LUCENE-8363: Interpolate should yield points on the ellipsoid.

lucene/spatial3d/src/java/org/apache/lucene/spatial3d/geom/GeoComplexPolygon.java

@@ -706,7 +706,7 @@ public Edge(final PlanetModel pm, final GeoPoint startPoint, final GeoPoint endP
       this.plane = new Plane(startPoint, endPoint);
       this.startPlane =  new SidedPlane(endPoint, plane, startPoint);
       this.endPlane = new SidedPlane(startPoint, plane, endPoint);
-      final GeoPoint interpolationPoint = plane.interpolate(startPoint, endPoint, halfProportions)[0];
+      final GeoPoint interpolationPoint = plane.interpolate(pm, startPoint, endPoint, halfProportions)[0];
       this.backingPlane = new SidedPlane(interpolationPoint, interpolationPoint, 0.0);
       this.planeBounds = new XYZBounds();
       this.planeBounds.addPoint(startPoint);

lucene/spatial3d/src/java/org/apache/lucene/spatial3d/geom/Plane.java

@@ -465,12 +465,13 @@ public double linearDistanceSquared(final PlanetModel planetModel, final double
    * Find points on the boundary of the intersection of a plane and the unit sphere,
    * given a starting point, and ending point, and a list of proportions of the arc (e.g. 0.25, 0.5, 0.75).
    * The angle between the starting point and ending point is assumed to be less than pi.
+   * @param planetModel is the planet model.
    * @param start is the start point.
    * @param end is the end point.
    * @param proportions is an array of fractional proportions measured between start and end.
    * @return an array of points corresponding to the proportions passed in.
    */
-  public GeoPoint[] interpolate(final GeoPoint start, final GeoPoint end, final double[] proportions) {
+  public GeoPoint[] interpolate(final PlanetModel planetModel, final GeoPoint start, final GeoPoint end, final double[] proportions) {
     // Steps:
     // (1) Translate (x0,y0,z0) of endpoints into origin-centered place:
     // x1 = x0 + D*A
@@ -595,7 +596,7 @@ public GeoPoint[] interpolate(final GeoPoint start, final GeoPoint end, final do
       final double sinNewAngle = Math.sin(newAngle);
       final double cosNewAngle = Math.cos(newAngle);
       final Vector newVector = new Vector(cosNewAngle * startMagnitude, sinNewAngle * startMagnitude, 0.0);
-      returnValues[i] = reverseModify(newVector, transX, transY, transZ, sinRA, cosRA, sinHA, cosHA);
+      returnValues[i] = reverseModify(planetModel, newVector, transX, transY, transZ, sinRA, cosRA, sinHA, cosHA);
     }
 
     return returnValues;
@@ -620,6 +621,7 @@ protected static Vector modify(final GeoPoint start, final double transX, final
 
   /**
    * Reverse modify a point to produce a GeoPoint in normal space.
+   * @param planetModel is the planet model.
    * @param point is the translated point.
    * @param transX is the translation x value.
    * @param transY is the translation y value.
@@ -630,10 +632,11 @@ protected static Vector modify(final GeoPoint start, final double transX, final
    * @param cosHA is the cosine of the height angle.
    * @return the original point.
    */
-  protected static GeoPoint reverseModify(final Vector point, final double transX, final double transY, final double transZ,
-                                          final double sinRA, final double cosRA, final double sinHA, final double cosHA) {
+  protected static GeoPoint reverseModify(final PlanetModel planetModel,
+    final Vector point, final double transX, final double transY, final double transZ,
+    final double sinRA, final double cosRA, final double sinHA, final double cosHA) {
     final Vector result = point.rotateXZ(-sinHA, cosHA).rotateXY(-sinRA, cosRA).translate(-transX, -transY, -transZ);
-    return new GeoPoint(result.x, result.y, result.z);
+    return planetModel.createSurfacePoint(result.x, result.y, result.z);
   }
 
   /**

lucene/spatial3d/src/test/org/apache/lucene/spatial3d/geom/PlaneTest.java

@@ -63,7 +63,7 @@ public void testInterpolation() {
     // [A=-0.6135342247741855, B=0.21504338363863665, C=0.28188192383666794, D=0.0, side=-1.0] internal? false;
     final Plane p = new Plane(-0.6135342247741855, 0.21504338363863665, 0.28188192383666794, 0.0);
 
-    final GeoPoint[] points = p.interpolate(start, end, new double[]{0.25, 0.50, 0.75});
+    final GeoPoint[] points = p.interpolate(PlanetModel.SPHERE, start, end, new double[]{0.25, 0.50, 0.75});
 
     for (GeoPoint point : points) {
       assertTrue(p.evaluateIsZero(point));