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<h1>The FreeType 2 FAQ</h1>
</h1>
<table width="80%" align="center">
<tr><td>
<hr>
<ul>
<li><a href="#general">General</a>
<ul><p>
<li>
<a href="#general-what">What is FreeType 2?</a>
</li>
<li>
<a href="#general-uses">What can I do with FreeType 2?</a>
</li>
<li>
<a href="#general-donts">What can I not do with
FreeType 2?</a>
</li>
<li>
<a href="#general-portability">How portable is
FreeType 2?</a>
</li>
<li>
<a href="#general-freetype1">What are the differences between
FreeType 1.x and FreeType 2?</a>
</li>
<li>
<a href="#general-ft1">Is FreeType 2 backwards compatible
to FreeType 1.x?</a>
</li>
<li>
<a href="#general-edit">Can I use FreeType 2 to edit fonts
or create new ones?</a>
</li>
</p></ul>
</li>
<li><a href="#builds">Compilation & Configuration</a>
<ul><p>
<li>
<a href="#builds-compile">How do I compile the FreeType 2
library?</a>
</li>
<li>
<a href="#builds-problems">I've tried to compile FreeType 2,
but I'm having problems. What can I do?</a>
</li>
<li>
<a href="#builds-config">How do I configure my library build?</a>
</li>
</p></ul>
</li>
<li>
<a href="#autohint">The FreeType 2 auto-hinter</a>
<ul><p>
<li>
<a href="#autohint-work">How does the auto-hinter work?
</li>
<li>
<a href="#autohint-other-scripts">Why doesn't the auto-hinter work
well with my script?</a>
</li>
</p></ul>
</li>
<li>
<a href="#other">Other questions</a>
<ul><p>
<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 2?</a>
</li>
<li>
<a href="#other-opentype">When will FreeType 2 support
OpenType?</a>
</li>
</p></ul>
</li>
</ul>
<p></p>
<table width="100%">
<tr bgcolor="#CCCCEE">
<td>
<a name="general"></a>
<h2 align=center>General questions & answers</h2>
</td>
</tr>
<tr>
<td>
<a name="general-what"></a>
<h3>
What is FreeType 2?
</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 30KB)
</p></li>
</ul>
<hr><a name="general-uses"></a>
<h3>
What can I do with FreeType 2?
</h3>
<p>FreeType 2 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
</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>
<hr><a name="general-donts"></a>
<h3>
What can I not do with FreeType 2?
</h3>
<p>FreeType 2 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 2 doesn't try to be a graphics library and thus
only supports two pixel formats when rendering glyphs:
monochrome 1-bit bitmaps, or 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 2
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 Type 1.</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 2 since version 2.0.1 which has become quite
stable at the time of this writing (version 2.2.1). 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.m are not
part of a <em>font service</em> in itself. They must be handled
one level higher.</p>
</li>
</ul>
<hr><a name="general-portability"></a>
<h3>
How portable is FreeType 2?
</h3>
<p>The FreeType 2 source code is <em>extremely</em> portable for the
following reasons:</p>
<ul>
<li>
Everything is written in standard ANSI C.
</li>
<li>
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.
</li>
<li>
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.
</li>
</ul>
<p>We have made great efforts to ensure that the library is
efficient, compact, and customizable.</p>
<hr><a name="general-freetype1"></a>
<h3>
What are the differences between FreeType 1.x and
FreeType 2?
</h3>
<p>The biggest differences are as follows.</p>
<ul>
<li>
<p>FreeType 1 only supports the TrueType format, while
FreeType 2 supports a lot more.</p>
</li>
<li>
<p>The FreeType 2 API is simpler as well as more powerful than the
FreeType 1 API.</p>
</li>
<li>
<p>FreeType 1 includes an extension to support OpenType
text layout processing. This support hasn't become part of
FreeType 2; a much improved version is now part of the <a
href="http:///www.pango.org">Pango<a/> library.</p>
</li>
</ul>
<hr><a name="general-ft1"></a>
<h3>
Is FreeType 2 backwards compatible with FreeType 1.x?
</h3>
<p>Short answer: No. However, transition from 1.x to 2 should
be rather straightforward.</p>
<p>The FreeType 2 API is a lot simpler than the one in 1.x
while being much more powerful. We thus encourage you to adapt your
source code to it as this should not involve much work.</p>
<hr><a name="general-edit"></a>
<h3>
Can I use FreeType 2 to edit fonts or create new ones?
</h3>
<p>No. The library has been 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="http://fontforge.sf.net">FontForge</a>.</p>
</td>
</tr>
</table>
<p></p>
<table width="100%">
<tr bgcolor="#CCCCEE">
<td>
<a name="builds"></a>
<h2 align=center>Compilation & Configuration</h2>
</td>
</tr>
<tr>
<td>
<a name="builds-compile"></a>
<h3>
How do I compile the FreeType 2 library?
</h3>
<p>The library can be compiled in various ways, and detailed
documentation is available in documentation directory of the
FreeType 2 source tree.</p>
<p>For compilation on the command line, GNU make is necessary; other
build tools won't work. The source bundle also comes with project
files for some graphical IDEs like Visual C; note, however, that
those files are sometimes not up to date since it is contributed
code not used by the core developers.</p>
<hr><a name="builds-config"></a>
<h3>
How do I configure my build of the library?
</h3>
<p>This is fully described in the file <tt>CUSTOMIZATION</tt> in
FreeType's documentation directory. Basically, you have to edit the
header file <tt>ftoption.h</tt> for compile-time options and to
select the modules with the file <tt>modules.cfg</tt>. Finally, it
is possible to replace the standard system interface (dealing with
memory allocation and stream I/O) with a custom one.</p>
</td>
</tr>
</table>
<p></p>
<table width="100%">
<tr bgcolor="#CCCCEE">
<td>
<a name="autohint"></a>
<h2 align=center>The FreeType 2 auto-hinter</h2>
</td>
</tr>
<tr>
<td>
<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 TUG-boat 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
<tt>FT_LOAD_FORCE_AUTOHINT</tt> bit flag when calling
<tt>FT_Load_Glyph()</tt>.</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><hr></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 Asian fonts, but not other complex scripts like Arabic.
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. Volunteers welcome!</p>
</td>
</tr>
</table>
<p></p>
<table width="100%">
<tr bgcolor="#CCCCEE">
<td>
<a name="other"></a>
<h2 align=center>Other questions</h2>
</td>
</tr>
<tr>
<td>
<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, note that 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 the 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>.
<font color="blue">
<pre>
FT_Raster_Params params;
FT_Outline outline;
... load vectorial glyph in ‘outline’ ...
params.flags = FT_RASTER_FLAG_AA | FT_RASTER_FLAG_DIRECT;
params.gray_spans = (FT_Raster_Span_Func)your_own_span_function;
params.user = your_own_data_pointer;
error = FT_Outline_Render( library, &outline, &params );</pre>
</font>
<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>
<hr><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>
<hr><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/consoles.
This assumption is not valid with most ‘modern’ font
formats, even 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 <tt>FT_Set_Char_Size()</tt> or <tt>FT_Set_Pixel_Sizes()</tt>
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</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 corresponded to the width of a
capital ‘M’ in Latin typefaces. 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>
<hr><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 which have considerably different
metrics. The TrueType format provides the (optional)
‘hdmx’ table which 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>
<hr><a name="other-antialias"></a>
<h3>
Which anti-aliasing algorithm is used by FreeType 2?
</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 2 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 2 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>
<hr><a name="other-opentype"></a>
<h3>
When will FreeType 2 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>FreeType 1 comes with a set of extensions that are used to
load and manage OpenType Layout tables. It even has a demonstration
program named <tt>ftstrtto</tt> to show its capabilities. However,
this code is no longer maintained, and we strongly advise to not use
it.</p>
<p>For FreeType 2, we have decided that the layout operations
provided through these tables are better placed in a specific
text-layout library like <a
href="http://www.pango.org">Pango</a>.</p>
</td>
</tr>
</table>
<p><hr></p>
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<p><font size=-3>Last update: 28-Jul-2006</font></p>
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