I want to display "basic" segments in a QGraphicsView with fixed width for the segments, zoom in/out feature and efficient bouding boxes.

The zoom is implemented that way:

Qt Code: 
  1. QMatrix vMatrix; 
  2. vMatrix.scale(HorizontalScale(), VerticalScale());
  3. setMatrix(vMatrix);
  4. scene()->update();
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And the segment that way:

Qt Code: 
  1. class Segment : public QGraphicsView {
  2.  
  3.     qreal mX1;
  4.     qreal mY1;
  5.     qreal mX2;
  6.     qreal mY2;
  7.  
  8.     void paint(QPainter* aPainter, const QStyleOptionGraphicsItem* aOption, QWidget* aWidget) {
  9.         QPen vPen(QColor(0, 0, 255));
  10.         vPen.setCosmetic(true);
  11.         vPen.setWidth(3);    
  12.  
  13.         QPaintDevice *vDevice = aPainter->device();
  14.         aPainter->setPen(vPen);
  15.         aPainter->drawLine(mX1, mY1, mX2, mY2);
  16.     }
  17.  
  18.     QRectF boundingRect() const {
  19.         qreal vHalfWidth = sPen.width() / 2.0f;
  20.         qreal vX = qMin(mX1, mX2) - vHalfWidth; 
  21.         qreal vY = qMin(mY1, mY2) - vHalfWidth; 
  22.         qreal vWidth = qAbs(mX2 - mX1) + vHalfWidth*2; 
  23.         qreal vHeight = qAbs(mY2 - mY1) + vHalfWidth*2; 
  24.  
  25.         return QRectF(vX, vY, vWidth, vHeight);
  26.     }
  27. }
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With the code above and an horizontal segment. The bouding box height is always returned to 3. But when zoomin, the segment always has its 3 pixels width (thanks to setCosmetic(true)) BUT the bounding box is stretched larger than the segment drawing.

Any idea on how to handle this ?