[QT3D] minExtent and maxExtent not giving the right values

Spazz

Hey there,

I'm trying to create a pretty simple .obj viewer rendering a mesh on a floor, like the default 3D viewer in Windows 10 illustrated below.

Here is my result for now :

As you can see, I'm having a problem with the plane which is not correctly positionned.

I'm using a Qt3DExtras::QPlaneMesh for it, and translating it using Qt3DCore::QTransform.

The problem is about this translation. What I'd like to do is to apply a translation (0.0f, minY, 0.0f) to the plane where minY is the lowest y value of the bounding box of the displayed mesh. As far as I understand the documentation, I need to use the minExtent function on the QGeometry associated to the QMesh. Let's say mesh is the mesh to display, I'm doing minY = mesh.geometry()->minExtent().y(). This is not working, minY is always 0.0f.

So basically I'm asking for help to compute this minY value :/

I made a minimal example, here is the code (the problem is in handleStatusChanged in myscene.cpp) :
main.cpp
myscene.h
myscene.cpp

Thanks for your help !

Spazz

aaaaand I found the solution :

As you can see, the floor origin is now perfectly positionned (and fits with any mesh).

Solution in short :

handle the statusChanged signal
once status == Qt3DRender::QMesh::Ready, the mesh is loaded BUT its bounding box is not (necessarily?) computed yet. I made this assumption and this was the mistake.
handle the minExtentChanged signal by connecting it to a function ONLY ONCE status == Qt3DRender::QMesh::Ready
update the plane position in this function

Here is the final code :

void MyScene::handleStatusChanged(const Qt3DRender::QMesh::Status &status)
{
    if(status == Qt3DRender::QMesh::Ready)
    {
          connect(mesh.geometry(), &Qt3DCore::QGeometry::minExtentChanged, this, &MyScene::handleMinExtentChanged);
    }
}

void MyScene::handleMinExtentChanged(const QVector3D & minExtent)
{
    Qt3DCore::QTransform * planeTransform = new Qt3DCore::QTransform(&floorEntity);

    planeTransform->setTranslation(QVector3D(0.0f,mesh.geometry()->minExtent().y(),0.0f));

    floorEntity.addComponent(planeTransform);
}

More details

I was mistaken by the Qt3DRender::QMesh::Ready status : the fact that the QMesh is flagged as "Ready" does not mean that its bounding box is computed.

Indeed, minExtent and maxExtent values are set by the function setExtent in qgeometry.cpp. By setting one breakpoint in setExtent and one breakpoint in the if statement of handleStatusChanged, you will observe that the if statement breakpoint is triggered the first. That is, even thought the QMesh is "Ready", it's extents are not yet set.

Furthermore, you have to wait for the QMesh to be "Ready" before connecting to the minExtentChanged signal because before that, the QGeometry associated to mesh is not set. You cannot connect to mesh.geometry() immediatly after a call to setSource because the QGeometry it points to is nullptr, and change only once the QMesh is "Ready". That's the reason why I'm connecting only in the if statement of handleStatusChanged

Hopefully this will help some people.

PS : I still do not understand how to use updateImplicitBounds(), but I'm done with my issue here.

Spazz

Still stucked, but I noticed that the call to mesh.updateImplicitBounds(); is returning false in myscene.cpp

Sadly, the documentation is not giving any detail about the possible reasons for such a thing :/

I'm going to debug the source code, I'll keep you informed.

Spazz

Reason why updateImplicitBounds() is returning false

I found out why mesh.updateImplicitBounds(); is returning false : mesh has no QGeometryView associated to it, and updateImplicitBounds() is defined in qboundingvolume.cpp as :

bool QBoundingVolume::updateImplicitBounds()
{
    Q_D(QBoundingVolume);
    if (!d->m_view) //Triggered since there is no view associated to mesh
        return false;

    auto data = BoundingVolumeComputeData::fromView(d->m_view);
    if (!data.valid())
        return false;

    auto res = data.compute();
    if (!res.valid())
        return false;

    d->setImplicitBounds(res.m_min, res.m_max, res.m_center, res.m_radius);
    return true;
}

The fact that this function always returns false makes sense because the first conditional is always triggered since I have no QGeometryView associated to mesh.

Now I have a new problem because I don't even understand what is a QGeometryView, which is defined in the documentation as "A GeometryView holds all the information necessary to handle a Geometry" which is way too fuzzy to be understandable.

Spazz