forked from minhngoc25a/freetype2
113 lines
3.9 KiB
HTML
113 lines
3.9 KiB
HTML
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<html>
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<head><title>The Design of FreeType 2 - Basic Design</title>
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<basefont face="Georgia, Arial, Helvetica, Geneva">
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<style content="text/css">
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P { text-align=justify }
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H1 { text-align=center }
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H2 { text-align=center }
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LI { text-align=justify }
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</style>
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</head>
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<body text=#000000 bgcolor=#ffffff>
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<center><table width="500"><tr><td>
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<center><h1>The Design of FreeType 2</h1></center>
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<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
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<h1>I. Components and APIs</h1>
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</td></tr></table>
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<p>It's better to describe FreeType 2 as a collection of
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<em>components</em>. Each one of them is a more or less abstract
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part of the library that is in charge of one specific task. We will
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now explicit the connections and relationships between them.</p>
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<p>A first brief description of this system of components could be:</p>
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<ul>
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<li><p>
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client applications typically call the FreeType 2 <b>high-level
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API</b>, whose functions are implemented in a single component
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called the <em>Base Layer</em>.
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</p></li>
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<li><p>
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depending on the context or the task, the base
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layer then calls one or more <em>module</em> components to
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perform the work. In most cases, the client application doesn't
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need to know what module was called.
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</p></li>
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<li><p>
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the base layer also contains a set of routines that are
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used for generic things like memory allocation, list
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processing, i/o stream parsing, fixed point computation,
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etc.. these functions can also be called by a module
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at any time, and they form what is called the <b>low-level
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base API</b>.
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</p></li>
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</ul>
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<p>This is illustrated by the following graphics (note that component
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entry points are represented as colored triangles):</p>
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<center><img src="basic-design.png" width="394" height="313"></center>
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<p>Now, a few additional things must be added to complete this picture:</p>
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<ul>
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<li><p>some parts of the base layer can be replaced for specific builds
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of the library, and can thus be considered as components themselves.
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this is the case for the <b>ftsystem</b> component, which is in
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charge of implementing memory management & input stream access,
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as well as the <b>ftinit</b>, which is in charge of library
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initialisation (i.e. implementing <tt>FT_Init_FreeType</tt>).
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</p></li>
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<li><p>
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FreeType 2 comes also with a set of <em>optional components</em>,
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which can be used either as a convenience for client applications
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(e.g. the <b>ftglyph</b> component, used to provide a simple API
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to manage glyph images independently of their internal representation),
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or to access format-specific features (e.g. the <b>ftmm</b> component
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used to access and manage Multiple Masters data in Type 1 fonts)
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</p></li>
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<li><p>
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Finally, a module is capable of calling functions provided by
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another module. This is very useful to share code and tables
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between several font driver modules (for example, the <tt>truetype</tt>
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and <tt>cff</tt> both use the routines provided by the <tt>sfnt</tt>
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module).
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</p></li>
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</ul>
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<p>Hence, a more complete picture would be:</p>
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<center><img src="detailed-design.png" width="390" height="429"></center>
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<p>Please take note of the following important points:</p>
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<ul>
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<li><p>
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an optional component can use either the high-level or base
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API. This is the case of <b>ftglyph</b> in the above picture.
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</p></li>
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<li><p>
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some optional component can use module-specific interfaces
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ignored by the base layer. In the above example, <b>ftmm</b>
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directly accesses the Type 1 module to set/query data
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</p></li>
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<li><p>
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a replacable component can provide a function of the high-level
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API. For example, <b>ftinit</b> provides <tt>FT_Init_FreeType</tt>
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to client applications.
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</p></li>
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</ul>
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</td></tr></table></center>
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</body>
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</html>
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