public IEnumerable<int> GetQuadsInFrustum(Frustum frustum, bool treestart = false) { if (treestart || frustum.Check(BoundingBox)) { if (SubContainer == null) return new List<int> { ContainedQuad }; return SubContainer.SelectMany(container => container.GetQuadsInFrustum(frustum)).Where(quadId => quadId != -1); } return new List<int> { -1 }; }