard tool. The source bundle also
comes with support files
for <a href="https://cmake.org">CMake</a> and
<a href="https://mesonbuild.org">meson</a> builds,
together with project files
for <a href="https://docs.microsoft.com/visualstudio/windows/">Visual
C++</a> and some other, older graphical IDEs. Note,
however, that those files are sometimes not up to date
since it is contributed code not used by the core
developers.</p>
<a name="builds-config"></a>
<h3>How do I configure my build of the library?</h3>
<p>This is fully described in the
file <a href="https://gitlab.freedesktop.org/freetype/freetype/-/blob/master/docs/CUSTOMIZE"><code>CUSTOMIZE</code></a>
in FreeType's documentation directory. Basically, you
have to edit the header
file <a href="https://gitlab.freedesktop.org/freetype/freetype/-/blob/master/include/freetype/config/ftoption.h"><code>ftoption.h</code></a>
for compile-time options and to select the modules with
the
file <a href="https://gitlab.freedesktop.org/freetype/freetype/-/blob/master/modules.cfg"><code>modules.cfg</code></a>.
Finally, it is possible to replace the standard system
interface (dealing with memory allocation and stream I/O)
with a custom one.</p>
<a name="builds-differences"></a>
<h3>Why does FreeType render differently on different
platforms?</h3>
<p>Different distributions compile FreeType with different
options. The developer version of a distribution's
FreeType package, which is needed to compile your program
against FreeType, includes the file
<a href="https://gitlab.freedesktop.org/freetype/freetype/-/blob/master/include/freetype/config/ftoption.h"><code>ftoption.h</code></a>.
Compare each platform's copy of
<code>ftoption.h</code> to find the differences.</p>
<a name="autohint"></a>
<h1>The FreeType auto-hinter</h1>
<a name="autohint-work"></a>
<h3>How does the auto-hinter work?</h3>
<p><em>Please note that the name of auto-hinter module is
<b>autofit</b>, which is a reimplementation of the old
‘autohint’ module.</em></p>
<p>A rather complete description of the hinting algorithm
(which is slightly out of date regarding the internal
structures) can be found in the TUGboat
article <a href="http://www.tug.org/TUGboat/Articles/tb24-3/lemberg.pdf">Real-Time
Grid Fitting of Typographic Outlines</a>.</p>
<p>The auto-hinter performs grid-fitting on scalable font
formats that use Bézier outlines as their primary
glyph image format (this means nearly all scalable font
formats today). If a given font driver doesn't provide
its own hinter, the auto-hinter is used by default. If a
format-specific hinter is provided, it is still possible
to use the auto-hinter using
the <a href="https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#ft_load_xxx"><code>FT_LOAD_FORCE_AUTOHINT</code></a>
bit flag when calling
function <a href="https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#ft_load_glyph"><code>FT_Load_Glyph</code></a>.</p>
<p>Currently, the auto-hinter doesn't use external hints to
do its job, as it automatically computes global metrics
(when it ‘opens’ a font for the first time)
and glyph ‘hints’ from their outline.</p>
<a name="autohint-other-scripts"></a>
<h3>Why doesn't the auto-hinter work well with my
script?</h3>
<p>The auto-hinter was first designed to manage and hint
Latin-based fonts, as they consist of most of the fonts
available today. It now supports almost all scripts
available in Unicode.</p>
<p>Hinting various scripts isn't really more difficult than
Latin, just different, with a set of different
constraints, which must be hard-coded into the autofit
module. If your particular script is still missing and
you are an expert for this script, we would be very glad
to add the necessary code with your help!</p>
<a name="other"></a>
<h1>Other questions</h1>
<a name="other-depth"></a>
<h3>Can I use FreeType to draw text on a pixmap with
arbitrary depth?</h3>
<p>Not directly, as FreeType is a font library, not a
general-purpose graphics library or text rendering
service. However, the anti-aliased renderer allows you to
convert a vectorial glyph outline into a list of
‘spans’ (i.e., horizontal pixel segments with
the same coverage) that can be rendered through
user-provided callbacks.</p>
<p>By providing an appropriate span callback, you can render
anti-aliased text to any kind of surface. You can also
use any colour, fill pattern or fill image if you want to.
This process is called <em>direct rendering</em>.</p>
<p>A complete example is given in
the <a href="tutorial/step3.html">FreeType tutorial</a>.
<p>Note that direct rendering is <em>not</em> available with
monochrome output, as the current renderer uses a two-pass
algorithm to generate glyphs with correct drop-out
control.</p>
<a name="other-color"></a>
<h3>How can I set the colour of text rendered by
FreeType?</h3>
<p>Basically, you can't do that, because FreeType is simply
a font library. In general, you need to use your favorite
graphics library to draw the FreeType glyphs with the
appropriate colour.</p>
<p>Note that for anti-aliased glyphs, you can ‘set the
colour’ by using <em>direct rendering</em> as
described in <a href="#other-depth">this answer</a>.</p>
<a name="other-size"></a>
<h3>I set the pixel size to 8×8, but the resulting
glyphs are larger (or smaller) than that. Why?</h3>
<p>A lot of people have difficulties to understand this
topic, because they think of glyphs as fixed-width or
fixed-height ‘cells’, like those of fonts used
in terminals or consoles. This assumption is not valid
with most ‘modern’ font formats, it even fails
for bitmapped-based ones like <b>PCF</b> or
<b>BDF</b>.</p>
<p>Be aware that the <em>character size</em> that is set
either through
function <a href="https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#ft_set_char_size"><code>FT_Set_Char_Size</code></a>
or <a href="https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#ft_set_pixel_sizes"><code>FT_Set_Pixel_Sizes()</code></a>
isn't directly related to the dimension of the generated
glyph bitmaps!</p>
<p>Rather, the character size is indeed the size of <em>an
abstract square</em>, called the <em>EM Square</em>,
used by typographers to design fonts. Scaling two
distinct fonts to the same character size, be it
expressed in points or pixels, generally results in
bitmaps with <em>distinct dimensions</em>!</p>
<p>Note that historically, the EM Square corresponded to the
width of a capital ‘M’ in Latin typefaces
(thus the name). However, later improvements in
typography led to designs that greatly deviate from this
rule. Today, it is not possible to connect the EM size to
a specific font ‘feature’ in a reliable
way.</p>
<a name="other-bbox"></a>
<h3>How can I compute the bounding box of a text string
without loading its glyphs before?</h3>
<p>This is not possible in general. Reason is that hinting
distorts the glyph shape for optimal rasterization, and
this process sometimes creates outlines that have
considerably different metrics. The TrueType format
provides the (optional) ‘hdmx’ table that
contains device horizontal metrics for selected pixel
sizes, but even here the vertical metrics are missing.</p>
<p>It is probably best to use both a glyph and a metrics
cache to avoid recomputation.</p>
<a name="other-antialias"></a>
<h3>Which anti-aliasing algorithm is used by FreeType?</h3>
<p>The algorithm has been specifically designed for
FreeType. It is based on ideas that were originally found
in the implementation of the
<a href="http://www.levien.com/libart/">libArt</a>
graphics library to compute the <em>exact pixel
coverage</em> of a vector image with no sub-sampling and
filtering.</p>
<p>However, these two implementations are radically distinct
and use vastly different models. The FreeType renderer is
optimized specifically for rendering small complex shapes,
like glyphs, at very high speed while using very few
memory. On the other hand, libArt has been designed for
general shape and polygon processing, especially large
ones.</p>
<p>The FreeType anti-aliasing renderer is indeed
<em>faster</em> than the monochrome renderer for small
character sizes (typically <20 pixels). The
reason is that the monochrome renderer must perform two
passes on the outline in order to perform drop-out control
according to the TrueType specification.</p>
<a name="other-opentype"></a>
<h3>When will FreeType support OpenType?</h3>
<p>Well, the engine already reads OpenType/CFF files
perfectly. What it doesn't do is handling ‘OpenType
Layout’ tables.</p>
<p>The predecessor of FreeType 2, the now defunct
FreeType 1 series, came with a set of extensions that
were used to load and manage OpenType Layout tables. It
even had a demonstration program
named <code>ftstrtto</code> to show its capabilities.
However, this code is no longer maintained, and we
strongly advise to not use it.</p>
<p>See <a href="#general-typesetting">this answer</a> for
more.</p>
<a name="other-vertical"></a>
<h3>How to implement Asian vertical writing text with
FreeType?</h3>
<p>FreeType 2 no longer supports it. Before the
general adoption
of <a href="https://docs.microsoft.com/en-us/typography/opentype/spec/">OpenType</a>
in the font industry, text renderers implemented many
hacks to handle vertical typesetting, like detection of
the vertical writing mode by relying on modified font
names, renderer-side rotation to obtain vertical glyph
representation forms, or putting pre-rotated glyphs at
non-standard <a href="http://www.unicode.org/faq/private_use.html">Unicode
PUA codepoints</a> in the font. Modern operating systems
and font developers switched to TrueType GX and later to
OpenType many years ago, and fonts designed for such
legacy technologies are no longer widely distributed.</p>
<p>See <a href="#general-typesetting">this answer</a> for
more.</p>
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<p>Last update: 16-May-2021</p>
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