汽车引擎是汽车的心脏，其决定了汽车的性能和稳定性，是人们在购车时相当关注的。而游戏中的物理引擎就如汽车的引擎一样，占据了非常重要的位置。一款好的物理引擎可以非常真实地模拟现实世界，使得游戏更加逼真，提供更好的娱乐体验。
一、JBox2D简介
JBox2D是开源物理引擎Box2D的Java版本，可以直接用于Android。由于JBox2D的图形渲染使用的是Processing库，因此在Android平台上使用JBox2D时，图形渲染工作只能自行开发。该引擎能够根据开发人员设定的参数，如重力、密度、摩擦系数和弹性系数等，自动地进行2D刚体物理运动的全方位模拟。
二、示例
1.小球弹跳进阶版
在第1节中小球的下落、碰撞、弹起都是用代码来维护的，下面使用物理引擎来实现，并且加入了刚体之间的碰撞。
(1)常量类Constant
package box2d.bheap;
public class Constant {
public static final float RATE=10; //屏幕与现实世界的比例
public static final boolean DRAW_THREAD_FLAG=true; //绘制线程工作标识位
public static final float TIME_STEP=2.0f/60.0f; //模拟的频率
public static final int ITERA=10; //迭代次数
public static int SCREEN_WIDTH; //屏幕宽度
public static int SCREEN_HEIGHT; //屏幕高度
}(2)抽象类MyBody
该类为自定义的抽象类，是所有自定义刚体类的基类。由于JBox2D中的刚体类对象仅具有物理仿真计算的功能，并没有提供Android平台下的绘制功能，直接使用不是很方便。因此，这里定义了MyBody对自定义刚体的绘制及JBox2D物理仿真对象进行了封装。
package box2d.bheap;
import org.jbox2d.dynamics.Body;
import android.graphics.Canvas;
import android.graphics.Paint;
public abstract class MyBody {
Body body; //JBox2D物理引擎中的刚体
int color; //刚体的颜色
public abstract void drawSelf(Canvas canvas,Paint paint); //绘制的方法
}(3)圆形刚体类MyCircleColor
package box2d.bheap;
import org.jbox2d.dynamics.Body;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Paint.Style;
import static box2d.bheap.Constant.*; //静态导入
public class MyCircleColor extends MyBody {
float radius; //圆形半径
public MyCircleColor(Body body,float radius,int color) {
this.body=body;
this.radius=radius;
this.color=color;
}
@Override
public void drawSelf(Canvas canvas, Paint paint) {
paint.setColor(color&0xCFFFFFF); //设置颜色
float x=body.getPosition().x*RATE;
float y=body.getPosition().y*RATE;
canvas.drawCircle(x, y, radius, paint); //画圆
paint.setStyle(Style.STROKE); //设置空心无填充
paint.setStrokeWidth(1);
paint.setColor(color); //画边
canvas.drawCircle(x, y, radius, paint);
paint.reset(); //恢复画笔设置
}
}(4)矩形刚体类MyRectColor
package box2d.bheap;
import static box2d.bheap.Constant.RATE;
import org.jbox2d.dynamics.Body;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.graphics.Paint;
public class MyRectColor extends MyBody {
float halfWidth;//半宽
float halfHeight;//半高
public MyRectColor(Body body,float halfWidth,float halfHeight,int color)
{
this.body=body;
this.halfWidth=halfWidth;
this.halfHeight=halfHeight;
this.color=color;
}
public void drawSelf(Canvas canvas,Paint paint)
{
paint.setColor(color&0x8CFFFFFF);
float x=body.getPosition().x*RATE;
float y=body.getPosition().y*RATE;
float angle=body.getAngle();
canvas.save();
Matrix m1=new Matrix();
m1.setRotate((float)Math.toDegrees(angle),x, y);
canvas.setMatrix(m1);
canvas.drawRect(x-halfWidth, y-halfHeight, x+halfWidth, y+halfHeight, paint);
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeWidth(1);//设置线条宽度
paint.setColor(color);
canvas.drawRect(x-halfWidth, y-halfHeight, x+halfWidth, y+halfHeight, paint);
paint.reset();
canvas.restore();
}
}(5)生成刚体形状的工具类Box2DUtil
package box2d.bheap;
import static box2d.bheap.Constant.RATE;
import org.jbox2d.collision.CircleDef;
import org.jbox2d.collision.PolygonDef;
import org.jbox2d.dynamics.Body;
import org.jbox2d.dynamics.BodyDef;
import org.jbox2d.dynamics.World;
public class Box2DUtil {
/**
* 创建矩形物体（颜色）
*/
public static MyRectColor createBox (
float x,
float y,
float halfWidth,
float halfHeight,
boolean isStatic, //是否为静止的
World world,
int color
) {
PolygonDef shape=new PolygonDef(); //创建多边形描述对象
if(isStatic) {
shape.density=0;
} else {
shape.density=1.0f;
}
shape.friction=0.0f; //设置摩擦系数
shape.restitution=0.6f; //设置能量损失率
shape.setAsBox(halfWidth/RATE, halfHeight/RATE);
BodyDef bodyDef=new BodyDef(); //创建刚体描述对象
bodyDef.position.set(x/RATE,y/RATE); //设置位置
Body bodyTemp=world.createBody(bodyDef); //在世界中创建刚体
bodyTemp.createShape(shape); //指定刚体形状
bodyTemp.setMassFromShapes(); //设置物体质量
return new MyRectColor(bodyTemp, halfWidth, halfHeight, color);
}
/**
* 创建圆形物体（颜色）
*/
public static MyCircleColor createCircle (
float x,
float y,
float radius,
World world,
int color
) {
CircleDef shape=new CircleDef(); //创建圆描述对象
shape.density=2; //设置密度
shape.friction=0.0f; //设置摩擦系数
shape.restitution=0.95f; //设置能量损失率
shape.radius=radius/RATE;//设置半径
BodyDef bodyDef=new BodyDef(); //创建刚体描述对象
bodyDef.position.set(x/RATE,y/RATE); //设置位置
Body bodyTemp=world.createBody(bodyDef); //在世界中创建刚体
bodyTemp.createShape(shape); //指定刚体形状
bodyTemp.setMassFromShapes(); //设置物体质量
return new MyCircleColor(bodyTemp, radius, color);
}
}(6)颜色工具类ColorUtil
package box2d.bheap;
public class ColorUtil {
static int[][] result=
{
{56,225,254},
{41,246,239},
{34,244,197},
{44,241,161},
{65,239,106},
{45,238,59},
{73,244,51},
{99,233,58},
{129,243,34},
{142,245,44},
{187,243,32},
{232,250,28},
{242,230,46},
{248,196,51},
{244,125,31},
{247,88,46},
{249,70,40},
{249,70,40},
{248,48,48},
{250,30,30},
{252,15,15},
{255,0,0},
};
public static int getColor(int index)
{
int[] rgb=result[index%result.length];
int result=0xff000000;
result=result|(rgb[0]<<16);
result=result|(rgb[1]<<8);
result=result|(rgb[2]);
return result;
}
}
(7)主控制类MyBox2dActivity
package box2d.bheap;
import java.util.ArrayList;
import java.util.Random;
import org.jbox2d.collision.AABB;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.World;
import android.app.Activity;
import android.content.pm.ActivityInfo;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.view.Window;
import android.view.WindowManager;
import static box2d.bheap.Constant.*;
public class MyBox2dActivity extends Activity
{
AABB worldAABB;//创建 一个管理碰撞的世界
World world;
Random random=new Random();
//物体列表
ArrayList<MyBody> bl=new ArrayList<MyBody>();
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
//设置为全屏
requestWindowFeature(Window.FEATURE_NO_TITLE);
getWindow().setFlags(WindowManager.LayoutParams. FLAG_FULLSCREEN ,
WindowManager.LayoutParams. FLAG_FULLSCREEN);
//设置为横屏模式
setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
//获取屏幕尺寸
DisplayMetrics dm=new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(dm);
if(dm.widthPixels<dm.heightPixels)
{
SCREEN_WIDTH=dm.widthPixels;
SCREEN_HEIGHT=dm.heightPixels;
}
else
{
SCREEN_WIDTH=dm.heightPixels;
SCREEN_HEIGHT=dm.widthPixels;
}
worldAABB = new AABB();
//上下界，以屏幕的左上方为 原点，如果创建的刚体到达屏幕的边缘的话，会停止模拟
worldAABB.lowerBound.set(-100.0f,-100.0f);
worldAABB.upperBound.set(100.0f, 100.0f);//注意这里使用的是现实世界的单位
Vec2 gravity = new Vec2(0.0f,10.0f);
boolean doSleep = true;
//创建世界
world = new World(worldAABB, gravity, doSleep);
//创建4边
final int kd=40;//宽度或高度
MyRectColor mrc=Box2DUtil.createBox(kd/4, SCREEN_HEIGHT/2, kd/4, SCREEN_HEIGHT/2, true,world,0xFFe6e4FF);
bl.add(mrc);
mrc=Box2DUtil.createBox(SCREEN_WIDTH-kd/4, SCREEN_HEIGHT/2, kd/4, SCREEN_HEIGHT/2, true,world,0xFFe6e4FF);
bl.add(mrc);
mrc=Box2DUtil.createBox(SCREEN_WIDTH/2, kd/4, SCREEN_WIDTH/2, kd/4, true,world,0xFFe6e4FF);
bl.add(mrc);
mrc=Box2DUtil.createBox(SCREEN_WIDTH/2, SCREEN_HEIGHT-kd/4, SCREEN_WIDTH/2, kd/4, true,world,0xFFe6e4FF);
bl.add(mrc);
//创建砖块
//砖块间距 行间距为20 模块宽度为10 最多一行为9块
final int bs=20;
final int bw=(int)((SCREEN_WIDTH-2*kd-11*bs)/18);
//============================================================
for(int i=2;i<10;i++)
{
if((i%2)==0)
{
//左侧蓝木块
for(int j=0;j<9-i;j++)
{
mrc=Box2DUtil.createBox
(
kd/2+bs+bw/2+i*(kd+5)/2+j*(kd+5)+3,
SCREEN_HEIGHT+bw-i*(bw+kd)/2,
bw/2,
kd/2,
false,
world,
ColorUtil.getColor(Math.abs(random.nextInt()))
);
bl.add(mrc);
}
//右侧蓝木块
for(int j=0;j<9-i;j++)
{
mrc=Box2DUtil.createBox
(
3*kd/2+bs-bw/2+i*(kd+5)/2+j*(kd+5)-3,
SCREEN_HEIGHT+bw-i*(bw+kd)/2,
bw/2,
kd/2,
false,
world,
ColorUtil.getColor(Math.abs(random.nextInt()))
);
bl.add(mrc);
}
}
if((i%2)!=0)
{
for(int j=0;j<10-i;j++)
{
mrc=Box2DUtil.createBox
(
kd/2+bs+kd/2+(i-1)*(kd+5)/2+j*(kd+5),
SCREEN_HEIGHT-(kd-bw)/2-(i-1)*(bw+kd)/2,
kd/2,
bw/2,
false,
world,
ColorUtil.getColor(Math.abs(random.nextInt()))
);
bl.add(mrc);
}
}
}
mrc=Box2DUtil.createBox
(
5*kd+bs+20,
SCREEN_HEIGHT-(kd+bw)*4-kd,
bw/2,
kd/2,
false,
world,
ColorUtil.getColor(Math.abs(random.nextInt()))
);
bl.add(mrc);
//创建球
MyCircleColor ball=Box2DUtil.createCircle(SCREEN_WIDTH/2-24, kd, kd/2, world,ColorUtil.getColor(Math.abs(random.nextInt())));
bl.add(ball);
ball.body.setLinearVelocity(new Vec2(0,50));
GameView gv= new GameView(this);
setContentView(gv);
}
}
(8)显示界面类GameView
package box2d.bheap;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.view.SurfaceHolder;
import android.view.SurfaceHolder.Callback;
import android.view.SurfaceView;
public class GameView extends SurfaceView implements Callback{
MyBox2dActivity activity;
Paint paint;
DrawThread dt;
public GameView(MyBox2dActivity activity) {
super(activity);
this.activity=activity;
this.getHolder().addCallback(this);
paint =new Paint();
paint.setAntiAlias(true);
dt=new DrawThread(this);
dt.start();
}
public void onDraw(Canvas canvas) {
if(canvas==null) {
return ;
}
canvas.drawARGB(255, 255, 255, 255); //设置背景颜色白色
for (MyBody mb : activity.bl) {
mb.drawSelf(canvas, paint);
}
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
int height) {
}
@Override
public void surfaceCreated(SurfaceHolder holder) {
repaint();
}
@Override
public void surfaceDestroyed(SurfaceHolder holder) {
}
public void repaint() {
SurfaceHolder holder=this.getHolder();
Canvas canvas=holder.lockCanvas();
try {
synchronized(holder){
onDraw(canvas);
}
} catch(Exception e){
e.printStackTrace();
} finally {
if(canvas!=null) {
holder.unlockCanvasAndPost(canvas);
}
}
}
}
(9)绘制线程类DrawThread
package box2d.bheap;
import static box2d.bheap.Constant.*;
//绘制线程
public class DrawThread extends Thread
{
GameView gv;
public DrawThread(GameView gv)
{
this.gv=gv;
}
@Override
public void run()
{
while(DRAW_THREAD_FLAG)
{
gv.activity.world.step(TIME_STEP, ITERA);//开始模拟
gv.repaint();
try
{
Thread.sleep(20);
} catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
}失败了，留坑待改进
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