I rotate the view to xy plane and try to draw a line by two points selected by mouse click.
I set Z1=Z2=0, the firt point P1(X1,Y1,0)、 second point
P2(X2,Y2,0) are got by "myView->Convert(point.x,point.y,Y1,Z1) ;"
However, after drawing several lines(about 3 or 4 lines), the error appears.
It's interesting that if I set the Z1=0,Z2=0.00001, the error disappears.
I wonder what's the reason.