glfw/docs/context.dox

/*!

@page context Context handling guide

@tableofcontents

The primary purpose of GLFW is to provide a simple interface to window
management and OpenGL and OpenGL ES context creation.  GLFW supports
multiple windows, each of which has its own context.


@section context_object Context handles

The @ref GLFWwindow object encapsulates both a window and a context.  They are
created with @ref glfwCreateWindow and destroyed with @ref glfwDestroyWindow (or
@ref glfwTerminate, if any remain).  As the window and context are inseparably
linked, the object pointer is used as both a context and window handle.


@section context_hints Context creation hints

There are a number of hints, specified using @ref glfwWindowHint, related to
what kind of context is created.  See
[context related hints](@ref window_hints_ctx) in the window handling guide.


@section context_sharing Context object sharing

When creating a window and context with @ref glfwCreateWindow, you can specify
another window whose context the new one should share its objects with.  OpenGL
object sharing is implemented by the operating system and graphics driver and is
described in the OpenGL documentation.  On platforms where it is possible to
choose which types of objects are shared, GLFW requests that all are shared.


@section context_current Current context

Before you can use the OpenGL or OpenGL ES APIs, you need to have a current
context of the proper type.  The context encapsulates all render state and all
objects like textures and shaders.

Note that a context can only be current for a single thread at a time, and
a thread can only have a single context at a time.

A context is made current with @ref glfwMakeContextCurrent.

@code
glfwMakeContextCurrent(window);
@endcode

The current context is returned by @ref glfwGetCurrentContext.

@code
GLFWwindow* window = glfwGetCurrentContext();
@endcode


@section context_swap Swapping buffers

See [swapping buffers](@ref window_swap) in the window handling guide.


@section context_glext OpenGL extension handling

One of the benefits of OpenGL is its extensibility.  Independent hardware
vendors (IHVs) may include functionality in their OpenGL implementations that
expand upon the OpenGL standard before that functionality is included in a new
version of the OpenGL specification.

An extension is defined by:

- An extension name (e.g. `GL_ARB_debug_output`)
- New OpenGL tokens (e.g. `GL_DEBUG_SEVERITY_HIGH_ARB`)
- New OpenGL functions (e.g. `glGetDebugMessageLogARB`)

Note the `ARB` affix, which stands for Architecture Review Board and is used
for official extensions.  There are many different affixes, depending on who
wrote the extension.  A list of extensions, together with their specifications,
can be found at the [OpenGL Registry](http://www.opengl.org/registry/).

To use a certain extension, you must first check whether the context supports
that extension and then, if it introduces new functions, retrieve the pointers
to those functions.

This can be done with GLFW, as will be described in this section, but usually
you will instead want to use a dedicated extension loading library such as
[GLEW](http://glew.sourceforge.net/).  This kind of library greatly reduces the
amount of work necessary to use both OpenGL extensions and modern versions of
the OpenGL API.  GLEW in particular has been extensively tested with and works
well with GLFW.


@subsection context_glext_header The glext.h header

The `glext.h` header is a continually updated file that defines the interfaces
for all OpenGL extensions.  The latest version of this can always be found at
the [OpenGL Registry](http://www.opengl.org/registry/).  It it strongly
recommended that you use your own copy, as the one shipped with your development
environment may be several years out of date and may not include the extensions
you wish to use.

The header defines function pointer types for all functions of all extensions it
supports.  These have names like `PFNGLGETDEBUGMESSAGELOGARB` (for
`glGetDebugMessageLogARB`), i.e. the name is made uppercase and `PFN` and `PROC`
are added to the ends.


@subsection context_glext_string Checking for extensions

A given machine may not actually support the extension (it may have older
drivers or a graphics card that lacks the necessary hardware features), so it
is necessary to check whether the context supports the extension.  This is done
with @ref glfwExtensionSupported.

@code
if (glfwExtensionSupported("GL_ARB_debug_output"))
{
    // The extension is supported by the current context
}
@endcode

The argument is a null terminated ASCII string with the extension name.  If the
extension is supported, @ref glfwExtensionSupported returns non-zero, otherwise
it returns zero.


@subsection context_glext_proc Fetching function pointers

Many extensions, though not all, require the use of new OpenGL functions.
These entry points are often not exposed by your link libraries, making
it necessary to fetch them at run time.  With @ref glfwGetProcAddress you can
retrieve the address of extension and non-extension OpenGL functions.

@code
PFNGLGETDEBUGMESSAGELOGARB pfnGetDebugMessageLog = glfwGetProcAddress("glGetDebugMessageLogARB");
@endcode

In general, you should avoid giving the function pointer variables the (exact)
same name as the function, as this may confuse your linker.  Instead, you can
use a different prefix, like above, or some other naming scheme.

Now that all the pieces have been introduced, here is what they might look like
when used together.

@code
#include "glext.h"

#define glGetDebugMessageLogARB pfnGetDebugMessageLog
PFNGLGETDEBUGMESSAGELOGARB pfnGetDebugMessageLog;

// Flag indicating whether the extension is supported
int has_debug_output = 0;

void load_extensions(void)
{
    if (glfwExtensionSupported("GL_ARB_debug_output"))
    {
        pfnGetDebugMessageLog = (PFNGLGETDEBUGMESSAGELOGARB) glfwGetProcAddress("glGetDebugMessageLogARB");
        if (pfnGetDebugMessageLog)
        {
            // Both the extension name and the function pointer are present
            has_debug_output = 1;
        }
    }
}

void some_function(void)
{
    // Now the extension function can be called as usual
    glGetDebugMessageLogARB(...);
}
@endcode

*/