[Interest] [Qt3D] Node's parent-child relationship clarification, please?

[Sean Harmer]

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
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