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]
}
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.
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 .