This is just an experiment I have been doing. It takes about 7 seconds to run with 200 points. The CPU and memory on the machine are not that great 454 MHz ARM9 processor with 128MB RAM, there might not be a way to speed this up, but I thought I'd ask.

Qt Code: 
  1. void HeatMap::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
  2. {
  3.     int x = 0;
  4.     int y = 0;
  5.     qreal intensity = 0;
  6.     qDebug() << QTime::currentTime();
  7.  
  8.     QImage image(m_width, m_height, QImage::Format_ARGB32_Premultiplied);
  9.     QPainter *paint = new QPainter(&image);
  10.  
  11.     foreach (QVariant v, m_data)
  12.     {
  13.         QStringList tmp = v.toString().split(",");
  14.  
  15.         x = tmp[0].toInt();
  16.         y = tmp[1].toInt();
  17.         intensity = tmp[2].toDouble();
  18.         QRadialGradient g(QPointF(x, y), m_radius, QPointF(x, y));
  19.         g.setColorAt(0.0, QColor::fromRgbF(0, 0, 0, intensity));
  20.         g.setColorAt(1.0, QColor::fromRgbF(0, 0, 0, 0.0));
  21.         paint->fillRect(QRectF(x - m_radius, y - m_radius, x + m_radius, y + m_radius), g);
  22.     }
  23.  
  24.     painter->drawImage(QPoint(0, 0), image);
  25.     qDebug() << QTime::currentTime();
  26. }
To copy to clipboard, switch view to plain text mode 

Thanks