如何在jbox2d中获得body的半宽半高

Question

所以我有一个矩形的身体。如何获得它的半宽半高？（我在其他地方找不到答案）

Answer 1

不幸的是，它不是完全直接的，因为Box2D（因此JBox2D）本身没有任何矩形的概念。矩形是PolygonShape，其形状可能是使用setAsBox(halfWidth, halfHeight)指定的。

要在创建Fixture后获得halfWidth和halfHeight，请考虑以下内容（请根据需要进行重构）：

public void checkOutThisFixture(Fixture fixture) {
    Shape fixtureShape = fixture.getShape();
    if (fixtureShape instanceof PolygonShape) {
        PolygonShape polygonShape = (PolygonShape) fixtureShape;
        Float minX = null;
        Float maxX = null;
        Float minY = null;
        Float maxY = null;
        for (int i = 0; i < polygonShape.getVertexCount(); i++) {
            Vec2 nextVertex = polygonShape.getVertex(i);
            float x = nextVertex.x;
            float y = nextVertex.y;
            if (minX == null || x < minX) {
                minX = x;
            }
            if (maxX == null || x > maxX) {
                maxX = x;
            }
            if (minY == null || y < minY) {
                minY = y;
            }
            if (maxY == null || y > maxY) {
                maxY = y;
            }
        }
        float width = maxX - minX;
        float height = maxY - minY;
        float halfWidth = width / 2;
        float halfHeight = height / 2;
        System.out.println("The polygon has half width & height of: " + halfWidth + " & " + halfHeight);
    } else if (fixtureShape instanceof CircleShape) {
        float radius = ((CircleShape) fixtureShape).m_radius;
        System.out.println("The circle has a radius of : " + radius);
    } else {
        // TODO handle other shapes
    }
}

要从Body获得此信息，请使用：

public void checkOutTheseFixtures(Body body) {
    for (Fixture fixture = body.getFixtureList(); fixture != null; fixture = fixture.getNext()) {
        checkOutThisFixture(fixture);
    }
}

还有一些测试：

World world = new World(new Vec2(0, 0), true);
Body body = world.createBody(new BodyDef());
// Add a circle
CircleShape circle = new CircleShape();
circle.m_radius = 20;
body.createFixture(circle, 5);
// Add a box
PolygonShape rectangle = new PolygonShape();
rectangle.setAsBox(137, 42);
body.createFixture(rectangle, 10);
// Add a more complex polygon
PolygonShape polygon = new PolygonShape();
Vec2[] vertices = new Vec2[5];
vertices[0] = new Vec2(-1, 2);
vertices[1] = new Vec2(-1, 0);
vertices[2] = new Vec2(0, -3);
vertices[3] = new Vec2(1, 0);
vertices[4] = new Vec2(1, 1);
polygon.set(vertices, 5);
body.createFixture(polygon, 10);
checkOutTheseFixtures(body);

打印：

多边形有一半宽度&高度：1.0&2.5

多边形有一半宽度&高度：137.0&42.0

圆的半径为：20.0

希望能有所帮助。

---

类似地，这里有一个简洁的方法来获得PolygonShape:的尺寸

  public static Vec2 getDimensions( final PolygonShape shape ) {
    float minX = Float.MAX_VALUE;
    float maxX = Float.MIN_VALUE;
    float minY = Float.MAX_VALUE;
    float maxY = Float.MIN_VALUE;
    
    final int vertices = shape.getVertexCount();
    
    for( int i = 0; i < vertices; i++ ) {
      final Vec2 v = shape.getVertex( i );
      
      minX = (v.x < minX) ? v.x : minX;
      maxX = (v.x > maxX) ? v.x : maxX;
      minY = (v.y < minY) ? v.y : minY;
      maxY = (v.y > maxY) ? v.y : maxY;
    }
        
    return new Vec2( maxX - minX, maxY - minY );
  }

将返回的Vec2的维度一分为二也将获得所需的值。