Hi, Entities maintain the classic scenegraph hierarchy as you call it. Any Entity that doesn't have a Transform component attached is assumed to have an identity transform as far as nesting transformations is concerned.
The fact that QEntity also happens to be a QNode which is in turn a QObject means that the parent-child relationship also manages lifetime too. Cheers, Sean On 29/09/2015 10:28, Émeric MASCHINO wrote: > Hi, > > In classical scenegraph approach, the path to a node gives the > relationships between the nodes. For example, the following path root > -> transform -> mesh means that the mesh node will be applied the > transform given by the transform node. The scenegraph's root is the > parent of the transform node, that in turn is the parent of the mesh > node. > > By contrast, there's no such parent-child relationship in ECS approach [1]. > > Although Qt3D 2.x is an ECS, Qt3D::QNode constructor can be passed a > parent. I initially thought the parent plays the same role as in > classical scenegraph approach. Then I saw Qt3D's simple C++ example > [2], where mesh transform and material are added "the ECS way", by > adding a transform component. Still, the material and torus entities > are passed the root entity as parent: > > // Root entity > Qt3D::QEntity *rootEntity = new Qt3D::QEntity(); > > <snip> > > // Material > Qt3D::QMaterial *material = new Qt3D::QPhongMaterial(rootEntity); > > // Torus > Qt3D::QEntity *torusEntity = new Qt3D::QEntity(rootEntity); > Qt3D::QTorusMesh *torusMesh = new Qt3D::QTorusMesh; > torusMesh->setRadius(5); > torusMesh->setMinorRadius(1); > torusMesh->setRings(100); > torusMesh->setSlices(20); > > Qt3D::QTransform *torusTransform = new Qt3D::QTransform; > Qt3D::QScaleTransform *torusScaleTransform = new Qt3D::QScaleTransform; > torusScaleTransform->setScale3D(QVector3D(1.5, 1, 0.5)); > > Qt3D::QRotateTransform *torusRotateTransform = new Qt3D::QRotateTransform; > torusRotateTransform->setAxis(QVector3D(1, 0, 0)); > torusRotateTransform->setAngleDeg(45); > > torusTransform->addTransform(torusScaleTransform); > torusTransform->addTransform(torusRotateTransform); > > torusEntity->addComponent(torusMesh); > torusEntity->addComponent(torusTransform); > torusEntity->addComponent(material); > > So, can anybody explain why passing the root entity to the material > and torus entities is required? What's the difference with _not_ > passing it? > > How to deal with "parent transform"? Is this the role of the parent > passed to Qt3D::QNode constructor? I mean, in the above example, if > the root entity was added a transform component, would this behave as > a parent transform also applied to the torus entity (and also the > material entity?), meaning that the complete transform applied to the > torus entity would be the one from the root entity and then the one > from the transform component added to the torus entity? > > Or is Qt3D::QNode's parent only for automatic memory management, i.e. > releasing an entity will also release its children entities? > > Thanks, > > Émeric > > > [1] > http://www.gamedev.net/topic/654122-entity-component-system-and-parent-relations/ > (for example, but there are many resources on this topic on the web) > [2] http://doc.qt.io/qt-5/qt3drenderer-simple-cpp-main-cpp.html > _______________________________________________ > Interest mailing list > Interest@qt-project.org > http://lists.qt-project.org/mailman/listinfo/interest _______________________________________________ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
