Position refPos = dragObject.getReferencePosition();
if (refPos == null)
return;
Vec4 refPoint = globe.computePointFromPosition(refPos);
Vec4 screenRefPoint = view.project(refPoint);
// Compute screen-coord delta since last event.
int dx = dragEvent.getPickPoint().x - dragEvent.getPreviousPickPoint().x;
int dy = dragEvent.getPickPoint().y - dragEvent.getPreviousPickPoint().y;