kc3-lang/freetype/docs/design/io-frames.html
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Hash : e35cac66
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Date : 2000-06-11T03:46:57
A complete revision of FreeType 2's GNU makefiles (of the library): Tons of unnecessary stuff have been removed; only the essential rules have been retained. The source files now depend on all header files in include/freetype, include/freetype/config, and include/freetype/internal. This is not optimal, I know, and I'll try to improve this, but it is better than before (namely no dependencies on `internal'). FTDEBUG_SRC has been added (similar to FTSYS_SRC) -- I don't know exactly whether this is really useful, but it doesn't harm. There is now more documentation in the makefiles itself. io-frames.html: Use of <th>, <code>, and <var> for better tagging. Reactivating of FT_DEBUG_LEVEL_xxx macros. Added a lot of #include directives to make `multi' builds possible -- note that currently the modules cid, t1, and t1z have clashing structures and functions which means that you can only use one of these three modules for a multi build. Added some missing function declarations to (local) header files. Renamed some T1_Open_Face() to CID_Open_Face() in the cid module -- a lot of other functions should be renamed also... Replaced many FT_xxx stuff with T1_xxx in t1z driver -- this isn't finished yet... Fixed FT_Free() to allow a NULL pointer without an assertion (this has always been a valid assumption in FreeType, at least in FT 1.x). A lot of other, minor fixes (mostly documentation).
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docs/design/io-frames.html
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<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <meta name="Author" content="David Turner"> <title>FreeType 2 Internals - I/O Frames</title> </head> <body text="#000000" bgcolor="#FFFFFF" link="#0000EF" vlink="#51188E" alink="#FF0000"> <h1 align=center> FreeType 2.0 I/O Frames </h1> <h3 align=center> © 2000 David Turner (<a href="mailto:david@freetype.org">david@freetype.org</a>)<br> © 2000 The FreeType Development Team (<a href="http://www.freetype.org">www.freetype.org</a>) </h3> <center> <table width="70%"> <tr><td> <hr> <h2> Introduction </h2> <p>This document explains the concept of I/O <b>frames</b> as used in the FreeType 2 source code. It also enumerates the various functions and macros that can be used to read them.</p> <p>It is targeted to FreeType hackers, or more simply to developers who would like a better understanding of the library's source code.</p> <hr> <h2> I. What frames are </h2> <p>Simply speaking, a frame is an array of bytes in a font file that is `preloaded' into memory in order to be rapidly parsed. Frames are useful to ensure that every `load' is checked against end-of-file overruns, and provides nice functions to extract data in a variety of distinct formats.</p> <p>But an example is certainly more meaningful than anything else. The following code</p> <font color="blue"> <pre> error = read_short( stream, &str.value1 ); if ( error ) goto ... error = read_ulong( stream, &str.value2 ); if ( error ) goto ... error = read_ulong( stream, &str.value3 ); if ( error ) goto ...</pre> </font> <p>can easily be replaced with</p> <font color="blue"> <pre> error = FT_Access_Frame( stream, 2 + 4 + 4 ); if ( error ) goto ... str.value1 = FT_Get_Short( stream ); str.value2 = FT_Get_ULong( stream ); str.value3 = FT_Get_ULong( stream ); FT_Forget_Frame( stream );</pre> </font> <p>Here, the call to <code>FT_Access_Frame()</code> will</p> <ul> <li> <p>Ensure that there are at least 2+4+4=10 bytes left in the stream.</p> </li> <li> <p>`Preload' (for disk-based streams) 10 bytes from the current stream position.</p> </li> <li> <p>Set the frame `cursor' to the first byte in the frame.</p> </li> </ul> <p>Each <code>FT_Get_Short()</code> or <code>FT_Get_ULong()</code> call will read a big-endian integer from the stream (2 bytes for <code>FT_Get_Short()</code>, 4 bytes for <code>FT_Get_ULong</code>) and advance the frame cursor accordingly.</p> <p><code>FT_Forget_Frame()</code> `releases' the frame from memory.</p> <p>There are several advantages to using frames:</p> <ul> <li> <p>Single-check when loading tables.</p> </li> <li> <p><em>Making code clearer</em> by providing simple parsing functions <em>while keeping code safe</em> from file over-runs and invalid offsets.</p> </li> </ul> <hr> <h2> II. Accessing and reading a frame with macros </h2> <p>By convention in the FreeType source code, macros are able to use two implicit variables named <var>error</var> and <var>stream</var>. This is useful because these two variables are extremely often used in the library, and doing this only reduces our typing requirements and make the source code much clearer.</p> <p>Note that <var>error</var> must be a local variable of type <code>FT_Error</code>, while <var>stream</var> must be a local variable or argument of type <code>FT_Stream</code>.</p> <p>The macro used to access a frame is <font color="purple"><code><b>ACCESS_Frame(_size_)</b></code></font>, it will translate to</p> <font color="blue"> <pre> ( error = FT_Access_Frame( stream, _size_ ) ) != FT_Err_Ok</pre> </font> <p>Similarly, the macro <font color="purple"><b><code>FORGET_Frame()</code></b></font> translates to</p> <font color="blue"> <pre> FT_Forget_Frame( stream )</pre> </font> <p>Extracting integers can be performed with the <code>GET_xxx()</code> macros, like</p> <table align=center cellpadding=5> <tr valign="top"> <th> Macro name </th> <th> Translation </th> <th> Description </th> </tr> <tr valign="top"> <td> <font color="purple"><code><b>GET_Byte()</b></code></font> </td> <td> <font color="blue"><code>FT_Get_Byte(stream)</code></font> </td> <td> <p>Reads an 8-bit unsigned byte.</p> </td> </tr> <tr valign="top"> <td> <font color="purple"><code><b>GET_Char()</b></code></font> </td> <td> <font color="blue"><code>(FT_Char)<br> FT_Get_Byte(stream)</code></font> </td> <td> <p>Reads an 8-bit <em>signed</em> byte.</p> </td> </tr> <tr valign="top"> <td> <font color="purple"><code><b>GET_Short()</b></code></font> </td> <td> <font color="blue"><code>FT_Get_Short(stream)</code></font> </td> <td> Reads a 16-bit signed big-endian integer. </td> </tr> <tr valign="top"> <td> <font color="purple"><code><b>GET_UShort()</b></code></font> </td> <td> <font color="blue"><code>(FT_UShort)<br> FT_Get_Short(stream)</code></font> </td> <td> Reads a 16-bit unsigned big-endian integer. </td> </tr> <tr valign="top"> <td> <font color="purple"><code><b>GET_Offset()</b></code></font> </td> <td> <font color="blue"><code>FT_Get_Offset(stream)</code></font> </td> <td> Reads a 24-bit signed big-endian integer. </td> </tr> <tr valign="top"> <td> <font color="purple"><code><b>GET_UOffset()</b></code></font> </td> <td> <font color="blue"><code>(FT_UOffset)<br> FT_Get_Offset(stream)</code></font> </td> <td> Reads a 24-bit unsigned big-endian integer. </td> </tr> <tr valign="top"> <td> <font color="purple"><code><b>GET_Long()</b></code></font> </td> <td> <font color="blue"><code>FT_Get_Long(stream)</code></font> </td> <td> Reads a 32-bit signed big-endian integer. </td> </tr> <tr valign="top"> <td> <font color="purple"><code><b>GET_ULong()</b></code></font> </td> <td> <font color="blue"><code>(FT_ULong)<br> FT_Get_Long(stream)</code></font> </td> <td> Reads a 32-bit unsigned big-endian integer. </td> </tr> </table> <p>(Note that an <b>Offset</b> is an integer stored with 3 bytes on the file.)</p> <p>All this means that the following code</p> <font color="blue"> <pre> error = FT_Access_Frame( stream, 2 + 4 + 4 ); if ( error ) goto ... str.value1 = FT_Get_Short( stream ); str.value2 = FT_Get_ULong( stream ); str.value3 = FT_Get_ULong( stream ); FT_Forget_Frame( stream );</pre> </font> <p>can be simplified with macros:</p> <font color="blue"> <pre> if ( ACCESS_Frame( 2 +4 + 4 ) ) goto ... str.value1 = GET_Short(); str.value2 = GET_ULong(); str.value3 = GET_ULong(); FORGET_Frame();</pre> </font> <p>Which is clearer. Notice that <var>error</var> and <var>stream</var> must be defined locally though for this code to work!</p> <hr> <h2> III. Alternatives </h2> <p>It is sometimes useful to read small integers from a font file without using a frame. Some functions have been introduced in FreeType 2 to do just that, and they are of the form <font color="blue"><code>FT_Read_xxxx</code></font>.</p> <p>For example, <font color="blue"><code>FT_Read_Short(stream, &error)</code></font> reads and returns a 2-byte big-endian integer from a <var>stream</var>, and places an error code in the <var>error</var> variable.</p> <p>Thus, reading a single big-endian integer is shorter than using a frame for it.</p> <p>Note that there are also macros <font color="purple"><code>READ_xxx()</code></font> which translate to</p> <font color="blue"> <pre> FT_Read_xxx( stream, &error ), error != FT_Err_Ok</pre> </font> <p>and can be used as in</p> <font color="blue"> <pre> if ( READ_UShort( variable1 ) || READ_ULong ( variable2 ) ) goto Fail;</pre> </font> <p>if <var>error</var> and <var>stream</var> are already defined locally.</p> </td></tr> </table> </center> </body> </html>