src/main/java/eu/svjatoslav/sixth/e3d/geometry/Polygon.java

   1 /*
   2  * Sixth 3D engine. Author: Svjatoslav Agejenko.
   3  * This project is released under Creative Commons Zero (CC0) license.
   4  */
   5 package eu.svjatoslav.sixth.e3d.geometry;
   6
   7 public class Polygon {
   8
   9     private static boolean intersectsLine(final Point2D point, Point2D p1,
  10                                           Point2D p2) {
  11
  12         if (p1.y > p2.y) {
  13             final Point2D tmp = p1;
  14             p1 = p2;
  15             p2 = tmp;
  16         }
  17
  18         if (point.y < p1.y)
  19             return false;
  20         if (point.y > p2.y)
  21             return false;
  22
  23         final double xp = p2.x - p1.x;
  24         final double yp = p2.y - p1.y;
  25
  26         final double yp2 = point.y - p1.y;
  27
  28         final double crossX = p1.x + ((xp * yp2) / yp);
  29
  30         return point.x >= crossX;
  31
  32     }
  33
  34     public static boolean pointWithinPolygon(final Point2D point,
  35                                              final Point2D p1, final Point2D p2, final Point2D p3) {
  36
  37         int intersectionCount = 0;
  38
  39         if (intersectsLine(point, p1, p2))
  40             intersectionCount++;
  41
  42         if (intersectsLine(point, p2, p3))
  43             intersectionCount++;
  44
  45         if (intersectsLine(point, p3, p1))
  46             intersectionCount++;
  47
  48         return intersectionCount == 1;
  49
  50     }
  51
  52 }