magnum/doc/plugins.dox

/*
    This file is part of Magnum.

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              Vladimír Vondruš <mosra@centrum.cz>

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namespace Magnum {
/** @page plugins Loading and using plugins
@brief Extending Magnum with additional functionality

@tableofcontents
@m_footernavigation

The base Magnum libraries contain math support, scene graph implementation and
are able to interact with graphics and audio hardware. However, they don't
provide any functions for dealing with specific file formats on their own, as
there are simply too many formats to include all of them directly. Instead, the
libraries provide plugin APIs using which you can load a plugin that's able to
understand a particular file format. Plugins are distributed as additional
libraries and loaded either dynamically from the filesystem or, in case of
embedded systems that don't support dynamic loading, built statically into the
executable. Depending on the target platform requirements, different set of
plugins can be deployed for different platforms.

The @ref building "Magnum repository" contains a few basic plugins for opening
simple formats, such as TGA images or WAV audio files. These plugins are
included because the file formats are so simple that no external library is
needed for loading their contents and thus they are suitable for quick demos
and prototyping without needing to deal with additional dependencies.

Additional plugins (such as importers for PNG and JPEG images, TrueType fonts
etc.) are available in the @ref building-plugins "Magnum Plugins" repository.
Majority of these plugins depends on external libraries, thus not all of them
might be available on all platforms.

@section plugins-types Plugin interfaces

Plugin functionality is exposed via an interface that's common for all plugins
of given type. Magnum provides these plugin interfaces:

-   @ref Trade::AbstractImporter --- importers for general 2D and 3D scene,
    mesh, material, texture and image data. See `*Importer` classes in the
    @ref Trade namespace for available importer plugins.
-   @ref Trade::AbstractImageConverter --- conversion among various image
    formats. See `*ImageConverter` classes in the @ref Trade namespace for
    available image converter plugins.
-   @ref Text::AbstractFont --- font loading and glyph layouting. See `*Font`
    classes in the @ref Text namespace for available font plugins.
-   @ref Text::AbstractFontConverter --- font and glyph cache conversion. See
    `*FontConverter` classes in the @ref Text namespace for available font
    converter plugins.
-   @ref Audio::AbstractImporter --- importers for audio formats. See
    `*Importer` classes in the @ref Audio namespace for available audio
    importer plugins.

@section plugins-loading Loading and instantiating plugins

To load a plugin, you need to instantiate @ref Corrade::PluginManager::Manager
for given plugin interface, for example for the @ref Trade::AbstractImporter.
In order to find plugins on the filesystem, plugin interfaces have a bunch of
plugin search paths hardcoded --- see documentation of the
@ref Trade::AbstractImporter::pluginSearchPaths() "pluginSearchPaths()"
functions provided by each plugin interface. It usually just works, but you can
also pass a plugin directory to the manager constructor explicitly.

Once the plugin manager is instantiated, you can start creating instances of
particular plugins. Keep in mind that the manager instance has to exist for
whole lifetime of all plugin instances created using it.

@snippet plugins.cpp loading

@section plugins-dependencies Plugin dependencies

Some plugins have dependencies, for example the @ref Trade::OpenGexImporter "OpenGexImporter"
plugin uses @ref Trade::AnyImageImporter "AnyImageImporter" to load texture
data. The dependency loading is done automatically, but in some cases it's
across plugin types (for example the @ref Text::MagnumFont "MagnumFont" plugin
depends on the @ref Trade::TgaImporter "TgaImporter" plugin, which is of a
different type) --- there you need to create manager instances for all required
plugin interfaces:

@snippet plugins.cpp dependencies

@section plugins-static Static plugins

By default, on desktop systems at least, all plugins are built as dynamic ones,
i.e. they are a separate binary which gets linked in at runtime. This is good
for reducing memory consumption, as the code is loaded in memory only for the
time it is actually needed. However, if you need to port to a platform which
does not support dynamic linking or you simply want to have the plugin loaded
at all times, you can use static plugins.

The plugins are built as static if the `BUILD_PLUGINS_STATIC` CMake parameter
is enabled (see @ref building and @ref building-plugins for more information).
The actual usage in the code is basically the same as above, but you need to
explicitly find the plugin and link it into the executable. If you use CMake,
it would look like this:

@code{.cmake}
find_package(MagnumPlugins REQUIRED
    TinyGltfImporter
    StbTrueTypeFont)

add_executable(my-app my-app.cpp)
target_link_libraries(my-app PRIVATE
    MagnumPlugins::TinyGltfImporter
    MagnumPlugins::StbTrueTypeFont)
@endcode

The only user-visible behavioral change in the code will be that
@ref Corrade::PluginManager::AbstractManager::load() "PluginManager::Manager::load()"
will return @ref Corrade::PluginManager::LoadState::Static "PluginManager::LoadState::Static"
instead of @ref Corrade::PluginManager::LoadState::Loaded "PluginManager::LoadState::Loaded",
but there is no need to change the above code, as it will work for both dynamic
and static case.

The static plugin needs to be explicitly registered so the plugin manager is
aware it's there. If you use CMake 3.1 and newer it's done automatically.
Otherwise, you can achieve the same if you explicitly @cpp #include @ce the
static plugin import file (named `importStaticPlugin.cpp`, residing in plugin's
include directory) in a source file that gets compiled into your main
application. The static import file contains just a simple call to
@ref CORRADE_PLUGIN_IMPORT(), see its documentation for further information.

@snippet plugins.cpp static-import

@section plugins-aliases Plugin aliases and "any" plugins

There's usually more than one plugin available to achieve a particular goal ---
for example in order to open a PNG file, you can choose among
@ref Trade::PngImporter "PngImporter", @ref Trade::StbImageImporter "StbImageImporter"
or @ref Trade::DevIlImageImporter "DevIlImageImporter" plugins. Rather than
this being an unnecessary redundancy, it allows you to pick a particular
performance / portability / feature tradeoff --- a plugin with no external
dependencies for a web build or, on the other hand, the fastest possible
implementation for a tool that does heavy image processing.

To make this simpler in the code and defer the decision to the buildsystem or
app deployment, all plugins that support a particular format provide a common
*alias* --- in case of PNG images, you can just load a @cpp "PngImporter" @ce
plugin and if  @ref Trade::PngImporter "PngImporter" is not available, it will
pick up any of the other candidates.

For greater control you can also use
@ref Corrade::PluginManager::AbstractManager::setPreferredPlugins() "setPreferredPlugins()",
giving a prioritized candidate list for a particular alias. This is especially
useful in case of font plugins, where you can get advanced layout capabilities
if the @ref Text::HarfBuzzFont "HarfBuzzFont" plugin is available or at least a
faster and smoother glyph rendering if you can get the @ref Text::FreeTypeFont "FreeTypeFont"
plugin. If none of the suggestions is available, it falls back to whatever is
left (which can be, for example, the @ref Text::StbTrueTypeFont "StbTrueTypeFont"
plugin).

@snippet plugins.cpp aliases

To make your life even simpler, there are various "any format" plugins ---
these detect format based on file extension, map it to a plugin alias and then
proxy the rest of the work to a corresponding plugin:

@snippet plugins.cpp anyimporter

Besides convenience for the end user, this allows for example the scene
importer plugins to load any image formats without needing to have a knowledge
about them or having an explicit dependency on multitude of image importer
plugins.

So far, the following plugins have the "any format" ability:

-   @ref Trade::AnyImageImporter "AnyImageImporter" --- imports any image
    format
-   @ref Trade::AnyImageConverter "AnyImageConverter" --- converts to any image
    format
-   @ref Trade::AnySceneImporter "AnySceneImporter" --- imports any scene
    format
-   @ref Audio::AnyImporter "AnyImporter" --- imports any audio format

@section plugins-configuration Plugin-specific configuration

Because it's not possible for a general statically typed plugin API to expose
all possible knobs and switches that a file format could support, the plugins
have a possibility to supply additional configuration via the
@ref Corrade::PluginManager::AbstractPlugin::configuration() "configuration()"
function. For example, in the @ref Trade-AssimpImporter-configuration "AssimpImporter"
plugin you can toggle various postprocessing steps that are applied to loaded
scene files:

@snippet plugins.cpp configuration

Besides affecting a single plugin instance, you can also change the
configuration globally via @ref Corrade::PluginManager::PluginMetadata::configuration() "PluginManager::PluginMetadata::configuration()".
That will affect all plugin instances created after. Resetting the global
configuration back to the initial state can be done by recreating the plugin
manager.

@section plugins-direct Direct usage of plugins

@todoc and also check that this is not already mentioned above

@section plugins-develop Developing your own plugins

See class documentation of particular plugin interfaces for more information
about subclassing. The Corrade's @ref plugin-management "plugin management tutorial"
contains more information about plugin compilation and registering.
*/
}