html5 canvas炫酷旋转银河系星空背景特效

这是一款html5 canvas炫酷旋转银河系星空背景特效。该特效通过canvas来绘制银河系星盘，并制作星系旋转的效果，非常炫酷。

使用方法

HTML结构

该旋转银河系星空背景特效的HTML结果只需要一个<canvas>元素即可。

<canvas id="canvas"></canvas>  
                

JavaScript

然后通过下面的js代码来在canvas上绘制银河系，以及制作动画效果。

"use strict";

var canvas = document.getElementById('canvas'),
  ctx = canvas.getContext('2d'),
  w = canvas.width = window.innerWidth,
  h = canvas.height = window.innerHeight,
    
  hue = 217,
  stars = [],
  count = 0,
  maxStars = 1400;

// Cache gradient
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, '#fff');
    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) {
  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;
}

var Star = function() {

  this.orbitRadius = random(w / 2 - 50);
  this.radius = random(100, this.orbitRadius) / 10;
  this.orbitX = w / 2;
  this.orbitY = h / 2;
  this.timePassed = random(0, maxStars);
  this.speed = random(this.orbitRadius) / 100000;
  this.alpha = random(2, 10) / 10;

  count++;
  stars[count] = this;
}

Star.prototype.draw = function() {
  var x = Math.sin(this.timePassed + 1) * this.orbitRadius + this.orbitX,
      y = Math.cos(this.timePassed) * this.orbitRadius/2 + this.orbitY,
      twinkle = random(10);

  if (twinkle === 1 && this.alpha > 0) {
    this.alpha -= 0.05;
  } else if (twinkle === 2 && this.alpha < 1) {
    this.alpha += 0.05;
  }

  ctx.globalAlpha = this.alpha;
    ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
  this.timePassed += this.speed;
}

for (var i = 0; i < maxStars; i++) {
  new Star();
}

function animation() {
    ctx.globalCompositeOperation = 'source-over';
    ctx.globalAlpha = 0.8;
    ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 1)';
    ctx.fillRect(0, 0, w, h)
  
  ctx.globalCompositeOperation = 'lighter';
  for (var i = 1, l = stars.length; i < l; i++) {
    stars[i].draw();
  };  
  
  window.requestAnimationFrame(animation);
}

animation();