QPaintedTextureImage in Qt3D (Qt 5.8) - c++

Question

I want to create an entity with Qt3D that has a custom image as texture. I came across the QPaintedTextureImage (link leads to Qt 5.9 version for details. Here ist doc for 5.8), which can be written with a QPainter but I don't understand how.
First, this is how I imagine the entity could look like:
[EDIT]: code is edited and works now!
planeEntity = new Qt3DCore::QEntity(rootEntity);
planeMesh = new Qt3DExtras::QPlaneMesh;
planeMesh->setWidth(2);
planeMesh->setHeight(2);
image = new TextureImage; //see below
image->setSize(QSize(100,100));
painter = new QPainter;
image->paint(painter)
planeMaterial = new Qt3DExtras::QDiffuseMapMaterial;
planeMaterial->diffuse()->addTextureImage(image);
planeEntity->addComponent(planeMesh);
planeEntity->addComponent(planeMaterial);
TextureImage is the subclassed QPaintedTextureImage with paint function:
class TextureImage : public Qt3DRender::QPaintedTextureImage
{
public:
void paint(QPainter* painter);
};
What does the QPainter, passed to paint function, need to do in the implementation of paint if I just want to draw a big circle to the planeEntity?
[Edit] Implementation:
void TextureImage::paint(QPainter* painter)
{
//hardcoded values because there was no device()->width/heigth
painter->fillRect(0, 0, 100, 100, QColor(255, 255, 255));
/* Set pen and brush to whatever you want. */
painter->setPen(QPen(QBrush(QColor(255, 0, 255)) ,10));
painter->setBrush(QColor(0, 0, 255));
/*
* Draw a circle (or an ellipse -- the outcome depends very much on
* the aspect ratio of the bounding rectangle amongst other things).
*/
painter->drawEllipse(0, 0, 100, 100);
}

Answer 1

The short answer is... use QPainter exactly the same way you would normally.
void TextureImage::paint (QPainter* painter)
{
int w = painter->device()->width();
int h = painter->device()->height();
/* Clear to white. */
painter->fillRect(0, 0, w, h, QColor(255, 255, 255));
/* Set pen and brush to whatever you want. */
painter->setPen(QPen(QBrush(QColor(0, 0, 0)) ,10));
painter->setBrush(QColor(0, 0, 255));
/*
* Draw a circle (or an ellipse -- the outcome depends very much on
* the aspect ratio of the bounding rectangle amongst other things).
*/
painter->drawEllipse(0, 0, w, h);
}
However, note that you really shouldn't invoke the paint method directly. Instead use update which will cause Qt to schedule a repaint, initialize a QPainter and invoke your overridden paint method with a pointer to that painter.

Answer 2

It might be simpler to dynamically load the image you need in QML.
I had to do it not so long ago and opened a question on SO for it:
Qt3D dynamic texture