magnum/doc/platform.dox

/*
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              Vladimír Vondruš <mosra@centrum.cz>

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namespace Magnum {
/** @page platform Platform support
@brief Integration into windowing toolkits and creation of windowless contexts.

@tableofcontents
@m_footernavigation

@ref Platform namespace contains classes integrating Magnum engine into
various toolkits, both windowed and windowless. Each class has slightly
different dependencies and platform requirements, see documentation of
@ref Platform namespace and particular `*Application` classes for more
information about building and usage with CMake.

All the classes have common API to achieve static polymorphism, so basically
you can use different toolkits on different platforms and the only thing you
need to change is the class name, everything else is the same. Basic usage is
to subclass the chosen `*Application` class and implement required methods.

@section platform-windowed Windowed applications

Windowed applications provide a window and keyboard and mouse handling. The
de-facto standard and most widely used toolkit is SDL2, which is implemented in
@ref Platform::Sdl2Application. As said above, the usage is similar for all
toolkits, you must provide one-argument constructor and implement at least
@ref Platform::Sdl2Application::drawEvent() "drawEvent()" function. The class
can be then used directly in @cpp main() @ce, but for convenience and
portability it's better to use @ref MAGNUM_SDL2APPLICATION_MAIN() macro.

To simplify the porting, the library provides @cpp Platform::Application @ce
typedef and @cpp MAGNUM_APPLICATION_MAIN() @ce macro (but only if only one
application header is included, to avoid ambiguity). Changing the code to use
different toolkit is then matter of replacing only the @cpp #include @ce
statement (and changing one line in CMake build script, as you see later).

Barebone application implementation which will just clear the window to dark
blue color is shown in the following code listing.

@note Fully contained base application along with CMake setup is available in
    `base` branch of [Magnum Bootstrap](https://github.com/mosra/magnum-bootstrap)
    repository.

@snippet Platform.cpp windowed

@subsection platform-windowed-viewport Responding to viewport size changes

By default the application doesn't respond to window size changes in any way,
as the window has fixed size in most cases. To respond to size change for
example by resizing the default framebuffer, you need to reimplement
@ref Platform::Sdl2Application::viewportEvent() "viewportEvent()" function and
pass the new size to the framebuffer:

@snippet Platform.cpp size

@section platform-windowless Windowless applications

Windowless applications provide just a context for ofscreen rendering or
performing tasks on GPU. There is not yet any platform-independent toolkit
which could handle this in portable way, thus you have to use platform-specific
ones. Magnum provides windowless applications for X11-based Unix, macOS and
Windows. To make things simple, as an example we will use only
@ref Platform::WindowlessEglApplication, see link for bootstrap application
below for fully portable example.

You need to implement just @ref Platform::WindowlessEglApplication::exec() "exec()"
function. The class can be then used directly in @cpp main() @ce, but again,
for convenience and portability it's better to use
@ref MAGNUM_WINDOWLESSEGLAPPLICATION_MAIN() macro.

Similarly as with windowed applications, to simplify the porting, the library
provides @cpp Platform::WindowlessApplication @ce typedef and
@cpp MAGNUM_WINDOWLESSAPPLICATION_MAIN() @ce macro, but only if just one
windowless application header is included. Changing the code to use different
toolkit is then matter of replacing only the @cpp #include @ce statement.
Aliases for windowless applications are separated from aliases for windowed
applications, because projects commonly contain both graphics application and
command-line tools (for data preparation etc.).

Barebone application which will just print out current OpenGL version and
renderer string and exits is in the following code listing.

@note Fully contained windowless application using @ref Platform::WindowlessCglApplication
    on macOS, @ref Platform::WindowlessGlxApplication on Unix and
    @ref Platform::WindowlessWglApplication on Windows along with CMake setup
    is available in `windowless` branch of [Magnum Bootstrap](https://github.com/mosra/magnum-bootstrap)
    repository.

@snippet Platform-windowless.cpp windowless

@section platform-compilation Compilation with CMake

Barebone compilation consists just of finding Magnum library with, for example,
`Sdl2Application` component, compilation of the executable and linking
`Magnum::Magnum` and `Magnum::Sdl2Application` to it.

Again, to simplify porting, you can also use generic `Magnum::Application`
aliases (or `Magnum::WindowlessApplication` for windowless applications), but
only if only one application (windowless application) component is requested to
avoid ambiguity. Changing the build script to use different toolkit is then
matter of replacing only the requested `*Application` component (and one
@cpp #include @ce line in the actual code, as said above).

@code{.cmake}
find_package(Magnum REQUIRED Sdl2Application)

add_executable(myapplication MyApplication.cpp)
target_link_libraries(myapplication
    Magnum::Magnum
    Magnum::Application)
@endcode

@section platform-configuration Specifying configuration

By default the application is created with some reasonable defaults (e.g.
window size 800x600 pixels). If you want something else, you can pass
@ref Platform::Sdl2Application::Configuration "Configuration" instance to
application constructor. Using method chaining it can be done conveniently like
this:

@snippet Platform.cpp configuration

@subsection platform-configuration-delayed Delayed context creation

Sometimes you may want to set up the application based on a configuration file
or system introspection, which can't really be done inside the base class
initializer. You can specify @ref NoCreate in the constructor instead and pass
the @relativeref{Platform::Sdl2Application,Configuration} later to a
@relativeref{Platform::Sdl2Application,create()} function:

@snippet Platform.cpp createcontext

If context creation in the constructor or in
@relativeref{Platform::Sdl2Application,create()} fails, the application prints
an error message to standard output and exits. While that frees you from having
to do explicit error handling, sometimes a more graceful behavior may be
desirable --- with @relativeref{Platform::Sdl2Application,tryCreate()} the
function returns @cpp false @ce instead of exiting and it's up to you whether
you abort the launch or retry with different configuration. You can for example
try enabling MSAA first, and if the context creation fails, fall back to no-AA
rendering:

@snippet Platform.cpp trycreatecontext

<b></b>

@m_class{m-block m-warning}

@par Implications for GL objects as class members
    Delaying GL context creation to
    @relativeref{Platform::Sdl2Application,create()} /
    @relativeref{Platform::Sdl2Application,tryCreate()} means that, at the
    point when class members get constructed, the context isn't available yet.
    If you have GL objects such as @ref GL::Mesh or @ref Shaders::PhongGL as
    class members, the application may hit the following assert during startup:
@par
    @code{.shell-session}
    GL::Context::current(): no current context
    @endcode
@par
    The solution is to construct the GL objects with @ref NoCreate constructors
    as well and populate them with live instances only after the context has
    been created. See @ref opengl-wrapping-instances-nocreate for more
    information.

@section platform-custom Using custom platform toolkits

In case you want to use some not-yet-supported toolkit or you don't want to use
the application wrappers in @ref Platform namespace, you can initialize Magnum
manually. First create OpenGL context and then create instance of
@ref Platform::GLContext class, which will take care of proper initialization
and feature detection. The instance must be alive for whole application
lifetime. Example @cpp main() @ce function with manual initialization is in the
following code listing.

@note Fully contained application using with manual Magnum initialization on
    top of Qt toolkit is available in `base-qt` branch of
    [Magnum Bootstrap](https://github.com/mosra/magnum-bootstrap/tree/base-qt)
    repository. Similar project, but for wxWidgets and gtkmm are in the
    [base-wxwidgets](https://github.com/mosra/magnum-bootstrap/tree/base-wxwidgets)
    and [base-gtkmm](https://github.com/mosra/magnum-bootstrap/tree/base-gtkmm)
    branches.
@note
    There's also an example showing @ref examples-triangle-plain-glfw "usage of plain GLFW to render a basic triangle".

@snippet Platform-custom.cpp custom

@attention The @ref Platform::GLContext instance is bound to a single OpenGL
    context, which must be always set as current when calling any Magnum APIs
    touching OpenGL state.

On majority of platforms the @ref Platform::GLContext class does GL function
pointer loading using platform-specific APIs. In that case you also need to
find particular `*Context` library, add its include dir and then link to it.
These platform-specific libraries are available:

-   `CglContext` --- CGL context (macOS)
-   `EglContext` --- EGL context (everywhere except Emscripten)
-   `GlxContext` --- GLX context (X11-based Unix)
-   `WglContext` --- WGL context (Windows)

Systems not listed here (such as Emscripten) don't need any `Context` library,
because dynamic function pointer loading is not available on these.

For example, when you create the OpenGL context using GLX, you need to find
`GlxContext` component, and link to `Magnum::GlxContext` target. Similarly to
application libraries, you can also use the generic `Magnum::GLContext` target,
providing you requested only one `*Context` component in the
@cmake find_package() @ce call. Complete example:

@code{.cmake}
find_package(Magnum REQUIRED GlxContext)

add_executable(myapplication MyCustomApplication.cpp)
target_link_libraries(myapplication
    Magnum::Magnum
    Magnum::GLContext)
@endcode

@section platform-windowless-contexts Manually managing windowless contexts

In case you need to manage windowless OpenGL contexts manually (for example
to use Magnum for data processing in a thread or when having more than one
OpenGL context), there is a possibility to directly use the context wrappers
from windowless applications. Each @ref Platform::WindowlessEglApplication "Platform::Windowless*Application"
is accompanied by a @ref Platform::WindowlessEglContext "Platform::Windowless*Context"
class that manages just GL context creation, making it current and destruction.
Similarly to using custom platform toolkits above, the workflow is to first
create a GL context instance, then making it current and finally instantiating
the @ref Platform::GLContext instance to initialize Magnum.

Similarly as with the applications, to simplify the porting, the library
provides @cpp Platform::WindowlessGLContext @ce typedef, but only if just one
windowless application header is included.

@attention With this approach it is possible to switch between different GL
    contexts, but make sure that Magnum is used only with its own OpenGL
    context.

@snippet Platform-windowless-custom.cpp custom

The main purpose of windowless contexts is threaded OpenGL, used for example
for background data processing. The workflow is to create the windowless
context on the main thread, but make it current in the worker thread. This way
the main thread state isn't affected so it can have any other GL context
current (for example for the main application rendering). See
@ref GL-Context-multithreading and @ref CORRADE_BUILD_MULTITHREADED for more
information.

@note Context creation is not thread safe on all platforms, that's why it still
    has to be done on the main thread.

@snippet Platform-windowless-thread.cpp thread
*/
}