957 lines
35 KiB
HTML
957 lines
35 KiB
HTML
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<html>
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<head>
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<meta http-equiv="Content-Type"
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content="text/html; charset=iso-8859-1">
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<meta name="Author"
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content="David Turner">
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<title>FreeType 2 Tutorial</title>
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</head>
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<body text="#000000"
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bgcolor="#FFFFFF"
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link="#0000EF"
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vlink="#51188E"
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alink="#FF0000">
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<h1 align=center>
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FreeType 2.0 Tutorial<br>
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Step 1 - simple glyph loading
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</h1>
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<h3 align=center>
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© 2000 David Turner
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(<a href="mailto:david@freetype.org">david@freetype.org</a>)<br>
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© 2000 The FreeType Development Team
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(<a href="http://www.freetype.org">www.freetype.org</a>)
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</h3>
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<center>
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<table width="70%">
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<tr><td>
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<hr>
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<h2>
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Introduction
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</h2>
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<p>This is the first section of the FreeType 2 tutorial. It will teach
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you to do the following:</p>
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<ul>
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<li>initialise the library</li>
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<li>open a font file by creating a new face object</li>
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<li>select a character size in points or in pixels</li>
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<li>load a single glyph image and convert it to a bitmap</li>
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<li>render a very simple string of text</li>
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<li>render a rotated string of text easily</li>
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</ul>
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<hr>
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<h3>
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1. Header files
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</h3>
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<p>To include the main FreeType header file, simply say</p>
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<font color="blue">
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<pre>
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#include <freetype/freetype.h></pre>
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</font>
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<p>in your application code. Note that other files are available in the
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FreeType include directory, most of them being included by
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<tt>"freetype.h"</tt>. They will be described later in this
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tutorial.</p>
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<hr>
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<h3>
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2. Initialize the library
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</h3>
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<p>Simply create a variable of type <tt>FT_Library</tt> named, for
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example, <tt>library</tt>, and call the function
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<tt>FT_Init_FreeType()</tt> as in</p>
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<font color="blue">
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<pre>
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#include <freetype/freetype.h>
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FT_Library library;
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...
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{
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...
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error = FT_Init_FreeType( &library );
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if ( error )
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{
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... an error occurred during library initialization ...
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}
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}</pre>
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</font>
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<p>This function is in charge of the following:</p>
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<ul>
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<li>
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<p>Creating a new instance of the FreeType 2 library, and set
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the handle <tt>library</tt> to it.</p>
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</li>
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<li>
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<p>Load each modules that FreeType knows about in the library.
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This means that by default, your new <tt>library</tt> object is able
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to handle TrueType, Type 1, CID-keyed & OpenType/CFF fonts
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gracefully.</p>
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</li>
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</ul>
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<p>As you can see, the function returns an error code, like most others
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in the FreeType API. An error code of 0 <em>always</em> means that
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the operation was successful; otherwise, the value describes the error,
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and <tt>library</tt> is set to NULL.</p>
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<hr>
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<h3>
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3. Load a font face
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</h3>
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<h4>
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a. From a font file
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</h4>
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<p>Create a new <em>face</em> object by calling <tt>FT_New_Face</tt>.
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A <em>face</em> describes a given typeface and style. For example,
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"Times New Roman Regular" and "Times New Roman Italic" correspond to
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two different faces.</p>
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<font color="blue">
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<pre>
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FT_Library library; /* handle to library */
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FT_Face face; /* handle to face object */
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error = FT_Init_FreeType( &library );
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if ( error ) { ... }
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error = FT_New_Face( library,
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"/usr/share/fonts/truetype/arial.ttf",
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0,
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&face );
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if ( error == FT_Err_Unknown_File_Format )
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{
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... the font file could be opened and read, but it appears
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... that its font format is unsupported
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}
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else if ( error )
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{
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... another error code means that the font file could not
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... be opened or read, or simply that it is broken...
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}</pre>
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</font>
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<p>As you can certainly imagine, <tt>FT_New_Face</tt> opens a font
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file, then tries to extract one face from it. Its parameters are</p>
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<table cellpadding=5>
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<tr valign="top">
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<td>
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<tt><b>library</b></tt>
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</td>
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<td>
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<p>handle to the FreeType library instance where the face object
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is created</p>
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</td>
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</tr>
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<tr valign="top">
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<td>
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<tt><b>filepathname</b></tt>
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</td>
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<td>
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<p>the font file pathname (standard C string).</p>
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</td>
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</tr>
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<tr valign="top">
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<td>
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<tt><b>face_index</b></tt>
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</td>
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<td>
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<p>Certain font formats allow several font faces to be embedded
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in a single file.</p>
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<p>This index tells which face you want to load. An error will
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be returned if its value is too large.</p>
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<p>Index 0 always work though.</p>
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</td>
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</tr>
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<tr valign="top">
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<td>
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<tt><b>face</b></tt>
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</td>
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<td>
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<p>A <em>pointer</em> to the handle that will be set to describe
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the new face object.</p>
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<p>It is set to NULL in case of error.</p>
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</td>
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</tr>
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</table>
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<p>To know how many faces a given font file contains, simply load its
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first face (use <tt>face_index</tt>=0), then see the value of
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<tt>face->num_faces</tt> which indicates how many faces are embedded
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in the font file.</p>
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<h4>
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b. From memory
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</h4>
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<p>In the case where you have already loaded the font file in memory,
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you can similarly create a new face object for it by calling
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<tt>FT_New_Memory_Face</tt> as in</p>
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<font color="blue">
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<pre>
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FT_Library library; /* handle to library */
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FT_Face face; /* handle to face object */
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error = FT_Init_FreeType( &library );
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if ( error ) { ... }
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error = FT_New_Memory_Face( library,
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buffer, /* first byte in memory */
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size, /* size in bytes */
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0, /* face_index */
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&face );
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if ( error ) { ... }</pre>
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</font>
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<p>As you can see, <tt>FT_New_Memory_Face()</tt> simply takes a
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pointer to the font file buffer and its size in bytes instead of a
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file pathname. Other than that, it has exactly the same semantics as
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<tt>FT_New_Face()</tt>.</p>
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<h4>
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c. From other sources (compressed files, network, etc.)
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</h4>
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<p>There are cases where using a file pathname or preloading the file
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in memory is simply not enough. With FreeType 2, it is possible
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to provide your own implementation of i/o routines.</p>
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<p>This is done through the <tt>FT_Open_Face()</tt> function, which
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can be used to open a new font face with a custom input stream, select
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a specific driver for opening, or even pass extra parameters to the
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font driver when creating the object. We advise you to refer to the
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FreeType 2 reference manual in order to learn how to use it.</p>
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<p>Note that providing a custom stream might also be used to access a
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TrueType font embedded in a Postscript Type 42 wrapper.</p>
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<hr>
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<h3>
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4. Accessing face content
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</h3>
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<p>A <em>face object</em> models all information that globally describes
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the face. Usually, this data can be accessed directly by dereferencing
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a handle, like</p>
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<table cellpadding=5>
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<tr valign="top">
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<td>
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<tt><b>face->num_glyphs</b></tt>
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</td>
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<td>
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<p>Gives the number of <em>glyphs</em> available in the font face.
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A glyph is simply a character image. It doesn't necessarily
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correspond to a <em>character code</em> though.</p>
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</td>
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</tr>
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<tr valign="top">
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<td>
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<tt><b>face->flags</b></tt>
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</td>
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<td>
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<p>A 32-bit integer containing bit flags used to describe some
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face properties. For example, the flag
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<tt>FT_FACE_FLAG_SCALABLE</tt> is used to indicate that the face's
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font format is scalable and that glyph images can be rendered for
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all character pixel sizes. For more information on face flags,
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please read the <a href="#">FreeType 2 API Reference</a>.</p>
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</td>
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</tr>
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<tr valign="top">
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<td>
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<tt><b>face->units_per_EM</b></tt>
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</td>
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<td>
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<p>This field is only valid for scalable formats (it is set to 0
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otherwise). It indicates the number of font units covered by the
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EM.</p>
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</td>
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</tr>
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<tr valign="top">
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<td>
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<tt><b>face->num_fixed_sizes</b></tt>
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</td>
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<td>
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<p>This field gives the number of embedded bitmap <em>strikes</em>
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in the current face. A <em>strike</em> is simply a series of
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glyph images for a given character pixel size. For example, a
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font face could include strikes for pixel sizes 10, 12
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and 14. Note that even scalable font formats can have
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embedded bitmap strikes!</p>
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</td>
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</tr>
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<tr valign="top">
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<td>
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<tt><b>face->fixed_sizes</b></tt>
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</td>
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<td>
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<p>this is a pointer to an array of <tt>FT_Bitmap_Size</tt>
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elements. Each <tt>FT_Bitmap_Size</tt> indicates the horizontal
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and vertical <em>pixel sizes</em> for each of the strikes that are
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present in the face.</p>
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</td>
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</tr>
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</table>
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<p>For a complete listing of all face properties and fields, please read
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the <a href="#">FreeType 2 API Reference</a>.<p>
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<hr>
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<h3>
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5. Setting the current pixel size
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</h3>
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<p>FreeType 2 uses "<em>size objects</em>" to model all
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information related to a given character size for a given face.
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For example, a size object will hold the value of certain metrics
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like the ascender or text height, expressed in 1/64th of a pixel,
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for a character size of 12 points.</p>
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<p>When the <tt>FT_New_Face</tt> function is called (or one of its
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cousins), it <b>automatically</b> creates a new size object for
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the returned face. This size object is directly accessible as
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<b><tt>face->size</tt></b>.</p>
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<p><em>NOTA BENE: a single face object can deal with one or more size
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objects at a time, however, this is something that few programmers
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really need to do. We have thus have decided to simplify the API for
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the most common use (i.e. one size per face), while keeping this
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feature available through additional fuctions.</em></p>
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<p>When a new face object is created, its size object defaults to the
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character size of 10 pixels (both horizontally and vertically) for
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scalable formats. For fixed-sizes formats, the size is more or less
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undefined, which is why you must set it before trying to load a
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glyph.</p>
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<p>To do that, simply call <tt>FT_Set_Char_Size()</tt>. Here is an
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example where the character size is set to 16pt for a 300x300 dpi
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device:</p>
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<font color="blue">
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<pre>
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error = FT_Set_Char_Size(
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face, /* handle to face object */
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0, /* char_width in 1/64th of points */
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16*64, /* char_height in 1/64th of points */
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300, /* horizontal device resolution */
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300 ); /* vertical device resolution */</pre>
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</font>
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<p>You will notice that:</p>
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<ul>
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<li>
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<p>The character width and heights are specified in 1/64th of
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points. A point is a <em>physical</em> distance, equaling 1/72th
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of an inch, it's not a pixel..<p>
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</li>
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<li>
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<p>The horizontal and vertical device resolutions are expressed in
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<em>dots-per-inch</em>, or <em>dpi</em>. You can use 72 or
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96 dpi for display devices like the screen. The resolution
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is used to compute the character pixel size from the character
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point size.</p>
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</li>
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<li>
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<p>A value of 0 for the character width means "<em>same as
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character height</em>", a value of 0 for the character height
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means "<em>same as character width</em>". Otherwise, it is possible
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to specify different char widths and heights.</p>
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</li>
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<li>
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<p>Using a value of 0 for the horizontal or vertical resolution means
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72 dpi, which is the default.</p>
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</li>
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<li>
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<p>The first argument is a handle to a face object, not a size
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object. That's normal, and must be seen as a convenience.</p>
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</li>
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</ul>
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<p>This function computes the character pixel size that corresponds to
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the character width and height and device resolutions. However, if you
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want to specify the pixel sizes yourself, you can simply call
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<tt>FT_Set_Pixel_Sizes()</tt>, as in</p>
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<font color="blue">
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<pre>
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error = FT_Set_Pixel_Sizes(
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face, /* handle to face object */
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0, /* pixel_width */
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16 ); /* pixel_height */</pre>
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</font>
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<p>This example will set the character pixel sizes to 16x16 pixels.
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As previously, a value of 0 for one of the dimensions means
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"<em>same as the other</em>".</p>
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<p>Note that both functions return an error code. Usually, an error
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occurs with a fixed-size font format (like FNT or PCF) when trying to
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set the pixel size to a value that is not listed in the
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<tt><b>face->fixed_sizes</b></tt> array.</p>
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<hr>
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<h3>
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6. Loading a glyph image
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</h3>
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<h4>
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a. Converting a character code into a glyph index
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</h4>
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<p>Usually, an application wants to load a glyph image based on its
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<em>character code</em>, which is a unique value that defines the
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character for a given <em>encoding</em>. For example, the character
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code 65 represents the `A' in ASCII encoding.</p>
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<p>A face object contains one or more tables, called
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<em>charmaps</em>, that are used to convert character codes to glyph
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indices. For example, most TrueType fonts contain two charmaps. One
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is used to convert Unicode character codes to glyph indices, the other
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is used to convert Apple Roman encoding into glyph indices. Such
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fonts can then be used either on Windows (which uses Unicode) and
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Macintosh (which uses Apple Roman, bwerk). Note also that a given
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charmap might not map to all the glyphs present in the font.</p>
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<p>By default, when a new face object is created, it lists all the
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charmaps contained in the font face and selects the one that supports
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Unicode character codes if it finds one. Otherwise, it tries to find
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support for Latin-1, then ASCII.</p>
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<p>We will describe later how to look for specific charmaps in a face.
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For now, we will assume that the face contains at least a Unicode
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charmap that was selected during <tt>FT_New_Face()</tt>. To convert a
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Unicode character code to a font glyph index, we use
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<tt>FT_Get_Char_Index()</tt> as in</p>
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<font color="blue">
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<pre>
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glyph_index = FT_Get_Char_Index( face, charcode );</pre>
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</font>
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<p>This will look the glyph index corresponding to the given
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<tt>charcode</tt> in the charmap that is currently selected for the
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face. If charmap is selected, the function simply returns the
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charcode.</p>
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<p>Note that this is one of the rare FreeType functions that do not
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return an error code. However, when a given character code has no
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glyph image in the face, the value 0 is returned. By convention,
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it always correspond to a special glyph image called the <b>missing
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glyph</b>, which usually is represented as a box or a space.</p>
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<h4>
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b. Loading a glyph from the face
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</h4>
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<p>Once you have a glyph index, you can load the corresponding glyph
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image. The latter can be stored in various formats within the font file.
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For fixed-size formats like FNT or PCF, each image is a bitmap. Scalable
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formats like TrueType or Type 1 use vectorial shapes, named "outlines"
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to describe each glyph. Some formats may have even more exotic ways
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of representing glyph (e.g. MetaFont). Fortunately, FreeType 2 is
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flexible enough to support any kind of glyph format through
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a simple API.</p>
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<p>The glyph image is always stored in a special object called a
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<em>glyph slot</em>. As its name suggests, a glyph slot is simply a
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container that is able to hold one glyph image at a time, be it a
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bitmap, an outline, or something else. Each face object has a single
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glyph slot object that can be accessed as
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<b><tt>face->glyph</tt></b>.</p>
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<p>Loading a glyph image into the slot is performed by calling
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<tt>FT_Load_Glyph()</tt> as in</p>
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<font color="blue">
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<pre>
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error = FT_Load_Glyph(
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face, /* handle to face object */
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glyph_index, /* glyph index */
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load_flags ); /* load flags, see below */</pre>
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</font>
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<p>The <tt>load_flags</tt> value is a set of bit flags used to
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indicate some special operations. The default value
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<tt>FT_LOAD_DEFAULT</tt> is 0.</p>
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<p>This function will try to load the corresponding glyph image
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from the face. Basically, this means that:</p>
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<ul>
|
|
<li>
|
|
<p>If a bitmap is found for the corresponding glyph and pixel
|
|
size, it will be loaded into the slot (embedded bitmaps are always
|
|
favored over native image formats, because we assume that
|
|
they are higher-quality versions of the same glyph. This
|
|
can be ignored by using the FT_LOAD_NO_BITMAP flag)</p>
|
|
</li>
|
|
|
|
<li>
|
|
<p>Otherwise, a native image for the glyph will be loaded.
|
|
It will also be scaled to the current pixel size, as
|
|
well as hinted for certain formats like TrueType and
|
|
Type1.</p>
|
|
</li>
|
|
</ul>
|
|
|
|
<p>The field <tt><b>glyph->format</b></tt> describe the format
|
|
used to store the glyph image in the slot. If it is not
|
|
<tt>ft_glyph_format_bitmap</tt>, one can immediately
|
|
convert it to a bitmap through <tt>FT_Render_Glyph</tt>,
|
|
as in:</p>
|
|
|
|
<font color="blue">
|
|
<pre>
|
|
error = FT_Render_Glyph(
|
|
face->glyph, /* glyph slot */
|
|
render_mode ); /* render mode */
|
|
</pre>
|
|
</font>
|
|
|
|
<p>The parameter <tt>render_mode</tt> is a set of bit flags used
|
|
to specify how to render the glyph image. Set it to 0, or the
|
|
equivalent <tt>ft_render_mode_normal</tt> to render a high-quality
|
|
anti-aliased (256 gray levels) bitmap, as this is the default.
|
|
You can alternatively use <tt>ft_render_mode_mono</tt> if you
|
|
want to generate a 1-bit monochrome bitmap.</p>
|
|
|
|
<p>Once you have a bitmapped glyph image, you can access it directly
|
|
through <tt><b>glyph->bitmap</b></tt> (a simple bitmap descriptor),
|
|
and position it through <tt><b>glyph->bitmap_left</b></tt> and
|
|
<tt><b>glyph->bitmap_top</b></tt>.</p>
|
|
|
|
<p>Note that <tt>bitmap_left</tt> is the horizontal distance from the
|
|
current pen position to the left-most border of the glyph bitmap,
|
|
while <tt>bitmap_top</tt> is the vertical distance from the
|
|
pen position (on the baseline) to the top-most border of the
|
|
glyph bitmap. <em>It is positive to indicate an upwards
|
|
distance</em>.</p>
|
|
|
|
<p>The next section will detail the content of a glyph slot and
|
|
how to access specific glyph information (including metrics).</p>
|
|
|
|
<h4>
|
|
c. Using other charmaps
|
|
</h4>
|
|
|
|
<p>As said before, when a new face object is created, it will look for
|
|
a Unicode, Latin-1, or ASCII charmap and select it. The currently
|
|
selected charmap is accessed via <b><tt>face->charmap</tt></b>. This
|
|
field is NULL when no charmap is selected, which typically happens
|
|
when you create a new <tt>FT_Face</tt> object from a font file that
|
|
doesn't contain an ASCII, Latin-1, or Unicode charmap (rare
|
|
stuff).</p>
|
|
|
|
<p>There are two ways to select a different charmap with FreeType 2.
|
|
The easiest is when the encoding you need already has a corresponding
|
|
enumeration defined in <tt><freetype/freetype.h></tt>, as
|
|
<tt>ft_encoding_big5</tt>. In this case, you can simply call
|
|
<tt>FT_Select_CharMap</tt> as in:</p>
|
|
|
|
<font color="blue"><pre>
|
|
error = FT_Select_CharMap(
|
|
face, /* target face object */
|
|
ft_encoding_big5 ); /* encoding.. */
|
|
</pre></font>
|
|
|
|
<p>Another way is to manually parse the list of charmaps for the
|
|
face, this is accessible through the fields
|
|
<tt><b>num_charmaps</b></tt> and <tt><b>charmaps</b></tt>
|
|
(notice the 's') of the face object. As you could expect,
|
|
the first is the number of charmaps in the face, while the
|
|
second is <em>a table of pointers to the charmaps</em>
|
|
embedded in the face.</p>
|
|
|
|
<p>Each charmap has a few visible fields used to describe it more
|
|
precisely. Mainly, one will look at
|
|
<tt><b>charmap->platform_id</b></tt> and
|
|
<tt><b>charmap->encoding_id</b></tt> that define a pair of
|
|
values that can be used to describe the charmap in a rather
|
|
generic way.</p>
|
|
|
|
<p>Each value pair corresponds to a given encoding. For example,
|
|
the pair (3,1) corresponds to Unicode. Their list is
|
|
defined in the TrueType specification but you can also use the
|
|
file <tt><freetype/ftnameid.h></tt> which defines several
|
|
helpful constants to deal with them..</p>
|
|
|
|
<p>To look for a specific encoding, you need to find a corresponding
|
|
value pair in the specification, then look for it in the charmaps
|
|
list. Don't forget that some encoding correspond to several
|
|
values pair (yes it's a real mess, but blame Apple and Microsoft
|
|
on such stupidity..). Here's some code to do it:</p>
|
|
|
|
<font color="blue">
|
|
<pre>
|
|
FT_CharMap found = 0;
|
|
FT_CharMap charmap;
|
|
int n;
|
|
|
|
for ( n = 0; n < face->num_charmaps; n++ )
|
|
{
|
|
charmap = face->charmaps[n];
|
|
if ( charmap->platform_id == my_platform_id &&
|
|
charmap->encoding_id == my_encoding_id )
|
|
{
|
|
found = charmap;
|
|
break;
|
|
}
|
|
}
|
|
|
|
if ( !found ) { ... }
|
|
|
|
/* now, select the charmap for the face object */
|
|
error = FT_Set_CharMap( face, found );
|
|
if ( error ) { ... }</pre>
|
|
</font>
|
|
|
|
<p>Once a charmap has been selected, either through
|
|
<tt>FT_Select_CharMap</tt> or <tt>FT_Set_CharMap</tt>,
|
|
it is used by all subsequent calls to
|
|
<tt>FT_Get_Char_Index()</tt>.</p>
|
|
|
|
|
|
<h4>
|
|
d. Glyph Transforms:
|
|
</h4>
|
|
|
|
<p>It is possible to specify an affine transformation to be applied
|
|
to glyph images when they're loaded. Of course, this will only
|
|
work for scalable (vectorial) font formats.</p>
|
|
|
|
<p>To do that, simply call <tt>FT_Set_Transform</tt>, as in:</p>
|
|
|
|
<font color="blue"><pre>
|
|
error = FT_Set_Transform(
|
|
face, /* target face object */
|
|
&matrix, /* pointer to 2x2 matrix */
|
|
&delta ); /* pointer to 2d vector */
|
|
</pre></font>
|
|
|
|
<p>This function will set the current transform for a given face
|
|
object. Its second parameter is a pointer to a simple
|
|
<tt>FT_Matrix</tt> structure that describes a 2x2 affine matrix.
|
|
The third parameter is a pointer to a <tt>FT_Vector</tt> structure
|
|
that describe a simple 2d vector that is used to translate the
|
|
glyph image <em>after</em> the 2x2 transform.</p>
|
|
|
|
<p>Note that the matrix pointer can be set to NULL, (in which case
|
|
the identity transform will be used). Coefficients of the matrix
|
|
are otherwise in 16.16 fixed float units.</p>
|
|
|
|
<p>The vector pointer can also be set to NULL (in which case a delta
|
|
of (0,0) will be used). The vector coordinates are expressed in
|
|
1/64th of a pixel (also known as 26.6 fixed floats).</p>
|
|
|
|
<font color="red">
|
|
<p>NOTA BENE: The transform is applied to every glyph that is loaded
|
|
through <tt>FT_Load_Glyph</tt> and is <b>completely independent
|
|
of any hinting process.</b> This means that you won't get the same
|
|
results if you load a glyph at the size of 24 pixels, or a glyph at
|
|
the size at 12 pixels scaled by 2 through a transform, because the
|
|
hints will have been computed differently (unless, of course you
|
|
disabled hints).</em></p></font>
|
|
|
|
<p>If you ever need to use a non-orthogonal transform with optimal
|
|
hints, you first need to decompose your transform into a scaling part
|
|
and a rotation/shearing part. Use the scaling part to compute a new
|
|
character pixel size, then the other one to call FT_Set_Transform.
|
|
This is explained in details in a later section of this tutorial.</p>
|
|
|
|
<p>Note also that loading a glyph bitmap with a non-identity transform
|
|
will produce an error..</p>
|
|
<hr>
|
|
|
|
<h3>
|
|
7. Simple Text Rendering:
|
|
</h3>
|
|
|
|
<p>We will now present you with a very simple example used to render
|
|
a string of 8-bit Latin-1 text, assuming a face that contains a
|
|
Unicode charmap</p>
|
|
|
|
<p>The idea is to create a loop that will, on each iteration, load one
|
|
glyph image, convert it to an anti-aliased bitmap, draw it on the
|
|
target surface, then increment the current pen position</p>
|
|
|
|
<h4>a. basic code :</h4>
|
|
|
|
<p>The following code performs our simple text rendering with the
|
|
functions previously described.</p>
|
|
|
|
<font color="blue"><pre>
|
|
FT_GlyphSlot slot = face->glyph; // a small shortcut
|
|
int pen_x, pen_y, n;
|
|
|
|
.. initialise library ..
|
|
.. create face object ..
|
|
.. set character size ..
|
|
|
|
pen_x = 300;
|
|
pen_y = 200;
|
|
|
|
for ( n = 0; n < num_chars; n++ )
|
|
{
|
|
FT_UInt glyph_index;
|
|
|
|
// retrieve glyph index from character code
|
|
glyph_index = FT_Get_Char_Index( face, text[n] );
|
|
|
|
// load glyph image into the slot (erase previous one)
|
|
error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT );
|
|
if (error) continue; // ignore errors
|
|
|
|
// convert to an anti-aliased bitmap
|
|
error = FT_Render_Glyph( face->glyph, ft_render_mode_normal );
|
|
if (error) continue;
|
|
|
|
// now, draw to our target surface
|
|
my_draw_bitmap( &slot->bitmap,
|
|
pen_x + slot->bitmap_left,
|
|
pen_y - slot->bitmap_top );
|
|
|
|
// increment pen position
|
|
pen_x += slot->advance.x >> 6;
|
|
pen_y += slot->advance.y >> 6; // unuseful for now..
|
|
}
|
|
</pre></font>
|
|
|
|
<p>This code needs a few explanations:</p>
|
|
<ul>
|
|
<li><p>
|
|
we define a handle named <tt>slot</tt> that points to the
|
|
face object's glyph slot. (the type <tt>FT_GlyphSlot</tt> is
|
|
a pointer). That's a convenience to avoid using
|
|
<tt>face->glyph->XXX</tt> every time.
|
|
</p></li>
|
|
|
|
<li><p>
|
|
we increment the pen position with the vector <tt>slot->advance</tt>,
|
|
which correspond to the glyph's <em>advance width</em> (also known
|
|
as its <em>escapement</em>). The advance vector is expressed in
|
|
64/th of pixels, and is truncated to integer pixels on each
|
|
iteration.</p>
|
|
</p></li>
|
|
|
|
<li><p>
|
|
The function <tt>my_draw_bitmap</tt> is not part of FreeType, but
|
|
must be provided by the application to draw the bitmap to the target
|
|
surface. In this example, it takes a pointer to a FT_Bitmap descriptor
|
|
and the position of its top-left corner as arguments.
|
|
</p></li>
|
|
|
|
<li><p>
|
|
The value of <tt>slot->bitmap_top</tt> is positive for an
|
|
<em>upwards</em> vertical distance. Assuming that the coordinates
|
|
taken by <tt>my_draw_bitmap</tt> use the opposite convention
|
|
(increasing Y corresponds to downwards scanlines), we substract
|
|
it to <tt>pen_y</tt>, instead of adding it..
|
|
</p></li>
|
|
|
|
</ul>
|
|
|
|
<h4>b. refined code:</h4>
|
|
|
|
<p>The following code is a refined version of the example above. It
|
|
uses features and functions of FreeType 2 that have not yet been
|
|
introduced, and they'll be explained below:</p>
|
|
|
|
<font color="blue"><pre>
|
|
FT_GlyphSlot slot = face->glyph; // a small shortcut
|
|
FT_UInt glyph_index;
|
|
int pen_x, pen_y, n;
|
|
|
|
.. initialise library ..
|
|
.. create face object ..
|
|
.. set character size ..
|
|
|
|
pen_x = 300;
|
|
pen_y = 200;
|
|
|
|
for ( n = 0; n < num_chars; n++ )
|
|
{
|
|
// load glyph image into the slot (erase previous one)
|
|
error = FT_Load_Char( face, text[n], FT_LOAD_RENDER );
|
|
if (error) continue; // ignore errors
|
|
|
|
// now, draw to our target surface
|
|
my_draw_bitmap( &slot->bitmap,
|
|
pen_x + slot->bitmap_left,
|
|
pen_y - slot->bitmap_top );
|
|
|
|
// increment pen position
|
|
pen_x += slot->advance.x >> 6;
|
|
}
|
|
</pre></font>
|
|
|
|
<p>We've reduced the size of our code, but it does exactly the same thing,
|
|
as:</p>
|
|
|
|
<ul>
|
|
<li><p>
|
|
We use the function <tt><b>FT_Load_Char</b></tt> instead of
|
|
<tt>FT_Load_Glyph</tt>. As you probably imagine, it's equivalent
|
|
to calling <tt>FT_Get_Char_Index</tt> then <tt>FT_Get_Load_Glyph</tt>.
|
|
</p></li>
|
|
|
|
<li><p>
|
|
We do not use <tt>FT_LOAD_DEFAULT</tt> for the loading mode, but
|
|
the bit flag <tt><b>FT_LOAD_RENDER</b></tt>. It indicates that
|
|
the glyph image must be immediately converted to an anti-aliased
|
|
bitmap. This is of course a shortcut that avoids calling
|
|
<tt>FT_Render_Glyph</tt> explicitely but is strictly equivalent.</p>
|
|
|
|
<p>
|
|
Note that you can also specify that you want a monochrome bitmap
|
|
instead by using the addition <tt><b>FT_LOAD_MONOCHROME</b></tt>
|
|
load flag.
|
|
</p></li>
|
|
</ul>
|
|
|
|
<h4>c. more advanced rendering:</h4>
|
|
|
|
<p>Let's try to render transformed text now (for example through a
|
|
rotation). We can do this using <tt>FT_Set_Transform</tt>. Here's
|
|
how to do it:</p>
|
|
|
|
<font color="blue"><pre>
|
|
FT_GlyphSlot slot = face->glyph; // a small shortcut
|
|
FT_Matrix matrix; // transformation matrix
|
|
FT_UInt glyph_index;
|
|
FT_Vector pen; // untransformed origin
|
|
int pen_x, pen_y, n;
|
|
|
|
.. initialise library ..
|
|
.. create face object ..
|
|
.. set character size ..
|
|
|
|
// set up matrix
|
|
matrix.xx = (FT_Fixed)( cos(angle)*0x10000);
|
|
matrix.xy = (FT_Fixed)(-sin(angle)*0x10000);
|
|
matrix.yx = (FT_Fixed)( sin(angle)*0x10000);
|
|
matrix.yy = (FT_Fixed)( cos(angle)*0x10000);
|
|
|
|
// the pen position in 26.6 cartesian space coordinates
|
|
pen.x = 300 * 64;
|
|
pen.y = ( my_target_height - 200 ) * 64;
|
|
|
|
for ( n = 0; n < num_chars; n++ )
|
|
{
|
|
// set transform
|
|
FT_Set_Transform( face, &matrix, &pen );
|
|
|
|
// load glyph image into the slot (erase previous one)
|
|
error = FT_Load_Char( face, text[n], FT_LOAD_RENDER );
|
|
if (error) continue; // ignore errors
|
|
|
|
// now, draw to our target surface (convert position)
|
|
my_draw_bitmap( &slot->bitmap,
|
|
slot->bitmap_left,
|
|
my_target_height - slot->bitmap_top );
|
|
|
|
// increment pen position
|
|
pen.x += slot->advance.x;
|
|
pen.y += slot->advance.y;
|
|
}
|
|
</pre></font>
|
|
|
|
<p>You'll notice that:</p>
|
|
|
|
<ul>
|
|
<li><p>
|
|
we now use a vector, of type <tt>FT_Vector</tt> to store the pen
|
|
position, with coordinates expressed as 1/64th of pixels, hence
|
|
a multiplication. The position is expressed in cartesian space.
|
|
</p></li>
|
|
|
|
<li><p>
|
|
glyph images are always loaded, transformed and described in the
|
|
cartesian coordinate system in FreeType (which means that
|
|
increasing Y corresponds to upper scanlines), unlike the system
|
|
typically used for bitmaps (where the top-most scanline has
|
|
coordinate 0). We must thus convert between the two systems
|
|
when we define the pen position, and when we compute the top-left
|
|
position of the bitmap.
|
|
</p></li>
|
|
|
|
<li><p>
|
|
we set the transform on each glyph, to indicate the rotation
|
|
matrix, as well as a delta that will move the transformed image
|
|
to the current pen position (in cartesian space, not bitmap space).
|
|
</p></li>
|
|
|
|
<li><p>
|
|
the advance is always returned transformed, which is why it can
|
|
be directly added to the current pen position. Note that it is
|
|
<b>not</b> rounded this time.
|
|
</p></li>
|
|
|
|
</ul>
|
|
|
|
<p>It is important to note that, while this example is a bit more
|
|
complex than the previous one, it is strictly equivalent
|
|
for the case where the transform is the identity.. Hence it can
|
|
be used as a replacement (but a more powerful one).</p>
|
|
|
|
<p>It has however a few short comings that we will explain, and solve,
|
|
in the next part of this tutorial.</p>
|
|
|
|
<hr>
|
|
|
|
<h3>
|
|
Conclusion
|
|
</h3>
|
|
|
|
<p>In this first section, you have learned the basics of FreeType 2,
|
|
as well as sufficient knowledge to know how to render rotated text.
|
|
Woww ! Congratulations..</p>
|
|
|
|
<p>The next section will dive into more details of the API in order
|
|
to let you access glyph metrics and images directly, as well as
|
|
how to deal with scaling, hinting, kerning, etc..</p>
|
|
|
|
<p>The third section will discuss issues like modules, caching and a
|
|
few other advanced topics like how to use multiple size objects
|
|
with a single face.
|
|
</p>
|
|
|
|
</td></tr>
|
|
</table>
|
|
</center>
|
|
|
|
</body>
|
|
</html>
|