/*******************************************************************************
* Copyright 2011 See AUTHORS file.
*
* 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 com.badlogic.gdx.math;
import com.badlogic.gdx.utils.FloatArray;
import com.badlogic.gdx.utils.IntArray;
import com.badlogic.gdx.utils.ShortArray;
/** A simple implementation of the ear cutting algorithm to triangulate simple polygons without holes. For more information:
* <ul>
* <li><a href="http://cgm.cs.mcgill.ca/~godfried/teaching/cg-projects/97/Ian/algorithm2.html">http://cgm.cs.mcgill.ca/~godfried/
* teaching/cg-projects/97/Ian/algorithm2.html</a></li>
* <li><a
* href="http://www.geometrictools.com/Documentation/TriangulationByEarClipping.pdf">http://www.geometrictools.com/Documentation
* /TriangulationByEarClipping.pdf</a></li>
* </ul>
* If the input polygon is not simple (self-intersects), there will be output but it is of unspecified quality (garbage in,
* garbage out).
* @author badlogicgames@gmail.com
* @author Nicolas Gramlich (optimizations, collinear edge support)
* @author Eric Spitz
* @author Thomas ten Cate (bugfixes, optimizations)
* @author Nathan Sweet (rewrite, return indices, no allocation, optimizations) */
public class EarClippingTriangulator {
static private final int CONCAVE = -1;
static private final int TANGENTIAL = 0;
static private final int CONVEX = 1;
private final ShortArray indicesArray = new ShortArray();
private short[] indices;
private float[] vertices;
private int vertexCount;
private final IntArray vertexTypes = new IntArray();
private final ShortArray triangles = new ShortArray();
/** @see #computeTriangles(float[], int, int) */
public ShortArray computeTriangles (FloatArray vertices) {
return computeTriangles(vertices.items, 0, vertices.size);
}
/** @see #computeTriangles(float[], int, int) */
public ShortArray computeTriangles (float[] vertices) {
return computeTriangles(vertices, 0, vertices.length);
}
/** Triangulates the given (convex or concave) simple polygon to a list of triangle vertices.
* @param vertices pairs describing vertices of the polygon, in either clockwise or counterclockwise order.
* @return triples of triangle indices in clockwise order. Note the returned array is reused for later calls to the same
* method. */
public ShortArray computeTriangles (float[] vertices, int offset, int count) {
this.vertices = vertices;
int vertexCount = this.vertexCount = count / 2;
ShortArray indicesArray = this.indicesArray;
indicesArray.clear();
indicesArray.ensureCapacity(vertexCount);
indicesArray.size = vertexCount;
short[] indices = this.indices = indicesArray.items;
if (areVerticesClockwise(vertices, offset, count)) {
for (short i = 0; i < vertexCount; i++)
indices[i] = i;
} else {
for (int i = 0, n = vertexCount - 1; i < vertexCount; i++)
indices[i] = (short)(n - i); // Reversed.
}
IntArray vertexTypes = this.vertexTypes;
vertexTypes.clear();
vertexTypes.ensureCapacity(vertexCount);
for (int i = 0, n = vertexCount; i < n; ++i)
vertexTypes.add(classifyVertex(i));
// A polygon with n vertices has a triangulation of n-2 triangles.
ShortArray triangles = this.triangles;
triangles.clear();
triangles.ensureCapacity(Math.max(0, vertexCount - 2) * 3);
triangulate();
return triangles;
}
private void triangulate () {
int[] vertexTypes = this.vertexTypes.items;
while (vertexCount > 3) {
int earTipIndex = findEarTip();
cutEarTip(earTipIndex);
// The type of the two vertices adjacent to the clipped vertex may have changed.
int previousIndex = previousIndex(earTipIndex);
int nextIndex = earTipIndex == vertexCount ? 0 : earTipIndex;
vertexTypes[previousIndex] = classifyVertex(previousIndex);
vertexTypes[nextIndex] = classifyVertex(nextIndex);
}
if (vertexCount == 3) {
ShortArray triangles = this.triangles;
short[] indices = this.indices;
triangles.add(indices[0]);
triangles.add(indices[1]);
triangles.add(indices[2]);
}
}
/** @return {@link #CONCAVE}, {@link #TANGENTIAL} or {@link #CONVEX} */
private int classifyVertex (int index) {
short[] indices = this.indices;
int previous = indices[previousIndex(index)] * 2;
int current = indices[index] * 2;
int next = indices[nextIndex(index)] * 2;
float[] vertices = this.vertices;
return computeSpannedAreaSign(vertices[previous], vertices[previous + 1], vertices[current], vertices[current + 1],
vertices[next], vertices[next + 1]);
}
private int findEarTip () {
int vertexCount = this.vertexCount;
for (int i = 0; i < vertexCount; i++)
if (isEarTip(i)) return i;
// Desperate mode: if no vertex is an ear tip, we are dealing with a degenerate polygon (e.g. nearly collinear).
// Note that the input was not necessarily degenerate, but we could have made it so by clipping some valid ears.
// Idea taken from Martin Held, "FIST: Fast industrial-strength triangulation of polygons", Algorithmica (1998),
// http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.115.291
// Return a convex or tangential vertex if one exists.
int[] vertexTypes = this.vertexTypes.items;
for (int i = 0; i < vertexCount; i++)
if (vertexTypes[i] != CONCAVE) return i;
return 0; // If all vertices are concave, just return the first one.
}
private boolean isEarTip (int earTipIndex) {
int[] vertexTypes = this.vertexTypes.items;
if (vertexTypes[earTipIndex] == CONCAVE) return false;
int previousIndex = previousIndex(earTipIndex);
int nextIndex = nextIndex(earTipIndex);
short[] indices = this.indices;
int p1 = indices[previousIndex] * 2;
int p2 = indices[earTipIndex] * 2;
int p3 = indices[nextIndex] * 2;
float[] vertices = this.vertices;
float p1x = vertices[p1], p1y = vertices[p1 + 1];
float p2x = vertices[p2], p2y = vertices[p2 + 1];
float p3x = vertices[p3], p3y = vertices[p3 + 1];
// Check if any point is inside the triangle formed by previous, current and next vertices.
// Only consider vertices that are not part of this triangle, or else we'll always find one inside.
for (int i = nextIndex(nextIndex); i != previousIndex; i = nextIndex(i)) {
// Concave vertices can obviously be inside the candidate ear, but so can tangential vertices
// if they coincide with one of the triangle's vertices.
if (vertexTypes[i] != CONVEX) {
int v = indices[i] * 2;
float vx = vertices[v];
float vy = vertices[v + 1];
// Because the polygon has clockwise winding order, the area sign will be positive if the point is strictly inside.
// It will be 0 on the edge, which we want to include as well.
// note: check the edge defined by p1->p3 first since this fails _far_ more then the other 2 checks.
if (computeSpannedAreaSign(p3x, p3y, p1x, p1y, vx, vy) >= 0) {
if (computeSpannedAreaSign(p1x, p1y, p2x, p2y, vx, vy) >= 0) {
if (computeSpannedAreaSign(p2x, p2y, p3x, p3y, vx, vy) >= 0) return false;
}
}
}
}
return true;
}
private void cutEarTip (int earTipIndex) {
short[] indices = this.indices;
ShortArray triangles = this.triangles;
triangles.add(indices[previousIndex(earTipIndex)]);
triangles.add(indices[earTipIndex]);
triangles.add(indices[nextIndex(earTipIndex)]);
indicesArray.removeIndex(earTipIndex);
vertexTypes.removeIndex(earTipIndex);
vertexCount--;
}
private int previousIndex (int index) {
return (index == 0 ? vertexCount : index) - 1;
}
private int nextIndex (int index) {
return (index + 1) % vertexCount;
}
static private boolean areVerticesClockwise (float[] vertices, int offset, int count) {
if (count <= 2) return false;
float area = 0, p1x, p1y, p2x, p2y;
for (int i = offset, n = offset + count - 3; i < n; i += 2) {
p1x = vertices[i];
p1y = vertices[i + 1];
p2x = vertices[i + 2];
p2y = vertices[i + 3];
area += p1x * p2y - p2x * p1y;
}
p1x = vertices[count - 2];
p1y = vertices[count - 1];
p2x = vertices[0];
p2y = vertices[1];
return area + p1x * p2y - p2x * p1y < 0;
}
static private int computeSpannedAreaSign (float p1x, float p1y, float p2x, float p2y, float p3x, float p3y) {
float area = p1x * (p3y - p2y);
area += p2x * (p1y - p3y);
area += p3x * (p2y - p1y);
return (int)Math.signum(area);
}
}