[Interest] Qt3D : QSceneLoader - determining scene level geometry bounding box

Mike Krus mike.krus at kdab.com
Tue Aug 2 14:55:58 CEST 2022


Hi

if you are using Qt6, then yes.

For a specific object, QGeometryRenderer derives from QBoundingVolume which has impliciteMinPoint and impliciteMaxPoint properties.

If you’re interested in the bounding volume of a portion of the scene graph, all a QBoundingVolume component at the root and the
same properties will be updated.

In Qt5, the QGeometry node associated with each render has similar min/max properties (note that they only valid if the mesh uses all the geometry).

Hope this helps,

Mike


> On 1 Aug 2022, at 16:05, Nicholas Yue <yue.nicholas at gmail.com> wrote:
> 
> Hi,
> 
>   I am diving into the geometry scene via QSceneLoader.
>   
>   Using the following code as a starting point, I wish to place the camera such that it automatically accommodates scenes of different sizes.
>   
>   One way I am thinking of doing is to base on the scene geometry bounding box.
>   
>   Is there a method or API which I can call to query the entire scene geometry bounding box ? Or do I have to walk the entire scene to build that up by querying the bounding box for every geometry in the scene ?
>   
>   Any sample code I can view to get me going ?
>  
> ```
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> 
> #include <QFileDialog>
> #include <QApplication>
> 
> #include <Qt3DRender/QCamera>
> #include <Qt3DCore/QEntity>
> #include <Qt3DCore/QAspectEngine>
> #include <Qt3DInput/QInputAspect>
> #include <Qt3DRender/QSceneLoader>
> #include <Qt3DRender/QRenderAspect>
> #include <Qt3DExtras/QForwardRenderer>
> #include <Qt3DExtras/qt3dwindow.h>
> // #include <Qt3DExtras/qfirstpersoncameracontroller.h>
> #include "MyCameraController.h"
> #include "TrackballCameraController.h"
> 
> class SceneWalker : public QObject
> {
> public:
>     SceneWalker(Qt3DRender::QSceneLoader *loader) : m_loader(loader) { }
> 
>     void onStatusChanged();
> 
> private:
>     void walkEntity(Qt3DCore::QEntity *e, int depth = 0);
> 
>     Qt3DRender::QSceneLoader *m_loader;
> };
> 
> void SceneWalker::onStatusChanged()
> {
>     qDebug() << "Status changed:" << m_loader->status();
>     if (m_loader->status() != Qt3DRender::QSceneLoader::Ready)
>         return;
> 
>     // The QSceneLoader instance is a component of an entity. The loaded scene
>     // tree is added under this entity.
>     QVector<Qt3DCore::QEntity *> entities = m_loader->entities();
> 
>     // Technically there could be multiple entities referencing the scene loader
>     // but sharing is discouraged, and in our case there will be one anyhow.
>     if (entities.isEmpty())
>         return;
>     Qt3DCore::QEntity *root = entities[0];
>     // Print the tree.
>     walkEntity(root);
> 
>     // To access a given node (like a named mesh in the scene), use QObject::findChild().
>     // The scene structure and names always depend on the asset.
>     Qt3DCore::QEntity *e = root->findChild<Qt3DCore::QEntity *>(QStringLiteral("PlanePropeller_mesh")); // toyplane.obj
>     if (e)
>         qDebug() << "Found propeller node" << e << "with components" << e->components();
> }
> 
> void SceneWalker::walkEntity(Qt3DCore::QEntity *e, int depth)
> {
>     Qt3DCore::QNodeVector nodes = e->childNodes();
>     for (int i = 0; i < nodes.count(); ++i) {
>         Qt3DCore::QNode *node = nodes[i];
>         Qt3DCore::QEntity *entity = qobject_cast<Qt3DCore::QEntity *>(node);
>         if (entity) {
>             QString indent;
>             indent.fill(' ', depth * 2);
>             qDebug().noquote() << indent << "Entity:" << entity << "Components:" << entity->components();
>             walkEntity(entity, depth + 1);
>         }
>     }
> }
> 
> int main(int ac, char **av)
> {
>     QApplication app(ac, av);
>     Qt3DExtras::Qt3DWindow view;
>     view.defaultFrameGraph()->setClearColor(Qt::black);
> 
>     // Root entity
>     Qt3DCore::QEntity *sceneRoot = new Qt3DCore::QEntity();
> 
>     // Scene Camera
>     Qt3DRender::QCamera *camera = view.camera();
>     camera->setProjectionType(Qt3DRender::QCameraLens::PerspectiveProjection);
>     camera->setViewCenter(QVector3D(0.0f, 3.5f, 0.0f));
>     camera->setPosition(QVector3D(0.0f, 3.5f, 25.0f));
>     camera->setNearPlane(0.001f);
>     camera->setFarPlane(10000.0f);
> 
>     // For camera controls
>     // Qt3DExtras::QFirstPersonCameraController *camController = new Qt3DExtras::QFirstPersonCameraController(sceneRoot);
>     TrackballCameraController *camController = new TrackballCameraController(sceneRoot);
>     camController->setWindowSize(QSize(view.width(),view.height())); // https://github.com/cjmdaixi/Qt3DTrackball/issues/1#issuecomment-544183228
>     camController->setCamera(camera);
> 
>     // Scene loader
>     Qt3DCore::QEntity *sceneLoaderEntity = new Qt3DCore::QEntity(sceneRoot);
>     Qt3DRender::QSceneLoader *sceneLoader = new Qt3DRender::QSceneLoader(sceneLoaderEntity);
>     SceneWalker sceneWalker(sceneLoader);
>     QObject::connect(sceneLoader, &Qt3DRender::QSceneLoader::statusChanged, &sceneWalker, &SceneWalker::onStatusChanged);
>     sceneLoaderEntity->addComponent(sceneLoader);
> 
>     QStringList args = QCoreApplication::arguments();
>     QUrl sourceFileName;
>     if (args.count() <= 1) {
>         QWidget *container = new QWidget();
>         QFileDialog dialog;
>         dialog.setFileMode(QFileDialog::AnyFile);
>         sourceFileName = dialog.getOpenFileUrl(container, QStringLiteral("Open a scene file"));
>     } else {
>         sourceFileName = QUrl::fromLocalFile(args[1]);
>     }
> 
>     if (sourceFileName.isEmpty())
>         return 0;
> 
>     sceneLoader->setSource(sourceFileName);
> 
>     view.setRootEntity(sceneRoot);
>     view.show();
> 
>     return app.exec();
> }
> ```  
> 
> Cheers
> --
> Nicholas Yue
> https://www.linkedin.com/in/nicholasyue/
> _______________________________________________
> Interest mailing list
> Interest at qt-project.org
> https://lists.qt-project.org/listinfo/interest

—
Mike Krus | mike.krus at kdab.com | Senior Software Engineer & Teamlead
KDAB (UK) Ltd., a KDAB Group company
Tel: UK Office +44 1625 809908   Mobile +44 7833 491941
KDAB - The Qt Experts, C++, OpenGL Experts


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