[Processing] 弹球

 PVector localPos = new PVector(0,0);//起始位置
 PVector velocity;//速度方向
 float speed = 20;//速度大小

 void setup()
 {
   size(800,600);
   velocity = new PVector(random(-1,1),random(-1,1));
   velocity.normalize();
   velocity.mult(speed);
 }

 void draw()
 {
   background(#CCCCCC);
   translate(width/2,height/2);
   stroke(#CBCB00);
   strokeWeight(8);
   point(0,0);//绘制中心点

   if(localPos.x>=width/2.0-5 || localPos.x<=-width/2.0+5)//超出范围则反转速度方向
   {
     velocity.x = -velocity.x;
   }
   if(localPos.y>=height/2.0-5 || localPos.y<=-height/2.0+5)
   {
     velocity.y = -velocity.y;
   }

   localPos.add(velocity);

   stroke(0,0,0);
   strokeWeight(5);
   ellipse(localPos.x,localPos.y,10,10);
 }