public void Render(int layer, int element_idx)
        {
            float layerZ = Grid.GetLayerZ(Grid.SceneLayer.Ground);

            layerZ -= 0.0001f * (float)element_idx;
            opaque.Render(new Vector3(0f, 0f, layerZ), layer);
            alpha.Render(new Vector3(0f, 0f, layerZ), layer);
        }