Qt3d. Draw transparent QSphereMesh over triangles

Question

I have a function that draws triangles through OpenGL

I draw two triangles by pressing a button (function on_drawMapPushButton_clicked()).

Then i draw a sphere that placed above these triangles. And now i see, that sphere is drawed correctly over first triangle, but second triangle drawed over the sphere and not vice versa.

If i press the button second time, then spehere is drawed correctly over first and second triangles.

When i press the button third time, then second triangle drawed over the sphere again.

When i press the button fourth time, then spehere is drawed correctly over first and second triangles and so on.

If i use in sphereMesh QPhongMaterial instead of QPhongAlphaMaterial, then spehere is drawed correctly over first and second triangles always. Like it must to be.

I can't understand what i do wrong to get my sphere is drawed always over the triangles.

Code, that draws transparent sphere:

selectModel_ = new Qt3DExtras::QSphereMesh(selectEntity_);
selectModel_->setRadius(75);
selectModel_->setSlices(150);

selectMaterial_ = new Qt3DExtras::QPhongAlphaMaterial(selectEntity_);
selectMaterial_->setAmbient(QColor(28, 61, 136));
selectMaterial_->setDiffuse(QColor(11, 56, 159));
selectMaterial_->setSpecular(QColor(10, 67, 199));
selectMaterial_->setShininess(0.8f);

selectEntity_->addComponent(selectModel_);
selectEntity_->addComponent(selectMaterial_);

Function drawTriangles:

void drawTriangles(QPolygonF triangles, QColor color){
    int numOfVertices = triangles.size();

    // Create and fill vertex buffer
    QByteArray bufferBytes;
    bufferBytes.resize(3 * numOfVertices * static_cast<int>(sizeof(float)));
    float *positions = reinterpret_cast<float*>(bufferBytes.data());

    for(auto point : triangles){
        *positions++ = static_cast<float>(point.x());
        *positions++ = 0.0f; //We need to drow only on the surface
        *positions++ = static_cast<float>(point.y());
    }

    geometry_ = new Qt3DRender::QGeometry(mapEntity_);
    auto *buf = new Qt3DRender::QBuffer(geometry_);
    buf->setData(bufferBytes);

    positionAttribute_ = new Qt3DRender::QAttribute(mapEntity_);
    positionAttribute_->setName(Qt3DRender::QAttribute::defaultPositionAttributeName());
    positionAttribute_->setVertexBaseType(Qt3DRender::QAttribute::Float); //In our buffer we will have only floats
    positionAttribute_->setVertexSize(3); // Size of a vertex
    positionAttribute_->setAttributeType(Qt3DRender::QAttribute::VertexAttribute); // Attribute type
    positionAttribute_->setByteStride(3 * sizeof(float));
    positionAttribute_->setBuffer(buf); 
    geometry_->addAttribute(positionAttribute_); // Add attribute to ours  Qt3DRender::QGeometry

    // Create and fill an index buffer
    QByteArray indexBytes;
    indexBytes.resize(numOfVertices * static_cast<int>(sizeof(unsigned int))); // start to end
    unsigned int *indices = reinterpret_cast<unsigned int*>(indexBytes.data());

    for(unsigned int i = 0; i < static_cast<unsigned int>(numOfVertices); ++i) {
        *indices++ = i;
    }

    auto *indexBuffer = new Qt3DRender::QBuffer(geometry_);
    indexBuffer->setData(indexBytes);

    indexAttribute_ = new Qt3DRender::QAttribute(geometry_);
    indexAttribute_->setVertexBaseType(Qt3DRender::QAttribute::UnsignedInt); //In our buffer we will have only unsigned ints
    indexAttribute_->setAttributeType(Qt3DRender::QAttribute::IndexAttribute); // Attribute type
    indexAttribute_->setBuffer(indexBuffer);
    indexAttribute_->setCount(static_cast<unsigned int>(numOfVertices)); // Set count of our vertices
    geometry_->addAttribute(indexAttribute_); // Add the attribute to ours Qt3DRender::QGeometry

    shape_ = new Qt3DRender::QGeometryRenderer(mapEntity_);
    shape_->setPrimitiveType(Qt3DRender::QGeometryRenderer::Triangles);
    shape_->setGeometry(geometry_); 

    //Create material
    material_ = new Qt3DExtras::QPhongMaterial(mapEntity_);
    material_->setAmbient(color);

    trianglesEntity_ = new Qt3DCore::QEntity(mapEntity_);
    trianglesEntity_->addComponent(shape_); 
    trianglesEntity_->addComponent(material_);
}

Press button handler on_drawMapPushButton_clicked():

void on_drawMapPushButton_clicked()
{
    clearMap(); //Implementation is above
    QPolygonF triangle1;
    triangle1 << QPointF( 0 ,-1000) << QPointF(0 ,1000) << QPointF(1000, -1000);
    drawTriangles(triangle1, Qt::black);

    QPolygonF triangle2;
    triangle2 << QPointF(-1000,-1000) << QPointF(-100,1000) << QPointF(-100,-1000);
    drawTriangles(triangle2, Qt::red);
}

Map clearing function clearMap():

void clearMap()
{
    if(mapEntity_){
        delete mapEntity_;
        mapEntity_ = nullptr;
        mapEntity_ = new Qt3DCore::QEntity(view3dRootEntity_);
    }
}

Answer 1

My mistake was that i did wrong order of creating and deletion of Triangles and Sphere entities.

In pseudo code right order is as follows:

clearTriangles();
clearSphere();       
drawTriangles();
drawSphere();