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, with each window having its own context.


@section context_object Context handles

The @ref GLFWwindow object encapsulates both a [window](@ref window) and
a context.  It is created with @ref glfwCreateWindow and destroyed with @ref
glfwDestroyWindow or @ref glfwTerminate.  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.  Object
sharing is implemented by the operating system and graphics driver and is
described in the OpenGL and OpenGL ES 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 make OpenGL or OpenGL ES calls, you need to have a current
context of the proper type.  The context encapsulates all render state and all
objects like textures and shaders.

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

@note A context must only be current for a single thread at a time, and a thread
must only have a single context current at a time.


@section context_swap Swapping buffers

Buffer swapping is part of the window and framebuffer, not the context.  See
@ref window_swap in the window handling guide.


@section context_glext OpenGL and OpenGL ES extensions

One of the benefits of OpenGL and OpenGL ES are their extensibility.
Hardware vendors may include extensions in their implementations that extend the
API before that functionality is included in a new version of the OpenGL or
OpenGL ES specification, and some extensions are never included and remain
as extensions until they become obsolete.

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.  The extension above was created by the ARB, but there
are many different affixes, like `NV` for Nvidia and `AMD` for, well, AMD.  Any
group may also use the generic `EXT` affix.  Lists of extensions, together with
their specifications, can be found at the
[OpenGL Registry](http://www.opengl.org/registry/) and
[OpenGL ES Registry](https://www.khronos.org/registry/gles/).


@subsection context_glext_auto Using an extension loader library

This is the easiest and best way to load extensions and newer versions of the
OpenGL or OpenGL ES API.  One such library is
[glad](https://github.com/Dav1dde/glad) and there are several others.  They will
take care of all the details of declaring and loading everything you need.

The following example will use glad, but other extension loader libraries work
similary.

First you need to generate the source files using the glad Python script.  This
example generates a loader for any version of OpenGL, which is the default for
both GLFW and glad, but loaders for OpenGL ES, as well as loaders for specific
API versions and extension sets can be generated.  The generated files are
written to the `output` directory.

@code{.sh}
python main.py --no-loader --out-path output
@endcode

@note The `--no-loader` option is used because GLFW already provides a function
for loading OpenGL and OpenGL ES function pointers and glad can use this instead
of having to add its own.

Add the generated `output/src/glad.c`, `output/include/glad/glad.h` and
`output/include/KHR/khrplatform.h` files to your build.  Then you need to
include the glad header file, which will replace the OpenGL header of your
development environment.

@code
#include <glad/glad.h>
#include <GLFW/glfw3.h>
@endcode

Finally you need to initialize glad once you have a matching current context.

@code
window = glfwCreateWindow(640, 480, "My Window", NULL, NULL);
if (!window)
{
    ...
}

glfwMakeContextCurrent(window);

gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
@endcode

Once glad has been loaded, you have access to all OpenGL core and extension
functions supported by the context you created and you are ready to start
rendering.

You can specify a minimum required OpenGL or OpenGL ES version with
[context hints](@ref window_hints_ctx).  If your needs are more complex, you can
check the actual OpenGL or OpenGL ES version with
[context attributes](@ref window_attribs_context), or you can check whether
a specific version is supported by the current context with the
`GLAD_GL_VERSION_x_x` booleans.

@code
if (GLAD_GL_VERSION_3_2)
{
    // Call OpenGL 3.2+ specific code
}
@endcode

To check whether a specific extension is supported, use the `GLAD_GL_xxx`
booleans.

@code
if (GLAD_GL_ARB_debug_output)
{
    // Use GL_ARB_debug_output
}
@endcode


@subsection context_glext_manual Loading extensions manually

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.  GLFW provides @ref glfwExtensionSupported and @ref
glfwGetProcAddress for manual loading of extensions and new API functions.

@note It is strongly recommended that you use an existing extension loader
library like [glad](https://github.com/Dav1dde/glad) instead of loading
manually.  Extension loading is a solved problem and you will gain nothing from
solving it again by hand.


@subsubsection 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` (pointer
to function) and `PROC` (procedure) are added to the ends.


@subsubsection 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.


@subsubsection context_glext_proc Fetching function pointers

Many extensions, though not all, require the use of new OpenGL functions.
These functions often do not have entry points in the client API libraries of
your operating system, making it necessary to fetch them at run time.  You can
retreive pointers to these functions with @ref glfwGetProcAddress.

@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

*/