The following code produces very different result, the only different is the precision (with "f" turn to float), can someone explain please? Many thanks!

Qt Code: 
  1. #include <QPolygonF>
  2. #include <QDebug>
  3.  
  4. int main()
  5. {
  6.   QPolygonF polygon1;
  7.   polygon1 << QPointF(-49.3670886075949369, 1930.1848049281315980 )
  8.            << QPointF(-34.7848101265822862, 1170.4312114989734255 )
  9.            << QPointF(-13.1139240506329173, 2084.1889117043124315 );
  10.  
  11.   QPolygonF polygon2;
  12.   polygon2 << QPointF(-41.6708860759493689, 1940.4517453798769111 )
  13.            << QPointF(-52.2025316455696142, 431.2114989733059929 )
  14.            << QPointF(-3.3924050632911431, 1509.2402464065710319 )
  15.            << QPointF(-26.8190801175864770, 1506.3079775881415117 )
  16.            << QPointF(-34.7848101265822862, 1170.4312114989734255 )
  17.            << QPointF(-41.1968964968032125, 1504.5083358315471287 );
  18.   qDebug() << "polygon1 - polygon2 = " << polygon1.subtracted( polygon2 );
  19.  
  20.   ////////////////////
  21.   QPolygonF polygon3;
  22.   polygon3 << QPointF(-49.3670886075949369, 1930.1848049281315980 )
  23.            << QPointF(-34.7848101265822862, 1170.4312114989734255 )
  24.            << QPointF(-13.1139240506329173, 2084.1889117043124315 );
  25.  
  26.   QPolygonF polygon4;
  27.   polygon4 << QPointF(-41.6708860759493689f, 1940.4517453798769111f )
  28.            << QPointF(-52.2025316455696142f, 431.2114989733059929f )
  29.            << QPointF(-3.3924050632911431f, 1509.2402464065710319f )
  30.            << QPointF(-26.8190801175864770f, 1506.3079775881415117f )
  31.            << QPointF(-34.7848101265822862f, 1170.4312114989734255f )
  32.            << QPointF(-41.1968964968032125f, 1504.5083358315471287f );
  33.   qDebug() << "polygon3 - polygon4 = " << polygon3.subtracted( polygon4 );
  34.  
  35.   ////////////////////
  36.   QPolygonF polygon5;
  37.   polygon5 << QPointF(-49.3670886075949369f, 1930.1848049281315980 )
  38.            << QPointF(-34.7848101265822862, 1170.4312114989734255 )
  39.            << QPointF(-13.1139240506329173, 2084.1889117043124315 );
  40.  
  41.   QPolygonF polygon6;
  42.   polygon6 << QPointF(-41.6708860759493689, 1940.4517453798769111 )
  43.            << QPointF(-52.2025316455696142, 431.2114989733059929 )
  44.            << QPointF(-3.3924050632911431, 1509.2402464065710319 )
  45.            << QPointF(-26.8190801175864770, 1506.3079775881415117 )
  46.            << QPointF(-34.7848101265822862, 1170.4312114989734255 )
  47.            << QPointF(-41.1968964968032125, 1504.5083358315471287 );
  48.   qDebug() << "polygon5 - polygon6 = " << polygon5.subtracted( polygon6 );
  49.  
  50. }
To copy to clipboard, switch view to plain text mode 

Output is:
Qt Code: 
  1. polygon1 - polygon2 =  QPolygonF() 
  2. polygon3 - polygon4 =  QPolygonF(QPointF(-49.3671, 1930.18)QPointF(-43.7755, 1638.85)QPointF(-41.6709, 1940.45)QPointF(-41.1969, 1504.51)QPointF(-37.0334, 1287.59)QPointF(-34.7848, 1170.43)QPointF(-27.884, 1461.4)QPointF(-26.8191, 1506.31)QPointF(-26.8191, 1506.31)QPointF(-13.1139, 2084.19)QPointF(-49.3671, 1930.18)) 
  3. polygon5 - polygon6 =  QPolygonF(QPointF(-43.7755, 1638.85)QPointF(-41.1969, 1504.51)QPointF(-41.6709, 1940.45)QPointF(-43.7755, 1638.85))
To copy to clipboard, switch view to plain text mode