代码片段：
(function() {
'use strict'
var c = document.getElementById('c');
var ctx = c.getContext('2d');
var w = c.width = window.innerWidth;
var h = c.height = window.innerHeight;
var cx = w / 2;
var cy = h / 2;
var Box = function(x, y, vx, color) {
this.color
