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+ FREETYPE 2 FAQ+ |
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| + General + Compilation & + Configuration + The FreeType 2 + auto-hinter + Other + | + FreeType 2 + documentation + | +
+ Table of Contents+ +
+ + + + General questions + ++ + ++ What is FreeType 2? ++ +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’: + +
+ + + + What can I do with FreeType 2? ++ +FreeType 2 is already used in many products. For example, it + serves as a font rendering engine + +
Generally speaking, the library allows you to access and manage + the contents of font files in a very easy way. + ++ + + + What can I not do with FreeType 2? ++ +FreeType 2 doesn't try to perform a number of sophisticated + things, because it focuses on being an excellent font + service. + +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: + +
+ + + + How portable is FreeType 2? ++ +The FreeType 2 source code is extremely portable for the + following reasons: + +
We have made great efforts to ensure that the library is efficient, + compact, and customizable. + ++ + + + What are the differences between FreeType 1.x and + FreeType 2? ++ +The biggest differences are as follows. + +
+ + + + Is FreeType 2 backwards compatible with FreeType 1.x? ++ +Short answer: No. However, transition from 1.x to 2 should be + rather straightforward. + +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. + ++ + + + Can I use FreeType 2 to edit fonts or create new ones? ++ +No. The library has been specifically designed to read + font files with small code size and very low memory usage. + +A good, freely available font editor is FontForge. + + + ++ + + + Compilation & Configuration + ++ + ++ How do I compile the FreeType 2 library? ++ +The library can be compiled in various ways, and detailed + documentation is available in documentation directory of the + FreeType 2 source tree. + +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. + ++ + + + How do I configure my build of the library? ++ +This is fully described in the file CUSTOMIZATION in + FreeType's documentation directory. Basically, you have to edit the + header file ftoption.h for compile-time options and to select + the modules with the file modules.cfg. Finally, it is + possible to replace the standard system interface (dealing with memory + allocation and stream I/O) with a custom one. + ++ + + + Why does FreeType render differently on different platforms? ++ +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 + ftoption.h. Compare each platform's copy of + ftoption.h to find the differences. + + + ++ + + + The FreeType 2 auto-hinter + ++ + ++ How does the auto-hinter work? ++ +Please note that the name of auto-hinter module is + autofit, which is a reimplementation of the old autohint + module. + +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 Real-Time + Grid Fitting of Typographic Outlines. + +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 FT_LOAD_FORCE_AUTOHINT bit flag + when calling FT_Load_Glyph(). + +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. + + + + + + Why doesn't the auto-hinter work well with my script? ++ +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! + + + ++ + + + Other questions + ++ + ++ Can I use FreeType to draw text on a pixmap with arbitrary 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 the 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 + colour, fill pattern or fill image if you want to. This process is + called direct rendering. + +A complete example is given at the end of the FreeType 2 tutorial. + + 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. + ++ + + + How can I set the colour of text rendered by FreeType? ++ +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. + +Note that for anti-aliased glyphs, you can ‘set the + colour’ by using direct rendering as described in this answer. + ++ + + + I set the pixel size to 8×8, 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 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 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 generated glyph + bitmaps! + +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, generally results 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 deviate from + this rule. Today, it is not possible to connect the EM size to a + specific font ‘feature’ in a reliable way. + ++ + + + How can I compute the bounding box of a text string without loading + its glyphs before? ++ +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. + +It is probably best to use both a glyph and a metrics cache to + avoid recomputation. + ++ + + + 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 no + sub-sampling and 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. On the other hand, libArt has + been designed for general shape and 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. + ++ + + + When will FreeType 2 support OpenType? ++ +Well, the engine already reads OpenType/CFF files perfectly. What + it doesn't do is handling ‘OpenType Layout’ tables. + +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. However, + this code is no longer maintained, and we strongly advise to not use + it. + +For FreeType 2, we have decided that the layout operations + provided through these tables are better placed in a specific + text-layout library like Pango. + + + |
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| + General + Compilation & + Configuration + The FreeType 2 + auto-hinter + Other + | + FreeType + documentation + | +
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+
- FreeType Glyph Conventions -
+ -- Version 2.1 -
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- Copyright 1998-2000 David Turner (david@freetype.org)
- Copyright 2000 The FreeType Development Team (devel@freetype.org)
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A font file contains a set of glyphs; each one can be + stored as a bitmap, a vector representation, or any + other scheme (most scalable formats use a combination + of mathematical representation and control + data/programs). These glyphs can be stored in any + order in the font file, and are typically accessed + through a simple glyph index.
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The font file contains one or more tables, called + character maps (also called + ‘charmaps’ or ‘cmaps’ for + short), which are used to convert character codes for + a given encoding (e.g. ASCII, Unicode, DBCS, Big5, + etc.) into glyph indices relative to the font file. A + single font face may contain several charmaps. For + example, many TrueType fonts contain an Apple-specific + charmap as well as a Unicode charmap, which makes them + usable on both Mac and Windows platforms.
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3. Character and font metrics
+ +Each glyph image is associated with various metrics which + describe how to place and manage it when rendering text; + see section III for more. + Metrics relate to glyph placement, cursor advances as well + as text layout. They are extremely important to compute + the flow of text when rendering a string of text.
+ +Each scalable format also contains some global metrics, + expressed in notional (i.e. font) units, to describe some + properties of all glyphs in the same face. Examples for + global metrics are the maximum glyph bounding box, the + ascender, descender, and text height of the font.
+ +Non-scalable formats contain metrics also. However, they + only apply to a set of given character dimensions and + resolutions, and are usually expressed in pixels then.
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