Custom QQuickItem: Updating Geometry

maxwell31

Hi,

I have a custom QQuickItem which I use to display a x,y graph coming from a measurement device. If the measurement device delivers new data, I need to reset the QSGGeometry. I wonder whether it is possible to only append points to the end of the geometry instead of resetting it completely.

IntruderExcluder

Isn't QSGGeometry::allocate is what you need?

maxwell31

Maybe :-). I tried it, but sometimes I receive a segfault. I should note that I guess I did not follow this rule:
"The rule of thumb is to only use classes with the "QSG" prefix inside the QQuickItem::updatePaintNode() function." from https://doc.qt.io/qt-5.9/qtquick-visualcanvas-scenegraph.html
So I guess the problem is that I might not call it in the right place
My approach:

The geometry is a member of the class.

class PlotCurve : public QQuickItem {
public:
  void updateGeometry(const std::vector<double>* xdata, const std::vector<double>* ydata);
private:
  QSGGeometry * _geometry;
  QSGGeometryNode * _curve;

When I had static x and y data, I set it in the constructor like this:

 _geometry = new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(),xdata->size());
  QSGGeometry::Point2D *points = _geometry->vertexDataAsPoint2D();
  for(int i=0;i<xdata->size();i++) {
    points[i].x = xdata->at(i);
    points[i].y = view_y_range-ydata->at(i)+_view_y_min;
  }

Here is my updateGeoemtry function

_geometry->allocate(xdata->size());
QSGGeometry::Point2D *points = _geometry->vertexDataAsPoint2D();
for(int i=0;i<xdata->size();i++) {
  points[i].x = xdata->at(i);
  points[i].y = (_view_y_max-_view_y_min)-ydata->at(i)+_view_y_min;
}
_curve->markDirty(QSGNode::DirtyGeometry);

and here my updatePaintNode

QSGNode *PlotCurve::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *) {

  QSGNode * mainNode = oldNode;
  if(!mainNode) {
    mainNode = new QSGNode;
  }
  if(!_transformNode) {
    _transformNode = new QSGTransformNode;
    mainNode->appendChildNode(_transformNode);
  }

  if(!_curve) {
    _geometry->setLineWidth(_curve_width);
    _geometry->setDrawingMode(GL_LINE_STRIP);
    _curve = new QSGGeometryNode;
    _curve->setFlag(QSGNode::OwnsMaterial,true);
    _curve->setFlag(QSGNode::OwnsGeometry,true);
    _curve->setGeometry(_geometry);
    QSGFlatColorMaterial* material = new QSGFlatColorMaterial;
    material->setColor(_curve_color);
    _curve->setMaterial(material);
    _transformNode->appendChildNode(_curve);
  }

  double transX = -_view_x_min;
  double transY = -(_view_y_max_init-_view_y_max);

  QMatrix4x4 m;
  m.scale(_scaleFactorX,_scaleFactorY);
  m.translate(transX, transY);
  _transformNode->setMatrix(m);

  return mainNode;

}

maxwell31

So what is the correct way to update the geometry of a QQuickItem? In my case, the xdata and ydata vectors would simply grow with time and the data before would remain the same

maxwell31

No one is creating his own QQuickItems with changing geometry?