7.Libgdx扩展学习之Box2D_距离关节旋转关节

文章中的概念来自《Box2D》中文手册

上一节中我们介绍了关节和鼠标关节的使用方法，本章中我们继续介绍距离关节和旋转关节

距离关节（DistanceJoint）

距离关节是两个物体上各有一点，两点之间的距离必须固定不变。当指定一个距离关节时，两个物体必须已在应有的位置上。之后指定世界坐标中的两个锚点。第一个锚点连接到物体1，第二个锚点连接到物体2。这两点隐含距离约束的长度。

// Define the distance joint
DistanceJointDef distanceJointDef = new DistanceJointDef();
// 距离关节连接的2个Body
distanceJointDef.bodyA=smallBall;
distanceJointDef.bodyB=bigBall;
// 是否允许两个Body碰撞
distanceJointDef.collideConnected=false;
// 两个Body之间的距离
distanceJointDef.length = 2.0f;

关节可以具有弹性，通过定义2个常数：频率(frequency)和阻尼率(damping ratio)。频率影响震动的快慢，典型情况下频率要小于时间步的一半。比如每秒执行60次时间步，距离关节的频率就要小于30。
阻尼率无单位，取值在「0，1」之间。当阻尼率设置为1时，没有振动。

        // 下面2个参数使关节具有弹性distanceJointDef.dampingRatio = 0.4f;distanceJointDef.frequencyHz = 4.0f;

下面是测代码

/*** 距离关节*/
public class DistanceJointTest extends ApplicationAdapter {World world;Box2DDebugRenderer box2DDebugRenderer;Body hitBody, groundBody;OrthographicCamera camera;Vector3 point = new Vector3();float scene_width = 12.8f;float scene_height = 7.2f;QueryCallback callback = new QueryCallback() {@Overridepublic boolean reportFixture(Fixture fixture) {if (fixture.testPoint(point.x, point.y)) {hitBody = fixture.getBody();return  false;} elsereturn true;}};@Overridepublic void create() {world = new World(new Vector2(0.0f, -9.8f), true);box2DDebugRenderer = new Box2DDebugRenderer();camera = new OrthographicCamera(scene_width, scene_height);camera.position.set(scene_width / 2, scene_height / 2, 0);camera.update();groundBody = createGroundWall();Gdx.input.setInputProcessor(new HandA());createDistanceJoint();}@Overridepublic void render() {world.step( 1/ 60f, 6, 2);Gdx.gl.glClearColor(0.39f, 0.58f, 0.92f, 1.0f);Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);box2DDebugRenderer.render(world, camera.combined);}@Overridepublic void dispose() {world.dispose();box2DDebugRenderer.dispose();}public void createDistanceJoint() {Body smallBall = createSphere(BodyDef.BodyType.DynamicBody, 0f, 3.75f, .8f, .8f, .4f, .25f);Body bigBall = createSphere(BodyDef.BodyType.DynamicBody, 3.0f, 4.5f, .8f, 1f, .4f, .5f);// Define the distance jointDistanceJointDef distanceJointDef = new DistanceJointDef();distanceJointDef.bodyA=smallBall;distanceJointDef.bodyB=bigBall;distanceJointDef.collideConnected=false;distanceJointDef.length = 5.0f;// 下面2个参数使关节具有弹性distanceJointDef.dampingRatio = 0.4f;distanceJointDef.frequencyHz = 4.0f;distanceJointDef.localAnchorA.set(0,0);distanceJointDef.localAnchorB.set(0,0);world.createJoint(distanceJointDef);}private Body createSphere(BodyDef.BodyType type, float x, float y, float d, float r, float f, float radius) {BodyDef bodyDef = new BodyDef();bodyDef.type = type;bodyDef.position.set(scene_width * 0.5f+x,y);bodyDef.angle=0;Body ball = world.createBody(bodyDef);FixtureDef fixtureDef=new FixtureDef();fixtureDef.density=d;fixtureDef.restitution=r;fixtureDef.friction=f;fixtureDef.shape=new CircleShape();fixtureDef.shape.setRadius(radius);ball.createFixture(fixtureDef);fixtureDef.shape.dispose();return ball;}public Body createGroundWall() {BodyDef bodyDef = new BodyDef();bodyDef.position.set(scene_width * 0.5f, 0.2f);Body body1 = world.createBody(bodyDef);PolygonShape polygonShape = new PolygonShape();polygonShape.setAsBox(scene_width * 0.5f, 0.2f);body1.createFixture(polygonShape, 0.0f);bodyDef.position.set(0.4f, scene_height * 0.5f);Body body2 = world.createBody(bodyDef);polygonShape.setAsBox(0.2f, scene_height * 0.5f);body2.createFixture(polygonShape, 0);bodyDef.position.set(12.4f, scene_height * 0.5f);Body body3 = world.createBody(bodyDef);polygonShape.setAsBox(0.2f, scene_height * 0.5f);body3.createFixture(polygonShape, 0);bodyDef.position.set(scene_width * 0.5f, 7.0f);Body body4 = world.createBody(bodyDef);polygonShape.setAsBox(scene_width * 0.5f, 0.2f);body4.createFixture(polygonShape, 0);polygonShape.dispose();return body1;}class HandA extends InputAdapter {MouseJoint mouseJoint;Vector2 target = new Vector2();@Overridepublic boolean touchDragged(int screenX, int screenY, int pointer) {if (mouseJoint != null) {camera.unproject(point.set(screenX, screenY, 0));mouseJoint.setTarget(target.set(point.x, point.y));}return false;}@Overridepublic boolean touchDown(int screenX, int screenY, int pointer, int button) {camera.unproject(point.set(screenX, screenY, 0));hitBody = null;world.QueryAABB(callback, point.x - 0.0001f, point.y - 0.0001f, point.x + 0.0001f, point.y + 0.0001f);if (hitBody == null || hitBody.equals(groundBody)) return false;MouseJointDef mouseJointDef = new MouseJointDef();mouseJointDef.bodyA = groundBody;mouseJointDef.bodyB = hitBody;mouseJointDef.collideConnected = true;mouseJointDef.target.set(point.x, point.y);mouseJointDef.maxForce = 1000.0f * hitBody.getMass();mouseJoint = (MouseJoint) world.createJoint(mouseJointDef);hitBody.setAwake(true);return false;}@Overridepublic boolean touchUp(int screenX, int screenY, int pointer, int button) {// 鼠标关节，不再使用时要销毁if (mouseJoint != null) {world.destroyJoint(mouseJoint);mouseJoint = null;}return false;}}
}

旋转关节（RevoluteJoint）

旋转关节会强制两个物体公用一个锚点。旋转关节只有一个自由度：两个物体相对旋转。这称之为关节角。
要指定一个旋转关节，需要提供两个物体以及世界坐标的一个锚点，可以参考下面定义:

// Define the revolute joint
RevoluteJointDef revoluteJointDef = new RevoluteJointDef();
revoluteJointDef.bodyA=smallBall;
revoluteJointDef.bodyB=bigBall;
revoluteJointDef.collideConnected=false;
// 指定锚点
revoluteJointDef.localAnchorA.set(0,0);
revoluteJointDef.localAnchorB.set(-2.0f,0);

在Box2D中默认是逆时针旋转的，此时关节角为正，而且旋转角也是弧度制的。在创建两个物体时物理当前的角度是怎样的，旋转关节角都为0。

每次执行step后，可以更新马达的参数。这样可以实现有些有趣的功能。可以在每个时间步中更新关节速度，使得它像正炫波或者任意一个想要的函数那样前后摆动

/*** 旋转关节*/
public class RevoluteJointTest extends ApplicationAdapter {World world;Box2DDebugRenderer box2DDebugRenderer;Body hitBody, groundBody;OrthographicCamera camera;Vector3 point = new Vector3();float scene_width = 12.8f;float scene_height = 7.2f;QueryCallback callback = new QueryCallback() {@Overridepublic boolean reportFixture(Fixture fixture) {if (fixture.testPoint(point.x, point.y)) {hitBody = fixture.getBody();return  false;} elsereturn true;}};@Overridepublic void create() {world = new World(new Vector2(0.0f, -9.8f), true);box2DDebugRenderer = new Box2DDebugRenderer();camera = new OrthographicCamera(scene_width, scene_height);camera.position.set(scene_width / 2, scene_height / 2, 0);camera.update();groundBody = createGroundWall();Gdx.input.setInputProcessor(new HandA());createRevoluteJoin();}@Overridepublic void render() {world.step( 1/ 60f, 6, 2);Gdx.gl.glClearColor(0.39f, 0.58f, 0.92f, 1.0f);Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);box2DDebugRenderer.render(world, camera.combined);}@Overridepublic void dispose() {world.dispose();box2DDebugRenderer.dispose();}public void createRevoluteJoin() {// 第一个Body要设置为Static才能保证第二个Body围绕第一个旋转Body smallBall = createSphere(BodyDef.BodyType.StaticBody, 0f, 3.75f, 1f, 1f, 0f, .25f);Body bigBall = createSphere(BodyDef.BodyType.DynamicBody, 0f, 3.75f, 1f, 1f, 0f, .5f);// Define the revolute jointRevoluteJointDef revoluteJointDef = new RevoluteJointDef();revoluteJointDef.bodyA=smallBall;revoluteJointDef.bodyB=bigBall;revoluteJointDef.collideConnected=false;revoluteJointDef.localAnchorA.set(0,0);revoluteJointDef.localAnchorB.set(-2.0f,0);revoluteJointDef.enableMotor=true;revoluteJointDef.maxMotorTorque=360;revoluteJointDef.motorSpeed=100f* MathUtils.degreesToRadians;world.createJoint(revoluteJointDef);}private Body createSphere(BodyDef.BodyType type, float x, float y, float d, float r, float f, float radius) {BodyDef bodyDef = new BodyDef();bodyDef.type = type;bodyDef.position.set(scene_width * 0.5f+x,y);bodyDef.angle=0;Body ball = world.createBody(bodyDef);FixtureDef fixtureDef=new FixtureDef();fixtureDef.density=d;fixtureDef.restitution=r;fixtureDef.friction=f;fixtureDef.shape=new CircleShape();fixtureDef.shape.setRadius(radius);ball.createFixture(fixtureDef);fixtureDef.shape.dispose();return ball;}public Body createGroundWall() {BodyDef bodyDef = new BodyDef();bodyDef.position.set(scene_width * 0.5f, 0.2f);Body body1 = world.createBody(bodyDef);PolygonShape polygonShape = new PolygonShape();polygonShape.setAsBox(scene_width * 0.5f, 0.2f);body1.createFixture(polygonShape, 0.0f);bodyDef.position.set(0.4f, scene_height * 0.5f);Body body2 = world.createBody(bodyDef);polygonShape.setAsBox(0.2f, scene_height * 0.5f);body2.createFixture(polygonShape, 0);bodyDef.position.set(12.4f, scene_height * 0.5f);Body body3 = world.createBody(bodyDef);polygonShape.setAsBox(0.2f, scene_height * 0.5f);body3.createFixture(polygonShape, 0);bodyDef.position.set(scene_width * 0.5f, 7.0f);Body body4 = world.createBody(bodyDef);polygonShape.setAsBox(scene_width * 0.5f, 0.2f);body4.createFixture(polygonShape, 0);polygonShape.dispose();return body1;}class HandA extends InputAdapter {MouseJoint mouseJoint;Vector2 target = new Vector2();@Overridepublic boolean touchDragged(int screenX, int screenY, int pointer) {if (mouseJoint != null) {camera.unproject(point.set(screenX, screenY, 0));mouseJoint.setTarget(target.set(point.x, point.y));}return false;}@Overridepublic boolean touchDown(int screenX, int screenY, int pointer, int button) {camera.unproject(point.set(screenX, screenY, 0));hitBody = null;world.QueryAABB(callback, point.x - 0.0001f, point.y - 0.0001f, point.x + 0.0001f, point.y + 0.0001f);if (hitBody == null || hitBody.equals(groundBody)) return false;MouseJointDef mouseJointDef = new MouseJointDef();mouseJointDef.bodyA = groundBody;mouseJointDef.bodyB = hitBody;mouseJointDef.collideConnected = true;mouseJointDef.target.set(point.x, point.y);mouseJointDef.maxForce = 1000.0f * hitBody.getMass();mouseJoint = (MouseJoint) world.createJoint(mouseJointDef);hitBody.setAwake(true);return false;}@Overridepublic boolean touchUp(int screenX, int screenY, int pointer, int button) {// 鼠标关节，不再使用时要销毁if (mouseJoint != null) {world.destroyJoint(mouseJoint);mouseJoint = null;}return false;}}
}

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7.Libgdx扩展学习之Box2D_距离关节旋转关节

距离关节（DistanceJoint）

旋转关节（RevoluteJoint）

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7.Libgdx扩展学习之Box2D_距离关节 旋转关节

距离关节（DistanceJoint）

旋转关节（RevoluteJoint）

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7.Libgdx扩展学习之Box2D_距离关节旋转关节

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