FreeType 2 FAQ
Author: David Turner

http://www.freetype.org

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# The FreeType 2 FAQ

Document index General I thought the FreeType project was dead. Is this true? Why did it take so long to release FreeType 2? Is FreeType 2 a Unix-only project? When will X11 support anti-aliased glyphs? Is FreeType 2 backwards compatible to FreeType 1.x? Can I use FreeType 2 to edit fonts or create new ones? Compilation & Configuration How do I compile the FreeType 2 library? How do I configure my library build? How do I select the modules I need? How do I compile all FreeType 2 files in a single directory? The FreeType 2 autohinter Under which license is the auto-hinter released? How does auto-hinting work in FreeType 2? Why doesn't the auto-hinter work well with CJK fonts? Other questions How can I set the color of text rendered by FreeType? Can I use FreeType to draw text on a pixmap with arbitrary depth? I set the pixel size to 8x8, but the resulting glyphs are larger than that. Why? How can I compute the bounding box of a text string without loading its glyphs? Which anti-aliasing algorithm is used in the FreeType 2 renderer? When will FreeType 2 support OpenType?
## General questions & answers
### I.1 I though the FreeType project was dead. Is this true? Well, not exactly :-) It's true that the TrueType patents issues have been less than a graceful event to handle but it didn't not really killed the project per se, as Apple hasn't made an official statement yet regarding the use of the patented "technology" in open source projects (or other products). We have thus continued updating FreeType 1.x, and started developing FreeType 2 with the idea of providing this time a completely patent free font engine. However, we largely preferred not to broadly communicate about it until we've got a satisfying implementation to show. ### I.2 Why did it take so long to release FreeType 2? Several factors come to mind. The first one is that FreeType 2 is a much more complex and dense project that was mostly developed during non-working hours. And surely some important changes in the life (like marriage, new jobs and kids) of some the FreeType developers cannot be ignored :-) A second one is that a first version of the library was designed one year ago (and already worked with a multitude of font drivers), though with a design that was judged by its authors as well as beta testers as not enough flexible or consistent. In short, it worked well but we were not exactly proud of it (call us perfectionists). It has then be significantly reworked to become what we are now distributing as FreeType 2 Finally, it would have been hard to distribute such a library without an alternative technology to replace the patented bytecode interpreter. This involved significant research work that could only be performed correctly full-time, and we had to found a company to fund such a development and still make it available under a BSD-like license. Huge thanks to Catharon Productions, Inc. for their commitment to this project. And of course, we added support for more font files, and we will continue to as long as the specifications are available and that we find an interest in it. For example, FreeType 2 is to date the only software library available on the market that supports the new Adobe "CEF" font format. ### I.3 Is FreeType 2 a Unix-only project? Absolutely not, even though many people still seem to think so :-) FreeType 2, just like version 1.x, can be compiled on any platform with an ANSI compiler. Some beta versions of the library are even heavily used in brand new OSes (see the AtheOS screenshots for examples). The library is itself mainly developed on several platforms (Windows & Linux, though a great deal has also been achieved on OS/2) and the code is highly generic and modular to adapt even the most strict environments like low-memory embedded systems. ### I.4 When will X11/XFree support anti-aliased text? This question isn't exactly related to FreeType as we have no direct connection to the XFree people, but we have been asked so frequently about it that it deserves a prominent place in this FAQ :-) FreeType has been capable of anti-aliasing since version 1.0. The reason why XFree doesn't support it is directly related to the limitations of the design and specification of X11. More specifically: X11 assumes that all glyph images are monochrome bitmaps, hence the X font library and server are unable to send anything else to the X server. Even if the X font library/server was able to generate anti-aliased bitmaps (and this has been already done through extensions), the X rendering model doesn't allow translucent composition of "gray" pixmaps onto an arbitrary drawable. As both the font and rendering models of X11 are limited, it is basically impossible to draw anti-aliased glyphs without performing huge hacks within the server. Note that Keith Packard, from XFree86 fame, has recently started working on a new rendering model for X11 in order to support new features (mainly transparency and anti-aliased fonts). This will be provided through protocol extensions. The question of knowing whether legacy X applications will be able to display anti-aliased text is still very uncertain. ### I.5 Is FreeType 2 backwards compatible with FreeType 1.x? Not directly, though we had the project to provide an optional binary compatibility layer on top of it in order to easily re-link applications with the new version. However, this idea has been dropped as it is possible to install and use the two versions independently on any system (read: no namespace conflicts). 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. ### I.6 Can I use FreeType 2 to edit fonts or create new ones? The answer is a definitive no, because the library was specifically designed to read font files with small code size and very low memory usage. We thus do not plan to support editing or creation in the font engine in any way, as this would imply a complete rewrite. This doesn't mean that we won't introduce a font editing/creation library in the future, as this really depends on how many people are asking for it (or how much they would be willing to pay for it), as well as the time of the FreeType developers. Do not expect anything in this direction until we officially announce something though. There are other axes of development for this project (like text-layout capabilities, glyph caching, etc.) that may be more important to us at the moment.
## Compilation & Configuration
### II.1 How do I compile the FreeType 2 library? The library can be compiled in various ways, and a detailed documentation is available in the file `freetype2/docs/BUILD`. However, we will summarize the process to a few cases: #### a. Using the command-line 2 build system The engine comes with a sophisticated build system that is used to configure and compile a build of the library. You will need GNU Make installed on your platform (Note: It will not work with other Make tools). Basically, you will need to invoke `make` a first time in the top-level FreeType 2 directory in order to set up the build. This will detect your current platform and choose a configuration sub-makefile to drive the build. A specific compiler can be selected on some platforms by providing an additional target. For example, on Win32: `make visualc` will select the Visual C++ compiler `make lcc` will select the Win32-lcc compiler Note that on Unix, when the first time make is called, a configure script located in `freetype2/builds/unix` will be run in order to automatically detect the platform & compiler. A summary will be displayed showing the detected platform and compiler selected. You will then be able to start the build by invoking `make` a second time. In case of problem, consult the `BUILD` document. #### b. Direct compilation You can also directly compile the library from the command line by using these simple rules: You should place the directories `freetype2/include` and `freetype2/src` in your include path in order to compile any component of the library. You can also add the system-specific build directory (i.e. `builds/system/`) in the case where an alternate implementation of some of the components is available there (e.g. the memory-mapped i/o implementation on some Unix systems). The components of the library are located in sub-directories of `src`, for example: `src/base`, `src/truetype`, etc. Each component is normally compiled through a single C file that wraps other sources in the component's directory. For example, you should build the TrueType font driver by compiling the file `src/truetype/truetype.c`. The list of C files to compile for a feature-complete build of the library is given in the `BUILD` document. #### c. Using a graphical IDE Well, the process is vastly similar to the one described in b., except that you need to set the include paths, source code paths, etc. in dialog boxes before running the compilation. ### II.2 How do I configure my build of the library? Each build of the library is configured through two header files located in `include/freetype/config`: `ftoption.h` This file contains various configuration macros whose definition can be toggled on a per-build basis. Each macro is heavily commented in this file's comment, and we invite you to refer to it directly. `ftmodule.h` This file contains the list of all the modules that are initially registered (added) when the function `FT_Init_FreeType()` is called. See the next answer to know how to change it and why it may be important. Alternatively, some specific implementations of some FreeType 2 components can be provided in a `builds/system/` directory (e.g. the Unix-specific `ftsystem.c` that uses memory-mapped file for i/o). ### II.3 How do I select the modules I need in my build? The function `FT_Init_FreeType()` creates a new instance of the FreeType 2 library and registers a set of "default" modules before returning to the calling application. Its default implementation is in the file `src/base/ftinit.c`. The list of default modules used by `ftinit.c` is located in the configuration file `include/freetype/config/ftmodule.h`. Normally, it is automatically generated by the build system by invoking the "`make modules`" command in the top level FreeType 2 directory (Note: this only works with GNU Make; you can edit the file by hand otherwise). It does so by parsing all sub-directories of `src` that contain a file named `module.mk`. Note that a specific port or project is free to provide its own implementation of `ftinit.c` in order to ensure a different initialization sequence. For example, one could do something like: Compile each module as a shared library (DLL or `.so`) with a common "entry point" to retrieve a pointer to its module class (there is already some code that allows this when compiling each module). Place these modules in a directory like `/usr/lib/freetype2/modules/`. Provide an implementation of `ftinit.c` that would scan the directory for valid modules. This example only emphasizes the flexibility that is left to developers when building the library. ### II.4 How do I compile all FreeType 2 files in a single directory? Some projects may need, for the sake of simplicity or ease of building, to compile the FreeType 2 library with all source files copied to a single directory. This is possible. To do so, you have to copy all source files located under `src` to your own directory (you must retain the include files in a distinct hierarchy though), then compile each of the FreeType 2 component with the macro `FT_FLAT_COMPILE`. This will change the way `#include` works during the build.
## The FreeType 2 auto-hinter
### III.1 Under which license is the FreeType 2 auto-hinter released? The auto-hinter was initially designed and implemented under contract for Catharon Productions, Inc which gladly accepted to released it under an open-source license compatible with the FreeType one. This license can be found in `src/autohint/CatharonLicense.txt` and requires that you cite Catharon Productions in your documentation (just like you do with FreeType) when using the auto-hinting module. Other than that, you still have the same freedom than with the good old FreeType license. Enjoy! ### III.2 How does the auto-hinter work? Well, a complete description would be difficult. Have a look at the dedicated auto-hinter pages on the FreeType site, as they describe most of its details with graphics and explanations. You could also look at the source code if you want to :-) To give a few details, the auto-hinter is used to perform 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 `FT_LOAD_FORCE_AUTOHINT` bit flag when calling `FT_Load_Glyph()`. The auto-hinter currently 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. Note that we plan the ability to specify external hints, given that it is based on a constraint system. That could be used to support native hints in Type 1/Type 2 fonts, for example. ### III.3 Why does the auto-hinter doesn't work correctly with CJK fonts? The auto-hinter was first designed to manage and hint Latin-based fonts, as they consist of most of the fonts available today. It doesn't hint Asian fonts, as well as a few other complex scripts, because we didn't put enough research on the topic yet. Hinting CJK isn't really more difficult than Latin, just different, with a set of different constraints (basically, more distortion of glyphs is acceptable as long as certain features like triple-stem positions are respected more strictly). We thus plan to handle such a case in the near future. Please be patient.
## Other questions
### IV.1 Can I use FreeType to draw text on a pixmap with arbitratry depth? 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 same coverage) that can be rendered through user-provided callbacks. By providing the appropriate span callback, you can render anti-aliased text to any kind of surface. You can also use any color or fill pattern/image if you want to. This process is called direct rendering. For more information, please read the documentation contained in the following files: `<freetype/ftimage.h>` contains the definition of the `FT_Raster_Params` type used with direct rendering. `<freetype/ftoutln.h>` contains the definition of the `FT_Outline_Render()` function that can be used to convert vectorial outlines to span lists. Here's some code that uses them:
        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_here;
        params.user       = your_own_data_pointer;

        error = FT_Outline_Render( library, &outline, &params );
Note that direct rendering is not available with monochrome output, as the current renderer uses a two-pass algorithm to generate glyphs with correct drop-out control. ### IV.2 How can I set the color of text rendered by FreeType? Basically, you can't do that, because FreeType is simply a font library. In general, you will need to use your favorite graphics library to draw the FreeType glyphs with the appropriate color. Note that for anti-aliased glyphs, you can "set the color" by using direct rendering as described in this answer. ### IV.3 I set the pixel size to 8x8, but the resulting glyphs are larger (or smaller) than that. Why? A lot of people have difficulties to understand this topic, because they think of glyphs as fixed-width resp. fixed-height "cells", like those of fonts used in terminals/consoles. This assumption is simply not valid with most "modern" font formats, even bitmapped-based ones like `PCF` or `BDF`. Be aware that the character size that is set either through `FT_Set_Char_Size()` or `FT_Set_Pixel_Sizes()` isn't directly related to the dimension of the glyph bitmaps generated. Rather, the character size is indeed the size of an abstract square, called the EM, used by typographers to design fonts. Scaling two distinct fonts to the same character size, be it expressed in points or pixels, will generally result in bitmaps with distinct dimensions! 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 detract from this rule. Today, it is not possible to connect the EM size to a specific font "feature" in a reliable way. ### IV.4 How can I compute the bounding box of a given string of text without loading its glyphs before? A lot of people want to be able to compute the size in pixels of a simple string of text with minimal overhead. For example, that can be useful to draw centered text within a button. (to be continued...) ### IV.5 Which anti-aliasing algorithm is used by FreeType 2? The algorithm has been specifically designed for FreeType. It is based on ideas that were originally found in the implementation of the libArt graphics library to compute the exact pixel coverage of a vector image with absolutely no sub-sampling/filtering. 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; while libArt shines at general shape/polygon processing, especially large ones. The FreeType 2 anti-aliasing renderer is indeed faster 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 (we could drop this requirement later though). We will try to document its design in a later document, though this is not a priority for now. ### IV.6 When will FreeType 2 support OpenType? Well, the engine already reads OpenType/CFF files perfectly. What it doesn't do is handle "OpenType Layout" tables yet. 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 `ftstrtto` to show its capabilities. For FreeType 2, we have decided that the layout operations provided through these tables are better placed in a specific text-layout library, (many people having asked for such a thing). This new engine will not depend on FreeType2 explicitly and will be developed as a separate project. We plan to announce it in a few weeks with all gory details, once the definitive 2.0 release of FreeType has been made.
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