js 实现星空背景

function initLoginBg() {
   
      var windowWidth = document.documentElement.clientWidth || document.body.clientWidth;
      var windowHeight = document.documentElement.clientHeight || document.body.clientHeight;
      // var windowWidth = window.clientWidth;
      // var windowHeight = window.clientHeight;
      var canvas = document.getElementById('canvas'),
        ctx = canvas.getContext('2d'),
        w = canvas.width = windowWidth,
        h = canvas.height = windowHeight,

        hue = 217,
        stars= [],
        count = 0,
        maxStars = 1500;//设置星星数量

      var canvas2 = document.createElement('canvas'),
        ctx2 = canvas2.getContext('2d');
      canvas2.width = 100;
      canvas2.height = 100;
      var half = canvas2.width / 2,
        gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
      gradient2.addColorStop(0.025, '#CCC');
      gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
      gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
      gradient2.addColorStop(1, 'transparent');

      ctx2.fillStyle = gradient2;
      ctx2.beginPath();
      ctx2.arc(half, half, half, 0, Math.PI * 2);
      ctx2.fill();

      // End cache

      function random(min, max = 0) {
   
        if (arguments.length < 2) {
   
          max = min;
          min = 0;
        }

        if (min > max) {
   
          var hold = max;
          max = min;
          min = hold;
        }

        return Math.floor(Math.random() * (max - min + 1)) + min;
      }