cosine_interpolate.cpp

  QPainterPath path;

  std::sort(points.begin(), points.end(),
            [](const QPointF& p1, const QPointF& p2) -> bool
  {
    return p1.x() < p2.x();
  });

  constexpr auto step = 100.0;

  for (size_t i = 0; i < points.size()-1; ++i) {
    auto x1 = points[i].x();
    auto y1 = points[i].y();
    auto x2 = points[i+1].x();
    auto y2 = points[i+1].y();

    auto diff = std::abs(x1 - x2);
    for (int j = 0; j < step; ++j) {
      auto mu = j / step;
      auto mu2 = (1-cos(mu*M_PI))/2;
      auto y = (y1*(1-mu2)+y2*mu2);
      auto x = (diff / step) * j + x1;
      QPointF pt(x, y);
      poly.append(pt);
    }
  }


  path.addPolygon(poly);
  painter.drawPath(path);