[Qt-interest] QPainter::drawLine performances

[Laurent Astart]

Hi,

I'm new to the list but I'm using Qt for some time now. Those days I
stumbled onto a problem which puzzled me. I'm working on a big software
using Qt 4.5.3 and experienced performances problems. Eventually I found
what the problem is and builded an example to try some things. I created a
widget (RenderArea) and my problem is entirely contained within the
paintEvent. Here is the function :

void RenderArea::paintEvent(QPaintEvent *event)
{
  QRect rect(QPoint(0, 0), QPoint(640, 480));
  QPainterPath path;
  path.addRect(rect);

  QPainter painter(this);
  painter.setPen(QPen(Qt::blue, 3, Qt::DashLine, Qt::SquareCap,
Qt::RoundJoin));
  painter.setBackground(QBrush(QColor(255, 255, 255), Qt::SolidPattern));
  painter.setRenderHint(QPainter::Antialiasing, true);
  painter.setClipRect(rect);
  //painter.setClipPath(path);
  bool hasClip = painter.hasClipping();

  painter.eraseRect(rect);

  CHighResTimeCounter counter;
  counter.Start();
  painter.drawLine(QPoint(-64000000, -48000000), QPoint(64000000,
48000000));
  int msCount = counter.Count();

  QString perf = QString("Perf: %1 ms (clipping
%2)").arg(msCount).arg(hasClip ? "enabled" : "disabled");
  emit PaintStatus(perf);
}

Whatever I do (clipping or not, rectangle or path, etc.), the render time is
around 5500 ms. When I reduce the line size by ten times in each direction,
I decrease the render time by ten. Moreover, when the line is entirely
outside the widget view, the render time is the same. Even when the bounding
box of the line doesn't intersect the widget view, the render time is the
same. I suspect the drawLine function to draw every pixel, then reject the
ones outside the port or the clipping.

My question is : Is there a way to reduce the drawing to a couple of ms
without surcharging the drawLine function ? I'd like to avoid calling a
precomputing function on my lines. The reasons are :
1) A lot of code is involved, but I use only one painter. It would be
simpler to configure it properly.
2) I'm not sure I will be able to create a good intersection function,
especially with dashed/dotted lines and antialiasing.

For the ones curious about why I'm trying to draw huge lines in comparison
of widget size, I'm working on a cartographic software. Some vector data are
sometimes huge (just imagine maritime routes or airways) and the user
sometimes zoom to a hundred meters zone. In this case, a 100 Nm line is huge
compared to the widget size.

Thanks in advance for your help.
Laurent
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