Apq
/
obs-studio
tükrözi: https://github.com/obsproject/obs-studio.git


			
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The OpenGL Extension Wrangler Library
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Copyright (C) 2002, Lev Povalahev
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<h1>The OpenGL Extension Wrangler Library</h1>

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<h2>Automatic Code Generation</h2>

<p>
Starting from release 1.1.0, the source code and parts of the
documentation are automatically generated from the extension
specifications in a two-step process.  In the first step,
specification files from the OpenGL registry are downloaded and
parsed. Skeleton descriptors are created for each extension.  These
descriptors contain all necessary information for creating the source
code and documentation in a simple and compact format, including the
name of the extension, url link to the specification, tokens, function
declarations, typedefs and struct definitions.  In the second step,
the header files as well as the library and glewinfo source are
generated from the descriptor files.  The code generation scripts are
located in the <tt>auto</tt> subdirectory.
</p>

<p>
The code generation scripts require GNU make, wget, and perl. On
Windows, the simplest way to get access to these tools is to install
<a href="http://www.cygwin.com/">Cygwin</a>, but make sure that the
root directory is mounted in binary mode. The makefile in the
<tt>auto</tt> directory provides the following build targets:
</p>

<table border=0 cellpadding=0 cellspacing=5>
<tr><td align="left" valign="top"><tt>make</tt></td>
<td align=left>Create the source files from the descriptors.<br/> If the
descriptors do not exist, create them from the spec files.<br/> If the spec
files do not exist, download them from the OpenGL repository.</td></tr>
<tr><td align="left" valign="top"><tt>make&nbsp;clean</tt></td>
<td align=left>Delete the source files.</td></tr>
<tr><td align="left" valign="top"><tt>make&nbsp;clobber</tt></td>
<td align=left>Delete the source files and the descriptors.</td></tr>
<tr><td align="left" valign="top"><tt>make&nbsp;destroy</tt></td>
<td align=left>Delete the source files, the descriptors, and the spec files.</td></tr>
<tr><td align="left" valign="top"><tt>make&nbsp;custom</tt></td>
<td align=left>Create the source files for the extensions
listed in <tt>auto/custom.txt</tt>.<br/> See "Custom Code
Generation" below for more details.</td></tr>
</table>

<h3>Adding a New Extension</h3>

<p>
To add a new extension, create a descriptor file for the extension in
<tt>auto/core</tt> and rerun the code generation scripts by typing
<tt>make clean; make</tt> in the <tt>auto</tt> directory.
</p>

<p>
The format of the descriptor file is given below. Items in
brackets are optional.
</p>

<p class="pre">
&lt;Extension Name&gt;<br>
[&lt;URL of Specification File&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;[&lt;Token Name&gt; &lt;Token Value&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;[&lt;Token Name&gt; &lt;Token Value&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;...<br>
&nbsp;&nbsp;&nbsp;&nbsp;[&lt;Typedef&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;[&lt;Typedef&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;...<br>
&nbsp;&nbsp;&nbsp;&nbsp;[&lt;Function Signature&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;[&lt;Function Signature&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;...<br>
<!-- &nbsp;&nbsp;&nbsp;&nbsp;[&lt;Function Definition&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;[&lt;Function Definition&gt;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;...<br> -->
</p>

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Note that <tt>Function Definitions</tt> are copied to the header files
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<p>
Take a look at one of the files in <tt>auto/core</tt> for an
example. Note that typedefs and function signatures should not be
terminated with a semicolon.
</p>

<h3>Custom Code Generation</h3>
<p>
Starting from GLEW 1.3.0, it is possible to control which extensions
to include in the libarary by specifying a list in
<tt>auto/custom.txt</tt>. This is useful when you do not need all the
extensions and would like to reduce the size of the source files.
Type <tt>make clean; make custom</tt> in the <tt>auto</tt> directory
to rerun the scripts with the custom list of extensions.
</p>

<p>
For example, the following is the list of extensions needed to get GLEW and the
utilities to compile.
</p>

<p class="pre">
WGL_ARB_extensions_string<br>
WGL_ARB_multisample<br>
WGL_ARB_pixel_format<br>
WGL_ARB_pbuffer<br>
WGL_EXT_extensions_string<br>
WGL_ATI_pixel_format_float<br>
WGL_NV_float_buffer<br>
</p>

<h2>Multiple Rendering Contexts (GLEW MX)</h2>

<p>Starting with release 1.2.0, thread-safe support for multiple
rendering contexts, possibly with different capabilities, is
available. Since this is not required by most users, it is not added
to the binary releases to maintain compatibility between different
versions. To include multi-context support, you have to do the
following:</p>
<ol>
<li>Compile and use GLEW with the <tt>GLEW_MX</tt> preprocessor token
defined.</li>
<li>For each rendering context, create a <tt>GLEWContext</tt> object
that will be available as long as the rendering context exists.</li>
<li>Define a macro or function called <tt>glewGetContext()</tt> that
returns a pointer to the <tt>GLEWContext</tt> object associated with
the rendering context from which OpenGL/WGL/GLX calls are issued. This
dispatch mechanism is primitive, but generic.
<li>Make sure that you call <tt>glewInit()</tt> after creating the
<tt>GLEWContext</tt> object in each rendering context.  Note, that the
<tt>GLEWContext</tt> pointer returned by <tt>glewGetContext()</tt> has
to reside in global or thread-local memory.
</ol>

<p>Note that according to the <a
href="http://msdn.microsoft.com/library/default.asp?url=/library/en-us/opengl/ntopnglr_6yer.asp">MSDN
WGL documentation</a>, you have to initialize the entry points for
every rendering context that use pixel formats with different
capabilities For example, the pixel formats provided by the generic
software OpenGL implementation by Microsoft vs. the hardware
accelerated pixel formats have different capabilities.  <b>GLEW by
default ignores this requirement, and does not define per-context
entry points (you can however do this using the steps described
above).</b> Assuming a global namespace for the entry points works in
most situations, because typically all hardware accelerated pixel
formats provide the same entry points and capabilities. This means
that unless you use the multi-context version of GLEW, you need to
call <tt>glewInit()</tt> only once in your program, or more precisely,
once per process.</p>

<h2>Separate Namespace</h2>

<p>
To avoid name clashes when linking with libraries that include the
same symbols, extension entry points are declared in a separate
namespace (release 1.1.0 and up). This is achieved by aliasing OpenGL
function names to their GLEW equivalents. For instance,
<tt>glFancyFunction</tt> is simply an alias to
<tt>glewFancyFunction</tt>.  The separate namespace does not effect
token and function pointer definitions.
</p>

<h2>Known Issues</h2>

<p>
GLEW requires GLX 1.2 for compatibility with GLUT.
</p>

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