Problem creating multiple viewports in Qt3D using C++ - c++

Question

I am trying to setup a Qt3DWindow with multiple viewports using C++. According to the documentation and the provided QML example, all I need to do is create a framegraph, where a main QViewport object branches into several RenderViews. The first RenderView contains a QClearBuffers object, and the remaining ones contain the tiled viewports and their corresponding camera selectors. Therefore, if I want N viewports, I need to create N+1 RenderViews.
However, if I follow that procedure, the main viewport displays some sort of "default" view, which appears on top of the window, spanning all the viewports. I don't know where this view comes from, since it doesn't correspond to any camera. Here is the output from the code posted below.
I found a solution, but I am not comfortable with it since it feels like some kind of hack: instead of making all the RenderViews branch from the main viewport, I attach one of the child viewports to the ClearBuffers object itself. Thus, for N viewports, I have N RenderViews instead of N+1. I don't quite understand the internals of the framegraph, so I would like to know if this solution is only wrong from an OCD perspective, or it can actually backfire at some point.
Here is a minimal example with two viewports sharing the default camera. If I make either viewPort1 or viewPort2 branch from clearBuffers instead of mainViewPort, everything works as expected:
#include <QGuiApplication>
#include <Qt3DCore/QTransform>
#include <Qt3DExtras/Qt3DWindow>
#include <Qt3DExtras/QTorusMesh>
#include <Qt3DExtras/QPhongMaterial>
#include <Qt3DRender/QCamera>
#include <Qt3DRender/QRenderSurfaceSelector>
#include <Qt3DRender/QViewport>
#include <Qt3DRender/QClearBuffers>
#include <Qt3DRender/QCameraSelector>
Qt3DCore::QEntity *createScene()
{
// Root entity
auto rootEntity = new Qt3DCore::QEntity;
// Torus
auto torusEntity = new Qt3DCore::QEntity(rootEntity);
auto torusMesh = new Qt3DExtras::QTorusMesh;
torusMesh->setRadius(0.5f);
torusMesh->setMinorRadius(0.1f);
torusMesh->setRings(100);
torusMesh->setSlices(20);
auto torusTransform = new Qt3DCore::QTransform;
torusTransform->setScale3D(QVector3D(1.2f, 1.f, 0.8f));
torusTransform->setRotation(QQuaternion::fromAxisAndAngle(QVector3D(1, 0, 0), 45.0f));
torusEntity->addComponent(torusMesh);
torusEntity->addComponent(torusTransform);
torusEntity->addComponent(new Qt3DExtras::QPhongMaterial(rootEntity));
return rootEntity;
}
int main(int argc, char* argv[])
{
QGuiApplication app(argc, argv);
Qt3DExtras::Qt3DWindow view;
// Set camera transform
view.camera()->setPosition(QVector3D(0, 4.0f, 0));
view.camera()->setViewCenter(QVector3D(0, 0, 0));
// Framegraph root node
auto surfaceSelector = new Qt3DRender::QRenderSurfaceSelector();
auto mainViewPort = new Qt3DRender::QViewport(surfaceSelector);
// First RenderView: clear buffers
auto clearBuffers = new Qt3DRender::QClearBuffers(mainViewPort);
clearBuffers->setBuffers(Qt3DRender::QClearBuffers::ColorDepthBuffer);
clearBuffers->setClearColor(Qt::white);
// Second RenderView: left viewport
auto viewPort1 = new Qt3DRender::QViewport(mainViewPort);
viewPort1->setNormalizedRect(QRectF(0.0f, 0.0f, 0.5f, 1.0f));
auto cameraSelector1 = new Qt3DRender::QCameraSelector(viewPort1);
cameraSelector1->setCamera(view.camera());
// Third RenderView: right viewport
auto viewPort2 = new Qt3DRender::QViewport(mainViewPort);
viewPort2->setNormalizedRect(QRectF(0.5f, 0.0f, 0.5f, 1.0f));
auto cameraSelector2= new Qt3DRender::QCameraSelector(viewPort2);
cameraSelector2->setCamera(view.camera());
// Add framegraph and scenegraph to viewer
view.setActiveFrameGraph(surfaceSelector);
view.setRootEntity(createScene());
view.show();
return app.exec();
}

Answer 1

I still think it's kind of a bug that it draws ontop of the viewports but you can put a QNoDraw node in your framegraph as a child of the QClearBuffers:
auto noDraw = new Qt3DRender::QNoDraw(clearBuffers);
That solves the rendering issue.