As I hold the mouse button down and move the vector around it's p1 anchor point, the grey image I am trying to draw sometimes shows up and when it dies, it is a partial draw.

No i am not calling update().

Qt Code: 
  1. void PhysGraphicsScene::drawBackground(QPainter *painter, const QRectF &rect) {
  2.     Q_UNUSED(rect);
  3.  
  4.     QMatrix mtx;
  5.     int w_2 = width() / 2;
  6.     int h_2 = height() / 2;
  7.  
  8.     painter -> setPen(Qt::blue);
  9.     painter -> drawLine(0, h_2, width(), h_2);     // X-Axis
  10.     painter -> drawLine(w_2, 0 , w_2, height());  // Y-Axis
  11.  
  12.     mtx.translate(w_2, h_2);
  13.     mtx.scale(1, -1);
  14.  
  15.     painter -> setMatrix(mtx);
  16.     painter -> setPen(Qt::NoPen);
  17.     painter -> setBrush(QBrush(Qt::blue, Qt::Dense4Pattern));
  18.     painter -> drawRect(-10, -10, 20, 20);
  19.  
  20.     QLineF y_axis(-10, -10, -10, 10);
  21.     QLineF x_axis(-10, -10, 10, -10);
  22.     QPen pen;
  23.     QLineF angleLine1, angleLine2;
  24.  
  25.     pen.setWidth(2);
  26.     pen.setColor(Qt::red);
  27.     painter -> setPen(pen);
  28.  
  29.     setupAngleLine(angleLine1, x_axis.p2(), x_axis.p1(), 135.0);
  30.     setupAngleLine(angleLine2, x_axis.p2(), x_axis.p1(), -135.0);
  31.     painter -> drawLine(x_axis);     // X-Axis
  32.     painter -> drawLine(angleLine1);
  33.     painter -> drawLine(angleLine2);
  34.  
  35.     pen.setColor(Qt::green);
  36.     setupAngleLine(angleLine1, y_axis.p2(), y_axis.p1(), 45.0);
  37.     setupAngleLine(angleLine2, y_axis.p2(), y_axis.p1(), 135.0);
  38.     painter -> setPen(pen);
  39.     painter -> drawLine(y_axis);  // Y-Axis
  40.     painter -> drawLine(angleLine1);
  41.     painter -> drawLine(angleLine2);
  42.  
  43.     if (m_Mode == InsertLine && m_pLine)
  44.         drawAngledPlane(painter);
  45. }
  46.  
  47. void PhysGraphicsScene::drawAngledPlane(QPainter *painter) {
  48.     QPointF lineMidPoint = findLineMidPoint(m_pLine ->line());
  49.     QRectF lineRect(m_pLine ->line().p1(), m_pLine ->line().p2());
  50.  
  51.     // Draw the temporary cartesian axis' origin'ed at p1 of the line
  52.     QPen pen(Qt::lightGray);
  53.     pen.setStyle(Qt::DashDotDotLine);
  54.     pen.setWidth(2);
  55.     painter ->setPen(pen);
  56.     QLineF xAxis, yAxis;
  57.     xAxis.setAngle(90);
  58.     xAxis.setP1(m_pLine ->line().p1()); xAxis.setP2(QPointF(m_pLine ->line().dx(), 0));
  59.     xAxis.setLength(m_pLine ->line().dx());
  60.  
  61.     yAxis.setAngle(0);
  62.     yAxis.setLength(m_pLine ->line().dy());
  63.  
  64.     painter -> drawLine(xAxis);   // x-axis
  65.     painter -> drawLine(yAxis);   // y-axis
  66.  
  67.  
  68.     // Draw the arc from the false cartesian axis to the line
  69.     double angle = m_pLine ->line().angle();
  70. }
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