1. Jason McKesson
  2. glLoadGen


glLoadGen / docs / Styles.xml

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    <para>The <literal>-style</literal> command line option defines how the header and source files
        are generated. They will always generate the same information (more or less); the difference
        between styles explains the form that this information takes. The information generated
            <para>Typedefs for OpenGL concepts (GLenum, etc).</para>
            <para>A means to tell whether each of the extensions specified by the user is loaded or
                not. This is usually exposed via a global variable who's name contains the extension
            <para>The enumerators for all of the extensions specified as well as any specified
                OpenGL versions/profiles (where applicable).</para>
            <para>Functions (whether function pointers or something else, that's up to the style)
                for the various extensions specified, as well as any specified OpenGL
                versions/profiles (where applicable).</para>
            <para>A function that will load all function pointers. Even if the style uses static
                linking, a function will still be provided. Until this function is called, you
                cannot use any of the mechanisms to test whether an extension is loaded, nor can you
                call any other functions.</para>
            <para>The function's return value will be a status code saying whether it succeeded.
                Success is defined solely in terms of loading the specified core OpenGL version;
                therefore, WGL or GLX loader functions will always <quote>succeed</quote>. Test for
                individual extensions if you want to know what happened there.</para>
            <para>Optionally, if using the OpenGL specification, the style will export a number of
                useful helper functions to query OpenGL version information. This is
    <para>The different types of styles will decide what form these take (enumerators could be
            <literal>const</literal> variables of some kind instead of the usual
            <literal>#define</literal>s, for example). But each style must provide this set of
        <title>Available styles</title>
        <para>There are a number of styles available, with documentation explaining exactly what
            they generate:</para>
                <para><link xlink:href="Style_Pointer_C"><literal>pointer_c</literal></link>: Used
                    for C-style header generation.</para>
                <para><link xlink:href="Style_Pointer_CPP"><literal>pointer_cpp</literal></link>:
                    Used for creating C++-style loaders, wrapping as much as possible in
                <para><link xlink:href="Style_No_Load_CPP"><literal>noload_cpp</literal></link>:
                    Used for creating C++-style loaders that you don't need to call an
                    initialization function on to get function pointers.</para>
        <title>Core Extensions</title>
        <para>OpenGL 3.0 introduced the concept of <quote>core extensions.</quote> Normally with
            extensions, even ARB extensions, the enumerators, functions and typedefs end in the
            extension type suffix: ARB, EXT, etc. This allows any extension that is to be inducted
            into the core to have its behavior modified where necessary.</para>
        <para>With GL 3.0, the ARB changed things by making certain extensions core extensions.
            These are extensions where their declarations don't have the extension suffix. This
            represents APIs that do not change between extension and core. As such, the
            enums/functions/etc are considered part of both the extension and a version of
        <para>Core extensions are special in that part of OpenGL is effectively in an extension. For
            example, ARB_uniform_buffer_object is a core extension; all of the functions/enums it
            defines are part of GL 3.1 as well as the extension.</para>
        <para>Because of this, it is possible to ask for GL 3.1 (which will provide those
            functions/enums) <emphasis>and</emphasis> ARB_uniform_buffer_object at the same time. Or
            to ask for GL 3.0 (where it isn't core) and ARB_uniform_buffer_object. Or to ask for GL
            3.1 <emphasis>without</emphasis> explicitly asking for ARB_uniform_buffer_object.</para>
        <para>The way this works is as follows. If you explicitly ask for an extension, the system
            will <emphasis>always</emphasis> provide you a way to query whether that extension is
            loaded. If you don't ask for the extension, but the version number effectively requires
            that extension (asking for GL 3.1+ requires ARB_uniform_buffer_object), you'll still get
            the enums and functions, but you <emphasis>won't</emphasis> get a way to query whether
            that extension specifically is loaded.</para>
        <para>In short, if you want GL 4.2, but you want to verify whether particular parts are
                <quote>available,</quote> (ie: if you only get GL version 4.1, but there are 4.2
            features exposed via extensions), you must explicitly request each extension.</para>
        <title>Compatibility profile</title>
        <para>The compatibility profile complicates code generation a bit. The system will do its
            best to cull inappropriate enumerators/functions based on core/compatibility.</para>
        <para>However, this may not be possible in every case. For example, take the
                <literal>GL_QUADS</literal> enumerator. This enumerator was defined way back in GL
            1.1. But 3.1 removed it, so if you ask for a 3.1 core header, you shouldn't get
        <para>The problem is that it didn't <emphasis>stay</emphasis> removed. GL
            4.0/ARB_tessellation_shader brought it back (though only as a tessellation target).
            Which means if you ask for a 3.1 core header it should be gone, but if you ask for a 3.1
            core header that includes ARB_tessellation_shader, it should return. As it should if you
            ask for a 4.1 core header (with or without ARB_tessellation_shader).</para>
        <para>This system cannot due that, primarily because the source of the OpenGL specification
            information (the .spec files, as processed through various scripts) does not provide
            enough information. The spec files only define what is core or compatibility in the
            current OpenGL version, not what <emphasis>used</emphasis> to be core for a while, then
            was only in compatibility, then came back into core.</para>
        <para>Therefore, the system errs on the side of being inclusive. If it came back into core,
            it is considered to have never <emphasis>left</emphasis> core OpenGL. Thus, if you ask
            the system for 3.1, core profiles, you will see <literal>GL_QUADS</literal>, as well as
            a few others. The number of these are rather few, so it should not be a problem.</para>
        <title>User-created styles</title>
        <para>The style mechanism is designed to be extensible. You should be able to write new
            styles relatively painlessly. This system is <link xlink:href="Style_Creation">covered
                in some detail</link>. The general idea is that you write a Lua script that exposes
            a number of functions. Then you hook this script into the build system by adding a line
            of code to one of the files. At which point, you're done. Making the style script is the
            hard part.</para>