LibGdx--PrismaticJoint移动关节

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package com.joye3g.joint;

import com.badlogic.gdx.ApplicationListener;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.GL10;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.physics.box2d.Body;
import com.badlogic.gdx.physics.box2d.BodyDef;
import com.badlogic.gdx.physics.box2d.BodyDef.BodyType;
import com.badlogic.gdx.physics.box2d.Box2DDebugRenderer;
import com.badlogic.gdx.physics.box2d.CircleShape;
import com.badlogic.gdx.physics.box2d.EdgeShape;
import com.badlogic.gdx.physics.box2d.FixtureDef;
import com.badlogic.gdx.physics.box2d.World;
import com.badlogic.gdx.physics.box2d.joints.PrismaticJointDef;
import com.joye3g.joint.util.Transform;

public class PrismaticJointDemo implements ApplicationListener{
    private Box2DDebugRenderer renderer;
    private OrthographicCamera camera;
    private static final float PXTM = 30;
    private World mWorld;
    @Override
    public void create() {
        float screenWidth = Gdx.graphics.getWidth();
        float screenHeight = Gdx.graphics.getHeight();
        renderer = new Box2DDebugRenderer();
        camera = new OrthographicCamera(screenWidth / PXTM, screenHeight / PXTM);
        mWorld = new World(new Vector2(0f, -10f), true);

        //绘制地面
        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyType.StaticBody;
        bodyDef.position.set(-screenWidth / PXTM / 2, -screenHeight / PXTM / 2);
        Body mLine = mWorld.createBody(bodyDef);
        EdgeShape edgeShape = new EdgeShape();
        edgeShape.set(new Vector2(0f, 1f), new Vector2(screenWidth / PXTM, 1f));
        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = edgeShape;
        mLine.createFixture(fixtureDef);

        //绘制一个半径为1米的圆形
        BodyDef mCircleBodyDef1 = new BodyDef();
        mCircleBodyDef1.type = BodyType.DynamicBody;
        //圆形的位置在屏幕(60,190)的位置
        Vector2 vector1 = Transform.ptm(60f, 190f, screenWidth, screenHeight, 0f, 0f, PXTM);
        mCircleBodyDef1.position.set(vector1);
        Body mCircle1 = mWorld.createBody(mCircleBodyDef1);
        CircleShape mCircleShape1 = new CircleShape();
        mCircleShape1.setRadius(1f);
        FixtureDef mCircleFix1 = new FixtureDef();
        mCircleFix1.shape = mCircleShape1;
        mCircle1.createFixture(mCircleFix1);

        //绘制一个半径为1米的圆形
        BodyDef mCircleBodyDef2 = new BodyDef();
        mCircleBodyDef2.type = BodyType.DynamicBody;
        //圆形的位置在屏幕(200,190)的位置
        Vector2 vector2 = Transform.ptm(200f, 190f, screenWidth, screenHeight, 0f, 0f, PXTM);
        mCircleBodyDef2.position.set(vector2);
        Body mCircle2 = mWorld.createBody(mCircleBodyDef2);
        CircleShape mCircleShape2 = new CircleShape();
        mCircleShape2.setRadius(1f);
        FixtureDef mCircleFix2 = new FixtureDef();
        mCircleFix2.shape = mCircleShape2;
        mCircle2.createFixture(mCircleFix2);

        //绘制一个半径为1米的圆形(静态)
        BodyDef mCircleBodyDef3 = new BodyDef();
        mCircleBodyDef3.type = BodyType.StaticBody;
        //圆形的位置在屏幕(60,100)的位置
        Vector2 vector3 = Transform.ptm(60f, 100f, screenWidth, screenHeight, 0f, 0f, PXTM);
        mCircleBodyDef3.position.set(vector3);
        Body mCircle3 = mWorld.createBody(mCircleBodyDef3);
        CircleShape mCircleShape3 = new CircleShape();
        mCircleShape3.setRadius(1f);
        FixtureDef mCircleFix3 = new FixtureDef();
        mCircleFix3.shape = mCircleShape3;
        mCircle3.createFixture(mCircleFix3);

        //创建一个移动关节
        PrismaticJointDef prismaticJointDef = new PrismaticJointDef();
        prismaticJointDef.bodyA = mCircle1;
        prismaticJointDef.bodyB = mCircle2;
        //最小位移偏移量
        prismaticJointDef.lowerTranslation = 3f;
        //启动限制
        prismaticJointDef.enableLimit = true;
        mWorld.createJoint(prismaticJointDef);
    }

    @Override
    public void resize(int width, int height) {
    }

    @Override
    public void render() {
        GL10 gl=Gdx.graphics.getGL10();
        gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
        mWorld.step(Gdx.graphics.getDeltaTime(), 8, 6);
        render

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