magnum/doc/opengl-wrapping.dox

/*
    This file is part of Magnum.

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              Vladimír Vondruš <mosra@centrum.cz>

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namespace Magnum {
/** @page opengl-wrapping OpenGL wrapping layer
@brief Overview of the base OpenGL wrapper API

-   Previous page: @ref plugins
-   Next page: @ref shaders

OpenGL wrapper classes are core part of Magnum. Their purpose is to simplify
interaction with the OpenGL API using type-safe C++11 features, abstracting
away extension and platform differences, tracking the state for optimum
performance and selecting the best available code path for given system.

@tableofcontents

Magnum provides wrappers for most native OpenGL objects like buffers, textures,
meshes, queries, transform feedback objects, shaders etc., but makes it
possible to use raw GL calls or combine Magnum with third-party OpenGL
libraries if the user wants to.

@section opengl-wrapping-instances OpenGL object wrapper instances

By default, all underlying OpenGL objects are created in wrapper class
constructor and deleted in wrapper class destructor. Constructing an object
using default constructor requires active @ref Context instance. All OpenGL
objects are movable (but not copyable), although for performance reasons (and
contrary to standard C++11 practice), the moved-from instance does *not* have
any associated OpenGL object and is thus in *invalid state*. Using instance in
moved-from state may result in OpenGL errors being generated, in some cases
even application crashes.

Besides the default behavior, it is possible to construct the object without
creating the underlying OpenGL object using the @ref NoCreate tag. Constructing
the object this way does not require any active context and its state is then
equivalent to the moved-from state. It is useful in case you need to construct
the object before creating context (such as class members) or if you know you
would overwrite it later with another object:
@code
Mesh mesh{NoCreate};
Buffer vertices{NoCreate}, indices{NoCreate};
std::tie(mesh, vertices, indices) = importSomeMesh();
@endcode

If you need to preserve the underlying OpenGL object after destruction, you can
call `release()`. It returns ID of the underlying object, the instance is then
equivalent to moved-from state and you are responsible for proper deletion
of the returned OpenGL object (note that it is possible to just query ID of the
underlying without releasing it using `id()`). It is also possible to do the
opposite -- wrapping existing OpenGL object ID into Magnum object instance
using `wrap()`.
@code
// Transferring the instance to external library
{
    Buffer buffer;
    buffer.setData(...);
    GLuint id = buffer.release();
    externallibrary.setSomeBuffer(id); // the library is responsible for deletion
}

// Acquiring an instance from external library
{
    GLuint id = externallibrary.someBuffer();
    Buffer buffer = Buffer::wrap(id, ObjectFlag::DeleteOnDestruction);
    // we are now responsible for deletion
}
@endcode

The `NoInit` constructor, `wrap()` and `release()` functions are available for
all OpenGL classes except @ref Shader and @ref AbstractShaderProgram, where
wrapping external instances makes less sense.

@section opengl-state-tracking State tracking and interaction with third-party code

It is possible (and encouraged) to combine Magnum with third-party libraries or
even raw OpenGL calls -- trying features that are not yet implemented in
Magnum, using some specialized GUI library etc. But bear in mind that to
improve performance and avoid redundant state changes, Magnum internally tracks
OpenGL state such as currently bound objects, activated renderer features etc.
When combining Magnum with third-party code, the internal state tracker may get
confused and you need to reset it using @ref Context::resetState():
@code
Buffer buffer;

// Raw OpenGL calls
glBindBuffer(GL_ARRAY_BUFFER, buffer.id());
glBufferStorage(GL_ARRAY_BUFFER, 32768, GL_MAP_READ_BIT|GL_MAP_WRITE_BIT);
// ...

// Reset buffer-related state tracker
Context::current()->resetState(Context::State::Buffers);

// Use the buffer through Magnum again
auto data = buffer.map<UnsignedInt>(...);
@endcode

Note that it is currently not possible to do the opposite -- reseting all state
touched by Magnum to previous values -- as it would involve impractically large
amount of queries and state switches with serious performance impact.

Magnum by default uses VAOs -- each time a @ref Mesh is drawn or configured,
its VAO is bound, but it is *not* unbound afterwards to avoid needless state
changes. This may introduce problems when using third-party OpenGL code that
does not use VAOs -- it will break internal state of Mesh that was used most
recently. The solution, besides state resetting, is to create a new VAO and
bind it every time the third-party OpenGL code is called.

@section opengl-wrapping-dsa Extension-dependent functionality

While the majority of Magnum API stays the same on all platforms and driver
capabilities, large portion of the functionality needs to be realized under the
hood using various different OpenGL API calls based on available extensions. If
required extension is not available, there are two possible outcomes -- either
given API is simply not available or it is emulated using older functionality.

In the first case, to avoid performance overhead, Magnum does not check that
you use only the APIs that are implemented in the driver -- you are expected to
do the checks. Documentation of each type, function and enum value explicitly
states whether the functionality is available everywhere or whether particular
GL version/extension is required. The information is also aggregated on
@ref opengl-required-extensions documentation page. Use
@ref Context::isVersionSupported() or @ref Context::isExtensionSupported():
@code
TextureFormat format;
if(Context::current()->isExtensionSupported<Extensions::GL::ARB_depth_buffer_float>())
    format = TextureFormat::DepthComponent32F;
else
    format = TextureFormat::DepthComponent24;
@endcode

Some functionality can be emulated by Magnum -- it detects available extensions
and selects best possible code path for optimal performance. On startup, the
application prints list of extensions that were used to improve the default
functionality. The most prominent feature is @extension{ARB,direct_state_access}
(part of OpenGL 4.3) and its predecessor @extension{EXT,direct_state_access}.
These extensions make it possible to modify OpenGL objects without explicitly
binding them, reducing the amount of needed API calls. Magnum API is designed
around direct state access as it is far easier to use and less error-prone, but
if these extensions are not available, the functionality is emulated through
classic bind-to-edit approach. Other examples of extension-dependent
functionality is @ref DebugMessage "debug output" which is simply no-op when
required extensions are not available, @ref Texture::setStorage() emulation on
platforms that don't support it etc. The goal is to abstract away the (mostly
unimportant) differences for easier porting.
@code
Texture2D texture;

// - on OpenGL 4.5+/ARB_direct_state_access this calls glTextureStorage2D()
// - if EXT_direct_state_access is available, calls glTextureStorage2DEXT()
// - on OpenGL 4.2+/ARB_texture_storage and OpenGL ES 3.0+ calls glTexStorage2D()
// - on OpenGL ES 2.0 with EXT_texture_storage calls glTexStorage2DEXT()
// - otherwise emulated using a sequence of four glTexImage2D() calls
texture.setStorage(4, TextureFormat::RGBA8, {256, 256});
@endcode

-   Previous page: @ref plugins
-   Next page: @ref shaders

*/
}