1   /* Copyright 2002-2025 CS GROUP
2    * Licensed to CS GROUP (CS) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * CS licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *   http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.orekit.models.earth.tessellation;
18  
19  import java.util.List;
20  
21  import org.hipparchus.geometry.euclidean.threed.Vector3D;
22  import org.orekit.bodies.GeodeticPoint;
23  
24  /** Interface defining the aiming direction of {@link Tile tiles}.
25   * @author Luc Maisonobe
26   */
27  public interface TileAiming {
28  
29      /** Get points at which aiming direction cannot be computed.
30       * <p>
31       * As per Brouwer's <a href="http://mathworld.wolfram.com/HairyBallTheorem.html">hairy
32       * ball theorem</a>, any vector field on the 2-sphere has at least one zero.
33       * This implies that any implementation of this interface has at least one point
34       * where the aiming direction cannot be computed. The most typical example is aiming
35       * always towards North pole, for which both poles are singular points.
36       * </p>
37       * @return a non-empty (as per hairy ball theorem) list of points where aiming direction
38       * is either zero or cannot be computed
39       * @since 10.0
40       */
41      List<GeodeticPoint> getSingularPoints();
42  
43      /** Find the along tile direction for tessellation at specified point.
44       * @param point point on the ellipsoid (Cartesian coordinates)
45       * @param gp point on the ellipsoid (geodetic coordinates)
46       * @return normalized along tile direction
47       */
48      Vector3D alongTileDirection(Vector3D point, GeodeticPoint gp);
49  
50  }