// Vec3Diff returns the rotation quaternion between two vectors.
func Vec3Diff(a, b *vec3.T) T {
cr := vec3.Cross(a, b)
sr := math.Sqrt(2 * (1 + vec3.Dot(a, b)))
oosr := 1 / sr
q := T{cr[0] * oosr, cr[1] * oosr, cr[2] * oosr, sr * 0.5}
return q.Normalized()
}
// Cross returns the cross product of two vectors.
func Cross(a, b *T) T {
a3 := a.Vec3DividedByW()
b3 := b.Vec3DividedByW()
c3 := vec3.Cross(&a3, &b3)
return T{c3[0], c3[1], c3[2], 1}
}