Stippled Pattern when Painting with mouse/tablet

**DanqueDynasty** · 22nd March 2014, 05:48

Hello all,

I am experience a problem where if I try to draw with pen/tablet input, the widget will draw a line along with circles at points periodically. Such as the following:
problem.png
The code(in context of QTabletEvent, QMouseEvent is coded the same way):

Qt Code:

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bool deviceDown = false;
QPoint drawPath[3];
 
void Editor::TabletEvent(QTabletEvent* event)
{
  if(event->type() == QEvent::TabletPress)
  { 
     deviceDown = true;
     drawPath[2] = drawPath[1] = drawPath[0] = event->pos();
  }
  if(event->type() == QEvent::TabletRelease)
  {
     if(deviceDown == true){deviceDown = false;}
  }
 
  if(event->type() == QEvent::TabletMove)
  {
     drawPath[2] = drawPath[1];
     drawPath[1] = drawPath[0];
     drawPath[0] = event->pos();
 
     /* DRAW CODE - Commented out due to actual code residing in another class/function. 
     QPainter painter(&pixmap);
     painter.drawLine(drawPath[1], event->pos());
     */
  }
}

bool deviceDown = false;
QPoint drawPath[3];

void Editor::TabletEvent(QTabletEvent* event)
{
  if(event->type() == QEvent::TabletPress)
  { 
     deviceDown = true;
     drawPath[2] = drawPath[1] = drawPath[0] = event->pos();
  }
  if(event->type() == QEvent::TabletRelease)
  {
     if(deviceDown == true){deviceDown = false;}
  }

  if(event->type() == QEvent::TabletMove)
  {
     drawPath[2] = drawPath[1];
     drawPath[1] = drawPath[0];
     drawPath[0] = event->pos();

     /* DRAW CODE - Commented out due to actual code residing in another class/function. 
     QPainter painter(&pixmap);
     painter.drawLine(drawPath[1], event->pos());
     */
  }
}

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I have attempted to use a single point as the last known point, but that failed. Using a QPoint array seams to be the only way to get the widget to draw any lines correctly.

**ChrisW67** · 24th March 2014, 04:47

You paint a translucent (alpha < 1.0) line from A to B, then B to C, C to D etc. The point B is painted twice, once for each line it is part of. The result is that the brush area on top of point B is now darker. Same goes for C etc.

Options:

Accumulate points from press to release and then draw a single poly-line (QPainter::drawPolyLine())
Accumulate points from press and draw a single polyline onto a transparent overlay image that you then compose with the base image onto the display. When the finger is raised draw the entire polyline into the base image.