872 lines
30 KiB
HTML
872 lines
30 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
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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 short tutorial will teach you how to use the FreeType 2
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library in your own applications.</p>
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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. Note that the glyph image can be in several formats. For
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example, it will be a bitmap for fixed-size formats like FNT, FON, or
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PCF. It will also be a scalable vector outline for formats like
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TrueType or Type 1. The glyph image can also be stored in an
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alternate way that is not known at the time of writing this
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documentation.</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>
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<li>
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<p>If a bitmap is found for the corresponding glyph and pixel
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size, it will in the slot (embedded bitmaps are always
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favored over native image formats, because we assume that
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they are higher-quality versions of the same image. This
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can be ignored by using the FT_LOAD_NO_BITMAP flag)</p>
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</li>
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<li>
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<p>Otherwise, a native image for the glyph will be loaded.
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It will also be scaled to the current pixel size, as
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well as hinted for certain formats like TrueType and
|
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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 to render
|
|
a monochrome bitmap, or to <tt>ft_render_mode_antialias</tt> to
|
|
generate a high-quality (256 gray levels) anti-aliased bitmap
|
|
from the glyph image.</p>
|
|
|
|
<p>Once you have a bitmap 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.</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 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>
|
|
|
|
<p><em>NOTA BENE: The transform is applied every glyph that is loaded
|
|
through <tt>FT_Load_Glyph</tt>. Note that loading a glyph bitmap
|
|
with a non-trivial transform will produce an error..</em></p>
|
|
|
|
<hr>
|
|
|
|
<h3>
|
|
7. Accessing glyph image data
|
|
</h3>
|
|
|
|
<p>Glyph image data is accessible through <tt><b>face->glyph</b></tt>.
|
|
See the definition of the <tt>FT_GlyphSlot</tt> type for more details.
|
|
As stated previously, each face has a single glyph slot, where
|
|
<em>one</em> glyph image <em>at a time</em> can be loaded. Each time
|
|
you call <tt>FT_Load_Glyph()</tt>, you erase the content of the glyph
|
|
slot with a new glyph image.</p>
|
|
|
|
<p>Note however that the glyph slot object itself doesn't change, only
|
|
its content, which means that you can perfectly create a "shortcut" to
|
|
access it as in</p>
|
|
|
|
<font color="blue">
|
|
<pre>
|
|
{
|
|
/* shortcut to glyph slot */
|
|
FT_GlyphSlot glyph = face->glyph;
|
|
|
|
for ( n = 0; n < face->num_glyphs; n++ )
|
|
{
|
|
... load glyph n ...
|
|
... access glyph data as glyph->xxxx
|
|
}
|
|
}</pre>
|
|
</font>
|
|
|
|
<p>The <tt>glyph</tt> variable will be valid until its parent
|
|
<tt>face</tt> is destroyed. Here are a few important fields of the
|
|
glyph slot:<p>
|
|
|
|
<table cellpadding=5>
|
|
<tr valign="top">
|
|
<td>
|
|
<tt><b>glyph->format</b></tt>
|
|
</td>
|
|
<td>
|
|
<p>Indicates the type of the loaded glyph image. Can be either
|
|
<tt>ft_glyph_format_bitmap</tt>, <tt>ft_glyph_format_outline</tt>,
|
|
or other values.</p>
|
|
</td>
|
|
</tr>
|
|
<tr valign="top">
|
|
<td>
|
|
<tt><b>glyph->metrics</b></tt>
|
|
</td>
|
|
<td>
|
|
<p>A simple structure used to hold the glyph image's metrics.
|
|
Note that <em>most distances are expressed in 1/64th of
|
|
pixels!</em> See the API reference or the user guide for a
|
|
description of the <tt>FT_Glyph_Metrics</tt> structure.</p>
|
|
</td>
|
|
</tr>
|
|
<tr valign="top">
|
|
<td>
|
|
<tt><b>glyph->bitmap</b></tt>
|
|
</td>
|
|
<td>
|
|
<p>If the glyph slot contains a bitmap, a simple
|
|
<tt>FT_Bitmap</tt> that describes it. See the API reference or
|
|
user guide for a description of the <tt>FT_Bitmap</tt>
|
|
structure.</p>
|
|
</td>
|
|
</tr>
|
|
<tr valign="top">
|
|
<td>
|
|
<tt><b>glyph->outline</b></tt>
|
|
</td>
|
|
<td>
|
|
<p>When the glyph slot contains a scalable outline, this structure
|
|
describes it. See the definition of the <tt>FT_Outline</tt>
|
|
structure.</p>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<h3>
|
|
8. Rendering glyph outlines into bitmaps
|
|
</h3>
|
|
|
|
<p>You can easily test the format of the glyph image by inspecting the
|
|
<tt>face->glyph->format</tt> variable. If its value is
|
|
<tt>ft_glyph_format_bitmap</tt>, the glyph image that was loaded is a
|
|
bitmap that can be directly blit to your own surfaces through your
|
|
favorite graphics library (FreeType 2 doesn't provide bitmap
|
|
blitting routines, as you may imagine :-)</p>
|
|
|
|
<p>If the format is <tt>ft_glyph_format_outline</tt> or something else,
|
|
the library provides a means to convert such glyph images to bitmaps
|
|
through what are called <b>rasters</b>.</p>
|
|
|
|
<p>On the other hand, if the image is a scalable outline or something
|
|
else, FreeType provides a function to convert the glyph image into a
|
|
pre-existing bitmap that you will handle to it, named
|
|
<tt>FT_Get_Glyph_Bitmap</tt>. Here's a <em>simple</em> example code
|
|
that renders an outline into a <b>monochrome</b> bitmap:</p>
|
|
|
|
<font color="blue">
|
|
<pre>
|
|
{
|
|
FT_GlyphSlot glyph;
|
|
|
|
... load glyph ...
|
|
|
|
glyph = face->glyph; /* shortcut to glyph data */
|
|
if ( glyph->format == ft_glyph_format_outline )
|
|
{
|
|
FT_Bitmap bit;
|
|
|
|
/* set-up a bitmap descriptor for our target bitmap */
|
|
bit.rows = bitmap_height;
|
|
bit.width = bitmap_width;
|
|
bit.pitch = bitmap_row_bytes;
|
|
/* render into a mono bitmap */
|
|
bit.pixel_mode = ft_pixel_mode_mono;
|
|
bit.buffer = bitmap_buffer;
|
|
|
|
/* render the outline directly into the bitmap */
|
|
error = FT_Get_Glyph_Bitmap( face, &bit );
|
|
if ( error ) { ... }
|
|
}
|
|
}</pre>
|
|
</font>
|
|
|
|
<p>You should note that <b><em><tt>FT_Get_Glyph_Bitmap()</tt> doesn't
|
|
create the bitmap</em></b>. It only needs a descriptor, of type
|
|
<tt>FT_Bitmap</tt>, and writes directly into it.</p>
|
|
|
|
<p>Note that the FreeType scan-converter for outlines can also generate
|
|
anti-aliased glyph bitmaps with 128 level of grays. For now, it is
|
|
restricted to rendering to 8-bit gray-level bitmaps, though this may
|
|
change in the future. Here is some code to do just that:</p>
|
|
|
|
<font color="blue">
|
|
<pre>
|
|
{
|
|
FT_GlyphSlot glyph;
|
|
|
|
... load glyph ...
|
|
|
|
glyph = face->glyph; /* shortcut to glyph data */
|
|
if ( glyph->format == ft_glyph_format_outline )
|
|
{
|
|
FT_Bitmap bit;
|
|
|
|
/* set-up a bitmap descriptor for our target bitmap */
|
|
bit.rows = bitmap_height;
|
|
bit.width = bitmap_width;
|
|
bit.pitch = bitmap_row_bytes;
|
|
/* 8-bit gray-level bitmap */
|
|
bit.pixel_mode = ft_pixel_mode_gray;
|
|
/* MUST be 128 for now */
|
|
bit.grays = 128;
|
|
bit.buffer = bitmap_buffer;
|
|
|
|
/* clean the bitmap - IMPORTANT */
|
|
memset( bit.buffer, 0, bit.rows*bit.pitch );
|
|
|
|
/* render the outline directly into the bitmap */
|
|
error = FT_Get_Glyph_Bitmap( face, &bit );
|
|
if ( error ) { ... }
|
|
}
|
|
}</pre>
|
|
</font>
|
|
|
|
<p>You will notice that</p>
|
|
|
|
<ul>
|
|
<li>
|
|
<p>As previously, <tt>FT_Get_Glyph_Bitmap()</tt> doesn't generate
|
|
the bitmap, it simply renders to it.</p>
|
|
</li>
|
|
<li>
|
|
<p>The target bitmap must be cleaned before calling the function.
|
|
This is a limitation of our current anti-aliasing algorithm and is
|
|
EXTREMELY important.</p>
|
|
</li>
|
|
<li>
|
|
<p>The anti-aliaser uses 128 levels of grays exclusively for
|
|
now (this will probably change in a near future). This means that
|
|
you <b>must</b> set <tt>bit.grays</tt> to 128. The generated
|
|
image uses values from 0 (back color) to 127 (foreground color).</p>
|
|
</li>
|
|
<li>
|
|
<p>It is <b>not</b> possible to render directly an anti-aliased
|
|
outline into a pre-existing gray-level bitmap, or even any
|
|
colored-format one (like RGB16 or paletted 8-bits). We will not
|
|
discuss this issue in great details here, but the reason is that we
|
|
do not want to deal with graphics composition (or alpha-blending)
|
|
within FreeType.<p/>
|
|
</li>
|
|
</ul>
|
|
</td></tr>
|
|
</table>
|
|
</center>
|
|
|
|
</body>
|
|
</html>
|