Feature guide » Using the scene graph

Basic concepts

The basic organization of a scene graph is as follows: a top-level scene object contains a hierarchy of objects . Each object has a transformation relative to its parent — usually a transformation matrix. The whole scene is rendered using a camera with a projection matrix . The projection matrix defines things like field-of-view, aspect ratio and near/far clipping planes. The final projective object transform , 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 its final transform is produced as follows:

The inverse camera transformation is called a camera matrix. It's useful for example to calculate light positions relative to a camera.

The objects themselves handle only parent/child relationship and transformation. Features add behavior to them. The camera is one of them, together with a drawable . 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 . 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.

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 SceneGraph are templated on the underlying type. However, in most cases 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:

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

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

typedef SceneGraph::Scene<SceneGraph::MatrixTransformation3D> Scene3D;
typedef SceneGraph::Object<SceneGraph::MatrixTransformation3D> Object3D;

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

Scene3D scene;

Object3D object;
object.setParent(&scene)
      .rotateY(15.0_degf)
      .translate(Vector3::xAxis(5.0f));

Scene hierarchy

Scene hierarchy is an essential part of the scene graph. In the root there is a SceneGraph::Scene, its children are 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 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 SceneGraph::Object::parent() and SceneGraph::Object::children().

Scene3D scene;

Object3D* first = new Object3D{&scene};
Object3D* second = new Object3D{first};

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

Scene3D scene;

Object3D& first = scene.addChild<Object3D>();
Object3D& second = first.addChild<Object3D>();

Object features

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

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

Object3D o;
new MyFeature{o, some, params};

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 SceneGraph::Object subclass without having to subclass each feature individually (and making the code overly verbose). A simplified example:

class BouncingBall: public Object3D, SceneGraph::Drawable3D, SceneGraph::Animable3D {
    public:
        explicit BouncingBall(Object3D* parent):
            Object3D{parent}, SceneGraph::Drawable3D{*this}, SceneGraph::Animable3D{*this} {}

    private:
        // drawing implementation for Drawable feature
        void draw(const Matrix4&, SceneGraph::Camera3D&) override;

        // animation step for Animable feature
        void animationStep(Float, Float) override;
};

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 construction and destruction order for information about possible issues. Also, there is the addFeature() counterpart to addChild():

Object3D o;
o.addFeature<MyFeature>(some, params);

class CachingObject: public Object3D, SceneGraph::AbstractFeature3D {
    public:
        explicit CachingObject(Object3D* parent): Object3D{parent}, SceneGraph::AbstractFeature3D{*this} {
            setCachedTransformations(SceneGraph::CachedTransformation::Absolute);
        }

    protected:
        void clean(const Matrix4& absoluteTransformation) override {
            _absolutePosition = absoluteTransformation.translation();
        }

    private:
        Vector3 _absolutePosition;
};

class TransformingFeature: public SceneGraph::AbstractFeature3D {
    public:
        template<class T> explicit TransformingFeature(SceneGraph::Object<T>& object):
            SceneGraph::AbstractFeature3D(object), _transformation(object) {}

        …

    private:
        SceneGraph::AbstractTranslationRotation3D& _transformation;
};

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.

{
    Scene3D scene;
    Object3D object(&scene);
}

{
    Object3D object;
    Scene3D scene;

    object.setParent(&scene);
} // crash!

class MyObject: public Object3D, MyFeature {
    public:
        MyObject(Object3D* parent): Object3D(parent), MyFeature{*this} {}
};

class MyObject: MyFeature, public Object3D {
    public:
        MyObject(Object3D* parent): MyFeature{*this}, Object3D(parent) {} // crash!
};

class MyObject: MyFeature, public Object3D {
    public:
        MyObject(Object3D* parent): Object3D(parent), MyFeature(*this) {}

        // crash on destruction!
};