本文地址：http://blog.csdn.net/you_and_me12/article/details/7961573
2012-09-10
从今天开始学习Box2D,在java中是JBox2D(Download)，包的导入就不用说了吧！
文章后面给出了我写的helloworld，仅供参考
笔记：Ⅰ）Box2D由世界，物体等组成，Box2D能自动计算物体的位置，然后我们还需要在Android中绘制出来；
Ⅱ）Box2D世界中的单位都是标准单位，秒、米、千克 etc. 需要按照比例缩放，不然速度很慢(代码中有转换)；
Ⅲ）Box2D每帧的位移是2m/frame,不能超过这个速度，所以要进行缩放，即可放大速度；
Ⅳ）注意坐标系的零点是在左上角，同时角度是以弧度制的；
坐标系 效果图
代码：
public class Hello_Box2d extends Activity { private World mWorld; private Handler mHandler; private View mView; private int screenH, screenW; private final float WORLD2PHONE_RATE = 5; private Runnable mRunnable = new Runnable() { @Override public void run() { mWorld.step(0.1f, 3, 3);// mView.invalidate(); mHandler.postDelayed(this, 100); } }; /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); this.requestWindowFeature(Window.FEATURE_NO_TITLE); //remove title getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);// full screen //get the window width and height DisplayMetrics metric = new DisplayMetrics(); getWindowManager().getDefaultDisplay().getMetrics(metric); screenH = metric.heightPixels; screenW = metric.widthPixels; //create view mView = new helloBox2dView(this); mView.setBackgroundColor(Color.WHITE); setContentView(mView); } @Override protected void onDestroy() { super.onDestroy(); mHandler.removeCallbacks(mRunnable); } class helloBox2dView extends View { private Canvas mCanvas; private Body mBody; public helloBox2dView(Context context) { super(context); /** Creating a World */ { Vec2 gravity = new Vec2(0f, 9.8f);// vector, the gravity direction, left and right, up and down mWorld = new World(gravity, false); } /** Creating a Ground Box */ { // Bodies are built using the following steps: // Step 1. Define a body with position, damping, etc. BodyDef groundBodyDef = new BodyDef(); groundBodyDef.position.set(0f, (screenH-screenW*3/4)/WORLD2PHONE_RATE); groundBodyDef.angle = (float) Math.asin(0.6); // Step 2. Use the world object to create the body. Body groundBody = mWorld.createBody(groundBodyDef); // Step 3. Define fixtures with a shape, friction, density, etc. PolygonShape groundBox = new PolygonShape(); groundBox.setAsBox(screenW*5/4/WORLD2PHONE_RATE, 1f/WORLD2PHONE_RATE); // Step 4. Create fixtures on the body. groundBody.createFixture(groundBox, 0.0f); //border BodyDef groundBodyDef2 = new BodyDef(); groundBodyDef2.position.set(screenW/WORLD2PHONE_RATE, 0f); groundBodyDef2.angle = (float) (0.5f*Math.PI); Body groundBody2 = mWorld.createBody(groundBodyDef2); PolygonShape groundBox2 = new PolygonShape(); groundBox2.setAsBox(screenH/WORLD2PHONE_RATE, 1f/WORLD2PHONE_RATE); groundBody2.createFixture(groundBox2, 0.0f); } /** Creating a Dynamic Body */ { BodyDef bodyDef = new BodyDef(); bodyDef.type = BodyType.DYNAMIC; bodyDef.position.set(100f/WORLD2PHONE_RATE, 0f); mBody = mWorld.createBody(bodyDef); CircleShape dynamicBox = new CircleShape(); dynamicBox.m_radius = 10/WORLD2PHONE_RATE; FixtureDef fixtureDef = new FixtureDef(); fixtureDef.shape = dynamicBox; fixtureDef.density = 500f; fixtureDef.friction = 0.5f; fixtureDef.restitution = 0.8f; mBody.createFixture(fixtureDef); } mHandler = new Handler(); mHandler.post(mRunnable); } @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); mCanvas = canvas; //draw ground drawGround(); //draw box Vec2 position = mBody.getPosition(); float angle = mBody.getAngle(); drawBox(position.x*WORLD2PHONE_RATE, position.y*WORLD2PHONE_RATE, angle); } public void drawBox(float x, float y, float angle){ Paint paint = new Paint(); paint.setAntiAlias(true); paint.setColor(Color.RED); mCanvas.drawCircle(x, y, 10, paint); mCanvas.drawLine(x, y, (float)(x+10*Math.cos(angle)), (float)(y+10*Math.sin(angle)), new Paint()); } public void drawGround(){ Paint paint = new Paint(); paint.setColor(Color.BLUE); paint.setStrokeWidth(10); mCanvas.drawLine(0, screenH-screenW*3/4, screenW, screenH, paint); mCanvas.drawLine(screenW, 0, screenW, screenH, paint); } }}