// face returns face ID from 0 to 5 containing the r. For points on the
// boundary between faces, the result is arbitrary but deterministic.
func face(r r3.Vector) int {
abs := r.Abs()
id := 0
value := r.X
if abs.Y > abs.X {
id = 1
value = r.Y
}
if abs.Z > math.Abs(value) {
id = 2
value = r.Z
}
if value < 0 {
id += 3
}
return id
}
