Triangulate.java

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/*
 * Copyright (C) 2014 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

package org.ros.android.view.visualization.shape;

import java.util.List;

public class Triangulate {

  private static final float EPSILON = 1e-9f;

  public static class Point {
    private final float x;
    private final float y;

    public Point(final float x, final float y) {
      this.x = x;
      this.y = y;
    }

    public float x() {
      return x;
    }

    public float y() {
      return y;
    }
  }

  public static boolean process(final Point[] contour, List<Point> result) {
    // Allocate and initialize list of Vertices in polygon.
    int n = contour.length;
    if (n < 3) {
      return false;
    }

    int[] V = new int[n];

    // We want a counter-clockwise polygon in V.
    if (0.0f < area(contour)) {
      for (int v = 0; v < n; v++) {
        V[v] = v;
      }
    } else {
      for (int v = 0; v < n; v++) {
        V[v] = (n - 1) - v;
      }
    }

    int nv = n;

    // Remove nv-2 Vertices, creating 1 triangle every time.
    int count = 2 * nv;  // error detection

    for (int m = 0, v = nv - 1; nv > 2; ) {
      // If we loop, it is probably a non-simple polygon.
      if (0 >= (count--)) {
        // Triangulate: ERROR - probable bad polygon!
        return false;
      }

      // Three consecutive vertices in current polygon, <u,v,w>
      int u = v;
      if (nv <= u) {
        u = 0;  // previous
      }
      v = u + 1;
      if (nv <= v) {
        v = 0;  // new v
      }
      int w = v + 1;
      if (nv <= w) {
        w = 0;  // next
      }

      if (snip(contour, u, v, w, nv, V)) {
        // True names of the vertices.
        final int a = V[u];
        final int b = V[v];
        final int c = V[w];

        // Output Triangle
        result.add(contour[a]);
        result.add(contour[b]);
        result.add(contour[c]);

        m++;

        // Remove v from remaining polygon.
        for (int s = v, t = v + 1; t < nv; s++, t++) {
          V[s] = V[t];
        }
        nv--;

        // Reset error detection counter.
        count = 2 * nv;
      }
    }

    return true;
  }

  public static float area(final Point[] contour) {
    final int n = contour.length;
    float A = 0.0f;
    for (int p = n - 1, q = 0; q < n; p = q++) {
      A += contour[p].x() * contour[q].y() - contour[q].x() * contour[p].y();
    }
    return A * 0.5f;
  }

  public static boolean isInsideTriangle(final float Ax, final float Ay,
                                         final float Bx, final float By,
                                         final float Cx, final float Cy,
                                         final float Px, final float Py) {
    final float ax = Cx - Bx;
    final float ay = Cy - By;
    final float bx = Ax - Cx;
    final float by = Ay - Cy;
    final float cx = Bx - Ax;
    final float cy = By - Ay;
    final float apx = Px - Ax;
    final float apy = Py - Ay;
    final float bpx = Px - Bx;
    final float bpy = Py - By;
    final float cpx = Px - Cx;
    final float cpy = Py - Cy;
    final float aCROSSbp = ax * bpy - ay * bpx;
    final float cCROSSap = cx * apy - cy * apx;
    final float bCROSScp = bx * cpy - by * cpx;
    return ((aCROSSbp >= 0.0f) && (bCROSScp >= 0.0f) && (cCROSSap >= 0.0f));
  }

  private static boolean snip(Point[] contour, int u, int v, int w, int n, int[] V) {
    final float Ax = contour[V[u]].x();
    final float Ay = contour[V[u]].y();
    final float Bx = contour[V[v]].x();
    final float By = contour[V[v]].y();
    final float Cx = contour[V[w]].x();
    final float Cy = contour[V[w]].y();

    if (EPSILON > (((Bx - Ax) * (Cy - Ay)) - ((By - Ay) * (Cx - Ax)))) {
      return false;
    }

    for (int p = 0; p < n; p++) {
      if ((p == u) || (p == v) || (p == w)) {
        continue;
      }
      final float Px = contour[V[p]].x();
      final float Py = contour[V[p]].y();
      if (isInsideTriangle(Ax, Ay, Bx, By, Cx, Cy, Px, Py)) {
        return false;
      }
    }
    return true;
  }
}