QT3D with C ++ and rotation - c++

Question

I hope someone can help me with this problem. I am new to working with QT3D and I need to make a Qt3D application using QT C++ for a task, the problem I have is that I need to rotate the figure from a specific point but it always rotates from the center point. How can I specify that the rotation is made from one of the pivots in the figure? I need it to be able to simulate the movement of a pendulum. Please help someone who tells me how I can solve this problem, here is my code.
void window::paint(){
arm1 = new Qt3DExtras::QCylinderMesh();
arm1->setRadius(0.5);
arm1->setLength(3);
arm1Transform = new Qt3DCore::QTransform();
arm1Transform->setTranslation(QVector3D(-3, 3, 0));
arm1Transform->setScale(1.5f);
Qt3DExtras::QPhongMaterial *arm1Material = new Qt3DExtras::QPhongMaterial();
arm1Material->setDiffuse(Qt::red);
arm1Entity = new Qt3DCore::QEntity(rootEntity);
arm1Entity->addComponent(arm1);
arm1Entity->addComponent(arm1Material);
arm1Entity->addComponent(arm1Transform);
}
void window::on_pushButton_clicked(){
arm1Transform->setRotation(QQuaternion::fromAxisAndAngle(QVector3D(1.0f, 0.0f, 0.0f), angle++));
}
Thanks in advance.

Answer 1

You can make arm1Entity to be a child of a new QEntity. This new QEntity will be positioned at the point where you want to rotate from. So if you rotate the new QEntity, his child arm1Entity will rotate from the new QEntity's position.
EDIT code (not tested):
void window::paint(){
arm1PivotEntity = new Qt3DCore::QEntity(rootEntity);
arm1PivotTransform = new Qt3DCore::QTransform(arm1PivotEntity);
arm1PivotTransform->setTranslation(QVector3D(/*desired pivot position*/));
arm1PivotEntity->addComponent(arm1PivotTransform);
arm1 = new Qt3DExtras::QCylinderMesh();
arm1->setRadius(0.5);
arm1->setLength(3);
arm1Transform = new Qt3DCore::QTransform();
// as arm1 is now child of arm1 pivot, you need to set the relative position
arm1Transform->setTranslation(QVector3D(/*relative position to arm1pivot position*/));
arm1Transform->setScale(1.5f);
Qt3DExtras::QPhongMaterial *arm1Material = new Qt3DExtras::QPhongMaterial();
arm1Material->setDiffuse(Qt::red);
// made arm1 child of arm1pivot so it will inherits its rotation
arm1Entity = new Qt3DCore::QEntity(arm1PivotEntity);
arm1Entity->addComponent(arm1);
arm1Entity->addComponent(arm1Material);
arm1Entity->addComponent(arm1Transform);
}
void window::on_pushButton_clicked(){
arm1PivotTransform->setRotation(angle++));
}