magnum/doc/scenegraph.dox

/*
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              Vladimír Vondruš <mosra@centrum.cz>

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namespace Magnum {
/** @page scenegraph Using the scene graph
@brief Overview of scene management capabilities.

The scene graph provides a way to hierarchically manage your objects, their
transformation, animation and rendering, among other things. The library is
contained in the @ref SceneGraph namespace, see its documentation for more
information about building and usage with CMake.

@tableofcontents
@m_footernavigation

There are naturally many possible feature combinations (2D vs. 3D, different
transformation representations, animated vs. static, object can have collision
shape, participate in physics events, have forward vs. deferred rendering...)
and to make everything possible without combinatiorial explosion and allow the
users to provide their own features, a scene graph in Magnum is composed of
three main components:

-   objects, providing parent/children hierarchy
-   transformations, implementing particular transformation type
-   features, providing rendering capabilities, audio, animation, physics etc.

@note Fully contained applications with initial scene graph setup are available
    in `scenegraph2D` and `scenegraph3D` branches of
    [Magnum Bootstrap](https://github.com/mosra/magnum-bootstrap) repository.

@section scenegraph-basic-concepts Basic concepts

@m_div{m-col-m-4 m-right-m}
@dotfile scenegraph-hierarchy.dot
@m_enddiv

The basic organization of a scene graph is as follows: a top-level scene object
@f$ \color{m-primary} s @f$ contains a hierarchy of objects @f$ o_i @f$. Each
object has a transformation @f$ \boldsymbol{T_i} @f$ relative to its parent ---
usually a transformation matrix. The whole scene is rendered using a camera
@f$ \color{m-primary} c @f$ with a projection matrix
@f$ \color{m-primary} \boldsymbol{P} @f$. The projection matrix defines things
like field-of-view, aspect ratio and near/far clipping planes. The final
projective object transform @f$ \boldsymbol{M_i} @f$, relative to camera, is
calculated as a combination of all relative transformations up to the scene
root (an absolute transformation), multiplied by the inverse of the camera's absolute
transformation. For the object @f$ o_3 @f$ its final transform @f$ \boldsymbol{M_3} @f$
is produced as follows: @f[
    \begin{array}{rcl}
        \boldsymbol{M_3} & = & {\color{m-primary} \boldsymbol{P}} ~
                (\color{m-success} \boldsymbol{T_4} ~ \boldsymbol{T_5})^{-1} ~
                {\color{m-warning} \boldsymbol{T_1} ~ \boldsymbol{T_3}} \\
            & = & {\color{m-primary} \boldsymbol{P}}
                \underbrace{\color{m-success} \boldsymbol{T_5}^{-1} ~ \boldsymbol{T_4}^{-1}}_{\boldsymbol{C}}
                {\color{m-warning} \boldsymbol{T_1} ~ \boldsymbol{T_3}}
    \end{array}
@f]

The inverse camera transformation @f$ \boldsymbol{C} @f$ is called a *camera
matrix*. It's useful for example to calculate light positions relative to a
camera.

@m_div{m-col-m-5 m-left-m}
@dotfile scenegraph-features.dot
@m_enddiv

The objects themselves handle only parent/child relationship and
transformation. *Features* add behavior to them. The camera
@f$ \color{m-primary} c @f$ is one of them, together with a *drawable*
@f$ \color{m-info} d_i @f$. A drawable makes it possible to draw things on
screen using a camera. It's not possible to just "draw the graph", the
drawables are grouped into a drawable group @f$ \color{m-success} g @f$. You
can have just one, drawing everything at once, or group the drawables by a
shader / transparency etc. It's also possible to have multiple cameras and
switch among them.

Besides drawables, there are other features for animation, audio, physics, etc.

@m_div{m-clearfix-m} @m_enddiv

@section scenegraph-transformation Transformations

A transformation handles object position, rotation, etc. Its basic property
is a dimension count (2D or 3D) and an underlying numeric type. All classes
in @ref SceneGraph are templated on the underlying type. However, in most cases
@ref Float is used and thus nearly all classes have convenience aliases so you
don't have to explicitly specify it.

Scene graph has various transformation implementations for both 2D and 3D. Each
implementation has its own advantages and disadvantages --- for example when
using matrices you can have nearly arbitrary transformations, but composing
transformations, calculating their inverse and accounting for floating-point
drift is a rather costly operation. On the other hand, quaternions for example
won't allow you to scale or shear objects, but have far better performance
characteristics.

It's also possible to implement your own transformation class for specific
needs, see the source of builtin transformation classes for more information.

Magnum provides the following transformation classes. See documentation of each
class for more detailed information:

-   @ref SceneGraph::BasicMatrixTransformation2D "SceneGraph::MatrixTransformation2D" ---
    arbitrary 2D transformations but with slow inverse transformations and no
    floating-point drift reduction
-   @ref SceneGraph::BasicMatrixTransformation3D "SceneGraph::MatrixTransformation3D" ---
    arbitrary 3D transformations but with slow inverse transformations and no
    floating-point drift reduction
-   @ref SceneGraph::BasicRigidMatrixTransformation2D "SceneGraph::RigidMatrixTransformation2D" ---
    2D translation, rotation and reflection (no scaling), with relatively fast
    inverse transformations and floating-point drift reduction
-   @ref SceneGraph::BasicRigidMatrixTransformation3D "SceneGraph::RigidMatrixTransformation3D" ---
    3D translation, rotation and reflection (no scaling), with relatively fast
    inverse transformations and floating-point drift reduction
-   @ref SceneGraph::BasicDualComplexTransformation "SceneGraph::DualComplexTransformation" ---
    2D translation and rotation with fast inverse transformations and
    floating-point drift reduction
-   @ref SceneGraph::BasicDualQuaternionTransformation "SceneGraph::DualQuaternionTransformation" ---
    3D translation and rotation with fast inverse transformation and
    floating-point drift reduction
-   @ref SceneGraph::BasicTranslationRotationScalingTransformation2D "SceneGraph::TranslationRotationScalingTransformation2D" ---
    2D transformations with separate translation, rotation and scaling. Usable
    for scenes with animation tracks that control these properties separately.
-   @ref SceneGraph::BasicTranslationRotationScalingTransformation3D "SceneGraph::TranslationRotationScalingTransformation3D" ---
    3D transformations with separate translation, rotation and scaling. Usable
    for scenes with animation tracks that control these properties separately.
-   @ref SceneGraph::TranslationTransformation "SceneGraph::TranslationTransformation*D" ---
    Just 2D/3D translation (no rotation, scaling or anything else)

Common usage of transformation classes is to typedef @ref SceneGraph::Scene and
@ref SceneGraph::Object with desired transformation type to save unnecessary
typing later:

@snippet MagnumSceneGraph.cpp typedef

@attention Note that you have to include both @ref Magnum/SceneGraph/Object.h
    and desired transformation class (e.g. @ref Magnum/SceneGraph/MatrixTransformation3D.h)
    to be able to use the resulting type.

The object type is subclassed from the transformation type and so the
@cpp Object3D @ce type will then contain all members from both @ref SceneGraph::Object
and @ref SceneGraph::MatrixTransformation3D. For convenience you can use method
chaining:

@snippet MagnumSceneGraph.cpp method-chaining

@section scenegraph-hierarchy Scene hierarchy

Scene hierarchy is an essential part of the scene graph. In the root there is
a @ref SceneGraph::Scene, its children are @ref SceneGraph::Object instances.
The whole hierarchy has a single transformation type, identical for all objects
(because for example having part of the tree in 2D and part in 3D just wouldn't
make sense).

Build the hierarchy by *parenting* one object to another. Parent object can be
either passed in the constructor or set using @ref SceneGraph::Object::setParent().
The scene is always a root object, so it naturally cannot have any parent or
transformation. Parent and children relationships can be observed through
@ref SceneGraph::Object::parent() and @ref SceneGraph::Object::children().

@snippet MagnumSceneGraph.cpp hierarchy

This hierarchy also takes care of memory management --- when an object is
destroyed, all its children are destroyed too. See detailed explanation of
@ref scenegraph-object-construction-order "construction and destruction order"
below for information about possible issues. To reflect the implicit memory
management in the code better, you can use @ref SceneGraph::Object::addChild()
instead of the naked @cpp new @ce call from the code above:

@snippet MagnumSceneGraph.cpp hierarchy-addChild

@section scenegraph-features Object features

Magnum provides the following builtin features. See documentation of each class
for more detailed information and usage examples:

-   @ref SceneGraph::Camera "SceneGraph::Camera*D" --- Handles projection
    matrix, aspect ratio correction etc.. Used for rendering parts of the
    scene.
-   @ref SceneGraph::Drawable "SceneGraph::Drawable*D" --- Adds drawing
    functionality to given object. Group of drawables can be then rendered
    using the camera feature.
-   @ref Audio::Listener "Audio::Listener*D" --- Handles audio listener
    properties like position and orientation. Audio equivalent of a camera.
-   @ref Audio::Playable "Audio::Playable*D" --- Handles audio source
    properties. Audio equivalent of a drawable.
-   @ref SceneGraph::Animable "SceneGraph::Animable*D" --- Adds animation
    functionality to given object. Group of animables can be then controlled
    using @ref SceneGraph::AnimableGroup "SceneGraph::AnimableGroup*D".
-   @ref DebugTools::ObjectRenderer "DebugTools::ObjectRenderer*D",
    @ref DebugTools::ForceRenderer "DebugTools::ForceRenderer*D" --- Visualize
    object properties, object shape or force vector for debugging purposes. See
    @ref debug-tools for more information.

Each feature takes a reference to *holder object* in its constructor, so adding
a feature to an object might look just like the following, as in some cases you
don't even need to keep the pointer to it. List of object features is
accessible through @ref SceneGraph::Object::features().

@snippet MagnumSceneGraph.cpp feature

Some features are passive, others active. Passive features can just be added to
an object, with no additional work except for possible configuration (for
example a debug renderer). Active features require the user to implement some
virtual function (for example to draw the object on screen or perform an animation
step). To make things convenient, features can be added directly to object
itself using multiple inheritance, so you can add all the active features you
want and implement functions you need in your own @ref SceneGraph::Object
subclass without having to subclass each feature individually (and making the
code overly verbose). A simplified example:

@snippet MagnumSceneGraph.cpp feature-inherit

From the outside there is no difference between features added "at runtime" and
features added using multiple inheritance, they can be both accessed from the
feature list.

Similarly to object hierarchy, when destroying object, all its features (both
member and inherited) are destroyed. See detailed explanation of
@ref scenegraph-feature-construction-order "construction and destruction order"
for information about possible issues. Also, there is the
@ref SceneGraph::AbstractObject::addFeature() "addFeature()" counterpart to
@ref SceneGraph::Object::addChild() "addChild()":

@snippet MagnumSceneGraph.cpp feature-addFeature

@subsection scenegraph-features-caching Transformation caching in features

Some features need to operate with absolute transformations and their
inversions --- for example the camera needs its inverse transformation (camera
matrix) to render the scene, collision detection needs to know about positions
of surrounding objects etc. To avoid computing the transformations from scratch
every time, the feature can cache them.

The cached data stay until the object is marked as *dirty* --- that is by
changing its transformation, its parent or by explicitly calling
@ref SceneGraph::Object::setDirty(). If the object is marked as dirty, all its
children are marked as dirty as well and @ref SceneGraph::AbstractFeature::markDirty()
is called on every feature attached to them. Calling
@ref SceneGraph::Object::setClean() cleans the dirty object and all its
dirty parents --- it goes through all object features and calls
@ref SceneGraph::AbstractFeature::clean() or
@ref SceneGraph::AbstractFeature::cleanInverted() depending on which caching is
enabled on given feature. If the object is already clean,
@ref SceneGraph::Object::setClean() does nothing.

Usually you will need caching in the @ref SceneGraph::Object itself --- which
doesn't support it on its own --- however you can take advantage of multiple
inheritance and implement it using @ref SceneGraph::AbstractFeature. In order
to have caching, you must enable it first, because by default caching is
disabled. You can enable it using @ref SceneGraph::AbstractFeature::setCachedTransformations()
and then implement the corresponding cleaning function(s):

@snippet MagnumSceneGraph.cpp caching

When you need to use the cached value, you can explicitly request the cleanup
by calling @ref SceneGraph::Object::setClean(). @ref SceneGraph::Camera, for
example, calls it automatically before it starts rendering, as it needs
up-to-date @ref SceneGraph::Camera::cameraMatrix() to properly draw all
objects.

@subsection scenegraph-features-transformation Polymorphic access to object transformation

Features by default have access only to @ref SceneGraph::AbstractObject, which
doesn't know about any particular transformation implementation. This has the
advantage that features don't have to be implemented for all possible
transformation implementations. But, as a consequence, it is impossible to
transform the object using only a pointer to @ref SceneGraph::AbstractObject.

To solve this, the transformation classes are subclassed from interfaces
sharing common functionality, so the feature can use that interface instead of
being specialized for all relevant transformation implementations. The
following interfaces are available, each having its own set of virtual
functions to control the transformation:

-   @ref SceneGraph::AbstractTransformation "SceneGraph::AbstractTransformation*D" ---
    base for all transformations
-   @ref SceneGraph::AbstractTranslation "SceneGraph::AbstractTranslation*D" ---
    base for all transformations providing translation
-   @ref SceneGraph::AbstractBasicTranslationRotation2D "SceneGraph::AbstractTranslationRotation2D",
    @ref SceneGraph::AbstractBasicTranslationRotation3D "SceneGraph::AbstractTranslationRotation3D" ---
    base for all transformations providing translation and rotation
-   @ref SceneGraph::AbstractBasicTranslationRotationScaling2D "SceneGraph::AbstractBasicTranslationRotationScaling2D",
    @ref SceneGraph::AbstractBasicTranslationRotationScaling3D "SceneGraph::AbstractBasicTranslationRotationScaling3D" ---
    base for all transformations providing translation, rotation and scaling

These interfaces provide virtual functions which can be used to modify object
transformations. The virtual calls are used only when calling through the
interface and not when using the concrete implementation directly to avoid
negative performance effects. There are no functions to retrieve object
transformation, you need to use the above transformation caching mechanism for
that.

In the following example we are able to get pointer to both the
@ref SceneGraph::AbstractObject and the needed transformation from a single
constructor parameter using a trick:

@snippet MagnumSceneGraph.cpp transformation

If we take for example @ref SceneGraph::Object "SceneGraph::Object<MatrixTransformation3D>",
it is derived from @ref SceneGraph::AbstractObject "SceneGraph::AbstractObject3D"
and @ref SceneGraph::BasicMatrixTransformation3D "SceneGraph::MatrixTransformation3D".
Thus the reference to @ref SceneGraph::AbstractBasicTranslationRotation3D "SceneGraph::AbstractTranslationRotation3D",
is automatically extracted from the reference in our constructor.

@section scenegraph-construction-order Construction and destruction order

There aren't any limitations and usage trade-offs of what you can and can't do
when working with objects and features, but there are two issues which you
should be aware of.

@subsection scenegraph-object-construction-order Object hierarchy

When objects are created on the heap (the preferred way, using @cpp new @ce),
they can be constructed in any order and they will be destroyed when their
parent is destroyed. When creating them on the stack, however, they will be
destroyed when they go out of scope. Normally, the natural order of creation is
not a problem:

@snippet MagnumSceneGraph.cpp construction-order

The `object` is created last, so it will be destroyed first, removing itself
from `scene`'s children list, causing no problems when destroying the `scene`
object later. However, if their order is swapped, it will cause problems:

@snippet MagnumSceneGraph.cpp construction-order-crash

The `scene` will be destroyed first, deleting all its children, which is wrong,
because `object` is created on stack. If this doesn't already crash, the
`object` destructor is called (again), making things even worse.

@subsection scenegraph-feature-construction-order Member and inherited features

When destroying the object, all its features are destroyed. For features added
as a member it's not an issue, however features added using multiple
inheritance must be inherited after the Object class:

@snippet MagnumSceneGraph.cpp feature-construction-order

When constructing `MyObject`, `Object3D` constructor is called first and then
`MyFeature` constructor adds itself to `Object3D`'s list of features. When
destroying `MyObject`, its destructor is called and then the destructors of
ancestor classes --- first `MyFeature` destructor, which will remove itself
from `Object3D`'s list, then `Object3D` destructor.

However, if we would inherit `MyFeature` first, it will cause problems:

@snippet MagnumSceneGraph.cpp feature-construction-order-crash

`MyFeature` tries to add itself to feature list in not-yet-constructed
`Object3D`, causing undefined behavior. Then, if this doesn't already crash,
`Object3D` is created, creating empty feature list, making the feature
invisible.

If we would construct them in swapped order (if it is even possible), it
wouldn't help either:

@snippet MagnumSceneGraph.cpp feature-construction-order-crash-destruction

On destruction, `Object3D` destructor is called first, deleting `MyFeature`,
which is wrong, because `MyFeature` is in the same object. After that (if the
program didn't already crash) destructor of `MyFeature` is called (again).
*/
}