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 /**
   8  * Utility class for polygon operations.
   9  */
  10 public class Polygon {
  11
  12     private static boolean intersectsLine(final Point2D point, Point2D p1,
  13                                           Point2D p2) {
  14
  15         if (p1.y > p2.y) {
  16             final Point2D tmp = p1;
  17             p1 = p2;
  18             p2 = tmp;
  19         }
  20
  21         if (point.y < p1.y)
  22             return false;
  23         if (point.y > p2.y)
  24             return false;
  25
  26         final double xp = p2.x - p1.x;
  27         final double yp = p2.y - p1.y;
  28
  29         final double yp2 = point.y - p1.y;
  30
  31         final double crossX = p1.x + ((xp * yp2) / yp);
  32
  33         return point.x >= crossX;
  34
  35     }
  36
  37     public static boolean pointWithinPolygon(final Point2D point,
  38                                              final Point2D p1, final Point2D p2, final Point2D p3) {
  39
  40         int intersectionCount = 0;
  41
  42         if (intersectsLine(point, p1, p2))
  43             intersectionCount++;
  44
  45         if (intersectsLine(point, p2, p3))
  46             intersectionCount++;
  47
  48         if (intersectsLine(point, p3, p1))
  49             intersectionCount++;
  50
  51         return intersectionCount == 1;
  52
  53     }
  54
  55 }