func (v Vec3) Perp2D(u Vec3) float32 {
	return v[2]*u[0] - v[0]*u[2]
}
func (v Vec3) Perp2D(u Vec3) float32 {
	return v[2]*u[0] - v[0]*u[2]
}
// Perp2D derives the xz-plane 2D perp product of the two vectors. (uz*vx - ux*vz)
//
// u is the LHV vector [(x, y, z)]
// v is the RHV vector [(x, y, z)]
//
// The vectors are projected onto the xz-plane, so the y-values are ignored.