public static clsPoint3d ProjectPointOntoPlaneAlongZ(clsPoint p, clsLine3d l1)
{
clsLine3d v1 = new clsLine3d();
double u = 0;
double r = 0;
double z = 0;
clsPoint3d p1 = new clsPoint3d();
v1 = new clsLine3d(new clsPoint3d(0, 0, 0), new clsPoint3d(0, 0, 1));
u = v1.Dot(l1);
//This is the length of the normal vector, measured in the vertical direction
p1 = new clsPoint3d(p.X - l1.X1, p.Y - l1.Y1, 0 - l1.Z1);
r = p1.Dot(l1.DP());
//r is the distance of our point to the nearest point on the plane
//r = v1.Dot(myV)
z = r / u;
//z is the vertical distance of our point to the plane. It is sign sensitive - positive is above the plane.
return(new clsPoint3d(p.X, p.Y, -z));
}
