magnum/doc/platform.dox

/*
    This file is part of Magnum.

    Copyright © 2010, 2011, 2012, 2013, 2014, 2015
              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.

-   Next page: @ref types

@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.

@tableofcontents

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 `main()`, but for convenience and portability it's
better to use @ref MAGNUM_SDL2APPLICATION_MAIN() macro.

To simplify the porting, the library provides `Platform::Application` typedef
and `MAGNUM_APPLICATION_MAIN()` 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 `#``include` 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.

@code
#include <Magnum/Color.h>
#include <Magnum/DefaultFramebuffer.h>
#include <Magnum/Renderer.h>
#include <Magnum/Platform/Sdl2Application.h>

using namespace Magnum;

class MyApplication: public Platform::Application {
    public:
        MyApplication(const Arguments& arguments);

    private:
        void drawEvent() override;
};

MyApplication::MyApplication(const Arguments& arguments): Platform::Application(arguments) {
    // Set clear color to dark blue
    Renderer::setClearColor({0.0f, 0.0f, 0.4f});
}

void MyApplication::drawEvent() {
    // Clear the window
    defaultFramebuffer.clear(FramebufferClear::Color);

    // The context is double-buffered, swap buffers
    swapBuffers();
}

// main() function implementation
MAGNUM_APPLICATION_MAIN(MyApplication)
@endcode

@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:
@code
class MyApplication: public Platform::Application {
    // ...

    private:
        void viewportEvent(const Vector2i& size) override;
};

// ...

void MyApplication::viewportEvent(const Vector2i& size) {
    defaultFramebuffer.setViewport({{}, size});
}
@endcode

@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, OS X and
Windows. To make things simple, as an example we will use only
@ref Platform::WindowlessGlxApplication, see link for bootstrap application
below for fully portable example.

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

Similarly as with windowed applications, to simplify the porting, the library
provides `Platform::WindowlessApplication` typedef and `MAGNUM_WINDOWLESSAPPLICATION_MAIN()`
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
`#``include` 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 OS X, @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.

@code
#include <Magnum/Context.h>
#include <Magnum/Platform/WindowlessGlxApplication.h>

using namespace Magnum;

class MyApplication: public Platform::WindowlessApplication {
    public:
        MyApplication(const Arguments& arguments);

        int exec() override;
};

MyApplication::MyApplication(const Arguments& arguments): Platform::WindowlessApplication(arguments) {}

int MyApplication::exec() {
    Debug() << "OpenGL version:" << Context::current()->versionString();
    Debug() << "OpenGL renderer:" << Context::current()->rendererString();

    // Exit with success
    return 0;
}

// main() function implementation
MAGNUM_WINDOWLESSAPPLICATION_MAIN(MyApplication)
@endcode

@section platform-compilation Compilation with CMake

Barebone compilation consists just of finding Magnum library with required
`*Application` component, adding Magnum's `${MAGNUM_INCLUDE_DIRS}` and
application-specific `${MAGNUM_SDL2APPLICATION_INCLUDE_DIRS}` to include path,
compilation of the executable and linking `${MAGNUM_LIBRARIES}` and
`${MAGNUM_SDL2APPLICATION_LIBRARIES}` to it.

Again, to simplify porting, you can also use generic `${MAGNUM_APPLICATION_INCLUDE_DIRS}`
and `${MAGNUM_APPLICATION_LIBRARIES}` aliases (or `${MAGNUM_WINDOWLESSAPPLICATION_INCLUDE_DIRS}`,
`${MAGNUM_WINDOWLESSAPPLICATION_LIBRARIES}` 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
`#``include` line in the actual code, as said above).

@code
find_package(Magnum REQUIRED Sdl2Application)

include_directories(${MAGNUM_INCLUDE_DIRS} ${MAGNUM_APPLICATION_INCLUDE_DIRS})

add_executable(myapplication MyApplication.cpp)
target_link_libraries(myapplication
    ${MAGNUM_LIBRARIES}
    ${MAGNUM_APPLICATION_LIBRARIES})
@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:
@code
MyApplication::MyApplication(int& argc, char** argv):
    Platform::Application(argc, argv, Configuration()
        .setTitle("My Application")
        .setSize({800, 600}))
{
    // ...
}
@endcode

However, sometimes you would need to configure the application based on some
configuration file or system introspection. In that case you can pass `nullptr`
instead of @ref Platform::Sdl2Application::Configuration "Configuration"
instance and then specify it later with @ref Platform::Sdl2Application::createContext() "createContext()":
@code
MyApplication::MyApplication(int& argc, char** argv): Platform::Application(argc, argv, nullptr) {
    // ...

    createContext(Configuration()
        .setTitle("My Application")
        .setSize(size));

    // ...
}
@endcode

If the context creation in constructor or @ref Platform::Sdl2Application::createContext() "createContext()"
fails, the application exits. However, it is also possible to negotiate the
context using @ref Platform::Sdl2Application::tryCreateContext() "tryCreateContext()".
The only difference is that this function returns `false` instead of exiting.
You can for example try enabling MSAA and if the context creation fails, fall
back to no-AA rendering:
@code
MyApplication::MyApplication(int& argc, char** argv): Platform::Application(argc, argv, nullptr) {
    // ...

    Configuration conf;
    conf.setTitle("My Application")
        .setSampleCount(16);

    if(!tryCreateContext(conf))
        createContext(conf.setSampleCount(0));

    // ...
}
@endcode

@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::Context class, which will take care of proper initialization and
feature detection. The instance must be alive for whole application lifetime.
Example `main()` function with manual initialization:
@code
int main(int argc, char** argv) {
    // Create OpenGL context ...

    {
        // Initialize Magnum
        Platform::Context context;

        // Open window, enter main loop ...

        // Magnum context gets destroyed
    }

    // Delete OpenGL context ...

    return 0;
}
@endcode

@attention Currently Magnum is limited to single OpenGL context, which must be
    always set as current.

On majority of platforms the @ref Platform::Context 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 (Mac OS X)
-   `EglContext` -- EGL context (everywhere except Emscripten and NaCl)
-   `GlxContext` -- GLX context (X11-based Unix)
-   `WglContext` -- WGL context (Windows)

Systems not listed here (such as Emscripten or NaCl) 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, include `${MAGNUM_GLCCONTEXT_INCLUDE_DIRS}` and link to
`${MAGNUM_GLXCONTEXT_LIBRARIES}`. Similarly to application libraries, you can
also use generic `${MAGNUM_CONTEXT_INCLUDE_DIRS}` and `${MAGNUM_CONTEXT_LIBRARIES}`,
providing you requested only one `*Context` component in the `find_package()`
call. Complete example:
@code
find_package(Magnum REQUIRED GlxContext)

include_directories(${MAGNUM_INCLUDE_DIRS} ${MAGNUM_CONTEXT_INCLUDE_DIRS})

add_executable(myapplication MyCustomApplication.cpp)
target_link_libraries(myapplication
    ${MAGNUM_LIBRARIES}
    ${MAGNUM_CONTEXT_LIBRARIES})
@endcode

-   Next page: @ref types
*/
}