Improve contour FindPointInPolygon() reliability (2).

src/Triangle/Tools/IntersectionHelper.cs

@@ -6,10 +6,42 @@
 
 namespace TriangleNet.Tools
 {
+    using System;
     using TriangleNet.Geometry;
 
     public static class IntersectionHelper
     {
+        /// <summary>
+        /// Check if a given test point lies on a segment.
+        /// </summary>
+        /// <param name="a">The segment start point.</param>
+        /// <param name="b">The segment end point.</param>
+        /// <param name="test">The point to test.</param>
+        /// <param name="eps">Threshold to test collinearity (default = 1e-12).</param>
+        /// <returns></returns>
+        public static bool IsPointOnSegment(Point a, Point b, Point test, double eps = 1e-12)
+        {
+            // The cross product.
+            double cross = (test.Y - a.Y) * (b.X - a.X) - (test.X - a.X) * (b.Y - a.Y);
+
+            // Check if points are collinear.
+            if (Math.Abs(cross) > eps) return false;
+
+            // The dot product (projection of test point onto segment).
+            double dot = (test.X - a.X) * (b.X - a.X) + (test.Y - a.Y) * (b.Y - a.Y);
+
+            // Check if test point is actually between a and b (left of a).
+            if (dot < 0) return false;
+
+            // Length of the segment.
+            double ab = (b.X - a.X) * (b.X - a.X) + (b.Y - a.Y) * (b.Y - a.Y);
+
+            // Check if test point is actually between a and b (right of b).
+            if (dot > ab) return false;
+
+            return true;
+        }
+
         /// <summary>
         /// Compute intersection of two segments.
         /// </summary>