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<!doctype html public "-//w3c//dtd html 4.0 transitional//en" "http://www.w3.org/TR/REC-html40/loose.dtd"><html><head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <meta name="Author" content="David Turner"> <title>The design of FreeType 2</title></head><body text="#000000" bgcolor="#FFFFFF" link="#0000EF" vlink="#51188E" alink="#FF0000"><h1 align=center> The design of FreeType 2</h1><h3 align=center> Copyright 1998-2000 David Turner (<a href="mailto:david@freetype.org">david@freetype.org</a>)<br> Copyright 2000 The FreeType Development Team (<a href="mailto:devel@freetype.org">devel@freetype.org</a>)</h3><center><table width="65%"><tr><td> <center> <table width="100%" border=0 cellpadding=5> <tr bgcolor="#CCFFCC" valign=center> <td align=center width="30%"> <a href="design-1.html">Previous</a> </td> <td align=center width="30%"> <a href="index.html">Contents</a> </td> <td align=center width="30%"> <a href="design-3.html">Next</a> </td> </tr> </table> </center> <p><hr></p> <table width="100%"> <tr bgcolor="#ccccee"><td> <h1> I. Components and APIs </h1> </td></tr> </table> <p>It's better to describe FreeType 2 as a collection of <em>components</em>. Each one of them is a more or less abstract part of the library that is in charge of one specific task. We will now explicit the connections and relationships between them.</p> <p>A first brief description of this system of components could be:</p> <ul> <li> <p>Client applications typically call the FreeType 2 <b>high-level API</b>, whose functions are implemented in a single component called the <em>Base Layer</em>.</p> </li> <li> <p>Depending on the context or the task, the base layer then calls one or more <em>module</em> components to perform the work. In most cases, the client application doesn't need to know which module was called.</p> </li> <li> <p>The base layer also contains a set of routines that are used for generic things like memory allocation, list processing, i/o stream parsing, fixed point computation, etc. these functions can also be called by a module at any time, and they form what is called the <b>low-level base API</b>.</p> </li> </ul> <p>This is illustrated by the following graphics (note that component entry points are represented as colored triangles):</p> <center> <img src="basic-design.png" width="394" height="313" alt="Basic FreeType design"> </center> <p>Now, a few additional things must be added to complete this picture:</p> <ul> <li> <p>Some parts of the base layer can be replaced for specific builds of the library, and can thus be considered as components themselves. This is the case for the <tt>ftsystem</tt> component, which is in charge of implementing memory management & input stream access, as well as <tt>ftinit</tt>, which is in charge of library initialization (i.e. implementing the <tt>FT_Init_FreeType()</tt> function).</p> </li> <li> <p>FreeType 2 comes also with a set of <em>optional components</em>, which can be used either as a convenience for client applications (e.g. the <tt>ftglyph</tt> component, used to provide a simple API to manage glyph images independently of their internal representation), or to access format-specific features (e.g. the <tt>ftmm</tt> component used to access and manage Multiple Masters data in Type 1 fonts).</p> </li> <li> <p>Finally, a module is capable of calling functions provided by another module. This is very useful to share code and tables between several font driver modules (for example, the <tt>truetype</tt> and <tt>cff</tt> modules both use the routines provided by the <tt>sfnt</tt> module).</p> </li> </ul> <p>Hence, a more complete picture would be:</p> <center> <img src="detailed-design.png" width="390" height="429" alt="Detailed FreeType design"> </center> <p>Please take note of the following important points:</p> <ul> <li> <p>An optional component can use either the high-level or base API. This is the case of <tt>ftglyph</tt> in the above picture.</p> </li> <li> <p>Some optional components can use module-specific interfaces ignored by the base layer. In the above example, <tt>ftmm</tt> directly accesses the Type 1 module to set/query data.</p> </li> <li> <p>A replacable component can provide a function of the high-level API. For example, <tt>ftinit</tt> provides <tt>FT_Init_FreeType()</tt> to client applications.</p> </li> </ul> <p><hr></p> <center> <table width="100%" border=0 cellpadding=5> <tr bgcolor="#CCFFCC" valign=center> <td align=center width="30%"> <a href="design-1.html">Previous</a> </td> <td align=center width="30%"> <a href="index.html">Contents</a> </td> <td align=center width="30%"> <a href="design-3.html">Next</a> </td> </tr> </table> </center></td></tr></table></center></body></html>⌨️ 快捷键说明
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