/* This file is part of Magnum. Copyright © 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022 Vladimír Vondruš Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. */ namespace Magnum { /** @page scenegraph Using the scene graph @brief Overview of scene management capabilities. @m_keywords{SceneGraph} 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", 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). */ }