com.kotcrab.vis.editor.module.physicseditor.util.PolygonUtils.java Source code

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Here is the source code for com.kotcrab.vis.editor.module.physicseditor.util.PolygonUtils.java

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/*
 * Copyright 2014-2015 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.kotcrab.vis.editor.module.physicseditor.util;

import com.badlogic.gdx.math.Vector2;

/**
 * @author Aurelien Ribon, Kotcrab
 */
public class PolygonUtils {
    private static final int B2_MAX_VERTICES = 8;
    private static final float B2_LINEAR_SLOP = 0.005f;

    public static float getPolygonSignedArea(Vector2[] points) {
        if (points.length < 3)
            return 0;

        float sum = 0;
        for (int i = 0; i < points.length; i++) {
            Vector2 p1 = points[i];
            Vector2 p2 = i != points.length - 1 ? points[i + 1] : points[0];
            sum += (p1.x * p2.y) - (p1.y * p2.x);
        }
        return 0.5f * sum;
    }

    public static float getPolygonArea(Vector2[] points) {
        return Math.abs(getPolygonSignedArea(points));
    }

    public static boolean isPolygonCCW(Vector2[] points) {
        return getPolygonSignedArea(points) > 0;
    }

    /** Checks whether polygon vertices will make degenerated box2d polygon or not */
    public static boolean isDegenerate(Vector2[] vertices) {
        // https://github.com/libgdx/libgdx/blob/master/extensions/gdx-box2d/gdx-box2d/jni/Box2D/Collision/Shapes/b2PolygonShape.cpp#L120-L226
        // it's not like I understand what is going on here

        int count = vertices.length;
        int n = Math.min(count, B2_MAX_VERTICES);

        // Perform welding and copy vertices into local buffer.
        Vector2[] ps = new Vector2[B2_MAX_VERTICES];
        int tempCount = 0;
        for (int i = 0; i < n; ++i) {
            Vector2 v = vertices[i];

            boolean unique = true;
            for (int j = 0; j < tempCount; ++j) {
                if (distanceSquared(v, ps[j]) < 0.5f * B2_LINEAR_SLOP) {
                    unique = false;
                    break;
                }
            }

            if (unique) {
                ps[tempCount++] = v;
            }
        }

        n = tempCount;
        if (n < 3) {
            // Polygon is degenerate.
            return true;
        }

        // Create the convex hull using the Gift wrapping algorithm
        // http://en.wikipedia.org/wiki/Gift_wrapping_algorithm

        // Find the right most point on the hull
        int i0 = 0;
        float x0 = ps[0].x;
        for (int i = 1; i < n; ++i) {
            float x = ps[i].x;
            if (x > x0 || (x == x0 && ps[i].y < ps[i0].y)) {
                i0 = i;
                x0 = x;
            }
        }

        int hull[] = new int[B2_MAX_VERTICES];
        int m = 0;
        int ih = i0;

        while (true) {
            hull[m] = ih;

            int ie = 0;
            for (int j = 1; j < n; ++j) {
                if (ie == ih) {
                    ie = j;
                    continue;
                }

                Vector2 r = ps[ie].cpy().sub(ps[hull[m]]);
                Vector2 v = ps[j].cpy().sub(ps[hull[m]]);

                float c = r.crs(v);
                if (c < 0.0f) {
                    ie = j;
                }

                // Collinearity check
                if (c == 0.0f && v.len2() > r.len2()) {
                    ie = j;
                }
            }

            ++m;
            ih = ie;

            if (ie == i0) {
                break;
            }
        }

        if (m < 3) {
            // Polygon is degenerate.
            return true;
        }

        return false;
    }

    private static float distanceSquared(Vector2 a, Vector2 b) {
        Vector2 c = a.cpy().sub(b);
        return c.dot(c);
    }
}