QPolygon transformation

Question

Need to transform polygon vertexes on the fly till origin point 0,0 (instead of center).

I have polygon ( 8x64 ) (with predefined vertex):

QPolygonF cursor;
cursor << QPointF(-4, 32); 
cursor << QPointF(-4, -32);
cursor << QPointF(4, -32);
cursor << QPointF(4, 32);

Then I draw it and use as cursor:

QPixmap pixmap( cursor.boundingRect().width()+1, cursor.boundingRect().height()+1 );
pixmap.fill( QColor( Qt::black) );

QPixmap alpha = pixmap.createMaskFromColor(QColor( Qt::black ),Qt::MaskOutColor);
pixmap.setAlphaChannel( alpha );

QPainter painter( &pixmap );
painter.setPen( QPen( Qt::green) );

// move to center, because polygon coordinated starts from center
painter.translate(cursor.boundingRect().width()/2, cursor.boundingRect().height()/2 );
painter.drawPolygon( cursor );

setCursor( pixmap );

But after mouse click, I have mouse position at center of this polygon, this is normal and logical.

Question, how to transform polygon on the fly ( transform matrix or such ) to 0,0 origin point and get after mouse click top, left coordinates (instead of center) ?