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

[Nicholas Yue]

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/
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