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179 lines
7.0 KiB
HTML
179 lines
7.0 KiB
HTML
<h1>The OpenGL Extension Wrangler Library</h1>
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<h2>Automatic Code Generation</h2>
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<p>
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Starting from release 1.1.0, the source code and parts of the
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documentation are automatically generated from the extension
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specifications in a two-step process. In the first step,
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specification files from the OpenGL registry are downloaded and
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parsed. Skeleton descriptors are created for each extension. These
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descriptors contain all necessary information for creating the source
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code and documentation in a simple and compact format, including the
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name of the extension, url link to the specification, tokens, function
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declarations, typedefs and struct definitions. In the second step,
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the header files as well as the library and glewinfo source are
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generated from the descriptor files. The code generation scripts are
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located in the <tt>auto</tt> subdirectory.
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</p>
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<p>
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The code generation scripts require GNU make, wget, and perl. On
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Windows, the simplest way to get access to these tools is to install
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<a href="http://www.cygwin.com/">Cygwin</a>, but make sure that the
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root directory is mounted in binary mode. The makefile in the
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<tt>auto</tt> directory provides the following build targets:
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</p>
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<center>
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<table border=0 cellpadding=0 cellspacing=0> <!-- bgcolor="#f0f0f0" -->
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<tr><td align="right" valign="top" width="25%"><tt>make</tt></td>
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<td> </td>
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<td align=left>Create the source files from the descriptors. If the
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descriptors do not exist, create them from the spec files. If the spec
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files do not exist, download them from the OpenGL repository.</td></tr>
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<tr><td align="right" valign="top"><tt>make clean</tt></td>
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<td> </td>
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<td align=left>Delete the source files.</td></tr>
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<tr><td align="right" valign="top"><tt>make clobber</tt></td>
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<td> </td>
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<td align=left>Delete the source files and the descriptors.</td></tr>
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<tr><td align="right" valign="top"><tt>make destroy</tt></td>
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<td> </td>
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<td align=left>Delete the source files, the descriptors, and the spec files.</td></tr>
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<tr><td align="right" valign="top"><tt>make custom</tt></td>
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<td> </td>
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<td align=left>Create the source files for the extensions
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listed in <tt>auto/custom.txt</tt>. See "Custom Code
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Generation" below for more details.</td></tr>
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</table>
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</center>
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<h3>Adding a New Extension</h3>
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<p>
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To add a new extension, create a descriptor file for the extension in
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<tt>auto/core</tt> and rerun the code generation scripts by typing
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<tt>make clean; make</tt> in the <tt>auto</tt> directory.
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</p>
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<p>
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The format of the descriptor file is given below. Items in
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brackets are optional.
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</p>
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<p class="pre">
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<Extension Name><br>
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[<URL of Specification File>]<br>
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[<Token Name> <Token Value>]<br>
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[<Token Name> <Token Value>]<br>
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...<br>
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[<Typedef>]<br>
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[<Typedef>]<br>
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...<br>
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[<Function Signature>]<br>
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[<Function Signature>]<br>
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...<br>
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<!-- [<Function Definition>]<br>
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[<Function Definition>]<br>
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...<br> -->
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</p>
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<!--
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<p>
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Note that <tt>Function Definitions</tt> are copied to the header files
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without changes and have to be terminated with a semicolon. In
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contrast, <tt>Tokens</tt>, <tt>Function signatures</tt>, and
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<tt>Typedefs</tt> should not be terminated with a semicolon.
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</p>
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-->
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<p>
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Take a look at one of the files in <tt>auto/core</tt> for an
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example. Note that typedefs and function signatures should not be
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terminated with a semicolon.
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</p>
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<h3>Custom Code Generation</h3>
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<p>
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Starting from GLEW 1.3.0, it is possible to control which extensions
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to include in the libarary by specifying a list in
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<tt>auto/custom.txt</tt>. This is useful when you do not need all the
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extensions and would like to reduce the size of the source files.
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Type <tt>make clean; make custom</tt> in the <tt>auto</tt> directory
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to rerun the scripts with the custom list of extensions.
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</p>
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<p>
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For example, the following is the list of extensions needed to get GLEW and the
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utilities to compile.
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</p>
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<p class="pre">
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WGL_ARB_extensions_string<br>
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WGL_ARB_multisample<br>
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WGL_ARB_pixel_format<br>
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WGL_ARB_pbuffer<br>
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WGL_EXT_extensions_string<br>
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WGL_ATI_pixel_format_float<br>
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WGL_NV_float_buffer<br>
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</p>
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<h2>Multiple Rendering Contexts (GLEW MX)</h2>
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<p>Starting with release 1.2.0, thread-safe support for multiple
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rendering contexts, possibly with different capabilities, is
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available. Since this is not required by most users, it is not added
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to the binary releases to maintain compatibility between different
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versions. To include multi-context support, you have to do the
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following:</p>
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<ol>
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<li>Compile and use GLEW with the <tt>GLEW_MX</tt> preprocessor token
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defined.</li>
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<li>For each rendering context, create a <tt>GLEWContext</tt> object
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that will be available as long as the rendering context exists.</li>
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<li>Define a macro or function called <tt>glewGetContext()</tt> that
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returns a pointer to the <tt>GLEWContext</tt> object associated with
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the rendering context from which OpenGL/WGL/GLX calls are issued. This
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dispatch mechanism is primitive, but generic.
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<li>Make sure that you call <tt>glewInit()</tt> after creating the
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<tt>GLEWContext</tt> object in each rendering context. Note, that the
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<tt>GLEWContext</tt> pointer returned by <tt>glewGetContext()</tt> has
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to reside in global or thread-local memory.
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</ol>
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<p>Note that according to the <a
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href="http://msdn.microsoft.com/library/default.asp?url=/library/en-us/opengl/ntopnglr_6yer.asp">MSDN
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WGL documentation</a>, you have to initialize the entry points for
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every rendering context that use pixel formats with different
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capabilities For example, the pixel formats provided by the generic
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software OpenGL implementation by Microsoft vs. the hardware
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accelerated pixel formats have different capabilities. <b>GLEW by
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default ignores this requirement, and does not define per-context
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entry points (you can however do this using the steps described
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above).</b> Assuming a global namespace for the entry points works in
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most situations, because typically all hardware accelerated pixel
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formats provide the same entry points and capabilities. This means
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that unless you use the multi-context version of GLEW, you need to
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call <tt>glewInit()</tt> only once in your program, or more precisely,
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once per process.</p>
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<h2>Separate Namespace</h2>
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<p>
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To avoid name clashes when linking with libraries that include the
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same symbols, extension entry points are declared in a separate
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namespace (release 1.1.0 and up). This is achieved by aliasing OpenGL
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function names to their GLEW equivalents. For instance,
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<tt>glFancyFunction</tt> is simply an alias to
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<tt>glewFancyFunction</tt>. The separate namespace does not effect
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token and function pointer definitions.
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</p>
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<h2>Known Issues</h2>
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<p>
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GLEW requires GLX 1.2 for compatibility with GLUT.
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</p>
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