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116 lines
4.1 KiB
Plaintext
116 lines
4.1 KiB
Plaintext
/*!
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@page glext OpenGL extension handling
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One of the benefits of OpenGL is its extensibility. Independent hardware
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vendors (IHVs) may include functionality in their OpenGL implementations that
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expand upon the OpenGL standard before that functionality is included in a new
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version of the OpenGL specification.
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An extension is defined by:
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- An extension name (e.g. `GL_ARB_debug_output`)
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- New OpenGL tokens (e.g. `GL_DEBUG_SEVERITY_HIGH_ARB`)
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- New OpenGL functions (e.g. `glGetDebugMessageLogARB`)
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Note the `ARB` affix, which stands for Architecture Review Board and is used
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for official extensions. There are many different affixes, depending on who
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wrote the extension. A list of extensions, together with their specifications,
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can be found at the
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[OpenGL Registry](http://www.opengl.org/registry/).
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To use a certain extension, you must first check whether the context exposes
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that extension and then, if it introduces new functions, retrieve the pointers
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to those functions.
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This can be done with GLFW, as will be described in this section, but usually
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you will instead want to use a dedicated extension loading library such as
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[GLEW](http://glew.sourceforge.net/). This kind of library greatly reduces the
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amount of work necessary to use both OpenGL extensions and modern versions of
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the OpenGL API. GLEW in particular has been extensively tested with and works
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well with GLFW.
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@section glext_header The glext.h header
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The `glext.h` header is a continually updated file that defines the interfaces
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for all OpenGL extensions. The latest version of this can always be found at
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the [OpenGL Registry](http://www.opengl.org/registry/). It it strongly
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recommended that you use your own copy, as the one shipped with your development
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environment may be several years out of date and may not include the extensions
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you wish to use.
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The header defines function pointer types for all functions of all extensions it
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supports. These have names like `PFNGLGETDEBUGMESSAGELOGARB` (for
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`glGetDebugMessageLogARB`), i.e. the name is made uppercase, the `gl` prefix is
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removed and `PFN` and `PROC` are added to the ends.
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@section glext_string Checking for extensions
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A given machine may not actually support the extension (it may have older
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drivers or a graphics card that lacks the necessary hardware features), so it
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is necessary to check whether the context exposes the extension. This is done
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with @ref glfwExtensionSupported.
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@code
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if (glfwExtensionSupported("GL_ARB_debug_output"))
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{
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// The extension is exposed by the current context
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}
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@endcode
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The argument is a null terminated ASCII string with the extension name. If the
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extension is exposed, @ref glfwExtensionSupported returns non-zero, otherwise it
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returns zero.
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@section glext_proc Fetching function pointers
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Many extensions, though not all, require the use of new OpenGL functions.
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These entry points are often not exposed by your link libraries, making
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it necessary to fetch them at run time. With @ref glfwGetProcAddress you can
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retrieve the address of extension and non-extension OpenGL functions.
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@code
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PFNGLGETDEBUGMESSAGELOGARB pfnGetDebugMessageLog = glfwGetProcAddress("glGetDebugMessageLogARB");
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@endcode
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In general, you should avoid giving the function pointer variables the (exact)
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same name as the function, as this may confuse your linker. Instead, you can
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use a different prefix, like above, or some other naming scheme.
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Now that all the pieces have been introduced, here is what they might look like
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when used together.
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@code
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#include "glext.h"
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#define glGetDebugMessageLogARB pfnGetDebugMessageLog
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PFNGLGETDEBUGMESSAGELOGARB pfnGetDebugMessageLog;
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// Flag indicating whether the extension is present
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int has_debug_output = 0;
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void load_extensions(void)
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{
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if (glfwExtensionSupported("GL_ARB_debug_output"))
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{
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pfnGetDebugMessageLog = (PFNGLGETDEBUGMESSAGELOGARB) glfwGetProcAddress("glGetDebugMessageLogARB");
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if (pfnGetDebugMessageLog)
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{
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// Both the extension name and the function pointer are present
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has_debug_output = 1;
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}
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}
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}
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void some_function(void)
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{
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// Now the extension function can be called as usual
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glGetDebugMessageLogARB(...);
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}
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@endcode
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*/
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