float rayAtDistanceFromOrigin(const Ray& r, float radius)
{
  const float* d = r.getDirection();
  if (d[0] == 0.0 && d[1] == 0.0 && d[2] == 0.0) return 0.0f;

  const float* p = r.getOrigin();
  const float a = d[0] * d[0] + d[1] * d[1] + d[2] * d[2];
  const float b = -(p[0] * d[0] + p[1] * d[1] + p[2] * d[2]);
  const float c = p[0] * p[0] + p[1] * p[1] + p[2] * p[2] - radius * radius;
  const float disc = b * b - a * c;
  if (disc < 0.0f) return -1.0f;
  const float d1_2 = sqrtf(disc);
  const float t0 = b + d1_2;
  const float t1 = b - d1_2;
  if (t0 < t1)
    if (t0 < 0.0f) return t1 / a;
    else return t0 / a;
  else
    if (t1 < 0.0) return t0 / a;
    else return t1 / a;
}