QGraphicsPathItem: paint elements differently

Question

How do I paint a QGraphicsPathItem in a way that it will fade out after n-elements at the end?

Every second I add a QPointF to the end of the path and wish to display the n-th element with 100% opacity. And n-1 with 90%, n-2 with 80% etc. So the path only show the last 10 (for example) and smoothly fades out. But how can I do that with qt5?

I figured how to change color of the entire path, but how do I change it on sub-item level... meaning here the path elements?

Answer 1

I would use separate QGraphicsLineItem for each segment and adjust their opacity independently:

class Object : public QObject {
  Q_OBJECT
public:
  Object();
  void add_point(QPointF point);
private:
  QGraphicsScene m_scene;
  QList<QGraphicsLineItem*> m_items;
  QPointF m_previous_point;
private slots:
  void timeout();
};

Object::Object() {
  QGraphicsView* view = new QGraphicsView();
  view->setRenderHint(QPainter::Antialiasing);
  view->setScene(&m_scene);
  view->scale(10, 10);
  view->resize(400, 200);
  view->show();
  QTimer* timer = new QTimer(this);
  timer->start(30);
  connect(timer, &QTimer::timeout, this, &Object::timeout);
}

void Object::add_point(QPointF point) {
  const int MAX_SEGMENTS = 100;
  QGraphicsLineItem* new_line = 
      m_scene.addLine(QLineF(m_previous_point, point));
  QPen pen;
  pen.setCosmetic(true);
  pen.setWidth(3);
  new_line->setPen(pen);
  m_items.append(new_line);
  while(m_items.count() > MAX_SEGMENTS) {
    delete m_items[0];
    m_items.removeFirst();
  }
  for(int i = 0; i < m_items.count(); i++) {
    m_items[i]->setOpacity(
        1.0 - 1.0 / MAX_SEGMENTS * (m_items.count() - 1 - i));
  }
  m_previous_point = point;
}

void Object::timeout() {
  double x = m_previous_point.x() + 0.1;
  add_point(QPointF(x, qSin(x)));
}