forked from minhngoc25a/freetype2
645 lines
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HTML
645 lines
24 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" content="text/html; charset=iso-8859-1">
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<meta name="Author" content="David Turner">
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<meta name="GENERATOR" content="Mozilla/4.5 [fr] (Win98; I) [Netscape]">
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<title>FreeType 2 Tutorial</title>
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</head>
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<body>
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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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<center>
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<h1>
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FreeType 2.0 Tutorial</h1></center>
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<center>
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<h3>
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© 2000 David Turner (<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></center>
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<p><br>
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<hr WIDTH="100%">
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<br>
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<h2>Introduction:</h2>
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<ul>
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This simple tutorial will teach you how to use the FreeType 2 library
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in your own applications.
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</ul>
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<p><hr><p>
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<h3>1. Header files :</h3>
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<ul>
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You only need to include the public header file named <tt>freetype.h</tt>
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in your source code. Depending on how the library was installed on your
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system, you might need to use :<p>
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<ul><font color="blue"><tt>
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#include <freetype.h><p>
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</tt></font></ul>
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or
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<ul><font color="blue"><tt>
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#include <freetype2/freetype.h><p>
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</tt></font></ul>
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in your application to include the public interface to FreeType.<p>
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</ul>
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<p><hr><p>
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<h3>2. Initialise the library:</h3>
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<ul>
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Simply create a variable of type <tt>FT_Library</tt> named, for example,
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<tt>library</tt>, and call the function <tt>FT_Init_FreeType</tt> as in:
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<font color="blue"><pre>
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#include <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) { .. an error occured during library initialisation .. }
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}
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</pre></font>
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<p>
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This function is in charge of the following:<p>
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<ul>
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<li>Creating a new instance of the FreeType 2 library, and set
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the handle <tt>library</tt> to it.
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<p>
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<li>Load each font driver that FreeType knows about in the library.
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This means that by default, your new <tt>library</tt> object is
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able to handle TrueType and Type 1 fonts gracefully.
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<p>
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</ul>
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<p>
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As you can see, the function returns an error code, like most others in the
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FreeType API. An error code of 0 <em>always</em> means that the operation
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was succesful; otherwise, the value describes the error, and <tt>library</tt>
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is set to NULL.
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</ul>
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<p><hr><p>
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<h3>3. Load a font face:</h3>
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<ul>
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<h4>a. From a font file:</h4>
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<ul>
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Create a new <em>face</em> object by calling <tt>FT_New_Face</tt>. A
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<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.
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<p>
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<font color="blue"><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, read or simply that it is broken..
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}
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</pre></font>
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<p>
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As you certainly imagine, <tt>FT_New_Face</tt> opens a font file then
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tries to extract one face from it. Its parameters are :<p>
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<ul>
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<table cellpadding=5>
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<tr valign="top"><td><tt><b>library</b></tt>
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<td>handle to the FreeType library instance where the face object is
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created
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<tr valign="top"><td><tt><b>filepathname</b></tt>
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<td>the font file pathname (standard C string).
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<tr valign="top"><td><tt><b>face_index</b></tt>
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<td>Certain font formats allow several font faces to be embedded in
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a single file.<br>
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This index tells which face you want to load. An
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error will be returned if its value is too large.<br>
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Index 0 always work though.
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<tr><td><tt><b>face</b></tt>
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<td>A <em>pointer</em> to the handle that will be set to
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describe the new face object.<br>
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It is set to NULL in case of error.
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</table>
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</ul>
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<p>
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To known 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 in
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the font file.
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</ul>
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<p>
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<h4>b. From memory:</h4>
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<ul>
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In the case where you have already loaded the font file in memory, you
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can similarly create a new face object for it by calling
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<tt>FT_New_Memory_Face</tt> as in:
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<p>
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<font color="blue"><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) { ... }
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</pre></font>
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<p>
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As you can see, <tt>FT_New_Memory_Face</tt> simply takes a pointer to
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the font file buffer and its size in bytes instead of a file pathname.
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Other than that, it has exactly the same semantics than
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<tt>FT_New_Face</tt>.
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</ul>
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<p>
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<h4>c. From other sources:</h4>
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<ul>
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There are cases where using a filepathname or preloading the file in
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memory is simply not enough. With FreeType 2, it is possible to provide
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your own implementation of i/o routines through the <tt>FT_Stream</tt>
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type.
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<p>
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Basically, one has to set up a <tt>FT_Stream</tt> object, according to
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the rules defined in the document named
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<a href="#">FreeType 2 System Interface</a>, then pass it to the function
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<tt>FT_Open_Face</tt> as in:
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<p>
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<font color="blue"><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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... set up stream object, with handle "stream" ...
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error = FT_Open_Face( library,
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stream, /* handle to stream objects */
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0, /* face_index */
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&face );
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if (error) { ... }
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</pre></font>
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<p>
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custom implementations of <tt>FT_Stream</tt> are great to provide advanced
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features like automatic support of compressed files, network transparency,
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using UTF-16 file pathnames, etc..
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<p>
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</ul>
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<p>
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</ul>
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<p><hr><p>
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<h3>4. Accessing face content:</h3>
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<ul>
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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 :
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<p>
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<table cellpadding=5>
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<tr valign="top">
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<td><tt><b>face−>num_glyphs</b></tt>
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<td>gives the number of <em>glyphs</em> available in the font face. A glyph
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is simply a character image. It doesn't necessarily correspond to
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a <em>character code</em> though.
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<tr valign="top">
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<td><tt><b>face−>flags</b></tt>
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<td>a 32-bit integer containing bit flags used to describe some face
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properties. For example, the flag <tt>FT_FACE_FLAG_SCALABLE</tt> is
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used to indicate that the face's font format is scalable and that
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glyph images can be rendered for all character pixel sizes. For more
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information on face flags, please read the <a href="#">FreeType API
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Reference</a>
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<tr valign="top">
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<td><tt><b>face−>units_per_EM</b></tt>
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<td>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.
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<tr valign="top">
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<td><tt><b>face−>num_fixed_sizes</b></tt>
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<td>this field gives the number of embedded bitmap <em>strikes</em> in
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the current face. A <em>strike</em> is simply a series of glyph
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images for a given character pixel size. For example, a font face
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could include strikes for pixel sizes 10, 12 and 14. Note that even
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scalable font formats can embedded bitmap strikes !
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<tr valign="top">
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<td><tt><b>face−>fixed_sizes</b></tt>
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<td>this is a pointer to an array of <tt>FT_Bitmap_Size</tt> element.
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each <tt>FT_Bitmap_Size</tt> indicates the horizontal and vertical
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<em>pixel sizes</em> for each of the strikes that are present in the face.
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</table>
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<p>
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For a complete listing of all face properties and fields, please read
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the <a href="#">FreeType 2 API Reference</a>.
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<p>
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</ul>
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<p><hr><p>
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<h3>5. Setting the current pixel size:</h3>
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<ul>
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A face object also holds a handle to a <em>size object</em> in its
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<tt>face->size</tt> field. The <em>size</em> object is used to model
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all information for the face that is relative to a given character
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size.
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<p>
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When a new face object is created, its size object defaults to the
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character size of 10 pixels (both horizontall 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.
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<p>
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To do that, simply call <tt>FT_Set_Char_Size</tt>. Here's an example
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where the character size is set to 16 pts for a 300x300 dpi device:
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<p>
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<font color="blue"><pre>
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error = FT_Set_Char_Size( 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 */
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</pre></font>
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<p>
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You'll notice that:<p>
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<ul>
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<li>The character width and heights are specified in 1/64th of points.<p>
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<li>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 96 dpi
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for display devices like the screen.<p>
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<li>A value of 0 for the character width means "<em>same as character
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height</em>", a value of 0 for the character height means
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"<em>same as character width</em>". Otherwise, it is possible to
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specify different char width and height.<p>
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<li>Using a value of 0 for the horizontal or vertical resolution means
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72 dpi, which is the default.
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<p>
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</ul>
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<p>
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This function computes the character pixel size that corresponds to the
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character width and height and device resolutions. However, if you want
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to specify the pixel sizes yourself, you can simply call
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<tt>FT_Set_Pixel_Sizes</tt>, as in:
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<p>
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<font color="blue"><pre>
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error = FT_Set_Pixel_Sizes( face, /* handle to face object */
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0, /* pixel_width */
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16 ); /* pixel_height */
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</pre></font>
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<p>
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This example will set the character pixel sizes to 16x16 pixels. As
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previously, a value of 0 for one of the dimensions means "<em>same as
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the other</em>".
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<p>
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Note that both functions return an error code. Usually, an error occurs
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with a fixed-size font format (like FNT or PCF) when trying to set the
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pixel size to a value that is not listed in the
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<tt><b>face->fixed_sizes</b></tt> array.
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</ul>
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<p><hr><p>
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<h3>6. Loading a glyph image:</h3>
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<ul>
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<h4>a. Converting a character code into a glyph index:</h4>
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<ul>
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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 the ASCII encoding.
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<p>
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A face object contains one or more tables, called <em>charmaps</em>,
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that are used to convert character codes to glyph indices. For example,
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most TrueType fonts contain two charmaps. One is used to convert Unicode
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character codes to glyph indices, the other is used to convert
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Apple Roman encoding into glyph indices. Such fonts can then be used
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either on Windows (which uses Unicode) and Macintosh (which uses
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Apple Roman, bwerk..). Note also that a given charmap might not map to all
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the glyphs present in the font.
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<p>
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By default, when a new face object is created, it lists all the charmaps
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contained in the font face and selects the one that supports Unicode
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character codes if it finds one. Otherwise, it tries to find support for
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Latin-1, then ASCII.
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<p>
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We'll describe later how to look for specific charmaps in a face. For
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now, we'll assume that the face contains at least a Unicode charmap that
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was selected during <tt>FT_New_Face</tt>. To convert a Unicode character
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code to a font glyph index, we use <tt>FT_Get_Char_Index</tt> as in:
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<p>
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<font color="blue"><pre>
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glyph_index = FT_Get_Char_Index( face, charcode );
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</pre></font>
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<p>
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This will look the glyph index corresponding to the given <tt>charcode</tt>
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in the charmap that is currently selected for the face. If charmap is
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selected, the function simply returns the charcode.
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<p>
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Note that this is one of the rare FreeType functions that do not return
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an error code. However, when a given character code has no glyph image in
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the face, the value 0 is returned. By convention, it always correspond to
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a special glyph image called the <b>missing glyph</b>, which usually is
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represented as a box or a space.
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<p>
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</ul>
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<p>
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<h4>b. Loading a glyph from the face:</h4>
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<ul>
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Once you have a glyph index, you can load the corresponding glyph image.
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Note that the glyph image can be in several formats. For example, it will
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be a bitmap for fixed-size formats like FNT, FON or PCF. It will also
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be a scalable vector outline for formats like TrueType or Type 1. The
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glyph image can also be stored in an alternate way that is not known
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at the time of writing this documentation.
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<p>
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The glyph image is always stored in a special object called a
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<em>glyph slot</em>. As it names 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 bitmap,
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an outline, or something else. Each face object has a single glyph
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object that can be accessed as <b><tt>face−>glyph</tt></b>.
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<p>
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Loading a glyph image into the slot is performed by calling
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<tt>FT_Load_Glyph</tt> as in:
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<p>
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<font color="blue"><pre>
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error = FT_Load_Glyph( face, /* handle to face object */
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glyph_index, /* glyph index */
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load_flags ); /* load flags, see below */
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</pre></font>
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<p>
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The <tt>load_flags</tt> value is a set of bit flags used to indicate
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some special operations. The default value <tt>FT_LOAD_DEFAULT</tt> is
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0. The function performs the following :<p>
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<ul>
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<li>if there is a bitmap for the corresponding glyph and size, load
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it in the glyph slot, unless the <tt>FT_LOAD_NO_BITMAP</tt> flag
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is set. This is even <em>true</em> for scalable formats (embedded
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bitmaps are favored over outlines as they usually correspond to
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higher-quality images of the same glyph).
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<p>
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<li>if there is an outline for the corresponding glyph, load it
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unless <tt>FT_LOAD_NO_OUTLINE</tt> is set. Otherwise, scale it
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to the current size, unless the <tt>FT_LOAD_NO_SCALE</tt> flag
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is set.
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<p>
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<li>if the outline was loaded and scaled, try to grid-fit it (which
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dramatically improves its quality) unless the flag
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<tt>FT_LOAD_NO_HINTING</tt> is set.
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</ul>
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<p>
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There are a few others <tt>FT_LOAD_xxx</tt> flags defined. For more
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details see the <a href="#">FreeType 2 API Reference</a>.
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</ul>
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<p>
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<h4>c. Using other charmaps:</h4>
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<ul>
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As said before, when a new face object is created, it will look for
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a Unicode, Latin-1 or ASCII charmap and select it. The currently
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selected charmap is accessed via <b><tt>face−>charmap</tt></b>. This
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field is NULL when no charmap is selected.
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<p>
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The field <b><tt>face−>num_charmaps</tt></b> and
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<b><tt>face−>charmaps</tt></b> (notice the 's') can be used by
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client applications to look at what charmaps are available in a given
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face.
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<p>
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<b><tt>face−charmaps</tt></b> is an array of <em>pointers</em>
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to the <tt><b>face−>num_charmaps</b></tt> charmaps contained in the
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font face.
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<p>
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Each charmap has a few visible fields used to describe it in more details.
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For example, <tt><b>charmap->encoding</b></tt> is an enumeration type
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that describes the charmap with FreeType codes. One can also look at
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<tt><b>charmap->platform_id</b></tt> and
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<tt><b>charmap->encoding_id</b></tt> for more exotic needs.
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<p>
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Here's an example code that looks for a chinese Big5 charmap then
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selects it via <tt>FT_Set_CharMap</tt>:
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<p>
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<font color="blue"><pre>
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FT_CharMap found = 0;
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FT_CharMap charmap;
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int n;
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for ( n = 0; n < face->num_charmaps; n++ )
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{
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charmap = face>charmaps[n];
|
|
if (charmap->encoding == ft_encoding_big5)
|
|
{
|
|
found = charmap;
|
|
break;
|
|
}
|
|
}
|
|
|
|
if (!found) { ... }
|
|
|
|
/* now, select the charmap for the face object */
|
|
error = FT_Set_CharMap( face, found );
|
|
if (error) { .... }
|
|
</pre></font>
|
|
<p>
|
|
One might now call <tt>FT_Get_Char_Index</tt> with Big5 character codes
|
|
to retrieve glyph indices.
|
|
<p>
|
|
</ul>
|
|
<p>
|
|
</ul>
|
|
|
|
<p><hr><p>
|
|
|
|
<h3>7. Accessing glyph image data:</h3>
|
|
<ul>
|
|
Glyph image data is accessible through <tt><b>face−glyph</b></tt>.
|
|
See the definition of the <tt>FT_GlyphSlot</tt> type on more details. You
|
|
can perfectly create a shortcut to the glyph slot as in:
|
|
<p>
|
|
<font color="blue"><pre>
|
|
{
|
|
FT_GlyphSlot glyph;
|
|
|
|
.... load glyph ...
|
|
|
|
glyph = face->glyph; /* shortcut to glyph data */
|
|
|
|
.... access glyph data as glyph->xxxx
|
|
}
|
|
</pre></font>
|
|
<p>
|
|
For example, one can access the following fields:
|
|
<p>
|
|
<table cellpadding=10>
|
|
<tr valign="top">
|
|
<td><tt><b>glyph−>format</b></tt>
|
|
<td>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.
|
|
|
|
<tr valign="top">
|
|
<td><tt><b>glyph−>metrics</b></tt>
|
|
<td>A simple structure used to hold the glyph image's metrics. Note
|
|
that <em>all distances are expressed in 1/64th of pixels !</em>
|
|
See the API reference or User Guide for a description of the
|
|
<tt>FT_Glyph_Metrics</tt> structure.
|
|
|
|
<tr valign="top">
|
|
<td><tt><b>glyph−>bitmap</b></tt>
|
|
<td>When 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.
|
|
|
|
<tr valign="top">
|
|
<td><tt><b>glyph−>outline</b></tt>
|
|
<td>When the glyph slot contains a scalable outline, this structure
|
|
describes it. See the definition of the <tt>FT_Outline</tt>
|
|
structure.
|
|
</table>
|
|
<p>
|
|
</ul>
|
|
|
|
<h3>8. Rendering glyph outlines into bitmaps:</h3>
|
|
<ul>
|
|
When the glyph image loaded in a glyph slot is a bitmap, you can use
|
|
your favorite graphics library to blit it to your own surfaces.
|
|
<p>
|
|
On the other hand, when 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'll 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;
|
|
bit.pixel_mode = ft_pixel_mode_mono; /* render into a mono bitmap */
|
|
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>
|
|
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's 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;
|
|
bit.pixel_mode = ft_pixel_mode_gray; /* 8-bit gray-level bitmap */
|
|
bit.grays = 128; /* MUST be 128 for now */
|
|
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'll notice that :<p>
|
|
<ul>
|
|
<li>As previously, <tt>FT_Get_Glyph_Bitmap</tt> doesn't generate the
|
|
bitmap, it simply renders to it.<p>
|
|
|
|
<li>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>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>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.
|
|
</ul>
|
|
<p>
|
|
</ul>
|
|
</ul>
|
|
|
|
<p><hr>
|
|
|