书名：代码本色：用编程模拟自然系统
作者：Daniel Shiffman
译者：周晗彬
ISBN：978-7-115-36947-5
目录

1.10　加速度的交互

一、示例代码1-11

  例子和引力还有个关键的不同点，引力的大小（加速度的大小）和距离成反比，也就是说，物体离鼠标越近，加速度越大。
  示例代码1-11　一组同时朝着鼠标加速的运动物体

Mover[] movers = new Mover[20];

void setup() {
  size(640,360);
  for (int i = 0; i < movers.length; i++) {
    movers[i] = new Mover(); 
  }
}

void draw() {
  
  background(255);

  for (int i = 0; i < movers.length; i++) {
    movers[i].update();
    movers[i].display(); 
  }
}
// mover.pde
class Mover {

  // The Mover tracks position, velocity, and acceleration 
  PVector position;
  PVector velocity;
  PVector acceleration;
  // The Mover's maximum speed
  float topspeed;

  Mover() {
    // Start in the center
    position = new PVector(random(width),random(height));
    velocity = new PVector(0,0);
    topspeed = 5;
  }

  void update() {
    
    // Compute a vector that points from position to mouse
    PVector mouse = new PVector(mouseX,mouseY);
    acceleration = PVector.sub(mouse,position);
    // Set magnitude of acceleration
    //acceleration.setMag(0.2);
    acceleration.normalize();
    acceleration.mult(0.2);
    
    // Velocity changes according to acceleration
    velocity.add(acceleration);
    // Limit the velocity by topspeed
    velocity.limit(topspeed);
    // position changes by velocity
    position.add(velocity);
  }

  void display() {
    stroke(0);
    strokeWeight(2);
    fill(127,200);
    ellipse(position.x,position.y,48,48);
  }

}